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

Abstract

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.

Nitrogen enrichment leads to changing fatty acid composition of phytoplankton and negatively affects zooplankton in a natural lake community

“…is predicted at seston C:P ratios >300 42 , as it was the case in our experiment. While previous studies have demonstrated increasing P limitation with N enrichment 14,43 , we were unable to detect signs of increasing P limitation with increasing N enrichment, such as increasing seston N:P ratios. Additionally we did not observe any correlation of seston stoichiometry with Daphnia biomass, which does not exclude the possibility that this mechanism could operate supplementary.…”

“…The causal relationship described between N enrichment and reduced biochemical food quality might also be relevant to other lake systems, although the qualitative effects of N enrichment could be masked by the quantitative effects of phytoplankton growth in N limited lake systems 39 and upscaling from mesocosms to lake systems may have its limitations 44 . However, an N related reduction in secondary production could even affect higher trophic levels, such as fish 43 . Cladoceran zooplankton are known to represent the primary food source for planktivorous fish in temperate lakes 45 , and cladoceran production is positively correlated to fish biomass in lakes e.g.…”

Increased Nitrogen Loading Boosts Summer Phytoplankton Growth by Alterations in Resource and Zooplankton Control: A Mesocosm Study

“…Excess N loading may also favor phytoplankton growth via reducing the fitness and biomass of herbivorous zooplankton (Trommer et al, 2017;Lorenz et al, 2019). For instance, mesocosm experiments found that N addition caused stoichiometric shifts in the biomass of primary producers towards higher N:P and C:P ratios, which lowered cladoceran and total zooplankton biomasses but favored the dominance of rotifers (Lorenz et al, 2019).…”

“…Excess N loading may also favor phytoplankton growth via reducing the fitness and biomass of herbivorous zooplankton (Trommer et al, 2017;Lorenz et al, 2019). For instance, mesocosm experiments found that N addition caused stoichiometric shifts in the biomass of primary producers towards higher N:P and C:P ratios, which lowered cladoceran and total zooplankton biomasses but favored the dominance of rotifers (Lorenz et al, 2019). In shallow lakes, although P has traditionally been considered the primary nutrient causing harmful cyanobacterial blooms, recent findings from laboratory studies and lakes throughout the world demonstrate that N can be important in controlling the timing, density, and toxicity of some nondiazotrophic cyanobacterial blooms (reviewed by Gobler et al, 2016).…”

Responses of lake phytoplankton communities to changing inorganic nitrogen supply forms

“…In the traditional context of ecological stoichiometry theory and nutrient limitation, attention has largely been focused on the general N:P supply ratio in ecosystems. The subject of N addition in N-excess systems has rarely been directly investigated (Poxleitner et al 2016;Trommer et al 2017;Lorenz et al 2019). Lakes are naturally co-limited by N and P, or are seasonally limited by either of the two nutrients (Elser et al 2007;Sterner et al 2008;Paerl et al 2016).…”