Structure in the Copepod Community of the North Pacific Central Gyre

Abstract

The North Pacific central gyre is a large, monotonous, and geologically old system. It (and other such gyres) is probably the nearest oceanic equivalent to an essentially self—regulating, climax ecosystem. We have taken a series of replicated, vertically stratified net tows for macrozooplankton and have replicated measures of a number of habitat variables. Copepods are the most numerous members of the macrozooplankton fraction of the community of the central gyre. It is likely that they are also the largest part of the biomass. There are at least 125 species regularly present and although we do not know what many of them eat, certain species can almost certainly be identified as herbivores or carnivores, and others as omnivores. We show that some species are significantly more frequent parts of each other's biotic environment than other species. These recurrent groups of species tend to occupy different depth zones. The members of the groups show strong concordance of abundance at the depth of high group coherence. Thus there is a dividing up of the water column and a clear tendency for vertical spatial structure in the copepod fauna. Within groups there is a significant constancy of dominance at the group's preferred depth over time. Thus there appears to be a large amount of species structure as well as spatial structure. A highly regulated species equilibrium is implied. There are congeners occurring within the same recurrent groups. The physical and biotic environment of each group differs with respect to both concentration (e.g., chlorophyll α) or magnitude (e.g., temperature) of habitat variables and with respect to the spatial variability of these. Our most diverse group tends to occupy a depth zone ranking high in primary productivity but does not have the greatest total environmental richness or greatest or least heterogeneity, and this seems contrary to what some current community theory predicts. Our evidence indicates that within groups many similar species coexist spatially and temporally. We speculate about the relative roles of competition and predation influencing the structure we see. Aspects of modern community theory, based on competitive equilibrium, seem inadequate to explain our results. Predator regulation of structure seems a more likely explanation. However, existing information indicates a lack of sufficient specialization of what we believe to be the main predators on copepods to account for the observed constancy of copepod species structure. We suggest that a more intensive study of the role of predation is in order.