In addition to the direct damage to coral, indirect effects seem to also play a role: ecological changes that have an impact on coral reefs over a distance of dozens of kilometres. One of the areas where ecologists from the Wagening Resource Ecology Group are seeing this in their research area in Indonesia. In the Beraugebied along the coast of East Kalimantan there is a ring of coral islands and reefs around an estuary, where the coastline is covered with dense mangrove forests.
Another important point is that mangroves act as a sort of filter between rivers and coastal zones. De Boer: “Eutrophication (algae invasion), that can be disastrous for coral, is less prevalent in areas with well-developed mangrove forests. In addition, fish that are able to mature in mangroves or seagrass fields are important for the health of the coral”. Miriam Schutter adds: "This is because of the non-symbiotic algae that could overrun the coral. Different species of fish eat the algae and ensure a balance. "
Dr. Fred de Boer: “It is becoming increasingly clear that mangroves as well as seagrass fields are playing an important role as a breeding ground for nearby coral reefs. It would seem that in areas where mangroves or seagrass fields are disappearing, some species of fish and the coral are shrinking or even disappearing.” The role that the mangroves or seagrass fields play is to protect against predators, and to provide food for growing fish. The fish migrate during different stages of their life for example from mangroves to seagrass fields and to areas of coral. “This link between coral reefs and mangroves can be seen over several dozens of kilometres and, in exceptional cases, even further”, says De Boer, who works among other with Paul van Zwieten of the Aquaculture and Fishing research group, within the Rescopar project. The project focuses among others on the interaction between mangroves, fish populations and coral reefs in Indonesia and Vietnam.
Not only is the conservation of the mangroves and seagrass fields important for the natural kingdom of nearby ecosystems, it is also of economic importance, as some commercially interesting fish are revealing negative effects when the mangrove areas shrink.