Nutrient cycling in impacted stream ecosystems: From microbes to watersheds

his dissertation explores the effects of disturbance on nutrient cycling in stream ecosystems at three spatial scales: the patch scale includes localized processes and assemblages, the reach scale encompasses tens to hundreds of meters of stream length, and the watershed scale consists of the hierarchical network of stream orders found in catchments. Additionally, the gut bacterial communities of three freshwater snail species were investigated to better understand the ecology and physiology of this important group of aquatic grazers. Snails are important regulators of periphyton growth, which plays an integral role in nutrient cycling in stream ecosystems. Freshwater snails are also intermediate hosts for a variety of parasites of medical and veterinary significance.Biofilm assemblages are patch scale communities which dominate the metabolism and biogeochemical cycles in stream ecosystems. To determine the effects of eutrophication, one of the most common disturbances to stream ecosystems, on the structure and function of heterotrophic stream biofilms, we created an enrichment gradient by amending darkened stream channel mesocosms with a stochiometrically balanced solution of sucrose, NH4, and PO 4. Biofilm community productivity and function responded exponentially to enrichment. The observed nonlinear increase in functional capacity suggests biofilms are highly responsive to resource availability likely due to the physical structures and synergistic social interactions found in biofilm assemblages.Domestic and native ungulate grazers significantly alter riparian areas, stream reaches, and catchment characteristics. We examined nutrient cycling linkages between riparian soils and adjacent streams and the impacts of ungulate grazing on these ecosystems and processes at six grazing exclosure sites in the Valles Caldera National Preserve, NM, USA. The results suggest ungulate grazing impacts terrestrial characteristics which are linked to ecosystem services provided by adjacent aquatic ecosystems. Management plans should carefully balance the positive effect of grazing on stream nutrient processing and retention reported here with the well documented grazing related loss of other ecosystem services such as decreased fish and aquatic invertebrate habitat and effects on water quality parameters such as turbidity and water temperature.Nutrient cycling in aridland catchments and rivers is controlled by a unique set of inputs and retention mechanisms. We investigated spatial and temporal variation in the sources and sinks of nutrients in the middle Rio Grande (MRG), a 300 km reach of aridland river in the southwestern United States that drains an agro-urban catchment experiencing rapid population growth. Wastewater treatment plant inputs were the dominant source of nutrients to the MRG, increasing loads of NO3-N, SRP, and NH4-N by 1000-2000% relative to upstream loading. The total retention of NO3-N and SRP inputs in the MRG corridor ranged from 6-99% and 34-99%, respectively. Retention was strongly and positively correlated with the percentage of water diverted from the MRG for agricultural irrigation.Little is known about the microbial gut flora of freshwater snails in spite of the important role gastropod mollusks play as grazers in freshwater ecosystems. This study used culture independent methods to describe the community composition and the variability of gut microbes within and among three species of planorbid snails, Helisoma duryi (North American species), Bulinus africanus (African species), and Biomphalaria pfeifferi (African species). Three hundred and fourteen unique bacterial operational taxonomic units (OTUs, DNA sequences with <98% similarity) were found in the guts of the three snail species. The results suggest the presence of highly diverse and relatively similar gut microbial communities in the three snail species in spite of varying levels of phylogenetic and geographic separation, and highlight the need for additional study to determine the roles gut microbes play in the physiology of these important intermediate hosts for digenetic trematodes of medical and veterinary significance. (Abstract shortened by UMI.)...