The distribution of temperature, salinity, dissolved inorganic nutrients, and chlorophyllais described along a transect in Pelorus Sound, as are phytoplankton populations and zoo‐plankton biomass. River water contributed dissolved inorganic nutrients to Pelorus Sound; silicon was a conservative property. Pelorus Sound was a sink, and the open sea a source for nitrate‐nitrogen. Havelock Arm and Cook Strait water appeared to be a source for ammonium‐nitrogen. Winter cooling at the head of Kenepuru Sound produced high‐density water which sank at the confluence with Pelorus Sound; this is a mechanism (in addition to internal tides) whereby low‐salinity high‐nutrient water is mixed downwards. Phytoplankton biomass was dominated by diatoms in July 1981. Zoo‐plankton biomass in outer Pelorus Sound in May 1982 was dominated byAmelia aurita. The distribution of chlorophyllais discussed in relation to the input of river water, inorganic nutrients, and variations in zooplankton grazing pressure. The proposition that more mussel farms would result in long‐term food limitation of cultured mussel production is discussed.