Microbial assemblage during the spring‐summer transition on the northeast continental shelf of New Zealand

Abstract

The northeast shelf of the North Island of New Zealand was sampled on four occasions from early spring to late summer in 1996–97. During this period, tight linkages between the physical and chemical forcings and the response from the microbial food‐web were observed. During bloom conditions in early spring the phytoplankton population was dominated by large diatoms with the picophytoplankton contributing an average of 8% of the phytoplankton biomass. The picophytoplankton fraction increased to contribute a mean of 57% of the phytoplankton biomass in late summer when upwelling had ceased, the water column was strongly stratified and dissolved inorganic nitrogen concentrations were negligible. The changes in the phytoplankton assemblage were accompanied by changes in the microzooplankton population. In early spring the microzooplankton population was dominated by larger species, including copepod nauplii and >20 μm aloricate ciliates, with copepod nauplii contributing up to 50% of the microzooplankton biomass in early and late spring. Peak microzooplankton biomasses of 880–2093 μg C m‐2 in the mixed layer occurred in early and late spring, depending on the station. The large ciliate Laboea strobila was an important component of the microzooplankton biomass, and increased substantially at three out of the five stations between early and late spring. Strong upwelling in early summer resulted in substantial decreases in picophytoplankton and microzooplankton abundance. In late summer the composition of microzooplankton population had changed substantially, with the <20 μm aloricate ciliates increasing from 35 to 90% of the total biomass. The overall increase in the importance of the microbial components of the food‐web between early spring and late summer was closely linked with changes in the physical and chemical drivers of the system.