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Abstract Spartina alterniflorahas a distinct flood‐adapted morphology, and its physiological responses are likely to vary with differences in tidal submergence. To understand these responses, we examined the impacts of tidal inundation on the efficiency of Photosystem II (φPSII) photochemistry and leaf‐level photosynthesis at different canopy heights through a combination of in situ chlorophyll fluorescence (ChlF), incident photosynthetically active radiation, and tide levels. Our result showed small declines (7%–8.3%) in φPSII for air‐exposed leaves when the bottom canopies were tidally submerged. Submerged leaves produced large reductions (30.3%–41%) in φPSII. Our results suggest that when submerged, PSII reaction centers inS. alternifloraleaves are still active and able to transfer electrons, but only at ∼20% of the typical daily rate. We attribute this reduction in φPSII to the decrease in the fraction of “open” PSII reaction centers (10% of the total) and the stomatal conductance rate caused by the tidal submergence. To our knowledge, this flooding induced leaf‐level reduction of φPSII forS. alterniflorain field settings has not been reported before. Our findings suggest that canopy‐level φPSII is dependent on the proportion of submerged versus emerged leaves and highlight the complexities involved in estimating the photosynthetic efficiency of tidal marshes. 

Abstract Light use efficiency (LUE) of salt marshes has not been well studied but is central to production efficiency models (PEMs) used for estimating gross primary production (GPP). Salt marshes are typically dominated by a species monoculture, resulting in large areas with distinct morphology and physiology. We measured eddy covariance atmospheric CO₂fluxes for two marshes dominated by a different species:Juncus roemerianusin Mississippi andSpartina alterniflorain Georgia. LUE for theJuncusmarsh (mean = 0.160 ± 0.004 g C mol⁻¹photon), reported here for the first time, was on average similar to theSpartinamarsh (mean = 0.164 ± 0.003 g C mol⁻¹photon). However,JuncusLUE had a greater range (0.073–0.49 g C mol⁻¹photon) and higher variability (15.2%) than theSpartinamarsh (range: 0.035–0.36 g C mol⁻¹photon; variability: 12.7%). We compared the responses of LUE across six environmental gradients.JuncusLUE was predominantly driven by cloudiness, photosynthetically active radiation (PAR), soil temperature, water table, and vapor pressure deficit.SpartinaLUE was driven by water table, air temperature, and cloudiness. We also tested how the definition of LUE (incident PAR vs. absorbed PAR) affected the magnitude of LUE and its response. We found LUE estimations using incident PAR underestimated LUE and masked day‐to‐day variability. Our findings suggest that salt marsh LUE parametrization should be species‐specific due to plant morphology and physiology and their geographic context. These findings can be used to improve PEMs for modeling blue carbon productivity.