A study published in the October 17 issue of the Proceedings of the National Academy of Sciences says that certain seaweeds around the Fiji islands are contributing to coral bleaching, which is helping to inhibit the growth of coral reefs around the islands.
Scientists have studied eight seaweed species and their effects on three corals and suggest that once corals decline due to other factors such as coastal pollution and warming oceans, some seaweed emit coral-harming hydrophobic compounds that inadvertently inhibit the growth of surviving corals on a reef as well as new coral larvae that begin growing on a reef. In addition to bleaching, these compounds caused decreased photosynthesis and in some cases, death of corals such as acropora.
The authors of the study, “Macroalgal terpenes function as allelopathic agents against reef corals” hypothesize that the seaweed only recently (in the last 30 to 40 years) began to produce compounds on their surfaces to fend off microbes or herbivorous animals, and it is these compounds that have negative effects on corals. The seaweeds are also growing at fast rates due to overfishing of fish and urchins that eat seaweed in spite of marine protected areas that ban fishing.
“There are two major forces affecting the abundance and distribution of seaweeds on reefs – herbivory (i.e. fish and urchins eating plant material) and nutrient enrichment (i.e. agricultural runoff and sewage outfall acting as fertilizers),” said Douglas Rasher, a doctoral candidate at the Georgia Institute of Technology in Atlanta and principal author of the study. “Herbivores reduce the abundance and distribution of seaweed, while nutrient enrichment stimulates the growth of seaweed. On reefs heavily used by people, overfishing of herbivores and nutrient enrichment often occurs simultaneously – both processes promoting the establishment, proliferation, and dominance of seaweeds on reefs.”
In the study, the authors note that within just a few kilometers of a protected zone near a Fijian village, where no fishing is allowed, sits an unprotected reef, where fishing is allowed. The taking of herbivorous animals allows the seaweed to grow unabated.
One solution, the authors say, is to institute a fishing ban on those species that are known algae eaters. This will increase the number of these animals to control the seaweed, and Fiji’s marine protected reserves can then introduce new coral larvae to help the reefs regenerate.
“Understanding which seaweeds harm corals, and which herbivorous fish and urchins consume these algae, can be used to develop a proactive fisheries management program that promotes grazing, thereby limiting community transitions to seaweed dominance,” Rasher told FishChannel. “By actively managing ecosystem processes such as herbivory, we may be able to maintain coral dominance on healthy reefs, or reverse the transitions to seaweed dominance that have occurred on many degraded reefs. Moreover, limiting nutrient input from sewage and agriculture may help to slow seaweed growth, and thus their competitive ability, on reefs.”
An abstract of the article “Macroalgal terpenes function as allelopathic agents against reef corals” can be found here