Impacts of increased atmospheric nitrogen deposition on plankton communities of phosphorus limited lake ecosystems

Gabriele Trommer1, Monika Poxleitner1, Patrick Lorenz1, Eleftherios Bitzilekis1, Alexandre Gogaladze1, Herwig Stibor1
1 Aquatic Ecology, Department Biologie II, Ludwigs-Maximilians-Universität München

O 10.5 in Links between the N cycle and other elements

17.07.2014, 15:50-16:10, H20

Until now, increased nitrogen supply was mainly investigated in lakes which are considered to be limited in nitrogen, since direct consequences like eutrophication can be relatively easy assessed by biomass measurements or oxygen consumption. On the contrary, in phosphorus limited lakes nitrogen is in excess available and implications of increased nitrogen supply are more concealed and difficult to quantify. Considering globally rising amounts of reactive nitrogen in the atmosphere, phosphorus limitation in lakes is predicted to increase in the future, with so far unknown consequences for lake ecosystems. In order to investigate the influence of increased atmospheric nitrogen deposition on freshwater plankton, we studied natural phyto- and zooplankton communities in three Upper Bavarian lakes in mesocosm experiments by applying a gradient of six increasing nitrogen treatments. Besides a control treatment without nitrogen addition, the applied nitrogen supply was based on the natural monitored wet deposition of nitrate and ammonium. Nitrogen treatments included 1-, 2-, 8-, 16- and 32-times the natural nitrogen wet deposition. We discovered considerable changes of phytoplankton as well as zooplankton biomass along the applied nitrogen gradient. Maximum chlorophyll a concentrations increased with increasing nitrogen addition in two and decreased in one lake experiment. Mesozooplankton densities decreased in all lake experiments, at least in the two highest nitrogen addition treatments. Under conditions of higher phosphorus limitation, we found especially smaller densities of Cladocerans as well as Copepods, suggesting worse conditions for growth and/or development of juveniles. Interestingly, rotifer densities appeared to benefit from high nitrogen conditions and increased in all lake experiments. The phytoplankton-zooplankton-interactions were affected by bottom-up as well as top-down effects depending on the lake ecosystem. The common confounding impact of increased atmospheric nitrogen deposition in the mesocosm experiments were decreasing mesozooplankton densities. In lake ecosystems, this would translate into a decreasing food resource for higher trophic levels such as fish.

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last modified 2014-04-03