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Impacts of increasing nitrogen:phosphorus ratios on zooplankton community composition and whitefish (Coregonus macrophthalmus) growth in a pre‐alpine lake

Lorenz, Patrick, Trommer, Gabriele, Stibor, Herwig
Freshwater biology 2019 v.64 no.6 pp. 1210-1225
Coregonus, Daphnia, Rotifera, algae, atmospheric deposition, biomass, community structure, ecosystems, empirical research, fish, food chain, food quality, herbivores, juveniles, lakes, nitrogen, phosphorus, phytoplankton, planktivores, pollution load, seston, surface water, zooplankton, Germany
The combination of increasing atmospheric depositions of reactive nitrogen (N) and the highly effective diminishing of external phosphorus (P) loadings can change key nutrient ratios in lake ecosystems. Consequently, ratios of dissolved inorganic N (DIN) to dissolved P (DP) in lakes are increasing. However, potential consequences for aquatic organisms are as yet far from understood. We formulated three hypotheses on the potential effects of rising DIN:DP ratios on a lake food web: (1) increasing DIN:DP ratios intensify the P limitation of phytoplankton communities and lower their food quality; (2) densities of P rich zooplankters (e.g. cladocerans) will be negatively affected by P‐limited food algae; (3) as result, planktivorous fish will experience a reduction of their main prey (especially Daphnia species) and respond with lowered growth. These hypotheses were tested in a mesocosm experiment conducted in a pre‐alpine lake in southern Germany, Lake Brunnensee. For 76 days, the natural phytoplankton and zooplankton communities were exposed to a wide gradient of DIN:DP ratios. At the end of the experiment, juvenile planktivorous whitefish (Coregonus macrophthalmus) were released into the mesocosms and allowed to feed on zooplankton communities for 72 hr. Along the gradient of DIN:DP ratios, we found evidence for a rising P limitation of autotroph growth, which was indicated by increasing ratios of N:P (15:1–157:1) and C:P (91:1–797:1) in seston biomass. The rising P limitation in algae reduced the nutritional food quality for the majority of herbivorous zooplankton. Both the total zooplankton biomass and the Daphnia biomass declined substantially with increasing DIN:DP ratios. In contrast, increasing DIN:DP ratios favoured rotifer species, showing strong positive correlation with rotifer biomass. Whitefish weights decreased with increasing rotifer biomass and increased with rising Daphnia biomass in zooplankton communities. In summary, our results provide an experimental demonstration that increasing DIN:DP ratios can cause stoichiometric shifts in the biomass of primary producers towards higher N:P and C:P ratios. Effects on zooplankton were changes in the taxonomical community composition towards lower cladoceran biomass (mainly Daphnia spp.). The reduction in Daphnia biomass in turn caused significantly reduced growth rates of whitefish in our experiment. Our experimental results therefore support the assumption that stoichiometric effects can travel up the food chain. General consequences of such multi‐trophic effects induced by altered nutrient ratios could be potentially visible during re‐oligotrophication of water bodies, often resulting in high N:P ratios. Further empirical studies could look for signatures of these effects on the yield of economically important species.