Non-additive effects of species interactions on aquatic ecosystems
responses to nutrient perturbation
Abstract
Nutrient perturbations are a threat to aquatic ecosystems worldwide.
Interactions among important species can influence the recovery of
aquatic ecosystems from nutrient perturbations, and the persistence in a
clear water state for a given regime of nutrient loading. In a factorial
manipulation of two foundation species (macrophytes and mussels), we
measured 16 large (15000L) ponds at high resolution (15 min interval)
over two years with two periods of nutrient perturbation. We found that
each foundation species had strong individual effects on multiple
ecosystem properties (e.g. phytoplankton densities and metabolism),
largely in line with our expectations (i.e. reduction of algal biomass
relative to control). However, when both species were present, we saw
dramatic increases in algal biomass and other ecosystem parameters,
indicating strong non-additive antagonistic effects. Overall, our
results demonstrate how interactions between foundation species can
cause surprisingly strong deviations from the expected responses of
aquatic ecosystems to nutrient additions.