Mohammed Mezied, Cédric G. Fichot, Sergio Fagherazzi
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Abstract
Saltmarsh estuaries influence land-ocean carbon exchange, yet inlet carbon fluxes remain poorly constrained by tidal variability and limited high-frequency observations. This study quantifies dissolved organic carbon (DOC) fluxes in a mesotidal saltmarsh estuary in the northeastern USA by integrating high-frequency field data with a two-dimensional hydrodynamic model. DOC concentrations were estimated from fluorescent dissolved organic matter and ancillary water-quality measurements collected by moored sondes during summers 2023 and 2024. DOC fluxes were computed by coupling modeled discharge with observation-derived concentrations. The tidal inlet exhibited the lowest DOC concentrations and largest DOC fluxes, indicating that tidal exchange volume exerts stronger control on carbon transport than concentration. DOC fluxes at the inlet switched from net export to the ocean in 2023 (outwelling) to net import into the estuary in 2024 (inwelling). The 2024 inwelling was associated with reduced riverine inputs and was consistent with greater relative influence of marine-derived DOC from the adjacent shelf, followed by partial degradation within the estuary. Phase analyses revealed systematic offsets between water level and DOC flux, consistent with delayed marsh drainage and tidal storage effects, especially at the inlet. Across years and cross sections, the spring-neap difference in DOC flux normalized to saltmarsh area was relatively consistent, providing a first-order proxy for marsh-associated DOC outwelling. Together, these results support a dynamic interpretation of the outwelling hypothesis, in which saltmarshes act as persistent DOC sources while the estuary alternates between net DOC export and import in response to fluvial and oceanic boundary conditions.
期刊介绍：
JGR-Biogeosciences focuses on biogeosciences of the Earth system in the past, present, and future and the extension of this research to planetary studies. The emerging field of biogeosciences spans the intellectual interface between biology and the geosciences and attempts to understand the functions of the Earth system across multiple spatial and temporal scales. Studies in biogeosciences may use multiple lines of evidence drawn from diverse fields to gain a holistic understanding of terrestrial, freshwater, and marine ecosystems and extreme environments. Specific topics within the scope of the section include process-based theoretical, experimental, and field studies of biogeochemistry, biogeophysics, atmosphere-, land-, and ocean-ecosystem interactions, biomineralization, life in extreme environments, astrobiology, microbial processes, geomicrobiology, and evolutionary geobiology
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