Intense Benthic Grazing on Phytoplankton in Coral Reefs Revealed Using the Control Volume Approach

Abstract

Grazing on phytoplankton by a section of the fringing coral reef in Eilat, the Gulf of Aqaba, Red Sea, was studied using the control-volume approach, a direct, nonintrusive technique common in engineering studies. Concentrations of chlorophyll a were measured using four vertical arrays of pumps, 10 pumps on each, that defined an imaginary box (i.e., the control volume), extending from the bed to surface, overlying a section of the sloping fore-reef .100 m 2 in size. Concurrent velocity measurements were made at the center of the box using a vertical array of acoustic current meters. Together, the concentration and velocity measurements allowed us to compute phytoplankton fluxes in and out of the control volume. While phytoplankton patchiness and other turbulent variations in concentration introduced significant variability, mass balances of fluxes into and out of the box indicate a flow-dependent grazing rate of approximately 10 m d 21 to 20 m d 21 . This value was larger than, but not statistically different from, values derived from an ''a priori estimate'' (calculated based on the density and feeding rates of benthic grazers) and an estimate computed from the observed near-bed gradient of Chl a. Phytoplankton grazing is a principal pathway through which allochthonous nutrients are imported to the reef community from the flowing water.

Near-surface enrichment of zooplankton over a shallow back reef: implications for coral reef food webs

“…Therefore, it does not seem that zooplankton are being attracted to high phytoplankton abundances in the water column. Although reef environments are typically depleted in phytoplankton near the bottom (Yahel et al 1998;Genin et al 2002), the benthic affinity of the diatom taxa at our study sites suggests some bottom resuspension of this taxon. While microzooplankton, another important food for zooplankton, were not quantified, the relatively homogeneous distribution of small flagellates, a major food source for microzooplankton, supports the notion that zooplankton were not responding to heterogeneous microzooplankton distributions.…”

High‐resolution carbon budgets on a Palau back‐reef modulated by interactions between hydrodynamics and reef metabolism

“…The CV method we use here was first described as an engineering fluid mechanics tool by Street (1996), and first used on reefs in 2002 (Genin et al 2002). Several studies have successfully used the CV approach to estimate NCC and NCP rates: Kaneohe Bay barrier reef, Hawaii (Falter et al 2008) and Ningaloo Reef, Great Barrier Reef .…”

“…Often in waters deeper than 3 m, the water column is sheared, so fluxes estimated from gradual changes in concentration using integration over the full depth of the environment can be erroneous. Water column separation was first noted as an important issue in Lagrangian and Eulerian reef studies by Genin et al (2002). Most recently, Falter et al (2013) have produced a highly detailed model elucidating the complexities of reef chemistry, relating reef metabolism effects and the importance of hydrodynamic regimes in modulating reef chemistry through wave forcing, depth, bathymetry, and benthic frictional resistance or rugosity.…”

A note on the potential transport of scalars and organisms by surface waves

“…Rosman unpubl. data 2003), as well as flows over fringing coral reefs (Genin et al 2002; R. Lowe unpubl. data) we find cross-shore flows that are generally less than 5 cm s Ϫ1 .…”

“…For example, consider the application of the control volume method, used by Genin et al (2002) to study grazing on a coral reef, to the wavy environment that typifies many nearshore kelp forests. In order to properly formulate the control volume for these flows, it is necessary to account for the net advection of scalars that accompanies the wave motion.…”