How the Impacts of N Loading on Resource Limitation, Functional Composition of Plankton, and Net Primary Production Influence Nitrate Uptake and Trophic Transfer in Lake Ecosystems

Abstract:

Nitrogen (N) loading in aquatic ecosystems can have a multitude of effects. Increased N availability often elevates primary production, but typically also alters community composition and trophic structure. How all the myriad impacts of N loading conspire to produce whole ecosystem responses to perturbation is not well understood. To characterize how whole ecosystems response to perturbation along a gradient of N loading, we added nitrate (and phosphate) to large in situ aquatic mesocosms at different rates over the course of three months and quantified biomass distributions across multiple size classes, plankton community composition (including functional traits), and N flow among size classes in both the epilimnion and the hypolimnion prior and subsequent to a one week shading perturbation. Increased N loading resulted in greater rates of light attenuation with depth, which in turn selected for species with higher tolerance to light limitation and low inorganic C availability, but also resulted in increased rates of primary production and top-down grazing pressure. Different degrees of N loading resulted in different rates of nitrate uptake and trophic transfer, as calculated from 15N pulse-chase additions, both prior and subsequent to the shading pertubation, with the loading effect diminished after the perturbation. N loading was positively associated with the rate of N transfer between the epilimnion, where the N was added, and the hypolimnion. A complex picture of whole ecosystem response to perturbation along a gradient of N loading emerges. N loading appears to simplify resource competition among phytoplankton by alleviating N limitation to an extent, and at the same time supports elevated production across trophic levels. Nitrate uptake rate is contingent on standing stock phytoplankton biomass and resource limitation status. Rates of nitrate removal from the water column depend on how N loading alters the abiotic environment (primarily light availability), which selects for particular functional trait combiations that influence uptake rates, and on overall rates of production. Thus, intermediate levels of N loading, which more likely result in co-limitation and moderate productivity, appear to stimulate the highest rates of nitrate uptake and trophic and vertical transfers.