Doctor of Philosophy (PhD)
School of Renewable Natural Resources
Rivers carry terrestrial nutrients and other materials to estuaries, affecting biogeochemical cycles and ecosystems in coastal margins. This dissertation research incorporated five studies to investigate the dynamics of carbon, nutrients, and trace metals in four major coastal rivers in southwest Louisiana – the Sabine, Calcasieu, Mermentau, and Vermilion Rivers. The overarching goal of this research was to broaden the knowledge and understanding of biogeochemical processes in freshwater – saltwater mixing zones. These studies employed decade-long monitoring records of water chemistry, utilized monthly field measurements conducted over a two-year period, and used analysis of over 200 water samples for carbon, nitrogen, phosphorus, and trace metals. Results gained from the studies showed that land use effects were clearly reflected in the much higher TOC, NO3+NO2, TKN, and TP yields from the Mermentau and Vermilion Rivers draining more agriculture-intensive watersheds, when compared with those of the Sabine and Calcasieu Rivers. The high average pCO2 value (2528 µatm) for the entire Calcasieu River reach suggests that an estuarine river functions as a CO2 source in the atmosphere. The DIC concentrations and δ13CDIC values increased rapidly with the increase of salinity. The DIC concentration appeared to be mostly controlled by conservative mixing. The δ13CDIC values during the study period were either close to or below those values suggested by the conservative mixing model, implying that an estuarine river can fluctuate from a balanced system to a heterotrophic system seasonally. The DOC concentration decreased with decreasing distance to the northern Gulf of Mexico, but to a much smaller degree. The depleted δ13CDOC values (-30.56‰ to -25.92‰) in the Calcasieu River suggest that the DOC discharged by this river was highly terrestrially derived. The DIP concentration versus salinity plots in the Calcasieu River suggest some sources were present throughout the Calcasieu Estuary. Such mixing dynamic were probably caused by desorption of DIP from suspended solids and river bed sediments, urban inputs, as well as stronger calcium carbonate and phosphorus co-precipitation at the marine endmember. The total Sr concentration and the Sr/Ca ratio both increased significantly with increasing salinity. In contrast, salinity has no effect on Ba concentration.
He, Songjie, "Changes in Carbon, Nutrient, and Trace Metal Concentrations During the Transition from Freshwater to Marine Environments - A Case Study in Southwest Louisiana" (2018). LSU Doctoral Dissertations. 4530.