A comparative-study of nitrogen and carbon uptake by phytoplankton in a coastal eutrophic ecosystem (bay of Brest, france)

Nitrogen (N-15) and carbon (C-14) uptake rates and hydrological, chemical and biological parameters were monitored at a single station during spring (March-April 1989) in a coastal eutrophic ecosystem of western Europe : the Bay of Brest. Before the spring growth of phytoplankton populations, total nitrogen and carbon uptake rates were low (almost-equal-to 0.3-mu-mol C.1(-1).h-1 and almost-equal-to 0.025-mu-mol N.l(-1).h-1). During the spring bloom, these uptake rates reached high values (0.5-mu-mol C.1(-1).h-1 and 0.28-mu-mol N.1(-1).h-1); at this time, nitrate was the main source of nitrogen production, although relative preference index (RPI) values showed that ammonium was the preferred nitrogen form for phytoplankton (RPI (NO3) 5). C/N assimilation ratios were higher than C/N composition ratios of the particulate matter. This difference is interpreted in terms of terrestrial detritus inputs from rivers and variation of the physiological state of phytoplankton populations.