CORAL REEF ECOSYSTEM DYNAMICS
- How does “nutrient” cycling work on the reefs?
- How do these systems respond to changes in “nutrient” levels?
- What is the significance of the form (solid/liquid) of the nutrients?
- What is the ecosystem response to the removal of organic biomass by fishing?
-Are fish equivalent to “nutrients?’
- Might their removal be equivalent to “nutrient” loss?
- Can fishing negatively affect “primary production?"
- How do the changing trends in coral reef ecosystem compare with those in other
marine systems?
1. Coral Reef Ecosystem dynamics.
At first glance, coral reef ecosystems seem to present something of an incongruity. A healthy coral reef
is a diverse, highly “productive” community of marine organisms, thriving in exceptionally “nutrient
poor” waters. “Productive” refers to the relatively high amount of “carbon fixation” that takes place in
these systems...measurements and calculations have been made to show that by this measure, coral
systems rank among the most productive of marine ecosystems anywhere. And this is accomplished in
the relative absence of dissolved nutrients (N and P) in the clear, “oligotrophic” water. Rather than
being a “poor” system, however, the coral reef is a “rich” living system that manages to have (and
keep) it’s nutrients largely tied up in solid, living matter. The standing stock of reef fish represents a
significant nutrient resevoir for a these systems.
The secret to the success of the coral reefs is
commonly believed to be the “tight recycling of
nutrients” in the system, particularly in the
corals, in which tiny plants and animals live
together in a symbiosis that conserves key
nutrients quite effectively. Algae
(zooxanthellae), living inside the tissues of the
cnidarian host, harness energy from sunlight and
“fix carbon” by photosynthesis. Energy from
this source is provided to the polyp host in
return for exclusive access to the waste-nutrients
produced by the host. These wastes (N and P)
function to fertilize the algae. The most
significant, and apparently “limiting” nutrient in
the picture is fixed nitrogen, a critical element in
the construction of all proteins. The symbiotic
arrangement allows the partners to avoid the loss
of fixed N to the water, a process that normally occurs in both free floating algae and solo-living
marine animal lifeforms. In larger marine systems, overall fixed N is essentially passed back and forth
between the plant and animal compartments of the community - corals have evolved a way to capture
and complete this loop inside their own special symbiosis. It’s a micro-model of what happens in the
bigger picture, and the same principle applies in the more “nutrient” rich systems of the temperate and
polar zones. Plants and animals perpetually passing the precious ball (fixed N) back and forth - it’s a
common theme in all living systems on the planet.
It is definitely an efficient plan for conserving nutrients, but the coral-algae symbiosis cannot live on
sunshine, CO2 and water alone. A net input of N and P is still essential to maintaining life and growth,
and all corals have feeding strategies in addition to deriving energy from their plant partners. The
partners have the ability to extract dissolved bio-available N from the water at low ambient levels, and
also they capture and consume microscopic prey (zooplankton) as well as bacteria and particles of
edible detritus that come into contact with their mucus layer. In short, corals also need to eat to live.
Like many organisms, corals are adept at storing food-energy for the lean times. Under favorable
feeding conditions they can become quite “fat.” This is important, since the food sources listed above
may not be consistently available. Of the dissolved forms of fixed N, ammonia is by far the most easily
available for uptake from the water by the symbionts, and this apparently is true for phytoplankton in
general. Since living fish constantly excrete ammonia from their gills....it suggests that removing major
amounts of fish from the system “might” ultimately deprive corals of needed nutrients. That’s the
hypothesis.
So, the coral reef ecosystem as a whole is characterized by a relatively low net exchange of nutrients
with the surrounding areas. How much do we know about the overall input-output patterns, or
“nutrient balance” in the system? The capture of plankton from oceanic water passing over the reefs is
thought to be one important input route, although this water is characteristically very “low” in
nutrients. Another significant input is the contribution of fixed N by the blue green algae - this group
of organisms does well in tropical water since they can out-compete other N-limited algae forms,
having the advantage of “making their own fertilizer.” As described in a basic marine ecology text:
“Atmospheric nitrogen is also fixed by blue-green algae such as Calothrix crustacea, which occurs on
intertidal reef flats in the Pacific as a thin, mono-specific film (it also occurs in other reef habitats in
different growth forms). Calothrix can fix nitrogen at the rate of 1.8 kg/ha/day -- two to five times the
rate achieved by fields of lucerne or alfalfa. Fixed nitrogen enters the food web through at least three
routes: (1) the blue-green algae are consumed by herbivores, especially by certain fish with low
assimilation efficiencies, so that the water over the reef gains nitrogen via the fish faeces; (2) in areas
subject to strong wave action, large peices of the Calothrix film are dislodged and washed over the
reef, where they will be available to consumers; (3) Calothrix releases about 50% of is fixed nitrogen
into solution, from which it may be taken up by other autotrophs.” (Barnes and Hughes, 1999, page
140).
Besides the capture of oceanic plankton, and the use of N fixed by the blue-greens, many coral reefs
derive significant nutrient input from terrestrial sources. (Although a normal feature in many
undisturbed areas, the “mangrove-seagrass-coral reef” type of system does not support all coral
communities. Undisturbed, the atoll reefs appear able to sustain themselves very nicely without a
current direct route for terrestrial source nutrients. Efficient recycling is the key.) Elsewhere, in the
“mangrove-seagrass-coral reef” systems, the mangroves grow at the shoreline, tolerant of relatively
high dissolved nutrient levels, very “productive,” and providing habitat for a wide range of other
organisms. The mangrove environment makes use of large amounts of dissolved nutrients, also
significant denitrification takes place there, so the natural effect of the mangrove growth is a
significant drop in nutrient levels in the water. Seaward of the mangroves are the seagrass beds,
another highly productive area that also removes a significant amount of dissolved nutrients from the
water. Seaward of the seagrass beds are the coral reefs, described as “oligotrophic systems least
tolerant of nutrient enrichment.” Mangrove forests and seagrass beds both provide ideal feeding and
shelter for a variety of juvenile reef fish, and they are well known to function as “nursery areas” for
these. From “Coral Reefs, Seagrass Beds and Mangroves” (UNESCO, 1983):
“The sheltered nature of these areas also contribute to make them important as nurseries. Mangrove
areas thus export protein to coastal areas in the form of aquatic organisms that use the mangrove
areas for their early development and then migrate offshore. Well known are the massive migrations of
mullets and shrimp from these areas. This is high quality protein that links mangroves directly to other
coastal systems like coral reefs, seagrass beds, and ultimately to man.” (Bak in Ogden and Gladfelter,
UNESCO Report, 1983).
What about natural “losses” of nutrients from coral reef systems? Some nutrients are inevitably swept
away in the seawater, and some will undergo denitrification in the seabed. Accurate quantification of
the amount of fixed N lost by the system in these ways is really not possible with the state of today’s
knowledge. Therefore, the annual amount of “extra” fixed N/protein in the reef system -- the amount
that could safely be removed without diminishing the system overall -- is unknown, but seems unlikely
to be a very great amount.
The other obvious mechanism of “nutrient loss” from tropical ecosystems is fishing. The removal of
fish from these systems by human fishing most likely represents the biggest net loss of nutrients. Large
quantities of “solid nutrients.” The argument offered in the temperate zones that “the food web can
always replace the fish that have been removed because the ocean contains a vast pool of bioavailable
nitrogen”....well, that particular line of reasoning will obviously not go far in the tropics, since it is
essentially referring to some great reserve of dissolved (liquid, non-living) nutrients. It is clear that the
tropical fish WERE a major fraction of the “nutrient pool” themselves. Is it likely or possible that the
N fixing activity of the blue-green algae can keep pace with human fishing removals? If so, they would
need to “pick up the pace” very significantly, and fix N at a greater rate than they did during the prefishing millenia of the reefs’ existence. Have they done this? It seems not, considering the current
depleted state of life on many reefs....many will say that’s only because they have been
“overfished”...but that’s only a term referring to recent fishing intensity, a more credible argument may
be that all-fishing, over the recent centuries of human exploitation, has greatly reduced the overall
quantity of life in the reef systems. It’s because we are the “unnatural predator,” the one who takes but
does not give back (anything useful)...and that is part of the essence of the whole problem with
disappearing marine life today. Mass coral bleaching (death of clean water corals by food starvation
during warm spells) is a truly new and ominous phenomenon, and it’s most likely to be merely the
ultimate end result of all the fishing that humans have done in the tropics.
3. “OVERFISHING” IS A MAJOR THREAT TO CORAL REEFS
This observation is commonly made today. But exactly what is the nature and consequences of this
threat?
How does fishing have a negative impact on coral reef communities?
1. Local community short-term impact of removing a particular species. Usually described as a “shift”
in species composition rather than being seen as a “loss.”
“Fishing activities are often aimed at a particular trophic level within a community or at one
particular species. By removing either predators or competitors from a system fishing can confer a
competitive advantage on species that were previously constrained.” (Barnes, p 242)
“Overexploitation affects the vast majority of the world’s reefs...At a minimum, overfishing results in
shifts in fish size, abundance, and species composition within reef communities. Evidence suggests that
removal of key herbivore and predator species may ultimately affect large-scale ecosystem changes.
For example, removal of triggerfish has been linked with explosions in burrowing urchin populations,
their prey, who subsequently accelerate reef erosion through feeding activities.” (World Resources
Institute, online article “Threats to coral reefs, coral ecosystems”).
Unusually large outbreaks of coral-eating “Crown of thorns starfish” have damaged reefs in quite a
few places in recent years, including the Great Barrier Reef of Australia. This seems to be a suspected
result of fishing removal of their predators -- “overfishing” the carnivorous reef fishes. And that much
is not hard to believe.
The removal of herbivores is similarly seen as resulting in increased algae presence on reefs: “In the
Caribbean, decades of overfishing has led, in many places, to very low levels of grazing fish species.
Because of this, herbivorous sea urchins (a nonburrowing species) have played an increasingly
important role in keeping down algal growth. In the early 1980s, huge numbers of these urchins
succumbed to disease. Without grazing fish or urchin populations, and spurred on in many areas by
organic pollution, algae quickly dominated the reefs, inhibiting coral settlement and sometimes
overgrowing living corals.” (WRI)
(The removal of herbivores plus the addition of organic pollution are seen as both contributing to the
increased growth of algae on the reefs. Entirely possible, of course.)
2. Destruction caused by fishing methods
Physical damage to habitat and non-targetted species. This is a major concern in many heavily
exploited reef systems. “Blast fishing, fishing with cyanide and other poisonous chemicals, muro-ami
netting (pounding reefs with weighted bags to scare fish out of crevices), and in deeper waters,
trawling directly damage corlas. Because these methods are generally nonselective, large numbers of
other species, along with undersized target species, may be swept up in nets or killed by poisons or
explosives in the process...As not all fishing methods are destructive, this is less of a widespread threat
than overexploitation.” (WRI)
3. Fewer fish.
“Overfishing” has one obvious and direct impact on reef fisheries: less fish become available for
human harvesting and consumption.
4. Reducing the standing stock of reef fish lessens the aesthetic appeal of the reefs thereby possibly
damaging their value to the tourism industry.
5. Occasionally, one will encounter the phrase “corals depend on fish” but it inevitably seems to end
up meaning that corals depend on fish to consume their algae competitors and thereby maintain a more
coral-friendly environment.
The prededing five points summarize what seems to be the usual thinking about how fishing degrades
or damages coral reef ecosystems. But there is one other mechanism of harm wrought by fishing here,
one not noticed because of its insidious nature perhaps; it seems most likely that fishing removal of
organic “biomass” leaves behind a generalized nutrient deficit...in a system with a known practice of
“tight nutrient recycling.” Have we somehow forgotten to state the obvious:
“FISHING REMOVES FOOD FROM THE SEA”...LOGICALLY RESULTING IN LESS
FOOD BEING AVAILABLE TO SUSTAIN THE LIVING MARINE SYSTEM?
Is there theoretical or proven evidence to support or to deny this particular hypothesis?
“One of the universal processes inherent in all ecosystems is the recycling of matter. Without this
process life on Earth cannot be sustained, as most of the 30-40 elements necessary for the growth and
development of living organisms are in finite supply.” (Barnes & Hughes, p 233)
Safe to say, similarly,.... “without this process life in the sea cannot be sustained?”
And regarding finite supply, fixed nitrogen is recognized as most often being the limiting nutrient in
marine systems. Therefore efficient and effective recycling of fixed nitrogen is a basic essential
requirement for the continuation of life, and scientific evidence for a real balance between fixed N
removed by fishing, and fixed N put back by humans...does not exist. (In contrast, it is a simple thing
to delineate the exact routes by which all natural predators in the sea efficiently recycle the fixed N and
other nutrients that they have consumed during their lives. They seem to operate on the simple
principle “keep most nutrient ‘inputs’ in solid form” as this keeps them “in the game” and doesn’t
trigger the systemic decompensation that kicks in when too much of the total is reduced to liquid form
at one time. For best results, fertilize the phytos only lightly...)
Regarding coral reefs this is a typical explanatory statement:
“High production with very low available nutrients is explained by high levels of nitrogen fixation plus
very intense nutrient recycling.”
The understood meanings of the words, “nutrients” and “production,” might be contributing to the
confusion. Reference has already been made to the problem with using the word “nutrient” to denote
those existing in liquid form only, and how this obscures the reality of how much “solid nutrient” has
disappeared. The word “production” can also cause confusion - it seems that it has a couple of
noticeably different meanings.
“Primary production” in marine biology refers to the amount of carbon fixed by the phytoplankton,
basically the rate of conversion of CO2 to food. Expressed this way, estimates of “productivity” on
coral reefs might be something like “7,000 gC/m(squared)/year.” A lot more carbon fixation takes
place on the reefs as compared to the surrounding areas. The potential problem is when the assumption
seems to be made that such a “highly productive” system can afford to give up a lot of fish
repeatedly....there seems to be an implication that there should be no trouble in replacing them...just
“producing” more. A lot of carbon may be fixed, but a lot of it may be reconverted to CO2 via
respiration of organisms...and a lot more may become rather permanently sequestered in the seabed. It
is not at all clear that there is any relationship between “grams of carbon” fixed on an annual basis, and
the potential for “sustainable” fisheries yield.
“Carbon fixed by photosynthesis on the reef is lost partly by offshore transportation but most is
accounted for by the metabolic activity of intermediate consumers. The production to respiration ratio
(P:R) therefore is close to unity and there is insufficient fixed carbon left to support high, sustainable
yields of large carnivores at the end of the food chain.” (Barnes and Hughes p 141)
“Close to unity?”....meaning that most of it is used up in that manner (respiration). So only a relatively
small amount would be left over to invest in building new fish flesh?
“...the end of the food chain.” -- That is another unfortunate phrase, since it’s a reflection of human
linear thinking...of the sort that has gotten us into this kind of trouble in the first place... “The food
chain” concept may serve to distract us from really seeing “the food cycle” -- a more accurate image
being of a CIRCLE, with no “end” -- that’s really what the recycling design was based on. The “food
chain” is circular, not linear! ...Acknowledging this is hard for us to do however, since it is rather
difficult to explain how it is that humans participate in the marine “circle” of perfectly balanced give
and take -- it’s undeniable that that was the ecosystem plan that worked to sustain marine communities
for eons. (We like to think of ourselves as just another top predator, happily glossing over the
differences between ourselves, seals and sharks...) The balance of species in the sea doubtless shifted
many times, and even the “total biomass” was subjected to natural fluctuations as the planet changed,
but over time, and largely due to the N-fixing work of the blue-green algae, a great wealth of organic
“nutrients” was accumulated in the sea. The most-valuable N was carefully conserved and recycled to
the greatest extent possible by the living organisms. And, any way you look at it, “Mother Ocean” is
just not looking particularly “wealthy” anymore...
“There are plenty of high-profile examples of marine species, such as whales, dolphins, seals, turtles
and seabirds, that are considered to be endangered. However, these organisms tend to be at the top of
the food chain, and so contribute little to the productivity of the ecosystem. Of more concern are the
abilities of human activities to affect animals from lower trophic levels, the populations of which are
considered, often over-optimistically, to be more resilient to exploitation.” (Barnes and Hughes, p 252,
1999)
There it is again, “the top of the food chain”...as
if there is nowhere to go from there, except
possibly “up” another notch to be consumed by
humans (?). And what exactly is the meaning to
be taken from the word “productivity” in that
sentence? There’s the myth again, that the
creatures at the higher trophic levels are not
particularly important to the functioning of the
ecosystem as a whole. But the truth is that they
were simply living there and practicing “efficient
recycling of nutrients,” just like all their
neighbours at the “lower trophic levels.”
That quote was taken from a university level
textbook on marine ecology published in 1999.
Granted, a lot of current literature uses the term
“food web” instead, and many diagrams exist of
“food webs” which are essentially a complex
intertwining of many upward tending “food
chains.” Occasionally the “downside” is
included, but usually only for lifeforms at the
very lowest levels, for instance the “microbial
loop” shows a circular movement pattern for nutrients. Usually the diagrams show all the top
organisms as “dead ends” for nutrients.....perhaps that’s because that’s the way that we unconsciously
“want” and “need” to see them. The fact is that besides feeding off their prey, predators also feed their
prey in more ways than one. It was always the secret to their continued health and success.
But the “productivity” concept gets mixed up with fisheries “production.” A totally different meaning,
“production” here means the amount of flesh extracted from the ecosystem by humans.
From a text on reef fisheries:
“Because industrial fishing gears are largely precluded, reef fisheries are the domain of small-scale
fishers....Production is governed primarily by two features: distance from centers of human
population, and population densities and hence demand for seafoods. Many remote Pacific atolls have
perhaps never been fished by anything more than a passing yacht.” (Polunin and Roberts, 1996)
By this definition the Pacific atoll reefs are not particularly “productive,” yet they are among the
healthiest in the world, with the best developed and thriving coral cover, a feature that should define
them as highly “productive.” It is evident that there are two differing definitions for the word, but it
seems that they clearly have the potential to cause a bit of confusion. One example is in the assumption
that since large amounts of fish have been removed from a reef in the past, that it should always
continue to be able to give up large amounts of fish. “Production” of coral reef fisheries has clearly
been declining, sharply in recent decades but a gradual decline reaching back much farther is also
apparent. Has “primary production” also been declining in these systems? Maybe. It seems possible
that it has been, but reefs tend to only have had single calculations done so no trends have been
revealed. Another question is whether just “primary production”/”carbon fixation by algae” is a good
enough indicator of overall health. It does not appear to be, but that seems to be how it is interpreted at
times. (Reassurances are given that "primary production is as healthy as ever," but they seem rather
shaky, and largely based on chlorophyll concentrations.) If “primary production” has gradually
declined, and the slowing of the growth of corals in clean waters seems to suggest that it has...it also
seems entirely possible that it has been an insidious, indirect result of “nutrient” depletion due to allfishing.
“Worldwide, the potential sustainable yield of fish, crustaceans, and molluscs from reefs represents
approximately 10% of the world’s annual fisheries take - worth billions of dollars.” (NOAA)
“...In general, it is clear that degradation has outpaced our comprehension of the problems at many
locations.” (same NOAA source)
Two seemingly contradictory statements - how can the first one really be made with any confidence?
The conclusions drawn about the coral reef system in a recent marine ecology text:
“Despite all the uncertainties about the relative contributions to the coral-reef ecosystems of different
kinds of autotroph, bacteria, dissolved organic matter, and internal vs. external inputs, it is clear that
the phenomenally high total productivity is in large measure due to the combination of a tremendous
surface area of photosynthetic tissue (either in the form of zooxanthellae or benthic algae and higher
plants), optimal light and temperature conditions for photosynthesis, and the tight recycling of
nutrients in an otherwise nutrient-poor environment. The efficient recycling of nutrients occurs both at
the level of the coral-zooxanthellae symbiosis and at the general level of the overall food web. Many
consumers are present in reefs and although in absolute terms primary production is high, relative to
the number of consumers food can be regarded as scarce. Hence food is consumed rapidly and
utilization is efficient. A high proportion of the environmental pool of nutrients is therefore maintained
within living tissues, so reducing opportunities for the loss of nutrients out of the system. Any such
losses are compensated by the slow accrual of nutrients from water masses passing over the reef and
by the nitrogen-fixing activities of blue-green algal associations on the reef or rhizomes of adjacent
sea-grasses.” (Barnes and Hughes, 1999, p 141)
The corals are bleaching, the fish are fewer, are there any other recent trends in the coral reef
ecosystems? Are there any other parallels with recorded changes in fish stocks elsewhere?
The stories of the tropical fisheries sound much like those in the rest of the world. Maybe relatively
lacking in recorded data (it’s hard to find “time series” data on anything), but the themes of a longterm gradual decline to a now very depleted state, declining trophic level of organisms, declining
abundance of fish and average size of fish, and declining size at sexual maturity...are the same in the
tropics as they are wherever we have exploited marine life. The “disappearance of the big ones” is a
major theme all over, and is noted in the reef fisheries as well. This is said to be because we have
preferentially targeted the big ones, and have thereby been killing them off more quickly than they can
replace themselves. But the explanation may not be as simple as that - for example, there’s
contradictory evidence from the North Atlantic where “big cod” are increasingly vanishing from an
unfished stock. Might another factor, affecting the system overall, be subtly “forcing” the bigger fish to
disappear?
I have only been to the tropics a few times, but I recall often eating fish while I was there. One thing I
remember, that seemed to be common practice, was avoiding the consumption of the larger specimens
of reef fish such as groupers. This is because the bigger ones tend to accumulate ciguatera - a toxin
endemic in tropical fisheries; known for centuries, it is capable of causing illness in human consumers.
I remember being told that the spear fishermen did not select fish above a certain size for this reason.
Eating the big ones just becomes too risky. I really don’t know whether or not the avoidance of large
fish would be strong enough to have an effect on the population - but it seems that “possibly,” this
practice should have spared a noticeable number of the bigger older fish, and “perhaps” they should
still be there. But they are not, the big groupers are definitely on the “missing list” for the fished out
areas. If systemic food depletion is contributing to the picture however, the higher trophic level fishes,
like the big grouper (and the big cod), will find themselves increasingly food limited, and this fact
alone could account for their disappearance. Has it? (To compare with the northern cod again, records
there show that the bigger, older fish showed a steeper declining trend in weight-at-age than did the
younger ones...and then the big ones just vanished from the data tables.)
In conclusion, there is at least one more reason why fishing should be considered as a threat to the
wider ecosystem. It seems that fish, even top carnivores, play important roles in nutrient cycling in
coral reef systems; therefore fishing needs to be examined as an activity that inevitably undermines
and decreases nutrient availability overall (...and concepts of “nutrient” need to move beyond the
liquid forms).
Therefore, the next section of this report examines the pathophysiology of coral bleaching, pursuing a
hunch that the problem might stem from a shortage of “nutrients.”