Expansion and homogeneity of the vertical distribution of zooplankton in a very deep mixed layer

During stratification over 70% of the zooplankton > 100 μm in the Gulf of Aqaba (Eilat), Red Sea, resided in the upper 100 m. In the course of winter mixing, the vertical pattern became more homogeneous, reaching an almost complete uniformity when the vertical mixing exceeded 500 m depth. The zooplankton expanded its vertical distribution throughout the mixed layer. However, full homogeneity, such as would be expected for a passive tracer and observed for phytoplankton, occurred for zooplankton only in the upper section of the mixed layer. Homogeneity occurred for zooplankton passing through 750 μm mesh but not for larger crustaceans. The copepod Pleuromamma indica retained its diel vertical migration regardless of mixing but expanded its nocturnal range under mixing conditions. The observed homogeneity of zooplankton could have been reached either by passive entrainment with the mixing water or by isotropic swimming of the animals, or both. The ecological implications of the expansion of zooplankton distribution for predator-prey relationships and particle fluxes are discussed.