SEASONAL-CHANGES IN FACTORS CONTROLLING PHYTOPLANKTON GROWTH IN BEATRIX BAY, NEW-ZEALAND

Phytoplankton biomass and growth rates were measured in Beatrix Bay, M arlborough Sounds, during 1994-95. In spring and summer, nitrate and c hlorophyll a concentrations in the mixed layer were low (typically < 1 mg N m(-3) and < 1 to 3 mg Chi. a m(-3), respectively), and phytoplan kton growth rates were moderate (average 0.3 d(-1)). Growth rates incr eased several-fold in response to the experimental addition of inorgan ic N (but not to inorganic phosphorus). Higher nitrate concentrations were found below the pycnocline. During autumn and winter, nitrate con centrations in the mixed layer were high (up to 60 mg m(-3)), but phyt oplankton growth rates were lower than in summer (average 0.2 d(-1)), and did not respond to added nutrients. Chlorophyll a concentrations, however, were highest during the winter (typically 3-6 mg m(-3)). Even so, low light levels meant integral photosynthesis per unit chlorophy ll a was low in winter. Phytoplankton growth appears to be controlled by the availability of nitrogen in summer and by light in winter.