Resilience of Lotic Ecosystems to a Light‐Elimination Disturbance

Resilience of eight laboratory stream ecosystems was evaluated following a 92—d light elimination disturbance. Prior to the disturbance, four treatments (snails/once—through flow, snails/recirculated flow, no snails/once—through flow, no snails/recirculated flow) were imposed on the streams, resulting in systems with different biomass levels, nutrient concentrations, and recycling indices. Based on results from models of ecosystem response to disturbance, we hypothesized a priori that once—through streams would recover more quickly than recirculated streams within each grazing regime and that grazed streams would recover more quickly than ungrazed streams within each flow regime. Our results indicated that once—through streams did have a higher resilience than recirculated streams when snails were absent, but not when snails were present. Indeed, most parameters recovered faster in streams without snails than those with them, irrespective of flow regime, in contrast to our prediction. Despite the faster initial recovery rates in once—through than recirculated streams without snails, final biomass levels were similar between these streams. Measurements of phosphorus recycling indices suggested that higher rates of nutrient recycling near the end of the experiment in recirculated streams compensated for the lower inputs of new nutrients in the incoming water, allowing biomass to reach levels similar to those in once—through streams. Results from this empirical work are consistent with the theory that resilience is positively associated with the rate of nutrient input to a nutrient—limited system, but are inconsistent with the idea that resilience is positively associated with the addition of a herbivore trophic level to the system. Grazing intensity (i.e., density), as well as behavioral (e.g., vagility) and life history characteristics may need to be considered in future models.