Coral Reefs vs. Poisonous Seaweed: overfishing gives dangerous seaweed an edge

Regarding coral reefs, it has been said that when water conditions are less than optimal and algae is allowed to run rampant - often the result of runoff and pollutants high in nitrogen - then corals can be pushed out or overtaken. A recent study, reported by SeaWeb, cites the danger to coral reefs from species of seaweed that are capable of poisoning the coral. The seaweed is normally kept in check by feeding fish, but when overfishing reduces the population of these important herbivores, then the corals are at risk.

Overfishing Allows Seaweed to Flourish, Killing Corals

Overfishing of herbivorous fish is allowing seaweed species (such as this green seaweed Chlorodesmis fastigiati, center) to thrive, killing corals with which they come into contact. Mark Hay

Researchers have found that several species of common seaweed contain chemicals that kill corals. In the Proceedings of the National Academy of Sciences, Douglas Rasher and Mark Hay of the Georgia Institute of Technology in Atlanta report that five of seven seaweed species caused bleaching, declines in photosynthesis and death of coral tissues when they came into direct contact with Porites [stony] corals off the Caribbean coast of Panama. Three of eight seaweed species had similar effects on Porites corals off Fiji.

The researchers noted that only the areas of the coral that came into direct contact with the seaweed were affected. Attempts to replicate the effect with plastic seaweed models did not produce the same results. However, when extracts from seaweed tissue were embedded in gel strips and placed on the corals, the results almost precisely replicated those from the seaweeds themselves, suggesting strongly that chemicals within the seaweeds were responsible for the damage.

The researchers note that when seaweeds were placed on corals in a marine protected area off Fiji, herbivorous fish that were prevalent within the area rapidly consumed them. However, when placed on an adjacent reef where fishing took place only 1,000 feet (more than 300 meters) away, the seaweeds were consumed far more slowly, if at all. They write that their study shows that in healthy reef systems, coral are protected from the impacts of seaweeds, and that even relatively small amount of fishing of species that graze on those seaweeds could have potentially disastrous effects on coral reefs.

Source: Rasher, D.B., and M.E. Hay. 2010. Chemically rich seaweeds poison corals when not controlled by herbivores. Proceedings of the National Academy of Sciences.

Contact: Mark Hay, Georgia Institute of Technology. E-mail: mark.hay@biology.gatech.edu