PHYTOPLANKTON PHOTOSYNTHESIS AND GROWTH IN CONTRASTING REGIONS OF MANUKAU HARBOR, NEW-ZEALAND

Contrasting environments for phytoplankton growth are found in Manukau Harbour. Nutrient levels are higher in parts of the inner harbour tha n elsewhere; and cells spend less time in aphotic waters there than in the outer harbour. Phytoplankton photosynthesis and growth in these r egions were determined during 1991-92. Average levels of biomass in th e inner regions were 2-4 times those in the outer regions, where chlor ophyll a averaged 2-3 mg m-3. However, the euphotic zones were about t wice as deep in the outer regions, so average rates of gross photosynt hesis were reasonably similar overall (344-557 mg C m-2 d-1). Respirat ion losses were greatest in the outer harbour, so net photosynthesis w as lower there. During a late-summer diatom bloom in the inner harbour , peak values of chlorophyll a and gross photosynthesis were 66 mg m-3 and 4100 mg C m-2 d-1, respectively. A simple equation involving biom ass, euphotic depth, and incident light accounted for most of the vari ability in euphotic zone photosynthesis. Average growth rates varied a cross the harbour: from about 0.4 d-1 in the inner harbour, to less th an half this in the outer regions where the aphotic zone was deeper.