''Seamount effects'' in the zooplankton community near Cobb Seamount

Oceanic seamounts often support large nektonic stocks. Since the mid-1950s it has been believed that this high productivity results, in part, from biological response to the physical interaction between oceanic currents and the abrupt topographic profiles represented by most seamounts. The ''classic theory'' for the production/maintenance of seamount nektonic stocks suggests that (i) the combination of localized upwelling and the trapping/concentrating action of closed anticyclonic vortices (i.e. Taylor cones) enhance local primary production, (ii) thereby promoting local secondary productivity that, (iii) supports local nektonic populations. Here we test one element of this theory: whether proximity to a shallow seamount is associated with changes in zooplankton abundance and species composition. Zooplankton samples were collected near Cobb Seamount, a shallow (<100 m) northeast Pacific seamount 50-km west of Vancouver Island. Both upwelling and closed recirculations occur at Cobb Seamount, but the latter is confined to depths >100 m and is not an effective retention mechanism. Total zooplankton abundance did not vary significantly on- versus off-seamount. However, using a variety of nonparametric multivariate techniques we demonstrate that a ''seamount effect'' on zooplankton-community composition is detectable up to 30 km from the seamount summit. This effect is superimposed on (and locally much stronger than) the expected slow decline in resemblance as between-sample geographic distance increases. Possible mechanisms by which this effect operates include: differential growth or reproduction, differential mortality and behavioral or migratory effects. The on-off seamount differences are accounted for largely by the increased relative abundances of two fast-growing opportunists, doliolids (Dolioletta sp.) and larvaceans (Oikopleura sp.), near Cobb Seamount. Predation pressure from seamount fish and active avoidance of the seamount by zooplankton may also play a role in generating the seamount effect. The absence of an effective trapping mechanism and the fact that total 2ooplankton abundance does not increase near the seamount lead us to conclude that the bottom-up model of localized energy transfer proposed under the ''classic hypothesis'' is incorrect for Cobb Seamount: nektonic stocks at Cobb Seamount (and, possibly, other shallow seamounts) are more likely supported by how-through (i.e. advected) rather than local production. Copyright (C) 1996 Elsevier Science Ltd.