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The influence of repeated chlorine additions on the resistance and resilience of laboratory stream ecosystems was investigated- Prior to the disturbance, four treatments were imposed on eight laboratory streams in a 2 x 2 factorial design, resulting in two replicate streams per treatment. Treatments included two circulation regimes (once-through or 98% recirculated) and two levels of grazer density (1000 individuals/m2 or 0 individuals/m2, using the snail Elimia clavaeformis).

Resistance of periphyton biomass to an initial set of chlorine additions was greater in once-through than recirculated streams. Periphyton resistance following a second set of chlorine additions was greater in streams without snails compared to those with snails. This appeared to be a biomass-mediated response; the greater biomass levels in no-snail streams provided a larger buffer to the oxidizing action of chlorine. Resilience of most structural parameters and area-specific carbon-fixation rates was not influenced to a significant degree either by flow regime or snail density. Resilience of chlorophyll-specific carbon fixation and exoproteolytic activity (an index of microbial activity) was greater in recirculated streams compared to once-through streams, but only when snails were present. Over the entire experimental period (15 wk), most structural parameters and area-specific carbon fixation rates were significantly greater in (1) streams without snails compared to those with them and (2) streams with once-through circulation compared to those that were recirculated, but only for those streams with snails. Our chlorine additions appeared to have relatively little influence on snails, and snail growth over the experiment was significantly greater in once-through compared to recirculated streams.

These data are consistent with other findings that total residual chlorine concentrations of < 1.0 mg/L can significantly affect periphyton structure and function. However, nutrient cycling rates appeared to be little affected by the chlorine perturbation, which allowed recirculated systems to recover faster than predicted. The findings from this study suggest that periphyton mat physiognomy and integrity strongly influence system resistance (due to buffering capacity) and resilience (because of nutrient cycling).