Flow effects on benthic grazing on phytoplankton by a Caribbean reef

Abstract

We present measurements of flows and fluxes of phytoplankton to Conch Reef, Florida, a Caribbean reef dominated by sponges and soft corals, located in 15 m of water offshore of Key Largo. Vertical profiles of chlorophyll a, a proxy for phytoplankton biomass, showed a near‐bed depletion, indicating the existence of concentration boundary layers. Along with simultaneous measurements of velocity profiles, near‐bed turbulence, and temperature stratification, these profiles were used to compute α, the mass transfer velocity of phytoplankton to the bed (i.e., the flux to the bed normalized by near‐bed concentration). The α value ranged from ‐40 to +130 m d−1, with a significant linear positive relationship with shear velocity. The median value of α = 48 ± 20 m d−1 is larger than would be expected, given the observed population of filter‐feeding sponges. Nonetheless, these large values of a are consistent with values found recently for another coral reef as well as for a soft bottom estuarine community. Taken as a whole, these measurements indicate that reefs with large roughness and/or energetic currents should be able to support higher biomasses of benthic organisms than would low relief reefs or reefs in sluggish waters.