Picophytoplankton in the Hauraki Gulf, New Zealand

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Abstract
Qualitative and quantitative data for picophytoplankton from the Hauraki Gulf, northeast New Zealand, were gathered from May 1986 to February 1987. Records for both whole and size‐fractionated samples (net‐, nano‐, and picophytoplankton) allowed the relative contribution by the picophytoplankton to total biomass and overall rates of carbon fixation to be estimated. Picophytoplankton represented a mean value of over 30% of the total chlorophyll a in winter and c. 20% in other seasons. In terms of total carbon fixation, values of 23% and 13% contribution were obtained. The picophytoplankton were composed of chroococcoid cyanobacterial cells, 0.7–1.2 μm in diameter, at densities between 103 and 104 ml‐1 together with larger (1–2 μm) eukaryote cells at densities up to 103 ml‐1. Winter counts were lower than those for summer. Cyanobacteria displayed subsurface peak densities higher in the water column than the eukaryotes. The cyanobacterial cells at depth tended to be larger and more brightly fluorescing than those in the surface waters.