JOURNAL OF PLANKTON RESEARCH | VOLUME 17 | NUMBER 6 | PAGES 1307-1324 | 1995
© Oxford University Press
research-article |
Seasonal nitrogen assimilation and carbon fixation in a fjordic sea loch
1 Plymouth Marine Laboratory Prospect Place, The Hoe, Plymouth PL1 3DH, UK 2Scottish Office Agriculture and Fisheries Department, Marine Laboratory Aberdeen, AB9 8DP, UK 3Present address: Department of Marine Sciences and Coastal Management, University of Newcastle Newcastle upon Tyne, NE1 7RU, UK
Received on August 22, 1994; accepted on February 13, 1995 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.