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Marine Ecology Progress Series

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MEPS 161:213-224 (1997)  -  doi:10.3354/meps161213

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)

Yolanda Del Amo1,*, Olivier Le Pape2, Paul Tréguer1, Bernard Quéguiner1, Alain Ménesguen2, Alain Aminot2

1UMR CNRS 6539 'Bioflux', Université de Bretagne Occidentale, Institut Universitaire Européen de la Mer, Technopole Brest-Iroise, Place Nicolas Copernic, F-29280 Plouzané, France
2Laboratoire 'Chimie et Modélisation des Cycles Naturels', Direction de l'Environnement Littoral, IFREMER, BP 70, F-29280 Plouzané, France
*Present address: Department of Ecology, Evolution and Marine Biology, Marine Science Institute, University of California, Santa Barbara, California 93106, USA. E-mail:

The chemical factors (inorganic nitrogen, phosphate, silicic acid) 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 µM, Si:N molar ratio as low as 0.2, i.e. among the lowest ever published). Intensive data collection and observations were carried 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) parameters and their impacts on the phytoplankton cycle (fluorescence, pigments, primary production). With insufficient PAR the winter stocks of nutrients were almost non-utilized 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 µg l-1; primary production maximum = 2.34 g C m-2 d-1) under high initial nutrient concentrations. 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 characterized the period of secondary blooms during summer and fall (successive surface chlorophyll a maxima = 3.5, 1.6, 1.8 and 1.0 µg l-1; primary production = 1.24, 1.18 and 0.35 g C m-2 d-1). Those secondary blooms developed under 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 N limited instead, this being a period of total nitrate depletion in almost the whole water column. Si limitation of spring blooms has become a common feature in coastal ecosystems that receive freshwater inputs with Si:N molar ratios <1. The peculiarity of Si limitation in the Bay of Brest is its extension through the summer period.

Coastal ecosystem · Phytoplankton dynamics · Macrotidal · Nutrient limitation · Silicon · Eutrophication

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