Could the death of some corals be due to nitrogen starvation?
If so, what can be done to improve the situation?

by Debbie MacKenzie

1. Introduction
                                                                                                     
2. 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?

3. 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?

4. PATHOPHYSIOLOGY OF MASS CORAL BLEACHING:
- A Look at the physiological changes at the level of the individual organisms, and the distribution patterns of the phenomenon.
- Are the observed changes in bleached corals consistent with what would predictably result from their nutrient starvation? (The short answer is “yes, very much so.”)

5. FISHING - IS IT A CAUSATIVE FACTOR IN MASS CORAL BLEACHING?

6. INVESTIGATIONS INTO THE CAUSE OF MASS CORAL BLEACHING - WHAT HAS BEEN DISCOVERED?
(the currently most favored “cause” is elevated water temperature, also considered are increased level of atmospheric CO2, and possibly UV radiation...but significant uncertainty remains.)

7. CONCLUSIONS
- IT’S TIME FOR AN ECOSYSTEM APPROACH
- IT’S TIME TO “THINK OUTSIDE THE BOXES”
- (and consider this one: BY UNDERMINING THE “BIOLOGICAL PUMP,” FISHING IS HAVING A DELETERIOUS EFFECT ON THE ATMOSPHERE)
Reference List


 INTRODUCTION:

Human fishing has reached the point of causing significant biomass depletion in marine ecosystems, a large amount of data on changes in northern temperate zone fisheries seems to back this up (see elsewhere on this website), but it is necessary also to study the stories from the tropics. If fishing causes systemic “food depletion” in one marine ecosystem, then the same pattern should be evident in all. This report looks at the recent developments and discoveries about the condition of coral reefs - primarily the "mass bleaching" of corals - and tries to justify the hypothesis that food limitation is one of the major causative factors that is driving these trends.

In recent years an alarming new trend has developed in coral reefs worldwide; large numbers are turning white, and many are dying. On some reefs the majority of corals have been eliminated, and many have been affected to lesser degrees.

The hypothesis that fishing has a cumulative effect of lowering overall biomass in the ecosystem would predict that the negative effects, including coral bleaching, be worse on reefs that have experienced the most fishing exploitation/nutrient extraction. (The "fishing loss" is spread throughout the ecosystem as a whole, similar to what happens in temperate fisheries and systems, as is argued in “The Marine Nutrient Cycle” - that’s why for a long time the fished stock seems able to replenish itself, but ultimately it weakens, along with the whole ecosystem.) The hypothesis also should help to explain inconsistencies in other proposed causes of coral bleaching, namely increasing water temperatures (sometimes attributed to global warming). And if food limitation is one of the driving forces behind changes in marine ecosystems today (as this author has argued for the temperate systems), then a close look at the physiological changes and patterns in the dying corals should be consistent with nutrient starvation as the primary cause. There needs to be a plausible mechanism proposed for the cause and effect (fishing removals causing overall nutrient loss to the system), and minor aspects of the puzzle should also start to fall into place (for example the frequently "patchy" nature of bleaching events on a given reef.)

The most interesting findings were these points:

- "Mass coral bleaching" appears to be a new phenomenon in the long history of corals. And signs of slowed growth and illness have preceded it.

- Coral reef ecosystems may be the best examples of the thrifty recycling of precious key nutrients in marine environments. The physical form of the "nutrient," solid or liquid, seems to be highly significant - should we be taking this into account when we are "feeding nutrients" to the sea? Coastal input of liquid fertilizer may not be enough.

- There appears to be no significant difference between the physiological changes in corals dying in mass bleaching events, and corals dying of simple food starvation. Is there a difference?

- Fishing exploitation is suggested as an independent risk factor for mass bleaching events on coral reefs.

- "High water temperature" and "high light exposure" are not adequate explanations for the coral bleaching phenomenon.

- Slowing of the "biological pump" in the ocean would predictably cause an increase in atmospheric CO2 levels on the planet. (The pump sequesters carbon in bottom sediment and calcium carbonate, basically seashells.) A decrease in the total living marine biomass would predictably cause a slowing of the rate of the "biological pump." Fishing, sustained for centuries, would predictably remove enough fish from the sea to cause a large drop in overall marine biomass...if nutrient replacements are inadequate (which they clearly are). Has fishing caused such a decrease in the "total marine biomass?" Yes...overwhelming evidence points in that direction, including the bleaching/starvation of tropical corals...and the historical records of fish extraction and rising CO2 fit together just a bit "too" well....

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