Carbon (C) fixation and nitrogen (N) assimilation rates have been estimated from 14C and 15N techniques for a 12 month period in a Scottish sea loch. The maximum rate of nitrogen assimilated (29.92 mmol N m-2 day-1) was in April at the most seaward station; similar high rates were experienced during May at the other stations. Carbon fixation rates were maximal (488-4047 mg C m-2day-1) at the time of high phytoplankton biomass (maximum 8.3 mg m-3 chlorophyll a) during May, whilst nitrate concentrations remained >0.7 μ.mol l-1. C:N assimilation ratios suggest nitrogen limitation only during the peak of the spring bloom, although at times nitrogen (nitrate and ammonium) concentration fell to 0.2 μmol l-1 in the following months. The vertical stability of the water column, influenced by tidal and riverine flushing, varied along the axis of the loch, resulting in marked differences between sampling stations. Although ammonium was preferentially assimilated by phytoplankton, >50% of production was supported by nitrate uptake and only during the summer months was the assimilation of ammonium quantitatively important.