Human activities are leading to increased stress on coastal ecosystems worldwide. On coral reefs, sediment runoff has been recognized as an important stressor for several decades. Manipulative experiments have repeatedly shown that sediment negatively affects coral and have highlighted the organism-scale consequences of artificially increasing sediment levels. However, little quantitative data exist on changes in sediment accumulation rates that have occurred over coral reefs concurrently with coastal development, and clear links between enhanced sediment load and changes in natural reef communities have been difficult to establish. In this thesis, I apply new methods to assess terrestrial sediment stress to coral reefs, quantify changes in sedimentation rates over reefs, and identify areas in upstream watersheds that contribute disproportionately to the coastal sediment yield. Using reef sediment composition as a proxy for terrestrial influence to a reef, I show associations between terrigenous sediment load and several metrics of reef health across two spatial scales: at multiple sites on the island of Saint Lucia and across multiple islands along the eastern Caribbean chain. The proportion of terrigenous material in surface reef sediment was a better predictor of both current coral cover and decadal change in coral cover than protection afforded by well-enforced marine protected areas. The proportion of terrigenous sediment increased over time on reefs located near obvious sources of sediment but not at sites with no such input, indicating that temporal increases are caused by local anthropogenic activities and are not likely the result of long-distance transport of sediment. Sediment accumulation rate, as estimated by short-lived radioisotope analyses, has at least doubled over coral reefs downstream of two watersheds in Saint Lucia during the past six to seven decades. Unpaved roads appear to be the most important contributing factor to downstream coastal sediment yield, and in particular a small number of steep road segments. These findings highlight that substantial changes have recently occurred in the sedimentary regime of several eastern Caribbean coral reefs, and show that increased runoff is likely an important driver of changes in coral reef communities in this region.
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Thesis advisor: Côté, Isabelle M.
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