From Ridge to Reef, Quantifying Sediment Pollution to Hawaiian Coral Reefs

Abstract:

Corals of the nearshore waters are increasingly under threat from a multitude of environmental changes. Amongst the most visible of these changes is arrival of large amounts of fine terrestrial sediment that degrade coral ecosystem's ability to capture sunlight and to reproduce. These muds and silts are products of accelerating landscape changes sweeping across the world's tropics in the last several hundred years of development. Unlike some of the more global threats to coral like ocean temperature increases, local communities can act to reduce this threat at human timescales. Yet restoring whole catchments is a daunting task. We use over 8 years of monitoring data to chart the sources, pathways and fate of fine sediment on its journey from the ridges of Moloka'i, Hawai'i to the reef below. Using imagery and topography from cm to meter-scale, instruments to capture and record the flow of water and sediment, and mapping that synthesizes these measurements, we show that ~ 1% of the landscape contributes over half the fine sediment pollution during a brief period of intense rainfall, ~ 0.1% of the time. Pockets of deep silt erode at rates that are proportional to the amount of time it rains at rates above saturated hydraulic conductivities (c. 10-20 mm/hr). Recent removal of many feral ungulates resulted in rapid vegetation regrowth which has reduced lowering in hotspots by almost an order of magnitude over the last 5 years, from ~10 mm/year to ~ 1-2 mm/year. We contrast these results with emerging evidence from West Maui that the current sediment budget of plumes there is dominated by erosion of legacy in-stream deposits, a challenging mitigation task. Although local communities cannot address every threat to their reefs, they can expect to significantly reduce some sediment pollution by targeted mitigation.