SEASONAL NITROGEN ASSIMILATION AND CARBON FIXATION IN A FJORDIC SEA LOCH

Carbon (C) fixation and nitrogen (N) assimilation rates have been esti mated from C-14 and N-15 techniques for a 12 month period in a Scottis h 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 rat es were experienced during May at the other stations. Carbon fixation rates were maximal (448-4047 mg C m(-2) day(-1)) at the time of high p hytoplankton biomass (maximum 8.3 mg m(-3) chlorophyll a) during May, whilst nitrate concentrations remained >0.7 mu mol l(-1). C:N assimila tion ratios suggest nitrogen limitation only during the peak of the sp ring bloom, although at times nitrogen (nitrate and ammonium) concentr ation fell to 0.2 mu mol l(-1) in the following months. The vertical s tability of the water column, influenced by tidal and riverine flushin g, varied along the axis of the loch, resulting in marked differences between sampling stations. Although ammonium was preferentially assimi lated by phytoplankton, >50% of production was supported by nitrate up take and only during the summer months was the assimilation of ammoniu m quantitatively important.