Abstract
Coral reefs worldwide are affected by excessive sediment and nutrient delivery from adjacent watersheds. Land cover and land use changes contribute to reef ecosystem degradation, which in turn,diminish many ecosystem services, including coastal protection, recreation, and food provisioning. The objectives of this work were to understand the role of coastal oceanic and biophysical processes in mediating the effects of sedimentation in shallow reef environments, and to assess the efficacy of land-based sediment remediation in the coastal areas near Maunalei reef, Lāna’i Island, Hawai’i. To the best of our knowledge, this was the first study of sediment dynamics on an east-facing (i.e., facing the trade winds) reef in the Hawaiian Islands. We developed ridge-to-reef monitoring systems at two paired stream bed-to-reef sites, where one of the reef sites was adjacent to a community stream sediment remediation project. We found that the two reef sites were characterized by different processes that affected the sediment removal rates; the two sites were also exposed to different amounts of sediment runoff. The community stream sediment remediation project appeared to keep at least 77 tonnes of sediment off the reef flat in one wet season. We found that resuspension of sediments on this reef was similar to that on north-facing and south-facing reefs that also are exposed to the trade winds. We posit that sites with slower sediment removal rates due to slower current velocities or high resuspension rates will require more-robust sediment capture systems on land to reduce sediment input rates and maximize potential for reef health recovery. This suggests that interventions such as local sediment remediation and watershed restoration may mitigate sediment delivery to coral reefs, but these interventions are more likely to be effective if they account for how adjacent coastal oceanographic processes distribute, accumulate, or advect sediment away from reefs. Our results on the effectiveness of gabion dam sediment capture may help guide scalable solutions for erosion control on islands.
Highlights
Coral reefs across the globe are affected by land-based sediment, which can increase turbidity and, in turn, smother corals and reduce light levels
Our research suggests that 1) the rate of sediment removal from the reef, i.e., reef exposure to sediment, depends on proximity to terrestrial sediment sources and coastal current dynamics; sediment resuspension can increase the time of local sediment retention from 35 days to 18 months, 2) the rate of net sediment accumulation at Site B-RF is higher than that in some other locations in the Hawaiian Islands, 3) reef sites in east Lāna’i within 1 km can have very different sediment and current dynamics, and 4) streambeds adjacent to the east Lāna’i coastline and within 1 km can have different hydrological characteristics, including exposure to winter flash floods
We found that a community-level effort to remediate sediment measurably reduced the amount of sediment reaching reefs
Summary
Coral reefs across the globe are affected by land-based sediment, which can increase turbidity and, in turn, smother corals and reduce light levels. Water circulation in coral reefs is highly variable in space and time, which leads to large variation in where land-sourced and reef-sourced sediments are deposited, how long sediment remains, how often it is resuspended, and its movement across the reef [13] These spatial and temporal variations affect the distributions of coral species that have different levels of tolerance to sediment exposure [14,15,16]. Requirements for mitigating effects of land-based sediments have been developed and implemented in such settings in Hawai’i through partnerships among state agencies, federal agencies, and local community watershed alliances [19]. These efforts identify major sources of terrestrial sediment and develop practices for land management to reduce sedimentation [20]
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