Habitat choice by marine zooplankton in a high-latitude ecosystem

Abstract

According to habitat choice theory, zooplanktonic prey should choose a depth habitat that maximizes food intake while minimizing predation risk. Body size affects competitive ability and vulnerability in zooplankton, thereby influencing the adaptive choice of depth habitat. To study size- dependent habitat choice in marine zooplankton, we sampled their vertical distributions on a fine- scale with an optical plankton counter (OPC) during a post-bloom condition in late spring on the con- tinental shelf off North Norway. Taxonomic information was obtained from net samples. We found that small and large zooplankton segregated along the water column under resource heterogeneity, with large zooplankton aggregating in the resource-rich habitat together with predators of small zoo- plankton, in accordance with multi-trophic level habitat choice theory. These patterns of habitat use differ from summer vertical distributions documented in Norwegian waters, when the distribution of small and large zooplankton is reversed. To account for the discrepancy, we propose a new habitat choice model that considers the different predation regimes experienced in spring vs. summer.