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An intensive study was made of greater Cook Strait in the summers of 1980 and 1981 to determine the availability of dissolved inorganic nutrients in surface waters and the influence this may have on phytoplankton production. Phytoplankton biomass in greater Cook Strait appears to be controlled mainly by zooplankton grazing and turbulence or advection. The relative influence of factors governing primary production on phytoplankton distribution was assessed for the tidal mixing area off Marlborough Sounds, Manawatu River water, the Kahurangi Point to Cape Farewell upwelling region, and eastern Cook Strait.

The surface distribution and metabolic uptake of nitrate and ammonium were investigated as they related to the movements of water in the upwelling zone off the north-west coast of South Island, New Zealand. Analyses were also made of chlorophyll a and paniculate nitrogen as indicators of phytoplankton biomass.The movement of water and resulting dispersion from the upwelling focus were shown to differ greatly from day to day; this could confound simple correlations between nutrient nitrate and phytoplankton. A suitable model to investigate metabolic processes due to influx of fertile upwelled water which could avoid this problem can be based upon assays of NO 3 and NH 4 uptake by the plankton. These assays, done with 15 NH 4 and I5 NO 3 , could provide an operational index for identifying the zone of nett production due to nutrient upwelling.

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