Non-conservative behavior of dissolved organic carbon in a Georgia salt marsh creek indicates summer outwelling

Salt marshes are highly productive ecosystems that efficiently accumulate carbon. However, it remains unclear how much carbon is exported from salt marshes (i.e., outwelled) through creeks as dissolved organic carbon (DOC). Addressing this uncertainty is critical for quantifying net carbon accumulation and determining the role marshes play in broader coastal carbon cycles. Consequently, this study sought to quantify net DOC fluxes through Groves Creek, the tidally-driven main creek channel of Groves Creek salt marsh on the Georgia coast (USA). To do this, near-continuous records of water flux, salt flux, and DOC flux were determined in Groves Creek's primary creek channel at 10-min resolution over 16 months. At our study site, we found that generally more water flowed onto the marsh through Groves Creek's primary creek channel during flood tide than left this route during ebb tide. The remaining unsampled water balance was assumed to leave the marsh via secondary marsh channels and over the marsh edge. Due to this limitation, the conservative nature of salt was utilized alongside flow and DOC to determine net DOC trends in Grove Creek's primary creek channel. Cumulative net DOC-salt relationships are linear for most of the study, indicating DOC largely behaved conservatively in the creek channel and wider marsh system. However, a period of non-conservative behavior in which the cumulative net DOC:salt ratio decreased indicated net DOC outwelling (4.1–30.1 g DOC m−2 y−1) during summer 2014. Results indicate DOC outwelling events, as observed here, may occur in pulses during highly productive summer months. Resolving these “hot” moments of DOC export at high-temporal resolution across larger salt marsh ecosystems is required to assess the true extent and quantitative significance of DOC outwelling to coastal carbon cycles and ecology. [Display omitted] •Dissolved organic carbon (DOC) flux was assessed for 16 months in a saltmarsh creek•For most of the year, DOC behaved conservatively•A pulse of DOC outwelling occurred in summer 2014•DOC outwelling is a significant term in the salt marsh carbon budget•Long-term, high temporal resolution data is required to capture outwelling events