Increased sediment loads over coral reefs in Saint Lucia in relation to land use change in contributing watersheds

• Sedimentation near Saint Lucia coral reefs increased 2-3 fold over several decades.
• Accumulation rates of both terrigenous and calcareous sediment have increased.
• Unpaved roads are the most important source of terrigenous sediment.

Increased sedimentation is widely acknowledged to be an important stressor for Caribbean coral reefs. However, for most locations we currently lack both accurate records of changes in sediment accumulation rates over reefs as well as a quantitative link between land-based sources of sediment and sediment delivery to coastal waters. This paper aims to address this gap in our quantitative understanding of these processes for two watersheds in the island of Saint Lucia in the West Indies. We used sediment cores collected near downstream coral reefs to examine changes in sediment composition and accumulation rate over the past several decades and relied upon a GIS-based sediment budget model to estimate recent sediment yields in the two focal watersheds. Analysis of sediment cores indicated that accumulation rates of terrigenous sediment, originating from the upstream watersheds, and calcareous sediment, likely arising from dead corals, have increased 2–3 fold over the last 3–4 decades. Model-estimated changes in sediment yields between 1995 and 2010 were associated with the expansion of the unpaved road network and were congruent with measured changes in terrigenous sediment accumulation rates near the reefs over the same period. The majority (83–95%) of sediment yield in the two watersheds was attributable to unpaved and degraded roads; in fact, just four or five road segments, representing <20% of the road network in each watershed, accounted for nearly half of the estimated sediment yield in 2010. Our results suggest that unpaved roads are major sediment sources in the two study watersheds and therefore merit closer attention when implementing erosion control measures intended to reduce sediment loading into reef-bearing coastal waters.