Unraveling sub-daily variability of Net Community Production in a Georgia Salt Marsh Estuary: Diel and Tidal forces.

Understanding the processes driving carbon fluxes in salt marsh estuaries is essential for quantifying their role in regional and global carbon budgets. While Net Community Production (NCP) in salt marsh estuaries has been widely studied on daily to seasonal scales, sub-daily variability remains poorly resolved and is a novelty in coastal Georgia. Here, we present the first estuarine deployment of the Pressure of In situ Gases Instrument (PIGI) to obtain high-frequency NCP measurements, validated against the diel oxygen method and in situ light-dark bottle incubations across three seasons. Our study site is the Wilmington River salt marsh in coastal Georgia, USA. Our results show that sub-daily NCP variability is influenced by tidal forcing and diel light cycles; with tidal effects, like advection, persisting under low light conditions. Sub-daily variability ranged between -30 to -200 mmol O 2 /m 3 /d (corresponding to -36 to -240 mmol C/m 3 /d), with lower variability in December and higher in June. Furthermore, PIGI and diel oxygen method yielded comparable whole ecosystem (benthic + planktonic) NCP estimates, while bottle incubations captured only the planktonic community metabolism, revealing benthic respiration as a major driver of the net heterotrophy seen in this study site. This methodological contrast also enables an indirect estimation of benthic NCP without benthic chambers, though further study is needed to validate this approach. These findings advance the mechanistic understanding of benthic–planktonic coupling in estuaries and highlight the need for high-frequency, multi-method observations to capture the dynamic nature of carbon cycling in coastal and estuarine environments.