DON’T BURY OUR BELOVED CORAL REEFS
(by Maggy Nugues, Chris Schelten and Callum Roberts)
If you take a look at the Soufriere bay after a heavy rain, you will see a cloud of brownish water coming from the river mouth. This is mud washed from the land by rain, and the sediment plume can extend up to Rachette Point. Underneath these brown waters, the mud drops onto the seabed covering the coral reef and jeopardizing the sensitive corals and sponges. After one or two days without rain, the sea regains its usual blue coloration but the mud remains on the reef. Over the last decade, reefs in St. Lucia have been exposed to a growing quantity of sediments and nutrients entering the sea from developed land. In 1994, Tropical storm Debbie washed large quantities of mud onto reefs, killing almost half the coral at the worst affected sites in Soufriere Bay and at Anse La Raye. The amount of mud reaching the sea was closely correlated with development on land. In 1996, a second storm brought heavy rainfall to the island dumping such a lot of mud onto the reef that it had to be removed from sponge by hand. Recovery from these events has been slow.
Corals are animals and can rid themselves of sediment by moving their tentacles and producing a thin layer of mucus to protect their surface. However rapid sediment deposition can overwhelm them resulting in burial and death. Getting rid of mud also costs energy and can stunt growth and prevent reproduction. Sponges find it more difficult to remove the mud that falls on them. Even if corals and sponges manage to temporarily remove mud from their bodies, it only gets moved somewhere else on the reef causing continuous stress.
With the support of the Department of Fisheries and the Soufriere Marine Management Area (SMMA), several research projects have been launched by a team of scientists from the University of York, England, to assess the severity of the problem. In September 1997, sediment traps were deployed throughout the SMMA to measure rates of sedimentation. These traps are regularly monitored by the SMMA rangers. In addition, the team is collaborating with Michael and Karyn Allard of Scuba St. Lucia who are recording daily rainfall and the SMMA which is taking underwater visibility measurements twice a day. Mud reduces underwater visibility so making the reefs less attractive for tourists.
One goal of the study is to find out how much damage sediment washed from land is causing to the reefs. Permanent photo stations have been established over a year to measure coral overgrowth by marine plants called algae and the associated mud trapped in algal mats. Increases in algae represents a serious threat to corals since algae compete for space and light. When input of mud and associated nutrients is high, algae compete better and overgrow corals. This problem is increased by the low numbers of algal feeding fish on the reefs, a result of heavy fishing. As a final blow, sediment-covered surfaces prevent settlement of young corals. If dead corals are not replaced by new colonies, sediment pollution will ultimately result in the disappearance of corals from many areas of the SMMA. This will impact badly on fishing and tourism.
Results from this ongoing research already show a clear negative effect of mud on the amount of coral present. Any further increase in sediment inputs from developed land will cause further loss in coral cover. The SMMA is an ambitious initiative to manage fisheries and protect reef resources via the use of marine reserves. However, there is an urgent need to control sediment and nutrient inputs from developed land if the reefs are to be kept in a healthy state for the benefits of fisheries and tourism. The reserve system is already helping to control damaging algal growth by restoring plant-eating fish populations. This must be accompanied by measures controlling pollution from land. Land clearance on shorelines and close to rivers should be undertaken only with a full appreciation of the harmful effects of such activities on coral reefs. One simple way that everybody can help protect the reefs is to stop cutting back vegetation to the bare soil. Plants keep the soil on the land. When you cut, leave some plants behind for healthier land and healthier reefs.