Does nutrient enrichment alleviate stoichiometric constraint on plankton trophic structure?

Stoichiometric mismatch between phytoplankton and zooplankton has implication for trophic transfer efficiency. Phosphorus (P) enrichment is expected to lower phytoplankton carbon (C) to P ratio (C : P) and thereby either alleviate P deficiency or induce excess P for zooplankton. However, the generality of zooplankton facing excess P and its effect on plankton trophic structure in natural systems are poorly understood. We analyzed the stoichiometry of seston and zooplankton, and plankton trophic structure in 32 (sub)tropical Chinese reservoirs. Our results showed that (1) stoichiometric mismatch between seston and zooplankton involved P or/and nitrogen (N) deficits in low-nutrient reservoirs and P excess in high-nutrient reservoirs; (2) at given seston C and phytoplankton compositional food quality levels, zooplankton to phytoplankton biomass ratio (Zoo : Phyto) showed a two-segment piecewise relationship to seston N : P with a maximum at a breakpoint of 12.6 and strong reductions toward the extremes of seston N : P gradient; (3) increasing stoichiometric mismatch between seston and consumers reduced the contribution of cladocerans to zooplankton biomass and increased the trophic position of copepods; and (4) chlorophyll a (Chl a) to total phosphorus (TP) ratio (Chl a : TP) increased with decreasing Zoo : Phyto, and at a given Zoo : Phyto, it was higher in reservoirs with zooplankton dominated by copepods than in reservoirs with zooplankton dominated by cladocerans. These findings suggest that nutrient enrichment might improve stoichiometric constraint on plankton trophic structure in low-nutrient reservoirs, but enhance negative effect of excess P in high-nutrient reservoirs. Thus, negative impact of excess P on zooplankton may be a mechanism partly contributing to low Zoo : Phyto and high Chl a : TP in eutrophic reservoirs.