Evolving Trends in Marine Algae Populations, seagrasses and other intertidal organisms:
Signs and Symptoms of a mounting Nitrogen Deficit in the Ocean?
by Debbie MacKenzie July, 2001
"Things are so strictly related, that according to the skill of the eye,
from any one object the parts and properties of any other may be predicted."
-- Ralph Waldo Emerson (1803-1882)
1.INTRODUCTION: Many signs today, in a wide range of marine organisms, point to dropping levels of fixed nitrogen availability in open ocean seawater. This DEMANDS investigation because the marine nitrogen level is critically important to the health of the global ecosystem. For example, stability in the amount of bioavailable nitrogen in the planetary ocean is one essential factor in the maintenance of the ocean-atmosphere carbon balance (atmospheric CO2 level) (Shaffer, 1993). In this paper, the ranges and conditions of intertidal organisms are suggested as useful indices of seawater nitrogen levels. A close look at individual species of seaweeds and intertidal organisms, repeatedly reveals the specific signs of nitrogen deficiency in their growth patterns and pigmentation changes. It appears that neither the fishing industry, nor the marine scientists that monitor the fish stocks, truly appreciates the dangerous global ecological destabilization that will result from the serious depletion of the marine biota. The fate of marine life will have impacts on human life that extend far beyond the fishing and tourism industries. 2.Nitrogen depletion in the sea- how to justify a hypothesis that seems directly contrary to current beliefs that the sea is "overloaded" with nitrogen due to increased terrestrial runoff. 3.The predicted pattern of changes induced in marine algae by declining nitrogen availability. 4.Changing patterns in the makeup of the global community of marine macroalgae and seagrasses - how do today's trends fit with changes that would be predicted to result from lowered nitrogen availability? 5.Changes noted in a community of intertidal organisms in one northern temperate zone over the last half century. 6.Comparison of records of intertidal organisms at Peggy's Cove between 1948 and 2001. 7.The decline of the barnacle belt on the rocky, exposed North West Atlantic shoreline of Nova Scotia. 8.The decline of rockweed. 9.The decline of Irish moss -- dramatic color changes in this red algae species tell a tale of declining availability of nitrogen-fertilizer in seawater. 10.THE RISE OF THE NITROGEN-FIXERS - the Blue-green algae, or cyanobacteria, why these organisms are becoming increasingly prominent in marine environments today...including discussion of these points:
- Harmful "algae" blooms. -Toxic cyanobacterial blooms in polluted estuaries- what is the nature of the cause and effect? Nutrient enhancement or nutrient depletion? (Can these conditions paradoxically coexist?) The relationship between "fishing/biomass removal," the resultant declining overall abundance of members of the living web, and the depletion of nutrients in surface waters during calm conditions...the lowering of the "toxic trigger" threshold of waterways and the concept of the complete marine life assemblage as a nutrient buffering system.
-The increasing prevalence of cyanobacterial forms in diverse marine ecosystems, from coral reefs to temperate coastlines and open ocean.
-A summary of key points regarding the nature of blue-green algae.
-The significance of "greeness" in the ocean, and the futility of attempts to use Chlorophyll a as a proxy indicator of seawater nutrient content. 11.Conclusions. 12.References.
(click on photo to see
large, high resolution view. File size: 972KB)
Above: Rocky exposed shoreline at Peggy's Cove, Nova Scotia. The blackish upper band of exposed seaweed is the rockweed belt. Below that is the Irish moss (chondrus crispus) belt, which shows a remarkable range of color variation as the picture is scanned from left to right. The emergence of this contrasting color pattern in seaweed here is a recent development. The color variation in the Irish moss clearly does not reflect a seasonal pattern in this algae, nor the effects of sunlight or temperature, nor is the seawater in this particular locality affected by polluted runoff. Analogous to "bleaching" such as that occurring in tropical corals, the color change in the Irish moss reflects the variability in the availability of nitrogen-fertilizer, which in this case has been induced by variable water flow. Over recent decades, however, Irish moss in this area as a whole has demonstrated the same gradual shift from a dark reddish or purplish brown color to a light yellowish-green one. This observation adds to the evidence for the gradual
decline in nitrogen availability in the open ocean seawater.
The ocean is in very deep trouble, worrisome indicators are appearing everywhere now, yet a clear explanation of exactly what is going wrong still eludes us. A careful examination of the plankton, the seaweed and the intertidal animal life will help to establish the correct diagnosis. Marked shifts, mostly downward, are occurring in marine fish, mammal and seabird populations worldwide, and these changes have been accompanied by a series of alterations in the marine algae community and small "unexploited" creatures as well. Most scientific and media attention, regarding changes in marine algae, has been turned to the recent increasing trend in the occurrence of
toxic algal blooms, as well as the proliferation of a couple of invasive green seaweeds, in diverse areas of the globe. Caulerpa taxifolia and
Codium fragile are perhaps the most prominent green "invasive" seaweeds. Multiple species are involved in the toxic algal blooms, but a common theme can be seen to
underlie all these changes: lower than usual levels of nitrogen are available to algae in the seawater, and this deals an advantage to today’s emerging dominant strains of marine algae. This paper considers the possibility that declining nitrogen
availability in seawater may be contributing to "forcing" these changing trends in algal growth. Caulerpa and Codium species both have unusual strategies for obtaining nitrogen in nutrient depleted waters, which gives them a clear competitive advantage over the previously dominant brown seaweeds. The microscopic toxin-producing algae that are involved in the toxic blooms, for example Gymnodinium species, frequently are found to belong to a class of organisms (cyanobacteria) that grows to advantage in LOW ambient nitrogen conditions due to their capacity for making use of atmospheric nitrogen (nitrogen fixation).
2. Nitrogen Deficiency in the Sea?
The hypothesis that nitrogen shortage has developed in the sea, however, goes directly against most current thinking on the topic. Due to the fact that humans are increasingly flushing great quantities of nitrogen (fertilizer and sewage) into the sea via rivers, the commonly accepted conclusion is that an overload of nitrogen, rather than a shortage, is at the root of the problem. Therefore, how can this author arrive at such a contrary conclusion?
The most important observation is that coastal DENITRIFICATION ultimately removes the vast majority of the nitrogen waste that we flush into the sea. This is so effective that it effectively "decouples" the terrestrial and marine nitrogen cycles. The nitrogen in our sewage does NOT enter the living marine food web, rather it ends up in the bottom sediment or returned to the atmosphere.
"Although nutrient loading has resulted in coastal eutrophication on a global scale, denitrification presently removes virtually all land-derived nitrogen before it can reach the open ocean. Coastal denitrification thus effectively decouples the terrestrial and oceanic nitrogen cycles." (Falkowski, 2000, Vitousek, 1999)
Therefore the oceanic system, which is generally N-limited, must rely on its own devices (nitrogen fixation) to replace nitrogen lost through human fishing. The critical role of the living marine biota in absorbing CO2 from the atmosphere and thereby stabilizing the atmospheric concentration, is fairly well appreciated by climate scientists (e.g. Shaffer, 1993, Trabalka,et al., 1985). Current carbon-cycle models assume that marine biological "primary production" has remained constant since preindustrial times, but all evidence points towards the conclusion that this is EXTREMELY unlikely to be true. ("Primary production" refers to the uptake of CO2 during photosynthesis, a process that relies directly on the fertilizing effect of nitrogen). "IF" fishing ultimately caused nitrogen-depletion in the sea, that nitrogen-depletion would inevitably result in a slowing of the rate of primary production...and therefore fishing would be directly implicated in
rising atmospheric CO2 levels.
These aspects of marine ecology have been argued in detail elsewhere on this website, it has been shown how human fishing/biomass removal has
likely interrupted the normal nitrogen cycling in the ocean ( The Marine Nutrient Cycle). The close temporal parallel between the history of the expansion of the fishing industry and rising atmospheric CO2 has also been described ( Effects of Fishing and Whaling on Atmospheric CO2). These points emphasize the critical importance of discovering/understanding whether or not there has been a decrease in the "inventory of bioavailable nitrogen" in the ocean. The purpose of the current paper is not to repeat those arguments, but rather to demonstrate how consistent today’s changing trends in
marine algae and intertidal lifeforms are with changes that would be predicted "IF" the ocean experienced a lowering of available nitrogen. The realization that available nitrogen is almost certainly dropping at sea allows us to draw together many seemingly unrelated threads in the story...and a close examination of the smaller details helps to bring the larger picture into clearer focus.
Considered as indicators of trends in nitrogen availability in seawater will be:
1. Changing patterns in the makeup of the global marine macroalgae (seaweed) community.
2. The global declining trend in sea grasses: eelgrass and salt marshes.
3. The decline of the barnacle belt on rocky exposed NW Atlantic shores in Nova Scotia.
4. Changing growth patterns and pigmentation in seaweeds along this same Nova Scotian coast.
5. The global increasing occurrence of harmful, often toxic, marine phytoplankton blooms.
Following is a brief global overview of the most remarkable changes in seaweed populations, plus a closer examination of the changes that have occurred in one northern temperate zone.
Today’s changing trends in macroalgae are compared to the changes that would be predicted to occur if the availability of N to algae growing in ocean waters was declining. Declines in oceanic levels of zooplankton (and also fish, and invertebrates) offer a clue as to why seaweeds may be experiencing nitrogen shortage. All of these living organisms excrete NH4, ammonia, which is the form of nitrogen fertilizer most readily assimilated by marine algae. Beyond dependence on bottom-to-top mixing patterns to make nutrients available in surface waters, the very fact of the existence of marine animals ensures and moderates the supply of fertilizer to these plants. Marine algae therefore depends on its normal coexistence with marine animal life. The affinity of many smaller organisms to living in seaweed habitat ensures the physical proximity of the fertilizer-excreters to the marine plants that thrive on it; this is one of many expressions of the natural interdependence of plants and animals.
Nitrogen is an essential fertilizer for plants, including all varieties of marine algae. If seawater gradually provided less usable N to these organisms, signs of increasing N-limitation or N-deficiency would be expected to occur in algae populations. Predictable shifts would be these:
1. Species that feed more heavily, requiring greater amounts of N and perhaps growing to larger sizes as individuals, may experience declines first. It appears that kelp and Irish moss have fairly high N-requirements. (Deduced from their relatively high minimum percentage of dry weight that is nitrogen -- Lobban and Harrison, 1994) Eelgrass beds, also "highly productive systems," require substantial amounts of nitrogen fertilizer to maintain their productivity. Kelps, Irish moss (Chondrus crispus) and eelgrass are in overall decline…seemingly due to often vague and mysterious "environmental changes" in the sea. A key to understanding the nature of the "environmental change" is the realization that nitrogen levels are dropping in the seawater.
(See photo galleries of kelp and
2. Perennials may feel the impact more so than annuals if the time and opportunity for storing food is lessened, and the period during which they are obliged to live off their stores is increased. An overall declining trend in nitrogen availability superimposed on normal seasonal fluctuations would be expected to increase the fasting period and thereby increase nutritional stress. (This is analogous to what the gray whales and tropical corals are facing today, for instance.) The
stress experienced during these natural lean times could also be amplified by an increasing warming trend if it results in more pronounced and prolonged thermal stratification and depletion of surface water nutrients.
3. Seaweeds and other intertidal organisms, such as barnacles, may contract their ranges, limiting themselves to a narrower zone in which their needs can be met in the face of less N-fertilizer (or food) in the seawater. Those individual organisms living at their other limiting extremes (e.g. light availability, number of hours exposed to air) may decline first under added nutritional stress, leaving the colony with a contracted vertical range. This change would contrast with the expected pattern if these organisms declined due to increased grazing pressure, for instance.
4. Plants with higher surface area/volume ratios will be naturally better equipped to derive the nitrogen that they need from seawater that contains less than it did previously. Thin, branching, filamentous shapes may increase in abundance. Conversely, plants with thicker flesh, and lower SA:V ratios may decrease. Increased "hair formation" on seaweeds grown in nitrogen-deficient medium has been noted in scientific studies (Lobban and Harrison, 1994). The variety and abundance of fluffy-looking, filamentous growth forms in the sea appears to be increasing overall.
(See gallery of 'fuzzy' seaweeds.)
5. Plants may show physical changes associated with nitrogen deficiency, specifically decreased pigment content and also possibly stunted or deformed growth patterns would be predicted (Lobban and Harrison, 1994). Plants may be observed to alter their morphology in ways which increase their SA:V ratio. Brown and red seaweeds would be increasingly expected to show a green color as their pigment content declined, since the green pigment, chlorophyll a, is the essential one for all photosynthesis. The other pigments that give algae colors other than green are secondary, and under nitrogen deprivation the secondary pigments will be lost first. (Lobban and Harrison, 1994, Lobban and Wynne, 1981).
(See gallery of rockweed photos.)
6. Seaplants living in closer proximity to significant sources of terrestrial N, near the mouths of rivers or sewage outfalls, specifically, may show more resistance to any changing trend that is being forced by N-depletion in the open sea. Also, coastal marine organisms living in areas where water flow patterns make nutrients relatively more available, plants and animals in these locations will thrive longer and appear better nourished than those in areas where flow patterns are less advantageous. This is a known pattern in the distribution of marine life, and the typical zonation and ranges that they occupy…but
negative shifts in colors and abundance will occur later in these naturally enriched areas. Patterns of pigmentation change will correlate with the relative availability of nitrogen to the plants -- this is well illustrated today in the Irish moss at
Peggy’s Cove, N.S.
Measurable biochemical indicators will help to verify or refute this author’s deduction that marine algae are now growing in relatively nitrogen depleted medium. Studies could reasonably be done now to compare the biological indices of nitrogen-nutritional status of seaplants today with earlier measurements that were done on these species decades ago.
Beyond the scope and abilities of this author at this time, these comparative studies would be very useful. (Examples would be C/N ratios, protein content, and perhaps DNA/RNA ratios on some organisms.)
7. Plants having alternate strategies for obtaining N (methods beyond direct absorption from the seawater into the flesh of the plant) - these will possibly increase in abundance. This includes plants with symbiotic associations with cyanobacteria (like
Codium fragile, and some species of Caulerpa) and plants with true roots, siphonophores that can draw up nutrients from the bottom substrate (like
Caulerpa taxifolia). Similarly, algae that can utilize a wider range of N-species will be at a competitive advantage (e.g.
Codium fragile, which, unlike many other algae, can uptake NO3-, NO2- or NH4+, making it a more versatile N-scavenger. (Lobban and Wynne, 1981)).
8. Algae naturally adapted to, or requiring for their growth, low N conditions (oligotrophic water) will compete more successfully for space. Those that thrive in oligotrophic tropical and sub-tropical waters may expand their ranges to higher latitudes as nutrient levels decline in these zones and fall more into line with what the tropical organisms are accustomed to. Temperature tolerance will undoubtedly limit the extent of their range expansion, but those tropical types with greater cold-tolerances will be expected to increase in abundance outside of the tropics. Specifically, changes in the relative abundance of various types of coralline algae might be observed. Coralline algae species vary in their reactions to changes in nutrient availability, some will be stimulated and some will be inhibited by nutrient enhancement of water (Johansen, 1981).
9. Cyanobacterial colonies, free floating types and benthic mats, will naturally become increasingly abundant as seawater declines in N-content. Very low N-availability is a requirement for the growth of these organisms. (Carpenter et al, 1991, Stal and Caumette, 1993, Lassus et al., 1995, Desikachary, 1972). As benthic habitats are abandoned by other organisms, cyanobacteria can be expected to colonize these "new," empty spaces. Cyanobacteria are the only class of marine organisms that can "fix" nitrogen. Therefore, under conditions of increasing nitrogen depletion, they will be increasingly be called upon to "fix" the nitrogen-starved condition of the system as a whole. An increase in the overall abundance of marine cyanobacteria is therefore predicted.
CHANGING PATTERNS IN THE MAKEUP OF THE GLOBAL MACROALGAE (SEAWEED) AND SEAGRASS COMMUNITIES
Global reports of changing seaweed communities reveal a few broad trends. And the changing trends are roughly consistent with the foregoing predictions. The overall impression received is not one of a shift toward a more nutritionally rich seaweed complex....which might have been expected
due to "overenrichment" of coastal waters with nutrients, and by "species replacement theory" since the biomass of fish in the sea has been greatly reduced. Rather, the shift seems to be in the opposite direction, towards a relatively nutritionally poor seaweed community. (Far from being very nutritious, many of the algal species that are increasing today are actually toxic to many consumers.) If there is a "regime shift" in marine life, it appears to be a broad shift toward those species, both plant and animal, that can make do with less. Smaller organisms, thinner organisms...this appears to be a common theme in marine life today...widely seen in marine mammals and fish, and even in seaweeds and plankton.
- Seagrass beds are in general decline. Unrelated to marine algae, seagrasses are actually flowering plants more closely related to the terrestrial versions. In this sense they are in a class by themselves in the ocean. Seagrasses have undergone a general decline in many places worldwide, and restoration projects have been surprisingly difficult and unsuccessful (Larkum, et al., 1989). The reason for the decline in seagrass remains unclear, but their disappearance has not generally been due to displacement by any other plant or algae in most cases. Exploring the places where the seagrass once grew, now in many cases reveals only a bare muddy or sandy bottom.
- Kelp is in decline. Atlantic coast, Pacific coast, Australian coast...kelp beds are seemingly in decline everywhere. Recognized as providers of critical habitat for a large range of marine creatures, the disappearance of kelp forests is a major cause of concern. The causes are not always clear, sometimes overharvesting or an overabundance of grazers is blamed - i.e. sea urchins - but this may or may not be the whole story. For a plant that can apparently grow many inches in a single day when it is healthy (giant kelp), it is hard to imagine that urchins could ever physically accumulate in numbers great enough to keep the kelp plants down. Kelp forests off the coast of California were reported to have deteriorated rapidly due to "nitrogen starvation" during the 1982-83 El Nino event (Lobban and Harrison, 1994, p 208). The giant kelp beds off Tasmania are reportedly in very steep decline, and casual mention is made in an Australian online news document (http://www.abc.net.au/oceans/jewel/kelp) that one possible cause in that case could be "the fall of dissolved nutrient levels in the ocean waters off Tasmania." The cause of the decline in nutrients in seawater is not discussed, but pressing this point usually finds it attributed to changing ocean currents, rather than physical removal
of nutrients by fishing. Today’s interpretation of declining trends in ocean nutrient availability seems always to blame it on changes in temperature and water movement patterns....and never on fishing. But fishing physically removes nitrogen/protein from the sea, a portion of the marine N-inventory that would otherwise have been naturally recycled by sea life…this is removed each time we transport a boatload of fish to shore. In the overall scheme, has this "protein mining" activity of ours ultimately contributed to the detriment of the
marine nitrogen balancing act? The growth of kelp directly depends upon the availability of ammonia in the water.....all fish, marine invertebrates and zooplankton excrete ammonia....humans (we must admit) took away most of the fish, and now the kelp doesn’t grow very well...somehow the N-inventory available for recycling has gradually been lowered…could the explanation possibly be as simple as that?
(See gallery of kelp photos.)
- Irish moss is in decline. Once the target of a substantial commercial harvesting effort, the abundance of Irish moss on the Atlantic Canadian coast is now significantly less than it was decades ago. For some reason, the beds of Irish moss are smaller and thinner than they were in the past, never fully recovering from the harvest, perhaps, although that has essentially been halted for about 20 years. Color changes have recently been evident in Irish moss. The plants living at the upper vertical limit of their natural range were always of a lighter hue than the deeper specimens, but recently large numbers have begun to appear green, greenish-yellow, then white, and plant growth in this region is much shorter than in the past. These changes are precisely what would be expected in Irish moss "if" nitrogen fertilizer was gradually withdrawn from the seawater in which it lives. It is interesting to note that an Irish moss aquaculture project has been operating in Nova Scotia for the last couple of decades, and that the operators routinely add commercial fertilizer to the seawater to obtain good growth of Irish moss. Unenriched seawater, it seems, produces only poor growth.
(See gallery of Irish moss photos.)
- Rockweed is in decline. There are not many long-term abundance records on rockweed, but there are a few. A study conducted in Prince William Sound between 1989 and 1996 reported a decline in rockweed cover and a simultaneous decline in the major algal grazers of rockweed. The focus of the study was the effects of an oil spill in the area, but of most interest to this author was the observations on the unoiled control study area, which revealed the unrelated background decline that occurred in both algae and their grazers. (http://response.restoration.noaa.gov/bat/transitn.html
) (See gallery of rockweed photos.)
Rockweed increasingly has heavy growth of filamentous brown epiphytes. These growths have been noted for a long time, but the prevalence and density of this stuff seems to be increasing. This could represent an early indication of trouble for rockeed. (A review of the sequence of events preceding the demise of many eelgrass beds reveals that they were heavily colonized by similar filamentous epiphytes (along with bryozoans and hydroids) before their demise. For example, see the records from Australia in the book Biology of Seagrasses, edited by Larkum, McComb and Shepherd, 1989. "Epiphytes" were considered to be the ultimate cause of the decline of the eelgrass.)
- Codium fragile, (pictured at right, in a photo taken July 28, 2001, at Sand Cove beach), has greatly expanded its range over the last century, most intensively over the last few decades. It has been an eye-catching and rapidly expanding addition to the seaweeds seen along North Atlantic coastlines, both eastern and western sides. A rare feature among the larger, fleshy seaweeds,
Codium hosts a form of cyanobacteria within its tissue that fixes atmospheric nitrogen and thereby partially provides the N needed by the algae. This feature gives
Codium a possible advantage over the other larger perennial seaweeds, such as kelp and rockweed, in a situation of N-shortage. As mentioned previously,
Codium fragile is also a more efficient N-scavenger from seawater, since it can use more different N-species than can most other seaweeds. But scavenging success is limited by the total amount of N in the seawater, and the facility for nitrogen-fixation by this plant can only supply about 7% of its total nitrogen requirement (Lobban and Harrison, 1994). Codium fragile has one final strategy to obtain the needed fertilizer, however.
"Nutrient shortage is known to cause hair formation in several species, including...Codium fragile..." - Lobban and Harrison, 1994, p 61
In the few years preceding 2001, specimens of
Codium fragile were more numerous on Sand Cove beach than they have been this year. The plant also had a noticeably deeper green color in previous years, and the "hair formation" was not noted until this summer. The photo clearly shows fine hair growth on this specimen of
Codium fragile, creating a fuzzy halo effect around each branch. The nitrogen deficiency in Atlantic ocean seawater appears to be growing more severe, judging by the changes observed in many seaweeds, including
Codium fragile, which may be experiencing a loss of what were until very recently ideal growth conditions.
- Caulerpa taxifolia, pictured at right in a photo from NOAA, has invaded and taken over large areas of coastal seabed in the Mediterranean, and has spread to areas of the North East Atlantic coast, Australian waters, and, recently, has been found off California. Apparently a mutant strain, of what was originally a tropical algae, was developed in the aquarium trade, and it has proven to be highly adept at colonizing today’s sub-tropical ocean. With a proven ability to thrive in nutrient-enriched and nutrient-depleted waters both, the mutant
Caulerpa poses a serious threat to biodiversity in the areas that it invades. Its ability to thrive in nutrient-depleted waters may prove to be
Caulerpa’s strongest asset in today’s marine environment. A "siphonophore,"
Caulerpa draws nutrients up from within the bottom sediment, in the style of land plants, rather than the usual macroalgae that is "rooted" only by a holdfast and must absorb all nutrients directly from the surrounding seawater. Caulerpa taxifolia may have yet another trump card in the competition for nitrogen, at least one species has been shown to host a nitrogen-fixing strain of cyanobacteria in its tissue (Williams, 2000).
- Lyngbya is increasing in abundance. This is a form of cyanobacteria that colonizes exposed surfaces, growth appearing in various colors of fine filaments. Lyngbya is well known as a nitrogen-fixer, the growth of which is best stimulated in conditions of low ambient bioavailable nitrogen concurrent with the ready availability of phosphorus. An increasing presence of Lyngbya in benthic communities has been noted in diverse geographic locations, from Hawaii to Australia (coments posted to coral-list June, 2001). Polluted coastal waters may stimulate increased growth of Lyngbya since the marine system has a natural strategy to remove excess dissolved nitrogen (denitrification) but not excess phosphorus. This alters the N:P ratio to favor higher growth of cyanobacteria, which are generally phosphorus-limited. Low N, high P, presents ideal growing conditions for blue-green algae.
- Cladophora is increasing. (Photo at right borrowed from http://www.turtles.org, an excellent website devoted to conservation of green sea turtles. Link to page with more pictures of cladophora and Lyngbya: http://www.turtles.org/01week2.htm) Cladophora appears as a fine, filamentous green algae affecting beaches off Mauii, in a coastal area affected by nutrient-rich effluent. (Again, this possibly reflects abnormally elevated phosphorus in the context of today's lowered oceanic nitrogen supply.)
(See gallery of 'fuzzy' seaweeds in Nova
- Changes in tropical reefs are occurring globally, the main features are the bleaching and death of corals and their "replacement" by cyanobacteria and algae -- this general pattern is also consistent with changes that would be predicted in these areas "if" the availability of nitrogen was gradually being lowered in ocean water. This has been described in detail elsewhere on this website: ""Mass Coral Bleaching - signs and symptoms of starvation in the tropics?"
CHANGES NOTED IN THE COMMUNITY OF INTERTIDAL ORGANISMS IN A NORTHERN TEMPERATE ZONE, OVER THE LAST HALF CENTURY
Latitude: approx. 44 degrees north Longitude: approx. 63 degrees west
These co-ordinates identify a point on the rocky coast of Atlantic Canada where this author spent her childhood, a very small fishing village with no major sources of pollution in the vicinity. This area experiences open exposure to the North West Atlantic ocean. The marine life in this intertidal zone is the group that I know best. A recent survey reveals some large changes, when compared with memories from the 1960s and 1970s. Even "serious" science these days, resorts to anecdotal information from human memory when rigorous scientific records are simply not available for comparison. It is a valid approach when it is all one has to work with. My comments on the relative abundance of species are therefore necessarily subjective.
In the 1960s-1970s the familiar seaweeds were predominantly rockweeds, which were in shades of brown and greenish brown to golden brown. Kelp and Irish moss grew in rocky exposed areas with more wave action. These two seaweeds were in shades of deep brown, the kelp showing a yellow-orange tint and the Irish moss tending from deep reddish brown to dark purple. The one green growth was the beds of eelgrass that grew in shallow sheltered areas just below the low tide margin. Periwinkles, blue mussels, barnacles, dogwhelks, anemones and starfish were abundant. The starfish was an orange variety that grew to quite a large size. Also common were crabs in a wide range of sizes, along with small fish, minnows, sculpins, and small flatfish. Sandy bottom was riddled with clam holes, and it was very easy to dig out the soft shelled white clams.
Today, the first change that one notices in the intertidal zone is the presence of much more "color" than was there in the past. For example, in areas that were traditionally covered in various shades of brown, there are now many more "greens" visible than there were a few decades ago, this despite the disappearance of the traditional green plant, the eelgrass. The green colors showing in the intertidal zone today are from cyanobacterial growths, new species like
Codium fragile, free-floating green filamentous growths, and abundant examples of
Ulva and Enteromorpha, two paper thin and delicate, bright green
annuals. Rockweed and Irish moss are also showing shades of green to various
extents. Also, bright yellow and golden tones are increasingly appearing among
the "brown" seaweeds. A particularly eye-catching change in some brown seaweeds
has been the recent appearance of RED patches.
(edit note Oct, 2002 - since this document was originally heavily overloaded
with pictures, most of them have now been removed and placed in the
Seaweed Photo Galleries.)
The most interesting observations can be made by comparing historical records from a single location. This sort of detailed record can be difficult to find, but it does exist for at least one popular and easily accessible bit of rocky coastline, one that is exposed to the open North Atlantic ocean: Peggy’s Cove, Nova Scotia. Most tourists and artists have focused on recording the crashing waves, but in 1948 marine scientist T. A. Stephenson visited Peggy’s Cove and meticulously recorded the intertidal life that he observed there at the time.Stephenson’s book "Life Between Tidemarks on Rocky Shores" (1972) describes his life work, elucidating the themes and patterns in organisms living on rocky shores worldwide. Stephenson illustrated the intertidal growth at Peggy’s Cove in 1948 in a painting (possibly due to the limitation of black and white photography in recording the subtle variations in the shades of brown?) The large granite slope that he focused on, on which he documented the prominent barnacle belt above the rockweed, will be referred to in this paper as "Stephenson’s rock." Comparing this painting to my recent photographs, taken 53 years later, reveals his close attention to detail in recording the morphology of the rock. Therefore one can probably conclude that Stephenson’s recording of the appearance of the seaweed and barnacles in 1948 was similarly accurate.
Stephenson, along with other scientific sources on exposed rocky coastlines, described the characteristic zonation of organisms that are found there. Life on the "extremely exposed" rocks at Peggy’s Cove illustrates this "zoning" tendency very well, and some remarkable changes have taken place over that last 50 years. From highest to lowest locations, the zones are barnacle belt, rockweed belt, Irish moss belt and kelp belt.
At first glance, the difference between the pictures from 1948 and 2001 seem to indicate that the "barnacle belt" above the rockweed zone has disappeared. When Stephenson visited Peggy’s Cove, a heavy encrustation of bright white barnacles extended to a level on the rock significantly higher than that inhabited by the rockweed (fucus). And marine biology texts consistently describe the upper limit of the barnacle zone as reaching above the rockweed zone. Heavy encrustation of barnacles can still be found on our rocky coasts, but it is generally necessary to look within the rockweed zone to find them. Growth of barnacles at a level higher than the upper rockweed has been practically eliminated in areas where it was once common.
Barnacles are small, permanently attached crustaceans that live by "suspension" feeding. They reach out and grab whatever edible particles they can from the water that passes over them. Their food is therefore largely plankton, and the recent decline in oceanic zooplankton would be expected to be reflected in a decline of barnacles as they become food limited. How closely the
barnacle zone approximates the high tide level on rocky, exposed coastlines, therefore, might present a useful index of the health and richness of the ocean water and marine ecosystem as a whole. A drop in food availability would force a gradual downward contraction in the range of the barnacle…and this appears to be exactly what has happened. Since barnacles can only feed when covered by water, those individual barnacles living highest on the rocks are the ones that endure the longest fasting period between high tides…and therefore they will predictably be the ones to disappear first as the sea becomes nutritionally impoverished. The other important quality of the seawater, from the point of view of the barnacle, is the fact that it is moving. Moving water presents a much more advantageous feeding scenario for a suspension feeder than does relatively still water. It is much easier for a barnacle to grab enough edible particles when they are moving by in a constant stream, than when the animal is limited to what it can reach in unmoving water. (Barnacles typically do not grow in sheltered areas with no wave action; the rocky exposed shoreline is their optimum natural habitat.)
So, the distance photo from 2001 initially appears to show no barnacle belt on Stephenson’s rock. But a closer look at the rock itself reveals that some barnacles do still live there above the rockweed zone. And the distribution of the few remaining barnacles in the former "belt" is interesting. Constant wave action is a major feature of this habitat. Areas with higher water flow patterns, specifically the crevices in the rocks, sections that natually drain off relatively more water than the flatter areas as the ocean waves recede, these are the spots on the rock where the few remaining upper barnacles live.
This new distribution pattern of barnacles is clearly related to volume and velocity of water flow and its feeding implications for the animal. If we were to entertain explanations such as "global warming" or "increased UV radiation" for the disappearance of the upper level barnacles, it would be very difficult to see how a distribution pattern such as this one would be produced in today’s
reduced barnacle population.
Stephenson described the rockweed (fucus) zone as "blackish," and it frequently still does give that color impression. A close comparison of the two pictures seems to indicate that the upper level of the fucus zone may have receded slightly, but only slightly, in the last 50 years. It is interesting to note that the spray zone (buff colored rock above the "barnacle belt" with patchy greyish coloration) appears to reach higher now. This is consistent with recent scientific claims that there has been a gradual increase in sea level. But the rise in sea level appears not to have been accompanied by a (possibly expected?) rise in the upper limit of the fucus zone.
The overall width of the fucus zone appears to be somewhat less now. This is most easily appreciated when looking at the profile of the outer slope of Stephenson’s rock. In 1948 the fucus belt appeared to be about three times as wide as the next highest belt, the Irish moss (chondrus crispus). Today the Irish moss belt appears to be significantly wider, a view of the same slope gives the impression that it now has approximately the same width as the fucus belt. This is most likely because the rockweed plants are shorter now, and therefore they no longer drape down and cover the Irish moss belt to the same extent at low tide. (Rockweeds are long lived perennials, single plants potentially living for decades; the difference in the length of the rockweed plants is therefore most unlikely to be due to the fact that my photos were taken in July while Stephenson’s record was made in September.)
While the typically rough water at Peggy’s Cove may normally contribute to a tendency to smaller growth of rockweed plants, it appears that they grew noticeably bigger and more abundantly in this same environment in the past. This supports the
hypothesis that the decrease in growth may be due to a decreasing availability of nutrients to these plants.
(See rockweed photo gallery, and updated
article on deteriorating rockweeds in Nova Scotia.)
Besides the dramatic loss of the barnacle belt, the most glaring difference in the two pictures lies in the changing color and growth pattern of the Irish moss. Stephenson describes the belt of Irish moss as "reddish brown." And his illustration shows a gradation of shading of this color, from a darker hue in the lower plants to a somewhat lighter hue in those living at higher points on the rock. (Well nourished, healthy Irish moss, usually found today in the relatively deeper locations, has a deep purplish-brown color with bluish phosphorescent glints when viewed underwater. As the level of nutrients (nitrogen) available to this plant become less, the color gradually lightens to a lighter reddish brown, then taking on shades of green and gold, and ultimately turning white if nutrient starvation is severe. This pattern of color change in Irish moss as an indicator of nutritional status is well documented in the seaweed literature. (e.g. Lobban and Harrison, 1994))
Today, Stephenson would need to mix a considerable amount of green, yellow and white into his palette to accurately reproduce the color of the Irish moss at Peggy‘s Cove.
The gradual increase in the greenish-yellow color of Irish moss was one thing that has become noticeable in other areas, but the picture in Peggy’s Cove turned out to be particularly revealing. The green color of Irish moss today contrasts markedly with my memories of thirty years ago as a teenaged Irish moss "raker." During the summer months at that time, people in this area, including this author, earned a few dollars harvesting, drying and selling Irish moss. The boatload of freshly harvested moss, however, never had a green color. It was consistently reddish to purplish brown. Those same moss beds now prominently exhibit bright shades of green and yellow…even to white in some areas… and the growth appears to be very scanty. Growth of Irish moss was much more lush in the past. Pulling the rake that we once used, through today’s Irish moss beds, would clearly produce only very sparse results. In some areas the growth of Irish moss is so minimal that it looks as if it may have already been "raked," as in the photo below, right. But this patch of moss has not been raked; the local harvest was stopped at least twenty years ago.
The typical top to bottom shading pattern is evident in the Irish moss at Peggy’s Cove today, although the range of colors overall is lighter and greener than it was 30 or 50 years ago. Beyond top to bottom shading gradations in the Irish moss, however, another shading gradient is evident at Peggy’s Cove. As you move in, away from the outer rocks of Peggy’s Point itself, the greenish Irish moss takes on an increasingly dark brown hue. It gives the appearance of being relatively better nourished. (It is an established fact that more highly fertilized Irish moss will attain a deeper color, more like the color that was "normal" for this plant over this whole area several decades ago.)
The darker Irish moss is actually better "fed" Irish moss.
Stephenson’s rock borders one side of a shallow cove that sits at the foot of a large funnel shape in the granite. The other side of the cove and funnel is bordered by Peggy’s Point, the rock that the famous lighthouse sits on. Peggy’s Point juts out into the ocean farther than Stephenson’s rock but the two granite formations define the outer reaches of the "funnel" at Peggy’s Cove.
(See photo of Peggy's Point at beginning of the article, or
large panoramic shot (972KB).) My photos and Stephenson’s painting depict an usually calm and quiet ocean state at Peggy’s Cove. When the tide and the ocean swells are higher, huge waves are channelled far up into the rocky funnel. The effect can be spectacular. Large volumes of seawater fill the funnel, waves rushing in and reaching surprising heights (and on several occasions sweeping unsuspecting people to their deaths in this spot). This natural funnel formation amplifies the water flow in this specific area. Encouraged by the long, relatively less steep rocky slope, and also the converging sides of the approach to the funnel, an unusually large volume of water rushes into this area with each swell and then drains out between waves. The volume and velocity of water that passes over the Irish moss growing on the rocks at the head of the little cove, therefore, is significantly more than the amount that washes the moss growing to either side of the funnel. This difference in volume of moving water, and therefore opportunity to absorb nutrients, is what is reflected by the green to brown color gradient that is displayed by the Irish moss in this area.
"The devil is in the details"…meaning, perhaps, that an explanatory theory needs to account for the smaller details, and also that important clues may be hidden therein.
The green to brown color gradient in the Irish moss at Peggy’s Cove can plausibly be explained on the basis of the variable nutrient availability due to variations in water flow. How else could it be explained? Just looking at the green moss in other localities, one might speculate that the color change had resulted from an increase in irradiance, for example (a variable that could theoretically cause the greening effect). But the brown moss at Peggy’s Cove receives the same solar radiation as does the nearby green moss…this helps to narrow down the answer to the one theory that best explains this gradual, long-term color change: a gradual, steady decline in the amount of bioavailable nitrogen in open ocean seawater.
If the SPATIAL shift in Irish moss color from brown to green, as illustrated in the recent photographs from Peggy’s Cove, can be attributed to a relative lack of nitrogen available to the greener section…then the TEMPORAL shift from brown to green, which has gradually occurred over recent decades as an overall trend, is also probably best explained by the same mechanism: a gradual lowering of the availability of nitrogen to fertilize the plant and produce pigment.
The wide color variation in Irish moss, the whole range from dark brownish purple to greenish-yellow and white; this represents another useful index of nitrogen availability in the seawater. Concerns about nutrient enrichment of oceanic water due to terrestrial runoff are overblown today. The impact of such enhancement is limited to the area proximal to the immediate point sources, and the effect of enhanced nutrient availability on brown and red seaweeds is the development of a deeper color rather than the loss of pigment, such as what Irish moss is demonstrating today in Atlantic Canada (Lobban and Harrison, 1994).
This signal of change in Irish moss fits perfectly with today’s larger marine picture of overall biomass depletion due to nitrogen loss.
(See photo gallery of Irish moss photos.)
THE RISE OF THE NITROGEN-FIXERS, THE "BLUE-GREEN ALGAE" OR
"Blue-green algae"....this interesting class of organisms appears to be
rising in abundance in marine systems today. The label "algae" applied
to these "blue-greens," however, may contribute to confusion regarding
their natural role in the ecosystem. True "algae" are plants, but
"blue-green algae" are not plants, rather they are forms of bacteria
that accomplish photosynthesis, or carbon-fixation. More properly
called cyanobacteria, the "blue-green algae" also accomplish
nitrogen-fixation, a process that no plants are capable of. Because
of this feature, their nutritional requirements are much lower than the
nutritional requirements of true plants. Plants require
bioavailable nitrogen to grow, but cyanobacteria do not, since they can
make use of atmospheric nitrogen. Unfortunately, since they are
commonly found living together, it seems that algae and cyanobacteria
often get lumped together in discussions of "marine algae," (e.g.
McCook, 1999) which implies that they respond rather similarly to
environmental conditions (especially nitrogen availability). An example
of this is the assumption that growth of both categories of organisms
is stimulated by the presence in seawater of "extra" nutrients, i.e.
sewage runoff. In fact, their responses to increasing levels of
nitrogen are markedly different. Growth of plants normally increases if
nitrogen becomes increasingly available (marine plants are generally
nitrogen-limited), but the growth of cyanobacteria is better stimulated
by a LACK of nitrogen. (Carpenter et al, 1991, Stal and Caumette, 1993,
Lassus et al., 1995, Desikachary, 1972) In fact, high levels of ambient
nitrogen appear to be toxic to cyanobacteria and will ultimately
inhibit their growth. Therefore, if a situation existed wherein the
availability of nitrogen in the sea was gradually lowered, a shifting
trend in the composition of the algae/cyanobacterial community could be
predicted. The expected pattern would be an overall increase in
cyanobacteria accompanied by a gradual decrease in plant algae.
Harmful "algal" blooms
As was briefly mentioned in the introduction, a large proportion of
"algae" species that are involved in marine toxic blooms fall under the
category of "nitrogen fixers." They are not algae, but cyanobacteria.
Their increasing abundance worldwide in the form of "red tides" is well
acknowledged (NOAA, Lassus, et al., 1995). The occurrence of these
blooms has been closely studied in recent years, and the common feature
preceding their formation is always a period of calm warm weather, a
time in which the water becomes increasingly thermally stratified and
the surface layer becomes extremely nutrient depleted. The virtual
absence of nitrogen in the immediate seawater appears to be the
necessary trigger to start up these algae blooms. (Carpenter et al,
1991, Stal and Caumette, 1993, Lassus et al., 1995, Desikachary, 1972)
Otherwise these organisms remain relatively dormant, and are much
smaller players in the marine algae picture. The difficulty in
culturing cyanobacteria in laboratories attests to this fact. Severely
nitrogen-depleted medium is a must to get a culture to start growing
(one source reporting success only when nutrients were reduced to
1/40th the level used in regular seawater medium - Lassus et al.,
1995). So, under extremely N-limited circumstances, these "blue-green"
organisms are positively triggered to grow, the same circumstances
under which true plant life (ordinary algae) is unable to grow as a
direct result of nitrogen-starvation. Similarly, when nitrogen levels
are high, plant algae are stimulated to grow while blue-green algae are
inhibited. It appears that, when the concentration of fixed N in the
environment reaches a particular threshold, the growth of blue-green
algae is thereby suppressed. (Carpenter et al, 1991, Stal and Caumette,
1993, Lassus et al., 1995, Desikachary, 1972) Thus, a natural feedback
mechanism limits their growth. These are a few of the fine details that
were built into the delicately tuned living web in the sea, that
allowed it to persist for millenia, and maintain relatively stable
conditions for life over that time. If concentrated nitrogen did NOT
shut off the activity of the N-fixers, runaway nitrogen-fixation would
probably have overwhelmed the ocean relatively early on.
BUT -- Toxic blooms of cyanobacteria DO occur in polluted estuaries.
This is true, so how can this observation be reconciled with the other
known fact that these same organisms are known to thrive only in
DOES sewage and fertilizer runoff trigger toxic algal blooms, OR NOT???
It seems that the best answer is, confusingly, "yes" and "no" both.
"Yes" because there is undeniably an observed coincidence of the two
phenomena in the same coastal waters.
"No" because growth of cyanobacteria is inhibited rather than
stimulated by high nitrogen levels in the water.
Examining nutrient pollution of waterways, let’s consider the
Mississippi River, and the load of nutrients that it delivers to the
Gulf of Mexico, as an example. The highest concentration of nutrients
is found in the murky (plant algae loaded) river itself. It is
interesting to point out that toxic blooms are not generally reported
in the river proper, this despite the fact that many species of
cyanobacteria will thrive in fresh water. It appears that the
turbulence of river water inhibits their growth, since they will bloom
only after they have been delivered to comparatively slack surface
waters in the Gulf. Still water, warm, calm weather, inefficient mixing
of the water column, these are the pre-requisites conditions for the
cyanobacterial blooms. Yet it appears to be reduced nutrients in the
surface layer, rather than lack of turbulence in itself, that triggers
the blooms. (This statement can be made since cyanobacteria have been
cultured in medium that is constantly flushed, as long as nutrient
levels, nitrogen in particular, remain at very low levels. (Lassus et
How is it possible that nutrient levels in the SURFACE of polluted
coastal waters can become so low?
The scientific study of nutrient pollution of water was initially
focused on freshwater systems, since the problem was more apparent
there, and since it was commonly believed that salt water systems
naturally have a greater capacity for "self-cleansing" of excess
nutrients. This is actually the truth. The first thing that the
polluted river water meets is the salt water - and the salinity
gradient itself causes the clumping together of a fair amount of
nutrient-loaded organic material which leads to its sinking to the
bottom (where denitrification is the most likely fate of the nitrogen,
if it’s not simply incorporated into the bottom sediment - regardless
of which one occurs, the nitrogen that sinks is reasonably effectively
removed from the water and doesn’t enter the nitrogen cycling pattern
in the food web.) Dissolved nutrients that arrive in the coastal water
do stimulate an abnormal increase in the growth and biomass of
phytoplankton (the types that like high nutrients). If they are not
consumed by their natural predators, the phytoplankton do not live
long, so larger than usual numbers of these die and also sink to the
bottom. Once on the bottom, bacterial decomposition of the dead plant
matter consumes the oxygen from the water, rendering the environment
hostile to animals that need oxygen (for example, all fish and
invertebrates). This is the chain of events that has been developing
annually in the Gulf of Mexico and resulting in the creation of the
huge "dead zone."
But is the nutrient load delivered to the Gulf the only culprit here?
Or has the adjustment of other variables in the system contributed to
the expansion of the dead zone, and the lowering of the ability of the
sea to "cleanse itself?"
The removal of the natural consumers of phytoplankton would clearly
exacerbate this problem. Has fishing-induced biomass depletion
contributed to a decline in zooplankton in this area of the ocean, as
it has in many others (both polluted and non-polluted)?
The strength of a marine system in being able to deal with, and absorb
into it’s living web, liquid nutrients that are added to the water - is
directly related to the number of fish that live in that system; the
total "standing stock" of live animals. An intact web, with healthy,
abundant organisms living at each natural trophic level, is much more
capable of absorbing added liquid fertilizer into itself, than is a
greatly diminished ("fished-out") living community. If one looks
back in time to the incredibly abundant sea life that once occupied the
bays and estuaries, it becomes evident that a great multitude of fish,
invertebrates, seabirds and marine mammals coexisted in those waters
without "polluting" them. Each of these living things excreted organic
waste into the water, ammonia, urea, feces...and the total quantity of
organic waste "added" to the water in this way would have been
substantial. It becomes intuitively obvious that adding the wastes of
all those animals to those same waterways, if the waterways contained
plankton only, would result in the murky overgrowth of phytoplankton,
as in today’s picture of "eutrophication." Removing the fish,
therefore, results in a lowering of the "eutrophication" threshold for
a given waterway. Greater fish assemblages have greater capacities to
absorb incremental additions to their "nitrogen inventories" than do
In a pristine marine system, with a healthy living web actively
recycling nutrients at a high level, an increase in phytoplankton
density would be met rather promptly by an increase in grazing by
zooplankton. Any resulting increase in zooplantkon density would be met
by an increase in grazing by their predators, and so on. Thus, the
"liquid nutrients" added at the bottom would be dispersed throughout
the living web as a whole, a subtle domino effect would ripple upward,
each creature feeding a bit more heavily as food became more abundant.
If a larger than usual flush of liquid nutrients was washed into the
bay, perhaps all the fish, birds and seals would just become
ever-so-slightly fatter within a short while. Rather than resorting to
accelerated denitrification, a fish-rich system could more readily
assimilate any available extra nitrogen into itself.
But the ability to absorb and "uptake" the added nutrients would still
have a natural upper limit. It would clearly be possible to add too
much sewage to a pristine marine system, no matter how healthy it was.
Once stimulation of phytoplankton exceeded the capacity of zooplankton,
etc., to eat it, the "eutrophication" chain of events would start to
occur. This is the ocean’s strategy to remove nitrogen when the
quantity that is free in the water becomes too high (N in dissolved
form, as opposed to swimming around in animate flesh).
Therefore, the absolute quantity of nitrogen that will trigger
eutrophication in a given body of water is directly related to the
extent and vitality of the living food web that occupies that water.
This implies that fish extraction from coastal waters actually lowers
the resistance of those waters to pollution, or their "eutrophication
threshold." Elevated nutrient runoff is associated with concentrations
of human populations. But so is fishing. Isolating the effects of
sewage pollution and fishing will be difficult since it would appear
that each waterway afflicted with human pollution has also been
subjected to human fishing. (The very depletion of fish from these
coastal waters attests to the fact that sewage nutrients do not
translate well into fish flesh...and for some reason that myth still
prevails, that human N-in (sewage) makes up for human N-out (fishing)).
Small amounts of additional nutrients seem to be acceptable (that’s
what occurs naturally due to the growth of cyanobacteria), but as soon
as the threshold is crossed, the waterway switches into clean-up mode (eutrophication/denitrification).
All of the natural nitrogen excretions of the members of the marine web
are acceptable to the system and easily recycled - their acceptability
determined by virtue of the fact that their living "source" is by
definition an active participant in the natural recycling pattern.
This variability of the "eutrophication threshold" of waterways, which
directly depends on the health of the living marine web, seems not to
be acknowledged in the literature (to the best of my knowledge). The
idea may help to explain, however, such problems as why the "dead zone"
in the Gulf of Mexico has shown a steady advance that is not really
paralleled by an increase in nutrient effluent from the Mississippi
River. Fishing is carried out continually in the Gulf of Mexico, as it
is elsewhere, and the gradual loss of fish abundance and overall marine
biomass that results from fishing is most likely causing the "eutrophication
threshold" of the Gulf to slowly drop to ever lower levels. Just
pointing the finger back toward the land is not sufficient to explain
away the problem of eutrophication in marine systems. The damage
inflicted on the living web by fishing is surely playing its part as
well, and exacerbating the problem. The query "how will the expansion
of the dead zone affect the fisheries in the Gulf?" is often
heard...yet another question that demands an answer is "how are the
fisheries in the Gulf impacting on the expansion of the dead zone?"
Backtracking to the question... "how could nutrient levels become
reduced to such low levels in the surface waters of the polluted
estuaries, low enough to trigger blooms of toxic cyanobacteria?" When
the water is still and warm, the plant algae (phytoplankton), will
continue to grow until they deplete the surface water of nutrients to a
very low level. Rather than consumption by zooplankton, the demise of
these plant-algae results in their sinking to the bottom and taking the
nitrogen down with them. In a situation of healthy grazing by
zooplankton, however, even in the calm weather, nitrogen levels in the
surface water would take a much longer time to sink to the
"toxic-trigger" level. This is because the zooplankton themselves
act to keep nitrogen in the surface water, they eat the algae but then
they excrete ammonia, for instance, which is easily available for
re-uptake by the phytoplankton. In this way, the very presence of the
living zooplankton (and by analogy the entire living web adds strength
to this) acts as a nitrogen-stabilizer or nitrogen-buffer that prevents
excessive N-depletion from developing in surface waters. A waterway
containing a rich living web could therefore withstand a significantly
longer and more intense warm, calm spell of weather before toxic
cyanobacteria would be seen to bloom on the surface.
The problem of toxic algae blooms is increasing worldwide, but the
worst examples are not always those that develop in the obviously
polluted estuaries. (Lassus et al, 1995) The more usual scenario is for
the cyanobacteria to bloom in offshore ocean waters when the surface
layer there becomes extremely nutrient depleted (always associated with
warm, calm spells). Winds may then drive the bloom toward shore where
toxicity to fish and mammals becomes evident. This particular scenario
is also apparently increasing worldwide. And it seems that this has
occurred throughout history, accounts of shellfish poisoning in
previous centuries are not unknown. Therefore, even the pristine (unfished)
marine system could be pushed down to the "toxic-trigger" nitrogen
level if doldrum conditions lasted long enough. The increasing
frequency of these bloom events seems to reflect the lowering of
another threshold, probably another system-destabilizing side-effect of
fishing. The ability of ecosystems to maintain surface nitrogen levels
above the "toxic-trigger" threshold during doldrum conditions, is also
dropping. And this is a direct result of the fact that there is now
simply less total life in the sea. The toxic blooms of cyanobacteria
are not new, they are just becoming an increasingly prominent feature
of today’s (nitrogen depleted) marine ecosystem.
Cyanobacteria are ancient organisms, they were the pioneering organisms
on the earth. Masters of living in hostile environments, their
advantage lies in their ability to fix atmospheric nitrogen and thereby
overcome the nitrogen-limitation that disqualifies the "plant-algae"
from initially colonizing these sorts of habitats. Cyanobacteria were a
very large feature in the development of the early ocean on earth, but
had apparently receded to the position of relatively minor players
until recently. Scientists today are realizing that cyanobacteria are
much more abundant in the ocean that they were previously believed to
be. Possibly an artifact of better investigative techniques, this
realization may well also reflect a true increase in the overall
abundance of this class of organisms over the timeframe of the
development of marine biology as a science. In an increasingly hostile,
approaching "sterile" nitrogen-depleted environment, the ocean system
now reverts to an old strategy: nitrogen-fixers are increasingly being
triggered in an attempt to regain stability in the face of the
nitrogen-draining effect of human fishing. The increasing incidence of
human-illness from toxin-containing seafood seems to reflect a true
increase in the abundance of the cyanobacteria in the sea, rather than
only an increase in the scientific awareness of their existence.
Besides the increasing frequency of the toxic, free-floating forms of
cyanobacteria, other organisms in the same class are also slowing
gaining the upper hand in various marine environments. Today’s
coral reefs offer one good example - the decline of corals accompanied
by an overgrowth of "turf algae" (often cyanobacteria dominated)
illustrates this trend. So does the recent emergence of coral diseases
that are directly caused by cyanobacteria. These involve species that
have historically coexisted on the reefs, but now some "change in the
environment" has dealt the upper hand to the cyanobacteria. The "red
band" and "black band" coral diseases are caused by "infections" of
cyanobacteria - the "disease" phenomenon is simply the overgrowth of
the living corals by the microbial mats. (Castenholz, 1994).
Cyanobacterial overgrowth is also often a prompt development in cases
of "mass coral bleaching." The one specific environmental change that
favors growth of these "blue-greens" over other organisms, as has been
discussed, is a LOWERED availability of nutrients in the water,
nitrogen in particular.
An increasing abundance of filamentous forms of "algae" is commonly
reported today. Whether it is the brown slime clogging the nets of
fishermen in the Northwest Atlantic or the green slime fouling the
beaches of Hawaii, a common feature of these is the fine, "hairy"
texture of the newly dominant organism. Characteristic of many
cyanobacterial colonies, the "filamentous algae" may be those, or if
they belong to the true plant-algae group, they are plant forms that
are adapted to take advantage of water containing low levels of
nitrogen. The filamentous form maximizes the surface area/volume ratio,
and thereby also the ability of the organism to take up whatever
dissolved nutrients are available in the water. This provides an
advantage over thicker-fleshed types of macroalgae that may
alternatively have dominated the environment.
The very toxic nature of the cyanobacterial blooms is an interesting
aspect of the story. Why would these organism invest the energy and raw
materials into the production of chemicals such as these toxins? The
toxins must serve a purpose. It has been demonstrated that, for some
strains of cyanobacteria, that the concentration of the toxin produced
increases as the availability of phosphorus declines in the
environment. In the overall scheme of things, the toxins appear
possibly to represent an attempt to correct the nutrient deficiency of
the environment. Especially the phosphorus deficiency that may
accompany the nitrogen deficiency, could potentially be compensated for
by the deliberate killing of organisms that contain the missing
Regarding the confusion that exists surrounding cyanobacteria, their
increasing presence in marine environments and the underlying reason
for that increase...besides the misleading tradition of referring to
them as "algae," here is a summary of the salient points:
-- Increasing levels of nitrogen in seawater does NOT stimulate the
growth of cyanobacteria. In fact the opposite is true, concentrated
fixed nitrogen is the specific inhibitor of growth of this class of
-- "Blue-green algae" must not be confused with plants (although they
may look like them). These organisms are the natural precursors of
plants, that effectively pave the way in hostile environments for the
eventual growth of plants. Growth of cyanobacteria results in a
gradual increase in the available nitrogen in the environment,
which results in a "new" habitat becoming increasingly plant-friendly
over time. An environment in which plant growth is giving way to
cyanobacterial growth is one that is experiencing the opposite of the
trend that predominated the history of the development and accumulation
of marine life. This is a compensatory response of the ocean system to
the gradual but relentless nitrogen depletion that has resulted from
-- Cyanobacterial blooms occur on the surface of polluted waterways
only when those waters become thermally stratified. Therefore their
growth occurs in close proximity to nitrogen pollution, but is not
directly stimulated by that pollution as such. The spatial proximity of
the occurrences may have led to confusion around this point - many
sources seem to indicate that nutrient enrichment of waters is a direct
cause of toxic blooms, but this idea is false.
-- Cyanobacteria can smell very much like sewage. Since they produce
ammonia, and also sulphur-containing compounds, cyanobacterial-dominated
communities often give off a characteristic stench. Since the smell
resembles that of human sewage, we may be quick to assume that sewage
is the cause of this particular "problem." It is not intuitively
obvious to us that the cause of this smell is actually the direct
opposite of sewage enrichment of the water. Nitrogen-depletion ends up
creating a smell that is very much like that of "nitrogen-overloaded"
effluent (containing sewage which gives off the same types of gases).
Whether the basic building blocks of proteins are being broken down (as
in sewage) or built anew (as in colonies of cyanobacteria), these
compounds smell the same to the human nose. This doubtless adds to our
confusion and difficulty in sorting out "what is going on here?"
The brown slimy mass that is exposed at low tide now definitely gives
off a "rotten" smell. This is another increasingly common phenomenon
along the Northwest Atlantic coast. The brown slime is mostly composed
of free floating filamentous colonies, and also a fine filamentous
epiphyte that has lately engulfed the now-stunted rockweed in the area.
This appears to be the result of increased growth of cyanobacterial
colonies...which, by their increasing presence, imply a declining trend
in the nitrogen availability in the seawater.
-- Cyanobacteria appear in many colors, including multiple shades of
green, this characteristic possibly leading to their original misnomer
as "blue-green algae." Cyanobacteria use the same green pigment as
do plants for the purpose of photosynthesis: called chlorophyll a, the
basic pigment is green. As in plants, sometimes the color of the
organism is not green due to the addition of other pigments, but the
green pigment is always there, underlying the others, and fairly often
it provides the dominant color. This "greeness" can add to the
confusion between cyanobacteria and plants...it is one point on which
the distinction can become blurred. The two very different types of
organisms may look a lot alike.
Specifically, a problem arises when conclusions about the
nutrient status of a marine environment are extrapolated from indices
of "greeness" or concentrations of chlorophyll a. The growth of
"green things" does not reliably provide information about the nutrient
status of the water, since high nutrients will stimulate "green growth"
and so will low nutrients stimulate "green growth." It’s a
significantly different type of growth, however, telling a very
different story about the nutrient status of the water....but the green
pigment remains the same. The amount of the pigment itself, the
concentration of chlorophyll a in the system, may not show a
changing trend even in the context of a steadily dropping availability
of nitrogen. The greeness might remain the same, but it would be
provided by an increasing ratio of cyanobacteria to plants. Therefore,
studies of trends in nutrient levels in seawater would be more useful
if they concentrated on the balance between the two types of green
organisms rather than simply on the "greeness" itself.
Monitoring programs are in place that record the "greeness" and
"chlorophyll a content" of seawater (e.g. those done by DFO, NMFS).
But the usefulness of their data becomes very questionable, (partly
because grazing pressure on phytoplankton is not factored in), but also
since without knowing the exact source of the "greeness," one can infer
NOTHING about the nutrient levels in the water. Unfortunately, it
is very common to find statements like "Phytoplankton biomass, as
estimated by chlorophyll concentration..."(DFO, 2000) Another
example of this same pitfall is provided by "Long-term monitoring of
chlorophyll in the Great Barrier Reef: an update." (Reef Research:
Volume 9, No. 1, March 1999) This research effort is seeking evidence
of "large-scale eutrophication in Great Barrier Reef waters," and
describes a program wherein "measurement of chlorophyll a
(the major algae pigment) concentration was chosen as a proxy indicator
of nutrient status." The analsyis reveals later that the "high
chlorophyll a concentrations were related to the presence of
Trichodesmium aggregations." Trichodesmium is not a form
of algae, rather it’s a very common marine form of cyanobacteria.....therefore
concluding on this basis that "nutrient status" in the seawater is also
"high," represents a rather large and significant error.
Due to the wide variability in the nutrient requirements of their
various sources, neither "greeness" nor "chlorophyll a
concentrations" can be used alone as proxy indicators of nutrient
status. Much more reliable indicators of "nutrient status" abound in
the intertidal zone...barnacles, mussels, seaweeds...
The usefulness of "unexploited" marine species as proxy indicators of
the overall health of the ocean has been largely overlooked. Each one
of these, if monitored closely, reflects the overall nutrient
availability in seawater and also the rate of marine primary
productivity. Intertidal organisms living in areas exposed to open
ocean water will provide more accurate information about trends in
marine productivity than will any elaborate mathematical attempts by
humans to factor in all of the variables. An examination of the
condition of these living "barometers" should bring to an end any
concern that open ocean waters today are afflicted with "nutrient
overload." The truth actually appears to be the exact opposite.
The decline of barnacles and mussels (and
tropical corals) attests to
the decreasing availability of their food, which is plankton, the
"base" of the entire marine ecosystem. Immobile once they have settled
and attached themselves to solid substrate, these types of small
filter-feeding organisms become ideal reflectors of the overall health
and productivity of the marine system.
Likewise, the evolving changes in the pigmentation and growth patterns
of seaweeds accurately reflect the degree to which the seawater is
fertilized with the essential nutrients. The evidence from these
sources is now overwhelming that the ocean has become markedly depleted
in available nitrogen. The rising occurrence of "nitrogen fixing"
organisms is clearly a natural consequence of this and a stabilizing
force in the system…will this mechanism, or can it, ever rise high
enough to achieve a "balance" with fishing removals by humans? Clearly
it has not done so up until this point, but a time can be foreseen when
the rising toxicity of seafood along with the difficulty in finding
more fish, will combine to reduce human fishing to a level that matches
the enhanced rate of "new" protein production in the ocean. Part of the
price that will be paid, however, is the climate destabilization that
is a direct consequence of such a lowering of the overall quantity or
"biomass" of the marine biota.
Many lines of evidence point to the certain conclusion that plankton
levels have dropped significantly in the ocean. Nutrient depletion will
naturally impact phytoplankton to a greater degree than it will
seaweeds. Commentary by Lobban and Harrison regarding comparisons of C
: N : P ratios:
"Extensive analysis of the chemical composition of marine plankton has
revealed that the ratio relating carbon, nitrogen, and phosphorus is
106 : 16 : 1 (by atoms) (i.e. C : N = 7 : 1 and N : P = 16 : 1). This
is commonly referred to as the Redfield ratio…The median ratio C : N :
P for seaweeds is about 550 : 30 : 1 …An important ramification of
these observations is that the amounts of nutrients required to support
a particular level of net production are much lower for macroalgae than
for phytoplankton…The high C : N : P ratios in seaweeds are thought
to be due to their large amounts of structural and storage carbon, with
vary taxonomically. Niell (1976) found higher C : N ratios in the
Phaeophyceae than in either the Chlorophyceae or Rhodophyceae. The
average carbohydrate and protein contents of seaweeds have been
estimated at about 80% and 15% respectively, of the ash-free dry weight
(Atkinson & Smith 1983). In contrast, the average carbohydrate and
protein contents of phytoplankton are 35% and 50% respectively (Parsons
et al. 1977)." (From Lobban and Harrison, 1994, pp 202-203)
In other words, as bad as the seaweeds appear to be doing in today’s
ocean water condition, it is an easily predictable conclusion that the
plankton is doing worse.
In the face of huge declines in multiple "unexploited" marine species,
both animal and plant, continued belief in "species replacement theory"
becomes impossible. The idea of "species replacement" is based on the
notion that total marine primary productivity basically remains
constant, and the hypothesis predicts that if the abundance of one
marine species is lowered (for instance, by fishing), that another will
rise to compensate, taking advantage of the "extra" food that will be
left over. "Species replacement" lines of thinking have led to the
prediction that once we have severely reduced the fish stocks the ocean
will be "full" of plankton. (This is not true, since cyanobacteria are
not usually included in the concept of "plankton" as it is used here,
meaning abundant food for "plankton feeders.") This thinking now
requires denial of the plain facts, and it becomes a dangerous delusion
if we continue to rely on "species replacement theory" to justify our
continued exploitation of marine species.
Recent scientific research into the historical decline in abundance of
marine life has indicated that the downturning trend is essentially as
old as human fishing. (See
"Old hunting, fishing blamed for today's coast woes") An
"unbelievable" abundance of marine life once inhabited the ocean, but
our individual memories are far too short to appreciate this. Therefore
we remain ignorant of the sheer magnitude of what has been lost, and of
the role played by our species in destroying it.
The time for humans to "wake up" is now. We must face the reality of
the cause of this disastrous crash of marine life, and learn to
appreciate its total implications for the future of all inhabitants of
the ocean-planet Earth. Believe it or not, people do not "need" to eat
fish to live…in fact, what we need to learn now is to stop eating fish
and find an effective way of giving back something to replace what we
have taken from the sea. And we need to appreciate the "ecological
service" that marine creatures have historically provided, nothing less
than the maintenance of a stable level of planetary atmospheric CO2.
The evidence offered by the seaweeds that marine bioavailable nitrogen
has become severely lowered in the ocean…the full appreciation of this
changes "everything" that we "know" about the workings of the
marine nutrient cycle and
global carbon cycling…
My thanks are due to
the people that assisted with the seaweed survey. Providers of boat
rides and physical and moral support were Michael LeBlanc, Bill Bell,
Samantha Bell, Kenneth Leccese, Nacho Chavarria Fuentes and Patrick
Also greatly appreciated was the support and
encouragement for this research offered by the late Dr. Don E.
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