COS 238-1 - Aquatic ecosystem response to nutrient loading disturbances mediated by benthic-pelagic coupling

Background/Question/Methods

Abrupt change within an ecosystem can be triggered by a large disturbance, such as an intense storm that leads to a pulse of nutrients entering a waterbody, potentially triggering an algal bloom. However, characteristics of the ecosystem, such as the food web structure, may mediate resilience to abrupt change by controlling the flow of energy and nutrients. In aquatic ecosystems, high connectivity between benthic and pelagic food chains (i.e., benthic-pelagic coupling) generates greater competition for nutrients and energy between food chains and strengthens top-down control. As such, we hypothesized that greater benthic-pelagic coupling increases resilience to algal blooms following a pulse disturbance of external nutrient loading. To test this hypothesis, we established three food web treatments – low, intermediate, and high benthic-pelagic coupling – in a set of experimental ponds, with each treatment replicated once. We then simulated a storm-induced disturbance by adding a pulse of nitrogen and phosphorus to one of the replicate ponds in each food web treatment, with the other pond serving as a reference. We compared the magnitude of response of algal biomass and net ecosystem production (NEP) to the disturbance among food web treatments and evaluated these state variables for evidence of a critical transition.

Results/Conclusions

The ponds with greater benthic-pelagic coupling exhibited higher resilience to the pulse nutrient disturbance. Following the nutrient addition there was no significant response in algal biomass or NEP; however, both the low and intermediate coupling treatments had significant increases in algal biomass following the disturbance. Furthermore, there was evidence that the increase in algal biomass within the low coupling treatment was a critical transition, which did not occur in the other food web treatments. We repeated the nutrient pulse disturbance later in the summer and again only observed a significant response in algal biomass in the low coupling treatment. There was not a significant response in NEP to the disturbances, regardless of food web structure. Together, these results support our hypothesis that greater benthic-pelagic coupling results in greater resilience to repeated pulses of nutrient loading. Analysis of the biomass at each trophic level in the ponds suggests that there was stronger trophic control over algal biomass with a higher degree of benthic-pelagic coupling. This experiment demonstrated that an ecosystem’s response to stochastic disturbances can be mediated by food web structure.