Size-dependent uptake of nitrate and ammonium as a function of light in well-mixed temperate coastal waters

The effect of light on nitrate (NO3−NO3−) and ammonium (NH4+NH4+) uptake by natural communities was investigated in relation to cell size (<10 and >10 μm) in the well-mixed coastal waters of the English Channel. Nitrogen (N) uptake kinetics as a function of irradiance were assessed using 15N tracer techniques, for a seasonal cycle of populations collected at 50% and 1% light penetration depth. The nitrogen uptake responses to irradiance can be represented by the formulation used to describe the photosynthesis versus irradiance relationships and modified by the addition of a dark uptake parameter. The response curves of two size fractions of phytoplankton collected at 50% and 1% of incident light did not differ significantly, which suggested that the physiological characteristics of N uptake were not affected by the light intensity at which the phytoplankton assemblages were sampled. The kinetics parameters indicated that the NO3−NO3− uptake system was more strongly dependent on light than the NH4+NH4+ uptake system. They also showed that N uptake was less limited by the light intensity in the small size fraction than it was in the large size fraction. At the mean light intensity in the water column, kinetics analysis predicted a NH4+NH4+ uptake that was on average 1.8 (±0.6) and 2.4 (±1.0) times greater than the NO3−NO3− uptake, for <10 and >10 μm size fractions, respectively. The kinetics also predicted, at the in situ mean light intensity, that the mean ability to take up nitrogen was twice as high (1.8±0.5) for the small than for the large cells when the N substrate was NH4+NH4+ and more than twice as high (2.6±1.5) when the substrate was NO3−NO3−. These results added to our understanding of the light effect on N uptake processes in well-mixed waters, and can largely explain the phytoplankton production, mainly regenerated and dominated by small cells, that has been observed in these waters.

► We examine the kinetics of N uptake as a function of light in the well mixed coastal waters.
► N uptake was limited by low light conditions induced to the strong hydrodynamism.
► The effect of low light explained why N taken up was mainly in the form of ammonium.
► Low light proves to be one of the reasons why N uptake was dominated by small cells.