Understanding effects of changes in land use, environmental parameters, habitat characteristics on the Great Basin aquatic invertebrates using ecological stoichiometric theory.

摘要

Many freshwater ecosystems in the Great Basin have undergone dramatic alteration over the past decades due to groundwater withdrawal, water diversion and changes in land use. These changes have affected benthic food web dynamics by altering the availability of chemical key elements for primary and secondary producers, by loss of biodiversity and extinction of endemic species. However, only few studies are available that addresses the link between changes in land use, habitat characteristics and its effects on benthic macroinvertebrates (BMI) communities comprehensively and little attention has been given to integrating physiochemical aspects to link ecosystem functions.;This dissertation analyzes the link between BMI communities and their habitat characteristics in the Walker River and in diverse spring ecosystems in the Great Basin by using ecological stoichiometry and secondary production assessment as tools and provides a knowledge base that can be used in stream and spring protection and restoration, respectively.;The dissertation includes three separate studies that are organized into three chapters that are being prepared for peer reviewed publication. In the first study, multiple regression analysis followed by corrected Akaike's Information Criterion (AICc) was used to determine predictor variable(s) that best explains the elemental composition of basal food resources (seston and periphyton) and BMI along an elevation gradient and land use type in the Walker River in California and Nevada. The elemental composition of basal food resources are best explained by land use and associated differences in soluble reactive phosphorus (SRP),dissolved organic carbon (DOC), and stream water nitrogen (N): phosphorus (P) ratios. In contrast, BMI elemental stoichiometry was correlated to taxonomic identity and body mass. Therefore, both parameters should always be taken into account when addressing land use effects on resource-consumer stoichiometry. Consumer-resource elemental imbalances in this study were less pronounced than those reported from the nutrient-poor streams. However, high spatial and temporal variability in food source elemental composition obscured clear spatial pattern in elemental imbalances between nutrient-poor upstream sites and nutrient-rich downstream sites.;Results from this study suggest that applying ecological stoichiometry is a useful tool to better understand the elemental dynamics of food components particularly in Great Basin watersheds with highly variable physiochemical conditions that such as Walker River.;The second project analyzed the link between the ecological health of BMI communities and habitat characteristics along the Walker River. Biotic indicesindicated a shift in BMI communities towards species-poor assemblages with few dominant taxa from upstream to downstream parts of Walker River. Further, the Hilsenhoff Biotic Index (HIB) indicated an increase in the dominance of pollutant-tolerant taxa at downstream parts of Walker River. These shifts in the BMI community compositionwere related to spatial changes in substrate size, stream width, stream temperature, SRP, and Total Suspended Solids (TSS).;Further, secondary production of Baetis mayflies, one of the most common genera in Walker River, was estimated between two sites differing in water quality to assess effects of nutrient enrichment on Baetis density, biomass and secondary production. Although mean individual body mass of Baetis was lower at the nutrient enriched site, secondary production was higher most likely due to the combination of higher growth rates and higher densities.;Based on these results I recommend that factors influencing nutrients, discharge, and stream temperature are the most important parameter to consider Walker River restoration.Further, the results from this study can be used to assess the success of stream restoration by documenting changes in BMI communities and secondary production.;In the third study, spring-dwelling snails and their potential food source were collected from twenty-three springs from the eastern part of the Great Basin to provide a basic understanding about the link of the snail's elemental composition to taxonomy, basal food resources and habitat characteristics. The results showed that body% phosphorus (P)of snails was highly variable among the four major snail families whereas body% nitrogen (N) and % carbon (C) varied less. Differences in the body elemental composition among the same species collected from multiple springs indicate that aquatic snails can deviate from strict homeostasis. The body %P content in snails belonging to Pyrgulopsiscollected from warm springs (water temperature ≥ 20ºC)was significantly lower compared to those collected from cold springs. Further, elemental imbalances between snails and there potential food sources were significantly higher in warm springs compared to cold springs.;These results indicating that water temperature is an important factor that constraints food web dynamics in long-term stable ecosystems, such as springs. Basic understandingabout food web dynamics in Great Basin springs is not only important to determinefurtherconsequenceson the nutrient cycling in ecosystems with strong temperature gradients. It can also be used to predict consequences of human alterations on the food web dynamic in these long-term stable ecosystems.