Why is it so important to monitor the sedimentation rate in our ocean?

(by Chris Schelten, PhD Student, University of York, UK)

Many of you might still remember me from my last trip to Soufriere in late 1998. Now back in Soufriere I am continuing my study of sedimentation rates in the SMMA area and south of Anse La Raye. Sedimentation is the mud that enters the ocean from river run-off. It consists of little particles and nutrients that are washed from the land into rivers by rainfall. What we do on land affects the coastal ocean. If the land is covered with trees and bushes, little soil will be washed into the rivers, but wherever soil is bare, it will be carried by the rainwater into the rivers. If it is not done with great care, land clearing and road building transforms soil from being the basis of life to being a pollutant that kills corals. Sedimentation probably damages coral reef communities more than all other human activities combined. When mud lands on corals it smothers them. Corals can free themselves from some particles, but if there is too much mud, they die because they use all their energy to remove it and cannot feed effectively. Corals also need light, because small plants live in coral tissues. They use light to produce energy, some of it going to their coral host Without this extra energy source a coral cannot survive. Mud in the water reduces the amount of light that reaches corals. You can see this after heavy rainfall because the visibility in the water is very low. If there is not enough light for the plants that live in corals to produce energy, the coral may die This happens first of all close to the river mouth. In Soufriere few corals grow close to Hummingbird, or Bat Cave and even the corals at Rachette Point suffer from the mud that runs out of the Soufriere River when there is heavy rainfall. Especially in the rain season, you can easily see the mud running into the Soufriere Bay covering the corals underneath.

As I said before, not only mud is running from the rivers into the ocean, but also nutrients. Nutrients come from fertilisers that are used on land to help the plants grow faster and bigger. Nutrients also come from sewage and leftover food that is put into the river. The problem with nutrients is that they let underwater plants called algae grow faster Corals grow slowly, and if there are a lot of nutrients in water, algae grow over corals and shade them. The part of the coral lying under the algae dies due to lack of light. After Hurricane Lenny, a lot of corals were broken, destroyed and, consequently, the coral communities weakened. Algae took advantage of this and grew over a lot of corals. There is now a lot of algal growth at Malgretoute, Petit Piton, Jalousie and Gros Piton. The problem is that this condition is quite difficult to reverse. Corals do not have the ability to overgrow algae. Our last hope for the recovery of the reefs is that there will be a lot of fish, like parrot fishes and surgeon fishes, that eat the algae. The marine reserves in the SMMA protect fish and consequently help to keep the reef healthy by controlling algal growth.

New young corals are not able to grow on loose particles like the ones coming from mud nor are small corals able to settle on algae. Mud deposition, even if it does not affect older, larger corals may be harmful and deadly for the smaller ones. Consequently, there will not be any new coral growth in areas with high sedimentation rates and the coral reefs will degrade and die. What we have to do is to think before we act: do not throw rubbish in the sea, think before cutting a tree or clearing grass from the sides of the road. People need to plan roads and other coastal and inland constructions with more concern for the effects they may have on coral reefs. This is very important because many people depend on the reefs. First of all coral reefs provide us with fish that depend highly on healthy corals. Fish use corals for food and shelter. And finally, corals are very old and they grow extremely slowly. If a big coral head that has a size of around 3 feet dies it will take more than 100 years to replace it.

Finally, you may ask how do we know how much mud is landing on corals (the sedimentation rate)? Well, we have placed several pieces of plastic pipe in the water in 15 and 45 feet deep at 14 different places (Gros Piton 3, Petit Piton 3, Soufriere Bay 5 and South of Anse La Raye 3). On the top of the pipes we put plastic tubes which trap falling mud. We change them every second week. Some pipes are longer than others. This is because I want to collect different kinds of sediment. The longer pipes collect the mud that comes freshly from the rivers whereas the smaller pipes collect the “old” mud and the new sediment input. If mud particles fall on coral reefs, they do not disappear. They will just be moved from one place to another by the currents and wave action. This is what I call the “old” mud. After changing the tubes, I filter the water that got collected in them. I dry the filter with the sediment particles on it and weigh them. In this way I know how much new mud runs on the reef and how much mud is already disturbing the coral reef communities in our ocean. If now something bad happens to the corals, for example a lot of them die at the same time, I am able to determine if the reason is a high input of sediment coming from the rivers. So this is why it is so important to monitor the sedimentation rate in our ocean!