Overlooked group of algae is stressing and killing Caribbean coral reefs
An increased abundance of fleshy algae is generally accepted as a tell-tale sign that a coral reef system is declining due to human impact. However, researchers from the University of Amsterdam (UvA) and Carmabi discovered that another group of algae that was overlooked until now is stressing and killing Caribbean coral reefs to a much larger extent.
In recent decades many researchers have observed that coral reefs all over the world are less and less populated by corals and that algae have become more abundant than ever before. Overharvesting of herbivores (e.g., parrot fishes) and eutrophication of reef waters (i.e. the introduction of excessive amounts of food sources) have caused rapid proliferation of fleshy algae on present day reefs. As a result, an increased abundance of fleshy algae is now one of the tell-tale signs that a reef system is declining and that conservation measures are required. However, while attention has focussed on “fleshy algae”, i.e., algae that are generally large and “plant-looking”, another group of algae, the much smaller turf algae, is hardly ever considered. In a recent joint study by the UvA’s Institute for Biodiversity and Ecosystem Dynamics (IBED) and the Curaçao based marine research station Carmabi showed that turf algae are stressing and killing nearby corals to an alarming extent. The research team was led by Dr. Mark Vermeij, who holds a shared position at UvA-IBED and Carmabi. Vermeij performed the study together with Dr. Petra Visser of UvA-IBED, as well as several MSc students of UvA-IBED’s MSc program Limnology and Oceanography.
Turf algae are communities of small algae that belong to a large number of different species. Due to their small size and often uncertain taxonomic status, they are hardly ever considered in traditional surveys and ecological studies on coral reefs. This is very surprising given the fact that these small algae are the most dominant algal group on present day coral reefs and by far exceed the surface area covered by fleshy algae. The study by UvA-IBED and Carmabi shows that the presence of nearby turf algae has severe consequences for corals. While the researchers found that corals can normally overgrow turf algae, eutrophication quickly reverses this balance and causes turf algae to rapidly overgrow corals. Vermeij: “I found it startling that the negative impacts of turf algae on coral reefs exceed those of traditionally considered fleshy or macro algae, yet no one seemed to have noticed this until now”.
Even more alarming is the finding of Vermeij and his team that the presence of herbivores that eat the turf algae could not reverse the dominance of turf algae over corals. This means that traditional conservation measures aimed at reducing algal abundance through measures such as increasing herbivore populations through the establishments of Marine Protected Areas or by tightening fishing regulations will not reduce the impact of turf algae on local coral communities as long as eutrophication persists. “Since nutrients seem to be foremost responsible for rapid expansion of turf algal communities, new conservation strategies should be focussed on reducing their influx to near shore waters in which coral communities occur”, Vermeij explains, “In this technological era, that shouldn’t be too difficult. I hope that The Netherlands in particular will work in this direction now that after the recent administrative reform of the country’s former Caribbean colonies, it has coral reefs herself”.
The study was published in the peer reviewed online journal PLoS ONE.