Phytoplankton biomass and growth rates were measured in Beatrix Bay, Marlborough Sounds, during 1994–95. In spring and summer, nitrate and chlorophyllaconcentrations in the mixed layer were low (typically < 1 mg N m3and < 1 to 3 mg Chi.am−3, respectively), and phytoplankton growth rates were moderate (average 0.3 d−1). Growth rates increased several‐fold in response to the experimental addition of inorganic N (but not to inorganic phosphorus). Higher nitrate concentrations were found below the pycnocline. During autumn and winter, nitrate concentrations in the mixed layer were high (up to 60 mg m3), but phytoplankton growth rates were lower than in summer (average 0.2 d−1), and did not respond to added nutrients. Chlorophyllaconcentrations, however, were highest during the winter (typically 3–6 mg m−3). Even so, low light levels meant integral photosynthesis per unit chlorophyllawas low in winter. Phytoplankton growth appears to be controlled by the availability of nitrogen in summer and by light in winter.