Landscape-level influences on community composition and ecosystem function in a large river ecosystem
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Riverine ecosystems are a vitally important link between terrestrial and aquatic ecosystems. Rivers are sites of major biogeochemical processes involved with the carbon (C), nitrogen (N), and phosphorous (P) cycles, providing critically important ecosystem services, and providing habitat for numerous groups of aquatic taxa. Although riverine systems have been a core component in human cultural and economic development, they have long served as a dumping ground for wastes and undesirable substances. Additionally, landscape development by humans has often happened without an understanding of the impact on riverine systems, and the ecological integrity of many river systems is increasingly threatened. However, critical gaps exist in our knowledge about river-system ecology and ecological function, and their link to terrestrial landscapes.
To better understand the ecology of riverine ecosystems, researchers need to study them in the context of the larger landscape. In-stream aquatic nutrients are influenced by a multitude of factors, both internal and external to the aquatic realm. Traditionally, much of the impact assessment has centered on the evaluation of land-use patterns in a catchment. However, land-use patterns are not independent of the physiographic context of a system (e.g. climate, topography, geology). Very few studies have attempted to parse out the independent influences of land-use versus physiographic context. Determining the independent effects of land-use and physiographic conditions, and at which scale they should be assessed (e.g., local, riparian, or watershed), has implications for monitoring and restoration programs. In Chapter 1 of this dissertation I report my investigation on the influence of varying scales of land use assessment and physiographic environmental gradients on aquatic nutrient dynamics. Also, I provide the first explicit assessment of the different influences of land-use and physiographic context on nutrients.
Understanding how different taxa in communities interact and how organisms are influenced by and interact with their environment is the central goal of the study of ecology. Therefore, it is important to focus research not only on how biological communities respond to changes in land use, water quality, or environmental gradients, but how the abundance of one taxonomic group responds to changes in the abundance of other taxonomic groups in the community. In addition to responding to environmental conditions, communities can be structured by predator-prey dynamics, competitive interactions and niche partitioning, as well as differing dispersal ability. Additionally, there is current controversy about whether community interactions or environmental conditions structure the spatial patterns of communities on a landscape. It is important to properly interpret which mechanisms are structuring biotic communities if we are to adapt conservation and management efforts to different scenarios of climate change or human alteration of the landscape. In Chapter 2 of this dissertation I integrated the physicochemical data from Chapter 1 with invertebrate and fish community data to investigate biogeographical patterns of community concordance in the Brazos River watershed, and the interactions with environmental and spatial gradients.
Bacteria are one of the most abundant and diverse forms of life on the planet. They are involved in and essential to nearly every biogeochemical cycle, including C processing and the cycles for N, P, and sulfur (S). Bacteria are also responsible for processing large amounts of non-living organic C and nutrients into forms that can be used by higher trophic level organisms. The relationship between bacterial production (the use of carbon for new tissue), bacterial respiration (the use of carbon for metabolism and cell maintenance), and total carbon consumption in riverine systems is relatively understudied. Terrestrially derived carbon is an important subsidy to many aquatic systems and bacterial production is often related to organic matter concentrations. Additionally, the interactions that bacterial community composition has with measures of function and nutrient conditions are also relatively understudied. Although advances in the understanding of bacterial ecology have been made, elucidating the specific role that bacterial community composition has in mediating ecosystem function remains a challenge. Recent developments in the areas of bioinformatics have greatly improved the detail at which we examine microbial communities, and are changing our understanding of environmental factors that drive patterns of biogeography in microbial communities. In Chapter 3 of this dissertation, I investigated landscape-level patterns of bacterial ecosystem function and bacterial community composition, and related both to nutrient and environmental conditions.
Using the Brazos River (TX) as my study system, the research presented in this dissertation addressed these gaps as they relate to nutrient cycling, community composition of major groups of biota (fish, invertebrate, and bacterial communities), and the function of bacteria in riverine systems. I found that nutrient conditions in large riverine systems are largely influenced by landscape-scale environmental gradients, with land-use/land-cover being a secondary influence. The landscape-influenced patterns of environmental conditions were additionally correlated with patterns of species distribution in macroinvertebrate, fish, and bacterial communities. However, there was little evidence that the widespread use of surrogate species in monitoring and restoration plans was justified, as the predictive ability between macroinvertebrates and fish was low. Finally, the patterns of carbon use by bacteria were very different that what has been found in other systems. Both production and respiration appeared to be supported largely by autochthonous production if organic matter. Together, this series of studies highlights the importance of considering both environmental controls and community interactions when assessing large-scale patterns of nutrients, community structure, or ecosystem function.