Impacts of high-nitrate freshwater inputs on macrotidal ecosystems. I. Seasonal evolution of nutrient limitation for the diatom-dominated phytoplankton of the Bay of Brest (France)

Abstract

The chemical factors (inorganic nitrogen, phosphate, silicic acld) that potentially or actually control primary production were determined for the Bay of Brest, France, a macrotidal ecosystem submitted to high-nitrate-loaded freshwater inputs (winter nitrate freshwater concentrations >700 ph4, SI:N molar ratio as low as 0.2, i e. among the lowest rver published). Intensive data collection and observations were carried out from February 1993 to March 1994 to determine the variations of physical [salinity, temperature, photosynthetirally ac!ive radiation (PAR), freshwater discharges] and chem-~c a l (oxygen and nutricnts) pardmeters and their impacts on the phytoplankton cycle (fluorescence, pigments, pnmary production). With insufficient PAR the winter stocks of nutrients were almost nonutilized and the nitrate excess was exported to the adjacent ocean, d u e to rapid tidal exchange. By early April, a diatom-dominated spring bloom developed (chlorophyll a maximum = 7.7 pg I-'; prlmary production maximum = 2.34 g C m-2 d-') under high initial nutrient concentrat~ons. Silicic acid was rapidly exhausted over the whole water column; it is inferred to be the primary limiting factor responsible for the collapse of the spring bloom by mid-May. Successive phytoplankton developments character~zed the period of secondary blooms during summer and fall (successive surface chlorophyll a maxima = 3.5, 1.6, 1.8 and 1.0 pg I-'; primary production = 1.24, 1.18 and 0.35 g C m-' d-l). Those secondary blooms developed undel-lower nutrient concentrations, mostly originating from nutrient recycling. Until August, SI and P most likely limited primary production, whereas the last stage of the productive period in September seemed to be K llmited instead, thls being a period of total nltrate depletion in almost the whole water column. Si limitation of spring blooms has become a common feature in coastal ecosystems that recelve freshwater inputs w ~t h Si:N molar ratlos < l The peculiarity of Si 1i.mitation in the Bay of Brest is its extension through the summer period.