The effect of repeated stressor episodes on algal communities in pasture streams
2009
Riddle, Cale J. | Matthaei, Christoph D. | Townsend, Colin R.
The effects in streams of anthropogenic stressors associated with agricultural intensification can be expected to vary with time, reflecting periodic movements of stock, fertilisation episodes and storm-related inputs. The present study focuses on the cumulative effects of augmented nutrients (N+P, three levels, for 1 month) coupled with episodes of increased bed sediment cover by comparing algal community responses to one, two or three stressor applications in nine sheep pasture streams in New Zealand. Algae were investigated for 1 month before and for 5 months during the manipulations. Algal taxonomic richness decreased with nutrient enrichment plus sediment addition, whereas the pollution-tolerant Achnanthidium increased, indicating reduced water quality. Sediment addition by itself had few negative effects on the algae, possibly because the sediment retention time during each 1-month experimental phase was reduced considerably (to 7-26 days) owing to floods. Algal communities (total algal cell density and densities of the common taxa Navicula spp. and filamentous green algae) reacted more strongly to multiple nutrient/sediment addition episodes than to rare or singular episodes. These results indicate a cumulative impact of anthropogenic stressors on a key component of the stream ecosystem. The effectiveness of management may be improved by focusing on the avoidance of repeated stressor events rather than the occurrence of rare episodes.
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