Near-bottom depletion of zooplankton over a coral reef 2: relationships with zooplankton swimming ability

Intense predation by corals and associated fauna can generate vertical gradients of their zooplankton prey. The goal of this study was to characterize the small-scale distribution of zooplankton above the coral reef of Eilat, Israel. Four vertical arrays, each consisting five underwater pumps attached 0.5-3 m apart on a taut mooring, were deployed for 10 days at two sites over the reef slope. A distinct layer of depleted zooplankton was repeatedly found in the 1.5 m high benthic boundary layer. The gradient was sharpest for strong swimmers (copepods and polychaeta), intermediate for weak swimmers (nauplii, molluscs and appendicularia), and lacking for passive taxa (eggs and foraminifera). Spatio-temporal changes in the abundance of copepods and polychaetes were highly correlated, more so in the water aloft (Pearson r > 0.9) than near the bottom (r > 0.8). The spatio-temporal correlations between weak swimmers and between passive taxa were much weaker (r < 0.8 and r < 0.4, respectively). Flow and shear stress did not affect the distribution patterns. The correspondence between the zooplankton distribution and their swimming ability indicates that bottom avoidance contribute to the formation of depleted layer over the reef.