Carbon Pools Differ in Source and Temporal Patterns in a Tidal Marsh Creek System of the York River, VA Estuary

Estuarine ecosystems, including tidal marsh creek systems, are hotspots of carbon cycling. Despite their biogeochemical importance, the magnitude and timescales at which carbon concentrations vary in these environments remain poorly characterized. To address this, our study identified temporal patterns in carbon pools in a tidal marsh creek with the goal of identifying their primary drivers. Specifically, we measured concentrations of particulate and dissolved organic carbon (POC, DOC), dissolved inorganic carbon (DIC), water quality parameters, and organic matter source proxies (δ 13 C, δ 15 N, and colored dissolved organic matter (CDOM)). We collected samples every 2.5 h over a tidal cycle monthly (October 2013–January 2015, intermittently thereafter). We characterized organic matter composition from marsh and estuarine endmembers using samples collected at low and high tides respectively. POC concentrations were highest at high tide (4.21 ± 0.40 mg/L), whereas DIC concentrations were highest at low tide (24.82 ± 0.74 mg/L), suggesting that the marsh was a sink for POC and source of DIC. Temporally, DIC concentrations were highest in fall (23.19 ± 1.30 mg/L); DOC and POC varied little across seasons. We used linear mixed effect models to identify potential processes that explained these temporal patterns. Suspended solids and chlorophyll-a were the best predictors of POC concentrations, while CDOM was the best predictor of DOC, and percent freshwater was the best predictor of DIC ( p < 0.001). Results from this study highlight the importance of measuring multiple carbon pools simultaneously, as each pool is influenced by different sources and processes and responds differently to drivers that change at tidal and seasonal timescales.