Altered food-web dynamics under increased nitrogen load in phosphorus deficient lakes

Atmospheric nitrogen deposition predominantly influences ecosystems by shifting their available nutrient budgets towards excess nitrogen conditions. In temperate lakes nitrogen is often naturally in excess and phosphorus is deficient, when compared with the optimal Redfield ratio of 16:1. To investigate effects of future increasing nitrogen conditions on lake plankton communities, we performed mesocosm experiments in three different nitrogen rich lakes, all characterised by high nitrogen to phosphorus ratios. In order to determine functional responses to increased nitrogen loading, we conducted six nitrogen fertilization treatments. Nitrogen fertilization was based upon existing nitrate and ammonium concentrations in natural wet deposition and multiple loadings of these concentrations. Despite the initial conditions of excess nitrogen, removal of additional nitrogen by the plankton community was observed in all of the lakes. In one lake, an increasing phosphorus limitation became visible in seston stoichiometry. Over all of the lakes and within each lake's experimental nitrogen gradient, we found evidence for decreased mesozooplankton due to nitrogen enrichment. The negative responses of mesozooplankton to N enrichment were mainly restricted to cladocerans and nauplii. The results indicate that nitrogen enrichment within the magnitudes of projected future atmospheric nitrogen depositions may lead to a long-term reduction of mesozooplankton in phosphorus deficient lakes. The transfer of nitrogen enrichment effects on lower food-web dynamics could have consequences for higher trophic levels, such as fish.