Pelagic ecosystem structure and functioning in the subtropical front region east of New Zealand in austral winter and spring 1993

Data are synthesized on biomass and fluxes of components of the pelagic food web, downward particulate flux, and associated physical and chemical oceanographic parameters in austral winter and spring in the Subtropical Front (STF) region east of New Zealand. All four food web types (sensu Legendre and Rassoulzadegan, Ophelia, 41, 153-172, 1995) are represented in this region. The STF food web is classified as 'multivorous' in spring and 'herbivorous'/'multivorous' in winter, even though grazing was dominated by microzooplankton. Based on relatively few data over each season, STF community biomass was dominated by large phytoplankton cells (>20 µm) in both seasons. Post- bloom conditions were sampled in spring and highest fucoxanthin vertical fluxes were recorded at this time. A phytoplankton sedimentation event may have occurred just after our mid-October sampling. Subtropical (ST) waters exhibited pronounced seasonality in all plankton compartments. The ST food web is classified as 'multivorous' in both winter and spring. Organic flux in sediment traps was highest in ST water relative to subantarctic (SA) and STF waters. Although the ST data coverage was limited, high spring algal specific growth rates, the large change in total community biomass between winter and spring, and small seasonal differences in early winter and spring macronutrient concentrations suggest that the structure and functioning of this ecosystem depended on the macronutrients entrained during deep winter mixing. Algal populations in SA high-nitrate-low-chlorophyll waters (HNLC) were dominated by small cells. Heterotrophic bacteria dominated community biomass in SA waters in winter and equalled phytoplankton biomass in spring. In contrast to Legendre and Rassoulzadegan's classification of the subarctic Pacific HNLC food web as 'multivorous', the SA food web is classified as 'microbial' tending towards 'microbial loop' in winter (i.e. there was greater carbon uptake by bacteria than phytoplankton) and 'microbial' in spring. Chlorophyll (Chl) a levels in spring were similar to those in winter, but because of a doubling of the C:Chl ratio, algal biomass (as carbon) was 2-fold higher in SA waters in spring, compared with winter. This seasonal increase in algal carbon biomass in spring occurred in spite of a >5-fold increase from winter to spring in microzooplankton biomass, challenging the notion of grazer control of algal biomass. Our findings are compared with Russian data collected in the South Pacific STF region in summer 1985. Aspects of ecosystem func- tioning in the STF region east of New Zealand may hold for other longitudes in the Southern Ocean, although it is likely that some characteristics are regional in nature.