Nutrient cycling in impacted stream ecosystems : from microbes to watersheds

Horn David Van

Abstract

The conditions found in stream ecosystems are an integration of watershed characteristics and processes. Anthropogenic disturbances are part of this integration and include direct inputs to streams, alteration of riparian areas, and modification of catchment properties which affect material inputs. Impacts to discrete portions of terrestrial watersheds combine as water moves down gradient, transporting the byproducts of catchment disturbances to streams including, nutrients, organic materials, particulates, and toxins. This 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 PO4. A total of ~2000 high quality bacterial partial 16S rRNA gene sequences yielded 381 unique phylotypes (<97% similarity). Significant differences (p<0.005) were detected between communities from all treatments, with increasing enrichment resulting in greater community divergence and decreased diversity. Biofilm community productivity and function responded exponentially to enrichment, with exponents of 1.5 for areal mass, 2.3 for live cell density, and 2.5-3.5 for the activities of 5 extracellular enzymes. 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 exclusion of native and domestic ungulate grazers for three years significantly increased the riparian aboveground biomass of standing vegetation (273 ± 155 vs. 400 ± 178 g m-2) and litter (56 ± 75 vs. 107 ± 77 g m-2) (p = 0.005 and 0.013, respectively). Soil nutrient values (0 to 15-cm depth) were minimally affected by grazing after five growing seasons, with significant increases in soil total phosphorus at three of the six sites. No connection was found between soil and stream nutrient availability or limitation. Stream geomorphology was not significantly altered by five years of grazing exclusion. The elimination of grazing suppressed instream nutrient processing with significantly longer NH4 uptake lengths (p = 0.02) and non-significant trends toward decreased NH4 uptake rates observed in exclosure reaches. These 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 (R2 = 0.86 and 0.80 for NO3-N and SRP, respectively). Irrigation diversions downstream of the urban wastewater inputs sequestered on average 480, 370 and 40 kg day-1 of NO3-N, SRP, and NH4-N, respectively, during the irrigation season. Within the river channel, retention was 129-906 kg day-1 for NO3-N and 56-779 kg day-1 for SRP, values similar to those measured in mesic systems. However, the combination of in-stream and irrigation network nutrient processing in the MRG adds up to catchment scale retention levels that are significantly higher than those found in mesic systems. 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. Some freshwater snail species are also responsible for the transmission of parasitic diseases including schistosomiasis, which affects ~ 200 million humans worldwide. 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. This diversity was distributed across 23 bacterial phyla with the largest number of OTUs found in the Proteobacteria and Bacteroidetes groups. A small percentage of bacterial clones from every snail species were related to opportunistic pathogens that infect a range of hosts including snails and humans. Measures of \u03b2 diversity revealed minimal divergence among the gut microbial communities both within and among the three planorbid species, with samples differing primarily in the abundance of sequences within bacterial lineages and not in the presence or absence of lineages. These 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. This investigation of nutrient cycling in stream ecosystems at three scales and under three disturbance regimes revealed anthropogenic impacts substantially alter this important ecosystem function; however, negative impacts are frequently moderated by other factors. Eutrophication was shown to decrease bacterial diversity at the local level, however, the functional capacity to process organic materials increased exponentially for microbial communities exposed to enrichment. At the reach scale domestic and native ungulate grazers negatively impact numerous stream characteristics but in this study were shown to enhance instream nutrient retention. At the watershed level urban wastewater inputs were the dominate source of stream enrichment, however, the use of nutrient enriched river water for irrigation removed substantial portions of these inputs. These findings highlight the importance of ecosystem based management plans to address the effects of disturbance on nutrient cycling in impacted stream ecosystems.