The role of geomorphology in river biogeochemical processing

Although once treated as simple downstream transporters of material, river networks are now recognized for their role in processing, or transformation, storage, and removal, of carbon and nutrients during transport. The linkages between channel geomorphic structure, biogeochemistry, and ecology of lotic ecosystems are important to their functioning. The overarching goal of this research was to develop a better understanding of the relationships among geomorphology, ecosystem metabolism, and carbon and nutrient dynamics in a larger river. This research exploited natural changes in river form associated with broad-scale geologic transitions in the Cahaba River, AL. Spatial and temporal patterns of ecosystem metabolism along the Cahaba indicated an important effect of regional geology. In the Coastal Plain this effect was expressed through strong seasonal changes in plankton productivity, driving high rates of gross primary production and ecosystem respiration. Anthropogenic activity associated with Birmingham, AL, causes elevated water-column nutrient concentrations. However, downstream nutrient levels are comparable to upstream of Birmingham. Corresponding to the location of this decline is a broad geomorphic transition creating shoal habitat, which supports expansive crops of macrophytes. We found whole ecosystem metabolism, and the biomass of macrophytes, could significantly contribute to water column nutrient declines. Recent models of organic matter export from rivers have identified the potential significance of seasonal pulses of autochthonous production and the dearth of knowledge regarding the fate of riverine algal production. Our evaluation of phytoplankton production, in the Coastal Plain, confirmed an important role in fueling water column processing and provided evidence of phytoplankton delivery to sediments. The identity and characteristics of primary producers appears to be constrained by broad geomorphologic transitions and has widespread ecological consequences including altering the timing and character of nutrient and carbon delivery downstream and the spatially and temporally dynamic nature of the relative importance of water-column and benthic processes. Overall, this research contributes to our understanding of river biogeochemistry by highlighting the complexity and variability of interactions among factors influencing rates of carbon and nutrient processing. Expanding the spatial and temporal scales of empirical measurements of ecosystem processes is critical to accurately predicting the biogeochemical activity of river networks. (Published By University of Alabama Libraries