Effects of ultraviolet‐B radiation and vertical mixing on nitrogen uptake by a natural planktonic community shifting from nitrate to silicic acid deficiency

We investigated the combined effects of mixing regimes and ultraviolet‐B radiation (UVBR: 280–320 nm) on the uptake of nitrate, ammonium, and urea by an estuarine phytoplankton community during a 10‐d mesocosm experiment in July 1997. The experiment was conducted in eight mesocosms (1,500 liters each) subjected to varying conditions of UVBR and mixing: (1) natural UVBR and fast mixing, (2) enhanced UVBR and fast mixing, (3) natural UVBR and slow mixing, and (4) enhanced UVBR and slow mixing. The study was carried out during a typical midsummer period of low nutrient concentrations. Throughout the experiment, the phytoplankton community was dominated by a mixed community composed mainly of centric diatoms, cyanobacteria, and prymnesiophytes, with larger algal cells (>5 µm) representing >70% of the total chlorophyll a biomass. The phytoplankton nutrient status changed over the experiment. Algal cells were nitrate‐deficient until the occurrence of a nitrification event on day 7, and then became silicic acid‐deficient. Mesocosms with different mixing rates showed significant differences in ambient nutrient concentrations, nitrogen uptake rates, and phytoplankton species composition. In general, no significant difference was detected between the N transport rates from samples incubated close to the surface and close to the bottom of the mesocosms, and no difference was detected between the UVBR treatments on all the other studied biological and chemical variables measured during the experiment. It is only under the slow mixing regime that the enhanced UVBR treatments significantly depress both the transport and the chlorophyll a—specific transport rates of dissolved organic nitrogen (urea). The UVBR effects observed only in the slow mixing regime could be a consequence of an inherent difference in the uptake capacity of the different communities under the two mixing regimes. Both nutrient conditions and mixing regime influenced the natural phytoplankton physiology and composition toward a community more sensitive to UVBR enhancement under a slower mixing regime.