Picture a coral reef. A manta ray glides by, flapping its wings majestically. Vibrantly colored fish hiding in the crevices of branching coral. Small patches of green seaweed called macroalgae begin to grow. They start small but begin to cover entire coral colonies, gradually cutting off their access to light. Eventually the seascape resembles a bumpy lawn of fleshy macroalgae.
Unfortunately this story is common, repeating itself around the world and especially in the Caribbean. On this expedition, we hope to collect samples to help us investigate what is causing the decline in coral reef health here. We may think of tropical seas as warm and delightful to swim in, but they are actually a very hard place for marine organisms to grow. Corals have adapted to the low nutrient levels in the tropics, working together with their symbiotic algae to cycle nutrients efficiently. In contrast, macroalgae have a lot of trouble growing in these conditions. However, if the environment becomes more hospitable for macroalgae, they can out-compete the corals and overrun the reef. Macroalgae aren’t all bad as they do provide food for herbivorous fish, but it’s important that all the inhabitants of the reef live in balance – not one taking over at the expense of the others.
It would be great if scientists could pinpoint one factor that favors macroalgae over corals but it appears that there are many factors, some of which are very difficult to address. Extra nutrients entering the reef through fertilizer runoff or human waste, as well as the decline of the macroalgae’s natural predators, herbivorous fish, can give macroalgae a leg up. In the Caribbean, disease has wiped out some of the faster-growing corals, and killed off sea urchins that eat macroalgae. Given this foothold, macroalgae can take advantage of opportunities such as further disease, nutrient input, or overfishing of their predators. In addition, stresses like rising temperatures and ocean acidification make it harder for corals to keep themselves above the macroalgal growth.
Many of these processes are related to human activity, but there are also natural influences impacting the coral reefs. My team is interested in how natural climate swings can also affect reef health. We will collect as many corals as we can from many sites in the Caribbean to investigate coral health in the context of natural climate variability. Here we go!
Written by Alice Alpert of Woods Hole Oceanographic Institute. See the original post here.