Structure and dynamics of the pelagic microbial food web of the Subtropical Convergence region east of New Zealand

Abstract

The Subtropical Convergence (STC) region to the east of New Zealand was sampled in both winter and spring to establish the food web structure and dynamics in 3 water masses. Subtropical (ST) waters to the north were warm with high salinity, and in spring macronutrient concentrations were potentially limiting to phytoplankton growth. Subantarctic (SA) waters to the south in both seasons were cold, with lower salinity and high macronutrient concentrations with chlorophyll a concentrations of 0.12 to 0.19 μg l-1. The frontal zone between these water masses (STC) had maximum chlorophyll a concentrations ranging from 0.39 to 3.42 μg l-1. In SA waters in both seasons and ST waters in spring the pelagic food web in the mixed layer was dominated by the microbial components with up to 98% of the phytoplankton biomass in the <20 μm size fraction and the bacterial C/phytoplankton C ratio ranging from 0.9 to 1.9. In comparison, in STC waters in both seasons and subtropical waters in winter the >20 μm phytoplankton dominated the phytoplankton biomass with ratios of 0.2 to 0.3 bacterial C/phytoplankton C. Despite significant differences in the food web structure, primary grazers of the phytoplankton in all water masses in both seasons were microzooplankton with 78 to 118% of the primary production being grazed on a daily basis. This suggests that the food web was dominated by recycling processes, and phytoplankton export fluxes from the upper mixed layer were low in all water masses at the time of sampling.