Seasonal dynamics and ecosystem impact of mesozooplankton at station ALOHA based on optical plankton counter measurements

Abstract

Abundances of mesozooplankton‐sized particles were measured at 45‐m depth at station ALOHA (22.75°N, 158°W) during 18 cruises from February 1995 through December 1996 with an optical plankton counter (OPC). Mesozooplankton were also sampled with oblique net tows to 155 m depth. Vertical OPC profiles showed uniform total abundance in the upper mixed layer, usually >45 m. OPC and net data agreed with respect to total abundance, size composition, abundance of individual size classes, and seasonal cycle of abundance. We found no evidence for significant contributions to OPC particle counts by diatom aggregates, Trichodesmium spp., or detritus. Variations in OPC estimates of abundance are well explained by diel behavior and seasonal cycles of species that dominate mesozooplankton abundance, of which 80% are copepods. The summer maximum in mesozooplankton abundance is due primarily to the increase of the six smallest OPC size classes (<1.15 mm equivalent spherical diameter), dominated by 14 nonmigrating copepod species that account for more than 95% of average copepod abundance. Seasonal cycles of zooplankton egestion estimated from OPC measurements were highly correlated, and comparable in magnitude, with observed sinking flux measurements of both C and N. Sinking flux at the base of the euphotic zone was 0.67 and 0.77 mol C m−2 yr−1 and 81 and 87 mmol N m−2 yr−1, for 1995 and 1995, respectively. The potential contribution of mesozooplankton egestion in the mixed layer, based on OPC measurements, accounts for 95% and 90% of C and 86% and 81% of N, respectively.