Zooplankton distribution around four eastern North Pacific seamounts

The effects of seamounts on the distribution of zooplankton were investigated at four seamounts in the northeastern Pacific. The following hypotheses were tested: (1) mesoscale gaps of reduced abundance of migrating zooplankton develop over seamounts every night; (2) fine-scale patchiness is augmented within these gaps and in the region downstream of seamounts; and (3) increased numbers of crustacean carcasses occur over seamounts. Gaps are expected because most zooplankton that descend over shallow topography at dawn are either eaten by resident predators or are advected off the summit, while fine-scale patchiness should result from lateral shear between the gap and the surrounding zooplankton-rich waters. Copepod carcasses should be more abundant over seamounts than the surrounding water because of the increased predation at seamounts. Zooplankton distributions were determined from net samples and acoustic records. Water column properties were measured with a CTD and hydrocasts, and currents by moored current meters, acoustic current profilers and drifter drogues. Zooplankton gaps were found over three of the four seamounts surveyed, but not on every survey of each seamount. Only three of the surveys provided the information necessary to test the patchiness hypothesis; on two of these increased patchiness and carcass abundance were found associated with gaps. When no gap was observed over a seamount, there was no evidence of increased carcass abundance or enhanced patchiness, indicating that the three phenomena are temporally and spatially linked. Copepod carcasses were found in the intestines of small fish sampled at the same time as the zooplankton. The fish, along with crustacean predators like euphausiids, are the likely source for the carcasses. Seamounts appear to be an important cause of enhanced zooplankton patchiness on scales ranging from 100s of meters to areas larger than the seamounts themselves.