Factors affecting spatiotemporal benthic macroinvertebrate diversity and secondary production in a semi-arid watershed

Spatial and temporal changes of the benthic macroinvertebrate (BMI) community composition and secondary production provide valuable information about the current 'health' of aquatic systems. We analyzed seasonal and inter-annual changes in BMI communities at five sites along an elevation and land use gradient in the Walker River, NV, USA. In addition, bimonthly collections of Baetis mayflies were made at two sites to assess the effect of nutrient enrichment on Baetis secondary production. BMI community composition changed spatially and temporally in response to the physicochemical environment. BMI diversity was positively correlated to substrate size and stream width but negatively correlated with stream temperature and organic pollutants. Cluster analysis based on BMI community composition revealed a similar community structure along the river, except at one site which was located below the outflow of a eutrophic reservoir. The BMI community in the river was dominated by collector-gatherers while shredders were absent through most of the downstream and impaired sites. Secondary production of Baetis was almost twice as high at the nutrient-enriched site (1.3g dry mass m(-2) yr(-1)) than at the undisturbed site (0.7g m(-2) yr(-1)), most likely the consequence of warmer water temperatures and superior food quality. Results from this study imply that a variety of anthropogenic activities have altered disparate reaches of the Walker River and that BMI community composition would change towards natural conditions with an increase in discharge and reduced water temperature and nutrients. This work also underscores the importance of considering biotic indices and secondary production to effectively assess stream ecosystem health and to design successful restoration strategies.