The Changjiang River provides the main source of sediment and terrestrial derived organic carbon (OC) to the Changjiang large delta-front estuary (LDE) in the East China Sea (ECS). This study analyzed bulk OC, biomarkers including lignin and plant pigment, black carbon (BC) on ECS sediments sampled in winter 2009 and 2010 in order to study the OC cycling under the influence of natural and anthropogenic disturbance. Low-oxygen tolerant foraminiferal microfossils were analyzed in another two sediment cores to study the historical hypoxia events in the Changjiang LDE. Bulk carbon to nitrogen (C/N) ratio and stable isotope δ13C in the surface sediment samples indicated a mixture source of terrestrial, deltaic and marine derived OC. Refractory BC and reworked marine OC seemed to comprise most of the OC pool with older, less reactive signatures as deduced from ∆14C, and BC analyses. Winter wind/wave energy and hydrodynamic sorting had a substantial winnowing effect on surface sediment OC redistribution. As a result, the highest lignin concentration shifted to the south during the 2010 cruise after the summer flood event. In addition, algal inputs from local deltaic lakes due to eutrophication and/or lateral transport likely caused the observed lack of benthic-pelagic coupling of pigment concentrations between the surface sediments and the water column after the summer flood in 2010. For the down-core sediment, the mass accumulation rate distribution followed the dispersal pathway of the ECS sediment. Terrestrial and marine derived OC showed significant spatial and temporal distribution. Lignin rich materials were better preserved in sediments closer to the coast while offshore sediments tended to be composed of lignin-poor, degraded OC, that were likely hydrodynamically sorted to a long distance during transport. Besides eutrophication, plant pigments indicated that marine-derived OC was mostly deposited in the sediment mixed layer with decay in the underlying sediment accumulation layer. The total OC standing stock since 1900 is approximately 1.62±1.15 kgC m^-2, about 1/10 of the total OC stock in all the middle and lower lakes in the Changjiang catchment. There has been an increase in the number of hypoxic bottom water events on the Changjiang LDE over the past 60 yrs indicated from the increases in low-oxygen tolerant foraminiferal microfossils due to excess deposition of OC and summer stratification.

<p>As the human population continues to increase, the effects of land use change on streams and their watersheds will be one of the central problems facing humanity, as we strive to find ways to preserve important ecosystem services, such as drinking water, irrigation, and wastewater processing. This dissertation explores the effects of land use change on watershed nitrate concentrations, and on several biogeochemical ecosystem functions in streams, including nitrate uptake, ecosystem metabolism, and heterotrophic carbon processing. </p><p>In a literature synthesis, I was able to conclude that nitrate concentrations in streams in forested watersheds tend to be correlated with soil solution and shallow groundwater nitrate concentrations in those watersheds. Watershed disturbances, such as ice storms or clear-cutting, did not alter this relationship. However both urban and agricultural land use change increased the nitrate concentrations in streams, soil solution, and groundwater, and altered the correlation between them, increasing the slope and intercept of the regression line. I conclude that although the correlation between these concentrations allows for predictions to be made, further research is needed to better understand the importance of dilution, removal, and transformation along the flowpaths from uplands to streams.</p><p>From a multi-site comparison of forested, urban, and urban restored streams, I demonstrated that ecosystem functions like nitrate uptake and ecosystem metabolism do not change in a linear unidirectional way with increasing urbanization. I also showed that Natural Channel Design stream restoration as practiced at my study sites had no net effect on ecosystem function, except those effects that came from clearing the riparian vegetation for restoration construction. This study suggested further consideration is needed of the ecosystem effects of stream restoration as it was practiced at these sites. It also suggested that more study was needed of the effects of urbanization on ecosystem metabolism and heterotrophic processes in streams.</p><p>In a 16-month study of ecosystem metabolism at four sites along an urbanization gradient, I demonstrated that ecosystem metabolism in urban streams may be controlled by multiple separate effects of urbanization, including eutrophication, light, temperature, hydrology, and geomorphology. One site, with high nutrients, high light, and stable substrate for periphyton growth but flashy hydrology, demonstrated a boom-bust cycle of gross primary production. At another site, high benthic organic matter standing stocks combined with low velocities and high depths to create hypoxic conditions when temperature increased. I propose a new conceptual framework representing different trajectories of these effects based on the balance of increases in scour, thermal energy and light, eutrophication, and carbon loading. </p><p>Finally, in a study of 50 watersheds across a landscape urbanization gradient, I show that urbanization is correlated with a decrease in particulate carbon stocks. I suggest that an increase in dissolved organic matter quality may serve to compensate for the loss of particulate carbon as fuel for heterotrophic microbial activity. Although I saw no differences among watershed landuses in microbial activity per gram of sediment, there was a strong increase in the efficiency of microbial activity per unit organic sediment with increasing watershed urbanization. Ultimately, I hope that this research contributes to our understanding of stream ecosystem functions and the way land use change can alter these functions, with the possibility of better environmental management of urban streams in the future.</p> / Dissertation