Dying Seaweed in Atlantic Canada reflects
Nutrient Depletion of Seawater
by Debbie MacKenzie, April 24, 2002
Red-black, 'burnt-looking' patches appearing on living seaweeds (and not associated with excessive heat) is the most obvious sign of trouble...e.g. photo below at right, taken April 2002...
A pattern of unusual breakdown is becoming evident in long-lived brown seaweeds along the unpolluted Atlantic coast of Nova Scotia. The pattern of deterioration and tissue loss is consistent with changes that would predictably occur in plants of these types if the availability of nutrients in seawater were lowered. The spatial distribution of the damage, and contrasting pathologies between affected species that live in close association, help to focus the highest suspicion on simple nutrient deficiency as the primary cause of the changes.
The affected plants are those living in areas of relatively lower natural nutrient availability, and the observed changes seem very difficult to explain by any cause other than nutrient depletion of the seawater. This pattern of decline in seaweeds fits into the larger theme of declining marine life, in which many species are showing increasing signs of food limitation.
Relatively simple laboratory testing could help to confirm or refute the hypothesis that the observed declining trend in seaweeds is due to nutrient starvation. The diagnosis can largely be established, however, on the basis of gross physical assessment of the individual seaweeds. Visual cues of low nutrient availability are clearly evident in these plants: this includes extremely low pigmentation levels, the pronounced development of "hairs" on some seaweeds, the unusual loss of large quantities of mature tissue, and an apparent loss of normal resistance to environmental stressors experienced at low tide.
A finding of declining nutrient availability in seawater, however, is in direct contradiction to mainstream scientific expectations in these days of intense concern regarding excessive nutrient loading of coastal waters yet, the simple fact remains that these seaweeds are in decline, and are clearly showing physical changes that are consistent with nutrient starvation. Due to the profound implications of such a finding, scientific investigation of this issue should become a priority.
The dominant brown seaweeds along major portions of the North Atlantic coastline, Ascophyllum nodosum and several species of Fucus seaweeds (hereinafter Ascophyllum and Fucus), have recently been noted to be showing unusual patterns of deterioration along the Atlantic coast of Nova Scotia. Ascophyllum ("knotted wrack") and Fucus ("bladder wrack") often grow in close proximity, and together they form the familiar brown seaweed belt. Related seaweeds, Ascophyllum and Fucus share the same types of pigmentation. Yellowish coloration in these plants, as opposed to the normal olive green, is a sign that the seaweeds are relatively poorly fertilized. (Lobban and Harrison, 1994) Ascophyllum and Fucus are relatively long-lived perennial seaweed species. This is especially true of Ascophyllum, with individual plants of this species being capable of surviving for many decades.
The relationship between the occurrence of yellowing and nutrient availability is easily discerned by observing the spatial distribution of darker (green) and lighter (yellow) seaweed specimens along the shoreline. Extreme yellowing is visible in areas characterized by relatively less water motion. Yellowing is also more extreme in plants living nearer the upper vertical limit of their range, as less time covered by seawater also tends to minimize nutrient availability, and to maximize the stressful effects of light and emersion. In contrast, seaweeds growing in close proximity to sewage outfalls (nutrient enhancement) are not exhibiting the yellowing or tissue breakdown pattern that can frequently now be seen in plants growing only a short distance away.
Most of the seaweed still maintains normal pigmentation, as a quick glance at the shoreline still reveals the presence of the familiar brown seaweed belt. But a closer look shows the increasing signs of stress and unusual breakdown in these plants. The extreme yellow hue (seen year round in stressed Ascophyllum) is increasingly noticeable in those plants growing high in sheltered inlets. It can often be observed that the seaweeds growing on the rocks that take the brunt of the small waves or ripples that enter an inlet, are normally pigmented, while nearby members of the same species, living in the calmer inside water are breaking down. In these cases, only a single environmental variable, the degree of water motion, seems to differ between affected plants and those that are not. The degree of water motion is directly related to the availability of dissolved nutrient in the seawater to the seaweeds. (Lobban and Harrison, 1994)
Between June, 2001 and April 2002, repeated observations were made of similar habitat type (high intertidal zones of clean, sheltered inlets) at various points along approximately 30 kilometres of shoreline (from Peggy's Cove to Prospect), and the described patterns of damage in Ascophyllum and Fucus were consistently found. Presumably a much wider area than this has been similarly affected.
Both brown seaweeds, Ascophyllum and Fucus, are showing pigmentation changes. Affected plants are exhibiting unusual degrees of yellowing, which, when it is extreme seems to render them increasingly vulnerable to turning red, a color which signals tissue death in brown seaweeds. The yellowing of Fucus is seasonal, however, while that in Ascophyllum appears to be permanent (it is visible year round).
The yellowing of these seaweeds has been there for years, it's hard to say how many people living nearby have grown accustomed
to the golden hue of the seaweed in these areas. And the yellow color generally does not strike anyone as being unusual.
(Nutrient deficiency of seawater seems most likely to have been an unsuspected and insidious problem that has gradually
developed over many years.) However, what is eye-catching and unusual looking now is the appearance of RED, burnt looking
tissue on the pale yellowed plants. This is new. This red color, the color of cell death, we are not accustomed to seeing
in living "brown" seaweeds.
(1) Ascophyllum (pictured at right). The immediate cause of tissue death in these plants seems to be desiccation. The affected seaweeds appear to have first changed from a green color to yellow, and then to red (dead). The plant tissues with the longest exposure times to air are affected first. The loss of 70% of its water content will cause cell death in Ascophyllum (DFO). Scientists have previously observed this dry, red form of tissue loss in Ascophyllum during unusually hot weather, which accelerates drying. They have attributed the damage to excessive "heat," and a similar pattern has been observed in Fucus. (Personal communication from several seaweed specialists.) However, the same phenomenon is occurring now in cold seasons (winter and spring) in Nova Scotia. Winds are higher and relative humidity is lower during winter and spring, so desiccation stress during these seasons may equal or exceed that occurring during summer. No significant change in weather patterns has occurred recently in the affected area, which makes "climate change" a most unlikely cause of the problem. And the majority of the Ascophyllum growing along this coastline, the more darkly pigmented plants living in locations with higher nutrient regimes, is tolerating the natural drying effects of the environment with no apparent difficulty.
(2) Fucus species (most noticeably F. spiralis and F. vesiculosus, but also involving the deeper living F. serratus).
The unusual breakdown pattern in Fucus can best be described as an extreme loss of mature tissue in which the older parts of the plant shrink from their normal flattened shape, with photosynthetically active "wings" on two sides of a central "midrib," to just the dark, thin midribs. The "wings" are completely lost.
Yellowing of Fucus seaweeds also occurs, and is apparent in this area on a seasonal basis. Some of these plants appear noticeably yellowed during summertime, but then regain darker pigmentation during the winter months. This reflects the well understood annual fluctuations in the nutrient availability in seawater, with nutrients dropping markedly in summertime in coastal areas such as this one, where the water column becomes thermally stratified in summer. (Lobban and Harrison, 1994)
The tips of Fucus plants still present a fairly normal appearance, but a marked change is obvious in the older tissue.
The normal physiology of Fucus plants helps to explain what is happening to them. Fucus species are one of a few types of seaweeds that transport nutrients internally toward the areas of greatest demand. New growth in Fucus occurs selectively at the tips of the fronds, and this is also where the reproductive structures form on these plants. By far the greatest demand for nutrients is therefore at the distal ends of Fucus plants, and the need is greater than what can normally be absorbed from seawater by the tips. (Lobban and Harrison, 1994) The entire surface of the Fucus plant can normally absorb nutrients from seawater, however, and is photosynthetically active. This continued activity of the older, non-growing tissue is used to support the growth of new tissues, as accumulated nutrients are actively transported from the older tissues to the tips.
Taken to an extreme degree, this tendency of the new growth to draw sustenance from the old, might be expected to result in weakening and deterioration of the older tissue. This appears to be the process that is occurring in Fucus seaweeds in Nova Scotia now, as normal-looking tips are seen on plants in which all of the older tissue has shrivelled to a thin, dark 'skeleton.'
Ascophyllum, in contrast to Fucus, does not have a strong tendency to translocate and concentrate nutrients
at the distal ends. In Ascophyllum, reproductive structures are formed along the entire length of the existing mature
tissue, and branching also does not occur at the tips of this plant as it does in Fucus.
The stressed tissue in Fucus is specifically the older tissue, and sometimes this can be seen to take on a red
color, which is reminiscent of the red/dead Ascophyllum. When observed together (as in the two photos immediately
below) a perfect reversal of color patterns is evident between the two species, with Fucus giving the impression
that it is 'burning' from the bottom up. This contrast offers additional evidence of the role of nutrients in the susceptibility
to tissue damage.
A review of standard reference texts on seaweed reveals that there are three common routes by which these perennial seaweeds normally lose tissue: grazing, reproductive cycles and physical breakage. (Is a fourth route of tissue loss in seaweeds nutrient starvation now also becoming evident?)
(1) Grazing - Direct consumption of living seaweeds by herbivores has a negligible effect on mature brown seaweeds in Nova Scotia. Crabs and snails consume minimal quantities and do not usually inflict significant damage on these plants, once they have grown to mature sizes. Juvenile members of these seaweed species are very commonly consumed, however, especially by grazing snails.
(2) Tissue shed through reproductive cycles - Mature brown seaweeds normally lose substantial quantities of their biomass through reproduction - reproductive structures form, which contain eggs and sperm, and these are then lost during the annual reproductive cycle. As mentioned above, the reproductive structures form at the distal tips in Fucus, and along the length of the mature thallus in Ascophyllum. The quantity of tissue lost is considerable. Ascophyllum may lose 50% of its biomass due to the shedding of reproductive structures. However, the entire surfaces of these plants are photosynthetically active and organic material is normally regained in this manner.
(3) Breakage due to wave action and ice scour - This includes broken pieces of fronds and ultimately entire plants - they are swept away, holdfast and all, by larger waves such as those occurring during storms. This is a normal occurrence, and one that would seem to pose an increasing risk as individual seaweeds grow older, larger and heavier. Removal by waves or ice is presumably - from all literature sources that I could find - the usual cause of death for these plants: they grow to a size such that the drag forces exerted by water motion increase to the point that the whole plant is removed when wave action is intensified. (This is presumably the reason why seaweeds grow to smaller sizes in areas exposed to more intense wave action.)
Beyond these normal loss mechanisms, "seaweeds rarely shed significant amounts of tissue " (Lobban and Harrison, 1994, p 158)
The existing literature on productivity and life cycles of seaweeds appears not to include descriptions of the extreme degrees of mature tissue loss that is now being seen in Fucus species in Nova Scotia. The picture of mature, living seaweeds withering away from the hold fast up, losing all of the "wing" tissue and leaving only the darkened midribs - this process of breakdown is nowhere described, or at least it was not found in this author's review of recent comprehensive textbooks on seaweed ecology and physiology. (see reference list)
The appearance of the 'skeletons' of dead Fucus seaweeds, still attached to the rocks on which they grew, after undergoing a process of gradual draining away of nutrients (and products of photosynthesis) from older to newer tissues, stands out as a starkly unusual sight to this experienced beachcomber. Yet, logically, once the physiological workings of the plant are considered, this is the sequence of events that might have been predicted had it been considered that these plants might undergo gradual nutrient starvation in nature.
In Fucus, tissue loss due to nutrient depletion reduces drag, and the skeleton may remain attached to the rock after the plant has died. In marked contrast to the classic scenario of larger, older seaweeds being ultimately torn off the rocks by heavy waves or ice - today some 'skeleton ' of Fucus still hang on with no live tissue left, though a great number of 'emaciated ' seaweeds are also broken off and cast ashore by the waves.
Several people have suggested to me that the deterioration of Ascophyllum might be a reflection of some other changing environmental variable, such as ultraviolet radiation. It is not possible to reject outright the hypothesis that increased ultraviolet radiation may have played a role, but the close correlation between nutrient availability patterns and the pattern of susceptibility to damage seem to make nutrient deficiency a much stronger suspect. Seaweeds receiving equal doses of UV but more nutrients (near to sewage outfalls or in areas of greater water motion) clearly have greater resistance to the described forms of breakdown.
Also, the selective breakdown of mature tissue in Fucus is very difficult to perceive as a possible effect of ultraviolet radiation on an otherwise healthy plant, since the portion of the seaweed that receives the most radiation (the floating tips) is the part that is not showing any damage.
Might this problem be due to "global warming?"
Simple thermal stress (too hot or too cold) is not a plausible cause of the observed pattern of deterioration in the seaweeds.
Nova Scotia is situated approximately at the center of a wide geographical range occupied by both Ascophyllum and
Fucus, both of these species being commonly found at 5 degrees of latitude (or more) south and north of the location
described in this paper. (Luning, 1990, p 52) Also, temperatures in Nova Scotia, both summer and winter, have recently
remained within the range of long term averages for the area. Neither unusually hot, nor unusually cold, weather has occurred.
Water temperatures have similarly not exceeded the tolerance range for these species (which is 0 - 28C, Luning, 1990, p
Once the index of suspicion is raised, this should not be difficult. Biochemical testing of seaweeds in laboratories has been done for decades, and the normal ranges of the chemical constituents of these seaweeds have been described in the past (e.g. Lobban and Wynne, 1981).
Specifically, three general responses of seaweeds to nutrient deficiency have been described in the literature (DeBoer in Lobban and Wynne, 1981, p 384):
(1) Decrease in content of photosynthetic pigments. (This can be grossly determined visually by observation of color change (i.e. yellowing) and confirmed by laboratory testing.)
(2) Accumulation of C-storage compounds. This has been especially noted in species of red algae, but a similar shift in carbon:nitrogen ratios would be predicted in all seaweeds and would be expected to show a relative depletion of nitrogen.
(2) Decrease in proteins and nucleic acids. (Laboratory testing will be necessary to determine these levels.)
The yellowed, mature Ascophyllum, that is now increasingly prone to taking on a red-black 'burnt ' look, may reveal the most useful information on lab testing. Due to the strong tendency for translocation of nutrients towards the growing tips in Fucus, newer tissues on these plants may yet be maintaining normal nutrient levels by virtue of having drained all that had been stored in the older tissue. Therefore, in the Fucus species, normal biochemical parameters at the growing tips - if a large amount of mature tissue has been drained to accomplish it - is not inconsistent with nutrient starvation of the organism as a whole. Simple, physical assessment of the gross condition of these plants actually tells the tale. (As with starving humans, biochemical parameters may be normal; the diagnosis of starvation in humans is based on an abnormally low weight:height ratio. This is likely also a telling sign in the Fucus and other seaweeds.)
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There seem to be two basic possibilities:
(1) It is a natural cycle.
The availability of dissolved nutrients in seawater in temperate zones is well known to fluctuate quite widely on an annual basis. This is especially true of the surface layer, the part of the water column that sustains the seaweeds in question. As the surface water warms in summer it is well known to become relatively stripped of dissolved nutrients, but the low temperatures and higher winds in winter induce mixing of the water column, which raises surface water nutrient content to its highest annual concentration. Since these physical factors (climate) that drive the cycle of vertical mixing of seawater have not changed, other possible causes of nutrient depletion must be considered.
(2) A second possibility is that a lowering of overall nutrient availability in the ocean has occurred because too many nutrients have been removed and possibly removed in the form of fish.
Might this decline in seaweeds be a manifestation of the extent of degradation that has occurred in the larger marine ecosystem as the ultimate result of centuries of fishing?
Systemic nutrient depletion could be a gradual, long term, cumulative effect of fishing removals, IF effective nutrient replacement has not occurred. This is not an unknown concept, and the possibility has been described in standard texts, as in this excerpt from "Biological Oceanographic Processes" (Parsons et al, 1984):
"The amount of fish taken out of any ecosystem must be proportional to the amount of production within that ecosystem. Since nitrogen is generally regarded as the rate-limiting element in many (but not all) ocean ecosystems, it is appropriate to consider first how much new nitrogen enters an ecosystem each year. The alternate source of this element is recycled nitrogen. However, recycled nitrogen is largely retained within the system so that removal of nitrogen from such a closed system in the form of fish results in an ultimate decimation of the nitrogen reserves of the ecosystem." (Parsons et al, 1984)
Production of marine life resulting from vertical mixing has traditionally been considered "new" production although looking at the wider picture does reveal that these nutrients have just been "recycled" via a longer route so an ecologically significant difference may not actually exist in the system between what we perceive as two different types of production ("new" and "recycled"). Sea life may simply have evolved to base all production on "recycled nutrients," just using a variety of recycling pathways.
Scientific thinking in fisheries management has traditionally been that "surplus production" exists (due to the continuous supply of "new" nitrogen) and that fish removal at an appropriate level will therefore not result in nutrient depletion or serious negative impact on the ecosystem. However, the ocean ecosystem appears to be in serious disequilibrium today, many marine populations are behaving "unpredictably," many are disappearing and maybe basic views like this one now need to be reassessed.
All of the fish removed by humans would have been "naturally recycled" had the fishing not occurred. We reassure ourselves however, that we have poured tons of nutrients into the sea as "replacements." The forms of replacement nutrients however, (mainly sewage and fertilizer), do not appear to be effectively cycled throughout the web that is subjected to fishing. (Vitousek et al, 1997) Therefore the distinct possibility exists that a reduction of the nutrient inventory of the marine food web has resulted directly from centuries of fishing ( and this has been argued in great detail elsewhere on this website).
Regardless of theoretical arguments that might be made against the hypothetical development of nutrient depletion in the open ocean, the simply observation remains that the common brown seaweeds are dying now in clean, sheltered inlets along the open North Atlantic coast. This is not "normal." And this needs to be explained. It is also a fact that the nitrogenous wastes excreted by living fish, invertebrates and zooplankton serve as the ideal fertilizer for these seaweeds. And that these forms of animate marine life in total are in marked decline in the ocean.
The decline of Ascophyllum and Fucus is not particularly surprising when viewed alongside other recent changes in coastal marine life. Multiple changes are occurring that seem to point in the same direction. Beyond the collapsing populations of fish, look at the change that has occurred in the population of barnacles in this area. The loss (contraction of vertical range) of these common intertidal organisms is very hard to account for by using other models of how the marine ecosystem works
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(1) Changing patterns in Ascophyllum and Fucus along the Atlantic coast of Nova Scotia are highly suggestive of an increasing nutrient deficiency of the seawater. Low levels of pigmentation, lowered resistance to normal environmental stressors and a strong correlation between the distribution pattern of damage and the pattern of natural nutrient availability - this much is visible to the naked eye. One additional thing that needs to be done now, is to test the hypothesis of nutrient starvation as the cause of these changes in these seaweeds, by biochemical testing (C:N ratios, protein and nucleic acid content, for instance) to compare the makeup of these deteriorating plants to the known normal ranges for the species.
(2) Seaweed communities have been considered in recent years to be useful indicators of nutrient enrichment of coastal waters. (Schramm and Nienhuis, 1996, Schubert, 1984) Their sessile, long lived nature provides several advantages as their condition integrates fluctuations in nutrient supply over long terms much better than shorter lived species such as phytoplankton. These same qualities will also make seaweeds excellent biomonitors for nutrient impoverishment of seawater. However, until now there has been no index of suspicion on this point, and research efforts to date have been largely focused on seaweeds in grossly polluted estuaries (Schramm and Nienhuis, 1996). It is important to look beyond "pollution" and "climate change" to explain today's changing trends in populations of unexploited marine organisms.
(3) The decline (or disappearance) of similar brown seaweeds, fucoid, has been noted in other parts of the world. There is good evidence that severe eutrophication can lead to the disappearance of some of these species (one well known example is their practical elimination from parts of the Baltic Sea (Schramm and Nienhuis, 1996)), but less dramatic declining trends have been noted in other areas and in less obviously nutrient enriched areas might nutrient depletion also be an unsuspected factor contributing to the demise of brown seaweeds in other parts of the world?
(4) There appears to have been no index of suspicion on the part of scientists that changes might be induced in intertidal communities by a declining availability of nutrients or by decreased nutrient cycling in the ocean. And practically no long term studies have been undertaken in unpolluted intertidal zones. In Atlantic Canada, for instance, monitoring of seaweeds is in its infancy (Bates et al, 2001, Chopin, undated). Early reports of recent baseline monitoring here have noted vague "symptoms of stress" in some seaweeds in the Bay of Fundy, but the interpretation has initially been posited as being related to increases in terrestrial-sourced organic input (Bates et al, 2001). Dramatic changes in intertidal communities have been recorded in well documented cases elsewhere in the world, but the changes are consistently discussed in relation to either nutrient enrichment or global warming (e.g. Barry et al, 1995, Schramm and Nienhuis, 1996)
(5) The nutrient starvation of seaweeds in small sheltered inlets that are flushed tidally by the unpolluted water of the North West Atlantic ocean may offer a very important clue as to the exact pathology of 'what is wrong' with our generally failing marine life. The assumption of a steady availability of nutrients and 'constant productivity ' of this ocean may no longer be justified.
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Barry, J. P., C. H. Baxter, R. D. Sagarin and S. E. Gilman. 1995. Climate-Related, Long-Term Faunal Changes in a California Rocky Intertidal Community. Science 267: 672-675.
Bates, Colin R, Thierry Chopin and Gary W. Saunders. 2001. Conservation in the Bay of Fundy: a macroalgal perspective. (pdf document online at: http://www.unb.ca/cemar/saunders/PDF%20files/conservation.pdf)
Chopin, Thierry. (undated) Marine Biodiversity Monitoring - Protocol for Monitoring Seaweeds, A Report by the Marine Biodiversity Monitoring Committee (Atlantic Maritime Ecological Science Cooperative, Huntsman Marine Science Center) to the Ecological Monitoring and Assessment Network of Environment Canada. (pdf document online at: http://eqb-dqe.cciw.ca/eman/ecotools/protocols/marine/seaweeds/seaweeds_marine_e.pdf)
Deboer, J. A. 1981. Chapter 10 - Nutrients (In Lobban and Wynne (eds) 1981. The Biology of Seaweeds)
DFO Science, 1997. Environmental Habitat Quality Requirements/Guidelines for Rockweed Ascophyllum nodosum. (online document posted at: http://www.mar.dfo-mpo.gc.ca/science/hab/e/rockweed.htm)
Lee, Robert Edward. 1989. Phycology (second edition). Cambridge University Press. (645 pp)
Levring, Tore, (ed) 1981. Xth International Seaweed Symposium Proceeding. Goteborg, Sweden, August 11-15, 1980. Berlin: Walter de Gruyter. (780 pp)
Lobban, Christopher S. and Paul J. Harrison. 1994. Seaweed Ecology and Physiology. Cambridge University Press. (366 pp)
Lobban, Christopher S. and Michael J. Wynne. 1981. The biology of Seaweeds. Botanical Monographs Vol 17, University of California Press. (786 pp)
Luning, Klaus. 1990. Seaweeds: Their Environment, Biogeography, and Ecophysiology. New York: John Wiley and Sons, Inc. (527 pp)
Parsons, T. R., M. Takahashi and B. Hargrave. 1994. Biological Oceanographic Processes (third edition). Pergamon Press.
Schramm , W. and P. H. Nienhuis (eds) 1996. Marine Benthic Vegetation, Recent Changes and the Effects of Eutrophication. Springer, Ecological Studies 123. (470pp)
Shubert, L. Elliot (ed). 1984. Algae as Ecological Indicators. London: Academic Press, Inc.
Vitousek, Peter M., John Aber, Robert W. Howarth, Gene E. Likens, Pamela A. Matson, David W. Schindler, William H. Schlesinger, and G. David Tilman. 1997. "Human Alteration of the Global Nitrogen Cycle: Causes and Consequences" http://esa.sdsc.edu/tilman.htm
For more detail on changing trends in seaweeds and more pictures of the seaweed changes described in this article, see the original seaweed article posted on this site last summer, and also seaweed updates posted in late December, 2001, and early April, 2002.