>When nutrients are added to a lake, river, estuary, lagoon or ocean, they invariably enhance the growth of some life forms (usually algae and sponges) at the expense of others; this is eutrophication. Eutrophication is the leading example of human activity degrading marine habitat in coastal waters, and it is always traceable to agricultural runoff, sewage (treated or otherwise) or both. Today the term is most frequently used to describe the effects on coral reefs from two groups of nutrients: phosphorus and nitrogen compounds.
>Some problems reefs face, such as storm damage, are the result of natural causes. Others are the result of human activity; overfishing, sedimentation from runoff and acidification are prime examples. When these stress factors are present, even a modest increase in phosphorus and nitrogen can spell the end of a coral ecosystem. Eutrophication is usually the final step in a shift from complex reef communities — desirable for so many human endeavors and economies — to simple, much less diverse communities. Simply put, a predominantly coral community becomes a predominantly algae community.
>Natural stressors such as coral bleaching and storms can be thought of as the forest fires of the reef. As bad as it looks right after the fire, the bleaching or the storm, the environment eventually benefits. Nature tends to periodically thin out areas choked with undergrowth, which stimulates resilience and new growth. Even coral reefs recover after a few years, possibly developing to even higher levels of complexity and community than before — provided the environment is fundamentally healthy.
>The elevated phosphates and nitrates that promote rapid growth of turf algae and macroalgae also encourage the expansion of encrusting and boring sponges, which break down the calcium carbonate structure of coral into fine powder. Some species of these destructive sponges have an utterly amazing capability for bioerosion. In fact, the scientific name for the Red Boring sponge is Cliona delitrix; the species name delitrix means "destroyer." This encrusting sponge advances over hard, nonliving surfaces at a rate of 2 inches per year, but it moves twice this fast over living coral. It excavates and burrows deep tunnels into the very core of a coral colony as it grows. Studies show that this and similar bioeroding sponges are capable of removing solid material from the underlying coral at a rate roughly twice as fast as the coral can grow. Healthy coral can defend itself against invading larva of these sponges, but stressed reefs are unable to repel the destroyers.
>Klaus Ruetzler, a research biologist at the Smithsonian Institution, confirms that in many coastal waters of the world sponges of the Cliona genus are destroying coral reefs at an unprecedented rate. The rate of reef destruction due to Cliona is directly related to eutrophication. After determining the percentage of reef covered by Cliona and other eutrophic organisms increases with proximity to sewage outfalls, some scientists have proposed using Cliona levels as a pollution index.
>Biologists tell us sponges have played an important role in reef systems throughout time, especially in providing critical support to reef health during times of biological stress. Perhaps sponges are more resistant to eutrophication and other hazards like rising sea temperatures and ocean acidification than corals are. If so, we could once again see sponges become the dominant animals on Earth's reefs instead of corals. It's already happening.
>© Alert Diver — Spring 2012