Phytoplankton grazing by epi- and infauna inhabiting exposed rocks in coral reefs

Exposed rocks with no visible macro-fauna are abundant in all coral reefs. Depletion of phytoplankton cells and pigments by the minute crypto fauna inhabiting the outer few centimeters of such rocks was experimentally studied over an annual cycle in the Gulf of Aqaba, Red Sea. Different substrata were introduced into small (3.6 L), well mixed, tanks that were fed by running seawater pumped directly from the reef at a rate of 11±1 L h −1 . A steady-state reduction in phytoplankton abundance and chlorophyll a concentration of 38±26% (mean ± 1 SD) was found for untreated rocks but not for sand, gravel, or killed controls. Average areal clearance rate by untreated rocks was 17.3±8.0 ml cm −2 h −1 . Conservative extrapolation of this rate to the whole reef community suggests that the fauna inhabiting exposed rocks clears 2.1±0.9 m 3 m −2 d −1 at Eilat. Phytoplankton removal by untreated rocks varied from 1.5 ng chlorophyll a cm −2 h −1 during the oligotrophic summer conditions to 6 ng chlorophyll a cm −2 h −1 during the spring bloom. These values correspond to a potential nitrogen gain of 1.3 and 5.2 mmol N m −2 day −1 , respectively. Cryptic reef-rock fauna can have a key role in the biogeochemical functioning of coral reef communities.