Cavity-dwelling suspension feeders in coral reefs - A new link in reef trophodynamics

Abstract

The small-scale distributions of chlorophyll a (chl a), pheopigments, oxygen and currents were determined in horizontal sections between coral reef cavities and benthic boundary layer waters (BBL) to estimate rates of grazing and respiration by cryptic cavity-dwelling (coelobite) suspension feeders. We investigated 0.5 to 5 m long and 0.1 to 1 m wide cavities in fringing reefs on the western coast of the Gulf of Aqaba (Red Sea), between 2 and 16 m depth. Within cavities net currents averaged 0.9 cm s-1, or 22% of the current speed measured in BBL ~2 m away from the reef. In spite of rapid flushing of cavity waters within a few minutes, we encountered significant chl a and oxygen depletions relative to BBL, particularly under oligotrophic conditions. Chl a depletions amounted, on average, to 0.10 ± 0.03 mg m-3 (median ± median absolute deviation [MAD]) or 54% (max. 86%) of BBL values and showed a positive relation to coelobite suspension feeder densities. Pheopigments, by contrast, remained remarkably constant, indicating selective grazing of the mainly picoplankton-sized food. Oxygen depletions were weak and mainly related to flushing. In sack-shaped cavities they amounted to 13.6 ± 6.1 mmol m-3 or 3 to 9% of BBL concentrations. Analyses of Water flow and chl a distributions show that under oligotrophic summer conditions 0.7 g C m-2 d-1 of phytoplankton disappears within the upper 1 m of cavernous reef framework. This conservative estimate is about 1 order of magnitude higher than grazing rates of coral-dominated communities on the exposed reef, rendering cryptofauna suspension feeding an important and new pathway of extrinsic organic matter into coral reefs.