A Field-Based Technique for Measuring Sediment Flux on Coral Reefs: Application to Turbid Reefs on the Great Barrier Reef
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Inshore turbid reefs on the Great Barrier Reef (GBR) are exposed to high and fluctuating sediment loads normally associated with poor reef growth, but many have high coral cover >30%) and diversity >50 species). Previous assessments of sediment regimes on these reefs have largely relied on sediment trap data, which overestimate sedimentation rates and may not accurately reflect sedimentary conditions. A new approach, based on paired sediment trays, is described here that allows the sedimentation rate, sediment resuspension, and total mass of mobile sediments transported on to and off of a site per unit time and area (termed the two-way total sediment flux) to be measured or calculated. The sediment trays were deployed on Middle Reef and Paluma Shoals, two inshore turbid reefs on the GBR where the two-way total sediment flux ranged from 34 g/m2/d in protected reef habitats to more than 640 g/m2/d in higher-energy settings. Mean sedimentation rates, calculated using data from four sites across these reefs, of less than 122 g/m2/d are considerably lower than published rates estimated for nearby coral reefs, largely because sediment traps limit sediment resuspension. At each tray installation, sediments were collected every 4 to 6 weeks to measure variations in net sedimentation through the year, and resuspension rates were calculated by comparing 100 g of preanalysed sediments placed on trays at deployment to sediments recovered 2 weeks later. These data demonstrate that despite high sediment delivery rates, net sedimentation may still be relatively low and potentially less of a threat to benthic communities on turbid reefs than previously assumed. Sediment trays provide a comprehensive assessment of sediment regimes that, together with ecological assessments of coral cover, improve our understanding of the sedimentary pressures affecting inshore turbid reefs and their ability to tolerate sedimentation.
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