Short-term changes in planktonic community structure and nitrogen transfers in a coastal upwelling system

Abstract Large changes were recorded in the mixed layer and deeper water column properties of a mid-shelf station in a coastal upwelling region (West Coast, South Island, New Zealand) after time intervals of 0·5 and 5·0 days. Throughout the period of sampling a food web based on small-celled organisms predominated. Most of the phytoplankton biomass and all of the measured nitrogen uptake and primary production were associated with particles that passed through a 20 μm filter. Species less than 2 μm accounted for at least 40% of the chlorophyll a , 60% of protein and polysaccharide biosynthesis, up to 67% of the ammonium uptake and up to 45% of the nitrate uptake. Excretion by zooplankton larger than 55 μm could account for only c . 5% of the uptake demand for ammonium, and grazing by these larger animals was equivalent to less than 4% of the average nitrate plus ammonium uptake. Large numbers of flagellates dominated the nacoplankton samples. Bacterial heterotrophy and nitrification rates, as well as picoplankton biomass and associated rate variables, greatly increased subsequent to a period of apparent upwelling. Microbial interactions (e.g. picoplankton—heterotrophic flagellates/ciliates) are likely to dominate the biological dynamics of this coastal shelf system.