Seamount Plankton Dynamics

The biomass of fishes and other zooplanktivores is unusually high over shallow, intermediate and, sometimes, deep seamounts. This cannot be explained in terms of 'bottomup control', which is driven by local upwelling and the ensuing augmentation of local primary production. Although upwelling does occasionally occur, it rarely penetrates the photic layer and has not been observed to remain over a seamount long enough to affect the growth of local zooplankton populations. In fact, the biomass of seamount zooplankton is, in many cases, lower above the summit than in the surrounding waters, especially over shallow seamounts. In contrast, the weight of the available evidence indicates that the trophic enrichment over seamounts is due to allochthonous inputs via (i) bottom trapping of vertically migrating zooplankton and (ii) greatly enhanced horizontal fluxes of suspended food. Bottom trapping of migrating zooplankton in the early morning is a major mechanism for accumulation and trophic focusing of zooplankton over seamounts at shallow and intermediate depths, the trapped zooplankters being readily consumed by fishes. Enhanced horizontal fluxes of planktonic prey appear to be a major pathway of trophic subsidy over deep seamounts. High fluxes are maintained due to substantial enhancement of currents and amplification of internal waves over the seamount topography. Biophysical interactions are the key mechanisms responsible for the maintenance of unusually high biomass at high trophic levels. Hence, the biological enrichment of seamount communities may be the product of a bottom-up pathway fueled by trophic subsidy to carnivores, rather than by local enhancement of primary production.