IMPACTS OF HIGH-NITRATE FRESH-WATER INPUTS ON MACROTIDAL ECOSYSTEMS -I - SEASONAL EVOLUTION OF NUTRIENT LIMITATION FOR THE DIATOM-DOMINATED PHYTOPLANKTON OF THE BAY-OF-BREST (FRANCE)

The chemical factors (inorganic nitrogen, phosphate, silicic acid) tha t potentially or actually control primary production were determined f or the Bay of Brest, France, a macrotidal ecosystem submitted to high- nitrate-loaded freshwater inputs (winter nitrate freshwater concentrat ions >700 mu M, Si:N molar ratio as low as 0.2, i.e. among the lowest ever published). Intensive data collection and observations were carri ed out from February 1993 to March 1994 to determine the variations of physical [salinity, temperature, photosynthetically active radiation (PAR), freshwater discharges] and chemical (oxygen and nutrients) para meters and their impacts on the phytoplankton cycle (fluorescence, pig ments, primary production). With insufficient PAR the winter stocks of nutrients were almost nonutilized and the nitrate excess was exported to the adjacent ocean, due to rapid tidal exchange. By early April, a diatom-dominated spring bloom developed (chlorophyll a maximum = 7.7 mu g l(-1); primary production maximum = 2.34 g C m(-2) d(-1)) under h igh initial nutrient concentrations. Silicic acid was rapidly exhauste d over the whole water column; it is inferred to be the primary limiti ng factor responsible for the collapse of the spring bloom by mid-May. Successive phytoplankton developments characterized the period of sec ondary blooms during summer and fall (successive surface chlorophyll a maxima = 3.5, 1.6, 1.8 and 1.0 mu g l(-1); primary production = 1.24, 1.18 and 0.35 g C m(-2) d(-1)). Those secondary blooms developed unde r lower nutrient concentrations, mostly originating from nutrient recy cling. Until August, Si and P most likely limited primary production, whereas the last stage of the productive period in September seemed to be N limited instead, this being a period of total nitrate depletion in almost the whole water column. Si limitation of spring blooms has b ecome a common feature in coastal ecosystems that receive freshwater i nputs with Si:N molar ratios <1. The peculiarity of Si Limitation in t he Bay of Brest is its extension through the summer period.