Abstract

Lake ecosystems are faced with increasing depositions of nitrogen (N), deriving from anthropogenic activities such as the use of fertilizers and fossil fuels. The consequences are, however, still far from being understood. Since the great majority of pre-alpine lakes are limited in phosphorus (P), one would not immediately expect that an increasing N input would have serious consequences for these lake ecosystems. However, there is evidence that the consequences of rising N depositions, especially for the trophic systems of these lakes, are more far-reaching than would be initially assumed. To investigate the influence of increasing N loading in pre-alpine lakes, I conducted various mesocosm and microcosm experiments and large-scale monitoring of 11 pre-alpine lakes. The mesocosm experiments were performed in three pre-alpine lakes in southern Germany. The natural phyto- and zooplankton communities and juvenile whitefish (Coregonus spec.) were exposed to different N concentrations or to gradients of dissolved inorganic nitrogen to total phosphorus (DIN:TP) ratios. These experiments were carried out in the spring, immediately after ice-break (March–May), for periods between 63 and 76 days. The aim of the microcosm experiments was to estimate the toxic thresholds of ammonium or ammonia (NH4+ or NH3) for the survival of a Daphnia hyalina complex. In these microcosm experiments, Daphnia were exposed to a NH4+ gradient with NH4+ concentrations between 0–87.8 mg L-1. In contrast, the large-scale lake-monitoring included 11 pre-alpine lakes in southern Germany. These lakes were located between Lake Königssee in the east and Lake Constance in the west of southern Germany. Over a period of three years, the lakes were sampled twice a year for nutrient concentrations, and different parameters concerning phytoplankton-, zooplankton- and whitefish communities (including whitefish of all age classes). I found that increasing N concentrations had significant effects on organisms from different trophic levels in the pelagic food-webs of pre-alpine lakes. Phytoplankton communities responded with measurable qualitative effects to increasing N inputs (changes in community composition, in biomass stoichiometry and in biochemical composition). In contrast, zooplankton communities and whitefish were both, qualitatively but also quantitatively affected. Observed effects on zooplankton levels were changes in community composition and decreasing Daphnia abundances. With increasing N, whitefish showed a decrease in growth and a lowered condition factor. These findings indicate a transfer of N-derived effects from the primary producers at the base of pelagic food-webs through all trophic levels up to top consumers such as planktivorous whitefish. However, rising N concentrations had no toxic effects on Daphnia survival, neither in the mesocosm experiments nor in natural lakes. For the survival of the Daphnia clone used in the microcosm experiments, I found a toxic threshold concentration of ammonium of NH4+ = 4.22 mg L-1. This is more than two times higher than the artificially established ammonium concentrations in the mesocosms (NH4+max = 1.88 mg L-1) and over 13 times higher than the natural ammonium concentrations found in the monitored lakes (NH4+max = 0.32 mg L-1). In conclusion, my results emphasize the importance of increasing N loading for organisms in pre-alpine lake ecosystems. This is especially true as the observed N-derived effects in pre-alpine lakes were clearly measurable, although the productivity of those lakes is known to be predominantly P-limited. Furthermore, the paradigm that P concentrations alone determine the functioning and transfer efficiency of lake food-webs comes into question. I suggest that for future lake-management programs not only the P concentrations but also N enrichment and consequent N:P ratios should be considered.