Ecology | 2021
Non-additive effects of foundation species determine the response of aquatic ecosystems to nutrient perturbation.
Abstract
Eutrophication is a persistent threat to aquatic ecosystems worldwide. Foundation species, namely those that play a central role in the structuring of communities and functioning of ecosystems, are likely important for the resilience of aquatic ecosystems in the face of disturbance. However, little is known about how interactions among such species influence ecosystem responses to nutrient perturbation. Here, using an array (N=20) of outdoor experimental pond ecosystems (15000 L), we manipulated the presence of two foundation species, the macrophyte Myriophyllum spicatum and the mussel Dreissena polymorpha, and quantified ecosystem responses to multiple nutrient disturbances, spread over two years. In the first year we added five nutrient pulses, ramping up from 10 to 50μg P/L over a 10-week period from mid-July to mid-October, and in the second year we added a single large pulse of 50μg P/L in mid-October. We used automated sondes to model whole-ecosystem metabolism, and to measure multiple ecosystems properties at high frequency (15min intervals), including phytoplankton and dissolved organic matter fluorescence. Overall, both foundation species strongly affected the ecosystem responses to nutrient perturbation, and, as expected, initially suppressed the increase in phytoplankton abundance following nutrient additions. However, when both species were present, phytoplankton biomass increased substantially relative to other treatment combinations: non-additivity was evident for multiple ecosystem metrics following the nutrient perturbations in both years but was diminished in the intervening months between our perturbations. Overall, these results demonstrate how interactions between foundation species can cause surprisingly strong deviations from the expected responses of aquatic ecosystems to nutrient additions.