The effect of repeated stressor episodes on algal communities in pasture streams
Cale J. Riddle A B , Christoph D. Matthaei A and Colin R. Townsend A CA Department of Zoology, University of Otago, PO Box 56, Dunedin 9001, New Zealand.
B New Zealand Environmental Ltd, 310 Kerikeri Road, PO Box 710, Kerikeri 0245, New Zealand.
C Corresponding author. Email: colin.townsend@stonebow.otago.ac.nz
Marine and Freshwater Research 60(5) 446-458 https://doi.org/10.1071/MF08179
Submitted: 13 June 2008 Accepted: 31 December 2008 Published: 25 May 2009
Abstract
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.
Additional keywords: Achnanthidium, agricultural intensification, anthropogenic stressors, cumulative response, Navicula, nitrate, phosphate.
Acknowledgements
Robin Holmes, Sebastian Uhlmann, Michael Meissle, Jay Piggott and Leni Wutte helped with the field work, Esben Kristensen provided initial assistance and guidance for the algal community analysis and Barbara Downes provided statistical advice. Funding for this research was provided by the Foundation for Research, Science and Technology. The comments of Andrew Boulton and two anonymous referees resulted in substantial improvements to this manuscript. We gratefully acknowledge numerous landowners for access to study sites.
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