Rain Alters Spartina Alterniflora Light-Use Efficiency Through Leaf Rinsing, Diffuse Light, and Salinity at North Inlet, South Carolina

Abstract In the absence of horizontal migration, salt marshes must build vertical elevation to persist under sea level rise by building soil through primary production and inorganic sediment deposition. Here we examine a phenomenon observed in a high salinity salt marsh estuary whereby leaf-surface coatings form on Spartina alterniflora (aka Sporobolus alterniflorus ) leaves and we investigate whether these coatings can inhibit photosynthesis. Using eddy covariance observations of carbon dioxide fluxes, chamber measurements of leaf level photosynthesis, and measurements of leaf and canopy phenology, we found that: during rain-free periods, leaf and canopy greenness declined as coatings developed, and rainfall rapidly rinsed leaves in proportion to rain amount; daily canopy light-use efficiency (LUE) was jointly controlled by clearness index, canopy greenness, and creek salinity, with the best-supported model including an interaction between clearness index and seasonal greenness, indicating that the positive greenness-LUE relationship was strongest under cloudier, more diffuse conditions; removing coatings increased leaf-level initial quantum yield by about 20%; and a simple GPP proxy indicated an average 10% enhancement in modeled productivity on days 1–3 after rain, with monthly median enhancement reaching 40% in October–November. These results identify a previously unrecognized mechanism by which tidal leaf coatings can suppress S. alterniflora photosynthesis. If climate change and sea-level rise enhance coating development or residence time through, for example, creek-bank erosion, sediment mobilization, or longer rain-free periods or drought, then this process may need to be incorporated into marsh elevation models.