Size-fractionated phytoplankton biomass and nitrogen uptake in response to high nutrient load in the North Biscay Bay in spring

The influence of continental nutrient inputs on nitrogen uptake (nitrate and ammonium) by size-fractionated plankton was investigated on the continental shelf of the North Biscay Bay during early spring. As a general trend, larger phytoplankton (>10 μm) contributed significantly to the biomass and N uptake in nitrate-repleted waters and this was particularly marked in the inshore waters where nitrate concentrations were above of 7–10 μmol L−1. In these waters, the relative contribution of nitrate to N uptake for the larger phytoplankton was generally higher than 50%, whereas most of the N used by the smaller cells (<10 μm) was in the form of ammonium. In the central part of the shelf and in the oceanic waters, the N nutrition of the two size fractions was more variable, but in almost all cases, the relative contribution of nitrate to total N uptake was greater for the large phytoplankton size fraction compared to the small one. Analysis of the nitrate taken up indicated that about 45% of the daily dissolved inorganic nitrogen inputs by the Loire estuary, mainly as nitrate, may be potentially trapped in the inshore waters in early spring and, thus should not reach the central part of the shelf.