HISTORY OF MASS CORAL BLEACHING
Where does this fit in the longer-term picture?
And how ominous a sign is this really?
Might it be just part of a natural cycle, a thing that corals have survived many times before?
...what do we know about the history of life on coral reefs?
First noted in the early 1980s, becoming more widespead and persistent in the years since, and generally coinciding with annual water temperature maximums, “mass coral bleaching” has emerged as a major concern, a rapidly expanding and and very poorly understood problem. Reef corals are dying off worldwide and scientists and others are now becoming alarmed by this...
A key question to be answered is “has mass coral bleaching occurred before, or is this truly a new phenomenon?” How far can we look back and what is revealed by the history of the reefs? There are some accessible records, and scientific investigations have revealed that the current global pattern of “mass coral bleaching events” on the reefs has never happened before.
Coral reefs are very old, living systems that co-existed with the dinosaurs, and they now appear to be collapsing ....and the primary cause (we must admit) appears to be the assaults that have been inflicted upon them by man. Co-existence with man is proving to be exceedingly tough for the corals and other sea creatures. Never before, in all the ages of the reefs’ existence, have the land animals forcibly removed great quantities of marine life from the sea. That is what has been accomplished in recent centuries. “Fishing removals” taken by man is the primary “new” development, and the one that has had the most significant impact on life on the reefs and other oceanic fish habitats. Essentially never practiced in a truly “sustainable” manner (one which would have included a pattern of equitable give and take), fishing has resulted in a gradual but steady depletion of nutrients/living biomass in the sea. The tropical reefs are just one more chapter in the wider story of worldwide extraction/elimation of sea life by humans. This effect has been a cumulative one, and it started to be felt centuries ago. In his recent book “Ocean’s End,” Colin Woodard quoted Jeremy Jackson, a reef scientist from the Scripp’s Institution of Oceanography; Jackson had reviewed historical records of marine life on Caribbean reefs:
“Jackson started wondering just how much deterioration had occurred in the decades and centuries before he started his work. Using historic records, he pieced together how the reef communities might have looked when Columbus arrived in 1492. ‘Think about the wildebeests and lions and all that on the plains of Africa,’ he said. ‘Well, there was a world out there in which the biomass of big animals amongst the reefs was greater than the biomass of the big mammals in the Serengeti plains. The absolute minimum number off 100-kilogram green (sea) turtles was something like 35 million. Think about that: 35 million 220-pound turtles grazing on crustaceans, sea grass, starfish, and mollusks. The productivity of those reefs must have been fantastic! The whole mind-set of scientists about what is a ‘pristine’ reef is completely wrong.’ The turtles were wiped out by the middle of the eighteenth century, hunted to near-extinction to provide cheap food for slaves on sugar plantations. All large vertebrates in Caribbean reefs systems had been decimated by 1800 to feed sailors and slaves, and there has been subsistence overfishing of smaller vertebrates and invertebrates since 1850. The latest declines are merely the latest wave, the decimation of all but the smallest grazers to feed expanding populations. ‘Coral reefs,’ Jackson concluded, ‘aren’t sustainable at even a tenth of current fishing rates.’” (Woodard, 2000, p 160-161)
“The latest declines are merely the latest wave..”...it seems the ripple effect of the “latest wave” is being felt by the corals themselves, damaging the very “base” of the reef systems. And what is being felt most acutely now is the LOSS of the great biomass of living animals that once participated in the endless recycling of life on the reef....and that loss translates into hunger and starvation...and results in what could be described as “clean water coral bleaching.”
The downfall occurred over recent centuries. The tropical marine ecosystem changed from a rich system teeming with life, including many large animals, to an impoverished system populated by the smallest, lowest feeding lifeforms....this history neatly parallels the stories of the North Atlantic, indeed of all other exploited oceans...
Scientists have been able to glean a certain amount of information about reef communities that existed millions of years ago, for example through the study of core samples from ancient reef sites. One worrisome finding concerns a type of coral that was formerly widespread in the Caribbean, but has been all but wiped out by coral bleaching in recent years. “Acropora,” is a fast growing, branching coral also known as “staghorn” or “elkhorn” coral. B. J. Greenstein, et.al. reported in 1998 that “the present Caribbean-wide decline of A. cervicornis is without historical precendent” and that it “contrasts with the long-term persistence of this taxon during Pleistocene and Holocene Optimum time..” The conclusion is that this coral species remained dominant throughout very long periods that spanned many changes in climate and sea level, changes that were much more extreme than those the planet has been experiencing lately. So Acropora must be a very resilient organism....a major die-off of this type of coral is unprecedented....but something essential has changed since mankind has become involved in the sea.... “although the A. cervicornis-dominated coral association persisted during the Pleistocene climatic fluctuations, it is apparently vulnerable to the array of perturbations currently being inflicted on it.” (Greenstein et al, 1998) The largest “perturbation” has been the physical removal of a large fraction of the living, organic biomass (= food) from the reefs. All living things, of course, are ultimately vulnerable to starvation...none of Acropora’s previous adaptive mechanisms that have successfully seen it through wide climate and environmental changes in the past, can be used to save it now. Today’s challenge is a new one. Acropora’s “fast-growing” nature has proven to be it’s downfall, now that food-limitation is an issue for corals. That is why the elkhorns and other faster-growing species are disappearing first. The increased vulnerability of the faster-growing coral species is one of the emerging patterns. Corals that are genetically programmed to grow more slowly, needing less nutrients, will be able to persist for a while yet. (In other marine systems, the faster-growing species are also feeling the pinch first, it is another theme in the wider pattern. And later in this discussion it will be shown how the physiological changes in the bleached corals are consistent with simple nutrient starvation.)
On a shorter time frame, scientific work has revealed recent patterns in growth rates of corals. Growth rates can be affected by many variables (temperature being an obvious one, and the usual trend is for organisms like corals is to grow more quickly at higher water temperatures), but one undeniable variable that also affects growth rate, is feeding success. It is not clear that this factor has been adequately considered in the explanations given for the coral growth trends that have been discovered.
Data on coral growth rates spanning recent centuries has been obtained from Florida and from the Great Barrier Reef in Australia. Periods of slower growth (calcification) have been associated in the past with unusually cold periods; a “cold shock” can also apparently induce coral bleaching. Both of these locations have shown decreased growth rates in corals in recent decades (...but no-one suggests that this time it’s because of cooler water, all the temperature “buzz” actually being about warmer water lately). Regarding the study in Florida, “Attention is drawn to nutrients from sewage outfalls as a possible contributing factor to observed growth rate decline since 1950.” (Hudson, et al, 1994)
A growth rate decline in corals has occurred over the last 50 years, and it is clearly NOT associated with a cooling climate trend? Has is been caused by an OVERSUPPLY of nutrients?
To cause a decline in coral growth, nutrient enhancement needs to be noticeable, because only a slight increase in nutrients will actually stimulate increased growth of corals, or at least their food-giving and color-giving partners, the zooxanthellae. This much has been proven. (Hoegh-Guldberg, 1994, and the article at http://www.marinebiology.org/coralbleaching.htm which states “Rather than causing coral bleaching, an increase in ambient elemental nutrient concentrations (e.g. ammonia and nitrate) actually increases zooxanthellae densities 2-3 times.”)
It is difficult to see how these particular Floridian reefs could have been so damaged by sewage outfalls, how they might have been subjected to anything more than a very slight nutrient enhancement (if that). The study was conducted in Biscayne National Park, a relatively pristine area described as having water that is “refreshingly clean, extraordinarily clear. Only the maintenance of the natural interplay between the mainland, Biscayne Bay, keys, reefs and the Florida Straits keeps it that way.” (park website) Located south of Miami, and including a long protected coastline, it seems that the natural effect of the Gulf Stream would be to move “nutrients from sewage outfalls” away from this large national park. The keys and reefs are located 10 miles or more from the also-protected mainland coast. The park is described as very “clean” and apparently it is a very popular location for recreational fishing. While it still contains fish, significant quantities of fish biomass have doubtless been removed from the Biscayne Park reefs in the centuries since humans arrived, allowing for the distinct possibility that the decline in the growth rate of coral there may be related to fishing-induced nutrient depletion of the system.
Attributing coral damage to nutrient enhancement is a debatable point these days in Florida, since many reefs are located well away from the shoreline and are thereby well flushed by open ocean water. Scientists disagree on this point. Some believe that nutrient input is one of the main stressors causing reef degradation, including mass bleaching, in Florida waters. But U.S. Environmental Protection Agency water quality expert, Bill Kruczynski was quoted recently in the media:
“Kruczynski and others believe runoff from cess pits and septic tanks is degrading sea water very close to the mainland of the Keys. But there’s little evidence that nitrogen is reaching the reefs that sit miles out, Kruczynski said. “In studies we’ve done, we’ve had very little information that says those sources of nutrients are actually making it out to the reef and causing the decline,” Kruczynski said. “If a molecule starts at land and starts seaward you have six miles of algae and seagrasses to collect it. It doesn’t get there.” (ENN news story 2001, from: http://www.enn.com/news/wire-stories/2001/02/02052001/ap_reef_41836.asp?=1 )
The argument about whether or not enhanced nutrient runoff into the sea is affecting offshore reefs and causing the decline - or not - is very significant. Because if the nutrients are NOT getting there, then they are NOT having any deleterious effect, and the 50 year decline in the growth rate of corals is due to some other factor (...possibly the fact that the nutrients are NOT getting there?). The half century of slowing growth may just have been the preamble to todays crisis, corals with weakened immune systems suffering from epidemics of infectious disease, mass coral bleaching and death. Could the decades of slowed growth and ultimate sickness and death now be telling the tale of an insidious, gradually worsening condition -- of malnutrition due to fishing induced nutrient depletion of the reefs in particular and the open ocean in general?
Still...nutrient depletion in the sea is one hypothesis that needs to be seriously investigated at this point if we ever hope to understand these critical problems.
Elsewhere in the tropics, an analysis of historical trends in shell abnormalities showed that deformities are now very common in the shells of “large foraminifers” following bleaching events. The same new set of environmental factors is apparently being felt by these ancient tiny organisms. Previously, “shell abnormalities were known in tidal pools or very shallow water with temperature, salinity, oxygen and pH excursions...or in small foraminifers in heavily polluted areas...but never in the recent past in pristine areas bathed by stable oceanic open waters. It proves that bleaching is a new phenomenon. Moreover...such high frequency of deformities was never observed in geololgical time. It indicates that bleaching is truly an unprecedented stress at planetary scale.” (Pecheux, 1992)
A perturbation so severe that it has seriously altered the nature of “stable oceanic open waters?” ...a development that is unprecedented in long historical records?
...that’s getting scary!
Also, it seems that the whole “mass coral bleaching” problem has taken scientists completely by surprise. No-one anticipated this problem. Scientific books and papers on corals before the early-mid 1980s make no mention of it, seemingly it just did not exist; it was never seen anywhere. Even now, in a current up-to-date book on coral reef fisheries, (Reef Fisheries, Polunin and Callum (eds) 1996) none of the contributors make mention of mass coral bleaching in their discussion of all the issues facing reef fisheries. This reluctance or inability to look at the whole picture is seriously handicapping our efforts to understand the changes in marine ecosystems. Also, along the same line, our natural reluctance to really objectively consider the whole extent of the negative impacts that fishing has had on marine life....this also prevents us from seeing the picture clearly.
We need to face up to the true history of fishing...and somehow overcome our habitual and sentimental attachment to it.
We cannot find the holy grail of “sustainability” because it simply doesn’t exist using out current approach. We essentially give nothing useful back to the sea. “Sustainable” fisheries in the past were undertakings that persisted for many years, seemingly without depletion of resources, but the reason that they appeared this way was because they were relatively light exploitation applied to a very rich resource (for example, subsistence fisheries by native peoples). The dents were perhaps too small to notice? Actually, historical records of commercial fisheries going back for centuries are full of reports of “declining resources.” The story of the history of fishing and whaling actually tells repeatedly of exploitation leading to depletion (the first dramatic examples being the marine mammals, whales and walruses, the seabirds and the turtles), then the human hunters moved on to new fishing grounds, or new target species, and the same sequence of events unfolded again, over and over. The total “resource” was always dropping to lower levels in the background. For many years, it was possible to increase overall fishing yields due to increasing effort and constant advances in technology, which greatly increased the efficiency of the fishermen. These factors effectively obscured the background theme, the thing that we are so afraid to acknowledge, the steady decline in the total available resources.
But now it is becoming increasingly clear from all sides that the well is running dry. We had better wake up soon....the mass coral bleaching, a mass of “white flags” from the tropics?...we had better take careful heed of their message:
=> THIS IS NEW, AND IT’S A TRULY OMINOUS SIGN.
=> IT IS NOT PART OF A “NORMAL” CYCLE, NOR IS IT A SIMPLE REFLECTION OF CHANGING TEMPERATURES.
=> SIGNS OF WEAKENING PRECEEDED THE MASS BLEACHING OF CORALS (EPIDEMICS OF INFECTIONS AND SLOWED GROWTH)
=> BEST GUESS: “FISHING” HAS “SOMETHING” TO DO WITH THIS PROBLEM.
We will have only one chance to establish the correct diagnosis, and there is clearly not a lot of time to be wasted, considering the rate at which corals are being lost. It may be later than we think.
And something REALLY BIG is going to have to change, we may just need to do an “about face” in our thinking about the “renewable” resources of the sea. It’s the “renewing” part that has proven to be the biggest problem...something’s missing that is a critical factor in the “renewal” process. Could it be the food? As someone else once said, “When theory conflicts with reality, must we point out that reality always wins?”