Microzooplankton grazing in different water masses associated with the subtropical convergence round the south island, New Zealand

Grazing rates by microzooplankton feeding on bacteria, picophytoplankton and total phytoplankton were measured in subtropical, Subtropical Convergence and subantarctic waters in winter and spring 1993. Water samples were collected simultaneously for determinations of nutrient concentrations and biological parameters. Bacteria, phytoplankton and microzooplankton abundance were generally higher in spring than winter. Picophytoplankton were a large proportion of the phytoplankton populations in subantarctic waters. Microzooplankton grazing impact on total chlorophyll a standing crop and phytoplankton production ranged from 10–92% to 71–194% in winter and 4–57% to 20–126% in spring, respectively. The only major difference between grazing impacts in winter and spring was in subtropical waters, where grazing impacts on both chlorophyll a and picophytoplankton were higher in winter. Grazing impact was generally greater in subantarctic water and on picophytoplankton than on total phytoplankton. Grazing balanced growth of phytoplankton in all water masses in winter, but in subtropical waters in spring, growth of total phytoplankton exceeded grazing and grazing exceeded growth in picophytoplankton populations. This decoupling of growth of the larger phytoplantion and grazing by microzooplankton contributed to the spring bloom and potential flux of carbon out of the surface waters in subtropical water masses. These findings are further discussed in relation to spatial and seasonal trends in other parts of the STC.