Disentangling the stream community impacts of Didymosphenia geminata: How are higher trophic levels affected?
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Human activities frequently result in either intentional or unintentional introductions of species to new locations, and freshwater environments worldwide are particularly vulnerable to species invasions. An introduced freshwater diatom, Didymosphenia geminata, was first discovered in New Zealand in 2004 but there was limited research available to predict the drivers of D. geminata biomass and how biomass variability might influence higher trophic levels (e.g. invertebrates and fish). We examined the effect of D. geminata biomass on benthic invertebrates, invertebrate drift and fish communities in 20 rivers in New Zealand with variable hydrology, physical habitat and water chemistry. Variation in D. geminata biomass was best explained by a model that showed D. geminata biomass increased with time since the last flow event exceeding three times the median annual discharge and decreasing concentration of dissolved reactive phosphorus. Analyses of biotic responses showed that high D. geminata biomass did not affect either invertebrate or fish diversity but altered the structure of benthic communities, changed the composition of drifting invertebrate communities and reduced fish biomass by 90 %, particularly trout. A partial least squares path model was used to disentangle both direct and indirect effects of D. geminata on fish communities and showed D. geminata had a significant negative direct effect on fish communities. This is the first study to show how the potential effects of the introduced diatom D. geminata can impact fish communities and has shown that D. geminata impacts fish both directly and indirectly through changes in their invertebrate prey community.
KeywordsFlow variability Benthic invertebrates Path model Fish Diversity Didymo
We thank Milen Marinov for assistance in the field and Hayley Stoddart for invertebrate processing. We are grateful to Kathy Walter (NIWA Christchurch) for her assistance in supplying flow data. Meridian Energy Ltd, Contact Energy Ltd, ECS Ltd, Environment Canterbury, Otago Regional Council and Environment Southland and are all thanked for supplying flow and/or water chemistry data. Cathy Kilroy provided helpful advice on an earlier version of this manuscript. This research was funded as part of the University of Canterbury V5 project initiative and surveys were conducted with the approval of the University of Canterbury Animal Ethics committee. Funding to PGJ by NIWA under Freshwater and Estuaries Programme 5, Freshwater Biosecurity (2015/16 SCI), assisted with manuscript preparation.
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