Role of Nutrients in Coral-Algal Symbiosis

In "classical" coral reef studies (a body of literature stretching, perhaps, from the 1950s to the 1970s), coral reefs are associated with nutrient poor waters (the "gin clear", high visibility waters that we are familiar with from natural history documentaries and glossy magazines). Reef corals are able to survive in these environments (the marine equivalents of terrestrial deserts) because corals conserve nitrogen by having low rates of protein catabolism, and instead break down translocated lipids and carbohydrates from their symbiotic algae.

That, at least, is the theory: In reality, coral reef waters today are more often than not clouded by sediments and fogged with microalgae fertilized by nutrients from terrestrial sources. While it is true that reef corals are exceptionally competitive in nutrient poor conditions, there is scant evidence that high nutrients are necessarily detrimental to corals in any direct sense. While phosphates, in particular, may act as "crystal poisons" that inhibit coral calcification, and while nutrients may cause uncontrolled proliferation of zooxanthellae, the seriously deleterious effects of high nutrients are probably more indirect.

High nutrients favor fast growing macro- and turf algae that smother and overgrow corals. If unchecked these algae upset the balance of the coral-dominated ecosystem and result in a "phase shift" from one community to another. Just such a shift (although not as simply explained by the scenario outlined here) has occurred over the last two to three decades in Jamaica.