Carbon Composition and Dynamics at the Marsh-Estuary Interface

Understanding the processes influencing the sources and fate of organic matter in estuaries is important for quantifying the contributions of carbon from land and rivers to the coastal ocean carbon budget. However, key components of the estuarine carbon budget such as exchange at the marsh-estuary interface remain poorly understood and require study at a variety of temporal and spatial scales. This study measured the amounts, sources, and composition of dissolved and particulate organic carbon exchanged at the marsh-estuarine interface of a representative temperate system, Taskinas Creek, VA. Water samples were collected over a tidal cycle on a monthly basis from Taskinas Creek over ~2 years, to determine the concentrations and sources of carbon pools over a range of timescales (e.g., tidal, monthly, seasonal). Concentrations of three carbon pools were determined: particulate organic carbon (POC), dissolved organic carbon (DOC), and dissolved inorganic carbon (DIC). Sources of POC were determined using lipid biomarkers, stable isotopes, and C:N ratios, while optical properties and C:N ratios were used to study the source, composition, and degradation state of DOC. Mean monthly concentrations of POC and DOC ranged from 1.59 to 6.98 mg/L and 4.16 to 7.11 mg/L, respectively. POC concentrations exhibited seasonal patterns with higher concentrations observed in the spring (Feb-Apr) while DOC concentrations did not vary seasonally. POC concentrations were generally higher during high tide while DOC and DIC were generally higher during low tide. This tidal variability suggests that the marsh acted as a sink for POC and a source for DOC and DIC. Carbon stable isotope values ranged from -16.88 to -31.16 per mil, indicating a mixture of sources; the most positive values occurred during spring (Feb-Apr), when chlorophyll-a concentrations were highest. Together, these data reinforce the need to better understand variations in carbon exchange in marsh and estuarine environments in order to estimate their roles as sources and sinks for carbon.