Vertical and horizontal structure of the picophytoplankton community in a stratified coastal system off New Zealand

Abstract

The distribution of the picophytoplankton community of the West Coast region of South Island, New Zealand, was investigated during a period of thermal stratification in February 1989. The < 2 urn size fraction was found to account for a significant proportion of the plankton biomass, with up to 73% of the particulate nitrogen and phosphorus occurring in this size class. The spatial variability of the picophytoplankton community was high with cell concentrations in the range 1.5-10.7 × 107 cells I‐1. The vertical distribution of cell numbers was highly variable, although the maximum cell concentrations occurred in the mixed layer in all but two of the 15 stations sampled. At these two stations the maximum cell concentration coincided with the deep chlorophyll a maxima in the region of the pycnocline. The composition of the picophytoplankton community was highly variable with the P:E ratio ranging from 5.7 to 162 within the mixed layer. Maximum concentration of eukaryotic cells occurred either just below or just above the pycnocline where NO3 concentrations were elevated. This study has illustrated the high degree of spatial variability in the composition of the picophytoplankton population of the West Coast region. Some of this variation can be attributed to physico‐chemical environmental factors such as nutrient supply impacting on growth rates. © The Royal Society of New Zealand 1994.