SedPods and Sedimentation

By Ceiteag Hennis
From 14 May 2018

Sedimentation pollution is known to be a major stressor of coral reefs worldwide. Because St. Lucia is such a steep island, sedimentation is especially significant. One of major parts of the research we are a part of is accurately measuring terrestrial sedimentation on the coral reefs outside of Soufrière. Understanding how much terrigenous sediment ends up on the reef and where it is coming from is important in making proper environmental management decisions in the future. In the past, marine biologists have used sediment traps to measure sedimentation in Soufriere. Sediment traps are simple devices made out of long tubes of PVC with a cap on the bottom, allowing sediments to enter through the opening at the top. The issue with sediment traps is that they actually trap whatever is inside and grossly overestimate the amount of sedimentation on a reef. Once the sediment gets in, it cannot easily come out. But in reality, wave and ciliary action can resuspend sediment on the surface of corals. We now know that they are not a good representative of sedimentation on coral because they do not measure net accumulation.
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SedPods, on the other hand, are a relatively new way of measuring sedimentation on coral reefs and are much more accurate.  Sedpods are made from PVC pipe with concrete poured inside to create an irregular surface in order to replicate the surface of many coral species. Dr. Begin installed 54 Sedpods along the 18 sights at 15m, making sure to have 3 Sedpods at each site. By allowing them to sit on the reefs for a month, we should be able to determine short-term sedimentation rates on coral. This past week, my group was tasked with processing some of the Sedpods. We first had to rinse the sides and cap of each Sedpod to make sure there was little contamination in our samples. We unsealed the cap and dumped the trapped sediments in a bucket. We then washed off any sediment that was trapped to the surface of the Sedpod and within the cap itself. Once all the sediment was in the bucket, we poured the sediment into a properly labeled measuring cup for each site. The bucket was then rinsed with water into the measuring cup to collect any remaining sediments in the bucket. The contents of each measuring cup was then poured into individual bags for further processing back in the States. With these samples, Dr. Begin will be able to run tests to determine what proportion is terrigenous sediments and what is calcareous sediments.