Seasonal nitrogen dynamics in a seepage lake receiving

11 pages, 6 figures, 2 tablesStudies on nitrogen dynamics in seepage lakes are seldom undertaken, yet our understanding of the complex pattern of the nitrogen (N) cycle is complicated by its temporal and spatial heterogeneity. This research investigated temporal variation inNconcentration, considering different forms ofN(NO−3 ,NO−2 ,NH+4 , dissolved organicN, particulate organic N) in a Spanish flowthrough seepage lake (Colgada Lake) receiving high N loads. The study was based on monthly data collected over the period 2003–2005 from lake inputs and outputs, vertical profiles at a single representative site in the middle of the lake and fluxes of NH+4 and NO−3 at the sediment–water interface. The distribution of total N (TN) inputs and outputs did not follow a clear temporal pattern. TN inputs varied from 27.70 to 125 tonnes N month−1, 75–84% of which is NO−3 .Temporal variation of concentration profiles for different N forms measured showed significant differences owing to stratification. Ammonium always entered the sediment, whereas sediments acted as either a sink or source of NO−3 . Fluctuation in N dynamics in this lake was more influenced by external factors, such as the input variability, than by the turnover of nitrogenous substances in the water body. Comparing seasonal N dynamics, there seems to be temporal differences between seepage lakes and drainage lakes. In seepage lakes, dissolved inorganic nitrogen (DIN) peaks were reached in early spring and after overturn, whereas in drainage lakes, the autumn minimum gradually increased to maxima in late winter and during the stratification periodREN 2002-00558 Proyecto del Ministerio de Educación y Ciencia de EspañaPeer reviewe

Nitrate uptake rates in freshwater plankton: the effect of food web structure

9 pages, 3 figure, 1 tableNitrate incorporation rates by primary producers and the transfer of nitrogen to upper planktonic food web levels in different seasons (spring and summer of different years) were studied using a microcosm experimental approach. The study communitieswere natural plankton communities from Colgada Lake (central Spain),which is heavily polluted by nitrate. Natural δ15N in phytoplankton and zooplankton was measured and experiments were performed on the 15N supply. Naturally derived δ15N varied from 7.4 to 8.6‰ and from 10.0 to 16.8‰ in phytoplankton and zooplankton respectively. Nitrogen incorporation rates ranged from 0.006 to 0.036μMh−1 and from 0.0004 to 0.0014μMh−1 in phytoplankton and zooplankton respectively. The differences in natural δ15N levels and nitrogen incorporation rates between plankton fractions from seasonally different communities reported in the present study suggested that the nitrogen uptake by planktonic communities in Colgada Lake depend on different combinations of dominant zooplankters and phytoplankton size structure. A higher level of nitrogen uptake by phytoplankton occurred when small algae were dominant without competitors (larger algae) or main predators (herbivorous zooplankters). This was because copepods, with the lowest zooplankton nitrogen uptake, were dominant. Phytoplankton nitrogen uptake was lower when big algae were dominantThe authors wish to thank the Spanish Ministry of Education and Science for the funding of the research projects REN-2002-558 and CGL2006-2346.We also acknowledge American Journal Experts for English language correctionPeer reviewe