Sediment characteristics in reef areas influenced by eutrophication-related alterations of benthic communities and bioerosion processes

This study investigated sediment deposits and suspended particulate matter in three reef areas of Reunion Island characterized by varying benthic communities and bioerosion processes, as a result of different eutrophication levels. The sediments were sampled using two different methods, hand-collecting directly on the reef bottom and sampling by traps. This approach allowed a comparison between the composition of reefal bottom sediments and suspended particles. In locations where algal communities prevailed over corals as a result of nutrification, a shift from coral to coralline algae dominated sediments could be observed. In addition, a decrease in sediment production and a prevalence of very fine sand to mud sized grains over medium to fine sands existed in the nutrient-enriched areas. This grain-size difference probably is caused by a decrease in grazing activity in the enriched areas. High proportions of coralline algal debris and sponge spicules were specifically found in the sediments within areas receiving high nutrient input. These two diagnostic components were abundant in both the suspended particulate matter and in the fine fractions (< 0.40 mm) of the bottom sediments. The implications for the interpretation of the fossil record of these bioerosion and sedimentation processes in recent reefal environments are discussed.