Nutrients and phytoplanktonic growth in the Bay of Seine, France.

As part of the Programme National d'Oceanographie Cotiere, the nutrient dyn amics of the Bay of Seine were studied between 1992 and 1994 in order to co mplement work on ecological modelling. Firstly, the River Seine's nutrient fluxes were established: 80 000-130 000 t a(-1) of dissolved inorganic nitr ogen, 6 400-8 400 t a(-1) of dissolved phosphorus and 20 000-77 000 t a(-1) of dissolved silicium. Estuarine processes were taken into account. Conseq uences of nutrient loading for the bay were then evaluated at the pelagic l evel (nutrient and chlorophyll enrichments) and the benthic level (distribu tion of various phosphorus fractions in superficial sediments). The large c ontinental inputs always induce concentration gradients in the water from t he mouth of the river to the northwest of the bay. The northward spreading of fine particulate matter controls the distribution of adsorbed phosphate and iron-aluminium bound phosphate in sediments. In contrast, calcium bound phosphate, the main fraction in sediments, is not affected by river inputs . Organic phosphorus in sediments is related to phytoplankton blooms, with increasing concentrations during productive periods; afterwards the fast re cycling prevents sedimentary accumulation. The nutrient depletions observed beyond the turbid plume during spring 1992 enabled the calculation of nutr ient uptake rates, and the comparison of these rates with phytoplankton bio mass (chlorophyll a + phaeopigments). Ratios of nutrient consumption to pig ment concentration were estimated at 1 mu mol mu g(-1) for nitrogen, 0.05 f or phosphorus and 0.5 for silicate. These values, as N/P and Si/N uptake ra tios (respectively 17.5 and 0.4), were similar to usual values. (C) Elsevie r, Paris.