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Depletion of suspended particulate matter over coastal reef communities dominated by zooxanthellate soft corals

Coastal reef communities dominated by zooxanthellate alcyonacean octocorals extract large quantities of suspended particulate matter (SPM) from the water column. Concentrations of SPM in water parcels, tracked by a curtain drogue, were measured before and after passing over 2 strips of soft coral dominated, near-shore reefs -200 m long, and over 2 adjacent sand-dominated strips for comparison. The reefs were covered with 50% zooxanthellate octocorals (total standing stock: -270 g AFDW m -2 ; mean live tissue volume: 701 m -2 ), 7% hard corals, 15 % turf algae with bioeroding sponges underneath, and <1% other filter feeders (sponges, tunicates, and bivalves). Downstream of the reef communities, chlorophyll, particulate organic carbon and particulate phosphorus were significantly depleted. The depletion of chlorophyll averaged 35% of the standing stock, whereas the net depletion of particulate organic carbon and particulate phosphorus was 15 and 23 %, respectively. Rates of depletion were similar for the 2 reef sites and 3 sampling periods, and were independent of upstream particle concentration. In contrast, concentrations of particulate nitrogen and phaeopigments were similar before and after passage across the reef sites. On the sandy sites, downstream concentrations of particulate nutrients, chlorophyll and phaeopigments were all similar to upstream concentrations. The net import of particulate organic carbon into the reef was estimated as 2.5 ± 1.1 g C m -2 d -1 . Less than 20% of this carbon import could be explained by area-specific rates of removal by sponges, tunicates, bivalves, and hard corals, suggesting that soft corals were the primary sink of carbon. The data suggest that detritus and other small SPM (<10 μm particle size) are an important food source for alcyoniid-dominated reef communities in high turbidity regimes.