Role of microzooplankton grazers in the subtropical and subantarctic waters to the east of New Zealand

Subtropical and subantarctic surface waters to the east of New Zealand were sampled in four seasons over a period of 3 years to evaluate the importance of the microzooplankton as grazers of the phytoplankton community. Subtropical waters (STW) to the north were warm with high salinity and seasonal macronutrient limitation. Subantarctic waters (SAW) to the south were colder, with lower salinity, high macronutrient concentrations and chlorophyll a concentrations between 0.20 and 0.28 μg litre‐ 1 . In the STW, phytoplankton showed a typical seasonal pattern for macronutrient‐limited waters, with biomass dominated by large phytoplankton and a chlorophyll a maximum of 1.4 μg litre– 1 in spring. Microzooplankton biomass varied from 4.5 μg carbon (C) litre ‐1 in the STW in winter, to 13.8 μg C litre ‐1 in the SAW in winter with the hetero‐trophic flagellates contributing between 22% and 78% of the biomass. In spring and winter in the STW, only 73% of the primary production was grazed by the microzooplankton compared with over 100% in autumn and summer. In contrast, over 100% of the primary production was consumed in all seasons by the microzooplankton in SAW. In the SAW in all seasons and the STW in summer and autumn, microzooplankton grazing on phytoplankton dominated the organic matter fluxes from the phytoplankton population. During these periods the picophytoplankton contributed a significant proportion of the phytoplankton biomass.