Mesozooplankton biomass, composition, and potential grazing pressure on phytoplankton during austral winter and spring 1993 in the Subtropical Convergence region near New Zealand

The biomass, composition, and grazing rates of three size fractions of meso zooplankton (200-500, 500-1000, and some >1000 mu m) were estimated in shel f waters and the water masses associated with Subtropical Convergence east of New Zealand, in the austral winter and spring of 1993, as part of a larg er New Zealand study of ocean carbon flux that contributes to the Joint Glo bal Ocean Flux Study (JGOFS). The total biomass was largest in spring in al l water types. It was similar to the biomass measurements made previously i n subantarctic and subtropical water masses in the Southwest Pacific and th ose from the North Atlantic, except for the spring biomass in subtropical w ater which was unusually large (86.5 and 101.3 mg m(-3) dry weight). Biomas s was concentrated in the upper 100 m, especially within the 0-25 or 25-50 m layers, both day and night. Night/day biomass ratios in the surface 100 m were often >2, and are presumed to be the result of sampling patchy popula tions as well as vertical migration. Biomass was greatest for the >1000 mu m fraction of the mesozooplankton population, followed by the 500-1000, and 200-500 mu m fractions, respectively.:The unusually small fraction of biom ass residing in the 200-500 mu m fraction is assumed to be the result of pr edation by larger mesozooplankton. The mesozooplankton community had maximu m gut fluorescence at night only at stations where chlorophyll a was >2 mg m(-3) and at many of the stations gut fluorescence was persistently low. Th is was probably the result of the poor feeding environment, since a large p roportion of the primary production resided in the <2 mu m fraction. The to tal mean ingestion of phytoplankton was calculated to be 1-40 mgC m(-2) d(- 1), based mainly on ingestion by the 200-500 and 500-1000 mu m fractions, w hich were dominated by herbivores or herbivores and omnivores. The heaviest prating pressure was in subtropical and Subtropical Convergence waters, in spring. Total grazing represented <1-4% of daily total integrated primary production. Phytoplankton carbon ingested usually met only a small fraction of the basic metabolic requirements of the mesozooplankton. These data, an d the fact that spring populations were apparently actively growing, since they contained a large proportion of developmental stages, imply that mesoz ooplankton diets were mainly microzooplankton. (C) 1998 Elsevier Science Lt d. All rights reserved.