Abstract |
Dissolved organic matter (DOM) is a critical component of aquatic environments and global carbon cycling; it has multiple sources including terrestrial runoff, riverine input, phytoplankton excretion, viral lysis, among others. These inputs have varying levels of contribution depending on temporal and spatial scales as well as environmental variables, making the characterization of the DOM increasingly complex. This dissertation used bulk (dissolved organic carbon - DOC), optical (chromophoric DOM - CDOM), molecular (FT-ICR MS) analyses as well as microbial incubation experiments to investigate changes in DOC concentration, DOM composition, and lability in coastal ecosystems in the southeastern U.S. In CHAPTER 2, changes in DOC concentration and DOM composition were analyzed monthly over a year at the Altamaha River and at the head of Sapelo Sound in coastal Georgia, USA. Results showed that river discharge was the primary driver that changed the DOM composition in both locations. In October 2016, the Georgia coast was hit by Hurricane Matthew, which increased the average DOC concentration by ~ 4 times and strongly augmented the terrigenous signature of DOM. In CHAPTER 3, changes in DOM composition and bacterial processing were investigated at fifteen sites across a riverine-estuarine gradient system as part of the GCE-LTER domain over four seasons. The terrigenous-marine gradient in organic matter sources explained the most variation in DOM composition throughout the year. Increased microbial degradation rates were observed for DOM that had a stronger terrigenous character, especially for samples collected ~ 30 days after Hurricane Irma had impacted the studied area. Finally, in CHAPTER 4, changes in DOC concentration and DOM composition of ambient seawater were characterized after interaction with a loggerhead sponge, Spheciospongia vesparium, in the Florida Bay, USA. The sponge-microbial holobiont removed small, oxygen-depleted, nitrogen-rich compounds and the DOM composition was significantly different than that of the ambient seawater. Microbial incubations suggested that sponge exhalent seawater was less labile than ambient seawater, possibly due to holobiont removal of nitrogen-rich compounds. Overall, this dissertation illuminates the merits of combining different chemical analyses and microbial experiments to better uncover and understand the dynamics of different DOM pools across complex coastal environments. |