Mechanisms of Blue Carbon Export in Blackwater River-dominated and Particle-dominated Estuaries

Modifications in land-use and climate change have resulted in shifts in organic matter (OM) transport from wetlands to coastal waters, thus, it is important to differentiate between riverine and wetland sources of OM in coastal areas. Here, we report on spatiotemporal changes in the composition and concentration of dissolved organic carbon (DOC), particulate organic carbon (POC), particulate nitrogen (PN), the stable isotopic composition of DOC and POC (δ¹³C-DOC and δ¹³C-POC, respectively), dissolved and particulate lignin phenols, and carbon normalized dissolved lignin phenol yields in surface waters of two bays in the Gulf of Mexico: Apalachicola (AP) and Barataria (BB). POC concentrations were higher at BB (x̅=253.8±14.9 µM) than those at AP (x̅=62.8±8.8 µM), but DOC concentrations were higher at AP (x̅=451.3±531.0 µM) than those at BB (x̅=380.0±134.7 µM). Lignin values suggested that BB had greater influence of wetlands and thus exported more blue carbon than AP. Discriminant analyses indicated that BB is a more particle-rich and blue-carbon dominated system compared to AP, which is likely due to a lack of direct river inputs and high prevalence of wetlands carbon sources (living vs. eroded/submerged). The primary parameters explaining the predominance of particulate and blue carbon sources in BB were total suspended sediment concentrations, δ¹³C-DOC, δ¹³C-POC, the ratio of cinnamyl to vanillyl phenols in DOC, and the ratio of syringyl to vanillyl phenols for both POC and DOC. In contrast, AP was separated by DOC, particulate C:N ratios, and the ratio of acid to aldehyde lignin phenols for both POC and DOC, indicating a more terrestrial source of organic matter and reflecting the importance of fluvial DOM inputs in this system. Total lignin export (sum of dissolved and particulate) was higher in BB (5.73×10⁵ kg yr^-1) than in AP (4.21×10⁵ kg yr^-1). Particulate lignin export from BB was greater than the export of dissolved lignin at either BB or AP, suggesting coastal marsh erosion may be driving this comparatively large export of particulate lignin.