Written by Sydney Grzep
Editted by Bobbie Renfro
Coral reefs are the most biodiverse and productive ecosystems in the world and utilize a complex system of nutrient cycling to maintain a balanced and healthy environment. However, in recent years, coral reefs have endured many pressures that have contributed to climate-driven reef degradation. This can be a big problem since coral reefs, a habitat that occupies 1% of ocean, supports 25% of marine life. One of the many symptoms that reefs exhibit during degradation is algal overgrowth.
In a healthy reef ecosystem, coral and algae release nutrients and form the base of a complicated food web, and algae will be consumed as quickly as it grows. However, a combination of overfishing removing too many reef herbivores and pollution providing excess nutrients has allowed algae populations to expand rapidly and overgrow on reefs. Corals prefer a low-nutrient environment, and in areas of high nutrient pollution, algae can quickly outcompete the coral. This poses an issue to coral because of increased competition for space as well as the relationship between algae and microbes. Algae is a source of dissolved organic carbons, which is released into the water columns and eaten by microbes. The microbes pose a threat to coral by reducing oxygen availability and by introducing diseases in the environment. This can lead to an increase in coral mortality, which in turn allows algae to take over more reef space and further buoy microbe populations. Andreas Haas, a biologist at SDSU, said that when reefs are dominated by algae, “most of the energy in the ecosystem goes to the microbes. It doesn’t support the variety of reef organisms that make up a healthy system.”
So, what do we do about algal overgrowth? In the Caribbean, one method that is being looked into is the reintroduction of a keystone herbivore, the long-spined sea urchin. Once abundant, the species suffered a massive die-off in 1983 throughout the Caribbean. This was the largest marine animal mass mortality event ever recorded! The mass die-off is thought to be due to a waterborne pathogen, and the urchin population has been slow to recover or entirely absent in many locations. The long-spined urchin played a significant role in reef ecosystems by controlling algal abundance. In subsequent years, many reefs, especially those in the Caribbean, have been unable to recover from persistent disturbances and have experienced long term phase shifts from coral to fleshy macroalgae. By attempting to reintroduce the long-spined sea urchin to its historic range, coral restoration projects may help slow or reverse reef degradation.
The possibility of reintroduction to coral reefs is already being studied. In one experiment, lab-reared urchins were reintroduced in Puerto Rico, where researchers placed the urchins in little corrals on algae dominated reefs and monitored them for two months. Prior to the start of the study, the substrate in the control and experimental plots were similar, both covered in algae. After the monitoring period, the plots with urchins had less algal cover while the control plots were largely unchanged. Although the change in algae cover was dependent on number of urchins and varied based on reef site, plot, and time of year, overall, the addition of sea urchins seems to significantly reduce the amount of algae overgrowth.
The implications of this are very interesting! Reintroducing long-spined urchins to their historic range has a very promising capability to help fight back against algae overgrowth, and with the added bonus of being a non-invasive native animal, helping to restore the original ecosystem! Efforts to restore urchin populations could be very useful when done alongside coral reef restoration projects, as the urchins could help reduce the competition new coral polyps would face from algae. There are many threats our reefs face and many possible solutions, and with the new, promising strategy of reintroduction, we could make meaningful progress in the ongoing effort to restore the reefs. There’s still a lot of work to be done, but with more research and understanding, we may even more ways to save our coral reefs.