Phytoplankton photosynthesis and growth in contrasting regions of Manukau harbour, New Zealand

Abstract

Contrasting environments for phytoplankton growth are found in Manukau Harbour. Nutrient levels are higher in parts of the inner harbour than elsewhere; and cells spend less time in aphotic waters there than in the outer harbour. Phytoplankton photosynthesis and growth in these regions were determined during 1991-92. Average levels of biomass in the inner regions were 2-4 times those in the outer regions, where chlorophyll a averaged 2-3 mg m-3. However, the euphotic zones were about twice as deep in the outer regions, so average rates of gross photosynthesis were reasonably similar overall (344-557 mg C m−2 d−1). Respiration losses were greatest in the outer harbour, so net photosynthesis was 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 biomass, euphotic depth, and incident light accounted for most of the variability in euphotic zone photosynthesis. Average growth rates varied across the harbour: from about 0.4 d−1 in the inner harbour, to less than half this in the outer regions where the aphotic zone was deeper. © The Royal Society of New Zealand 1993.