Mesozooplankton community response during the SERIES iron enrichment experiment in the subarctic NE Pacific

The response of the mesozooplankton community was examined during the development and early decline of the Subarctic Ecosystem Response to Iron Enrichment Study diatom bloom in July 2002. No significant changes in species composition were observed during this period. In terms of abundance, the community was dominated by small copepods (Oithona spp. and Pseudocalanus spp.). The abundance of large herbivorous calanoids of the genus Neocalanus (N. plumchrus, N. cristatus and N. flemingeri) was already low at the start of the experiment as most of these animals had already descended to their overwintering depth. Mesozooplankton biomass was dominated by the calanoids Eucalanus bungii, Calanus pacificus, Metridia pacifica and, to a lesser extent, late stage copepodites (CIV, CV) of Neocalanus spp. In general, the abundance of all copepod species tended to increase inside the patch (relative to conditions outside the patch) during the experiment. However, mean species-specific abundances were not significantly different inside and outside of the patch, except for a significant increase of E. bungii (p<0.05) that occurred between Days 10 and 13 inside the iron-fertilized patch. By Day 8, most of the E. bungii population had shifted its vertical distribution into the surface mixed layer. We hypothesize that a wind event displacing the surface mixed layer coincident with continued immigration from below the thermocline facilitated this increase in abundance. Here we address this higher trophic level response to pulsed primary production in terms of potential mechanisms and their potential impact on the diatom bloom.