Sediment coatings reduce leaf and canopy scale photosynthesis in a salt marsh: a novel soil-plant-atmosphere linkage

Salt marshes gain vertical elevation to persist under sea level rise by building soil through primary production and trapping inorganic sediments.  Current models assume inorganic sediments contribute positively to marsh elevation, and that plants facilitate deposition and accretion through sediment trapping, suggesting rates of sediment trapping may be positively related to primary productivity.  Here we examine a phenomenon observed in a high salinity salt marsh estuary whereby inorganic sediments contribute to coating the Spartina alterniflora canopy and we investigate whether these coatings can inhibit photosynthesis.  Using eddy covariance observations of carbon dioxide flux, chamber measurements of leaf level photosynthesis, and measurements of leaf and canopy phenology we determined that 1) during rainless periods leaf and canopy greenness decline due to coating development, which is rinsed by rain proportionally to rain amount, 2) canopy light use efficiency declines as coatings develop for up to six days, 3) leaf level quantum use efficiency increases when coatings are removed, 4) canopy light use efficiency is weakly inversely correlated with creek salinity, 5) rinsing leaves amplifies the enhancement of canopy photosynthesis by diffuse light.  This study identifies a new mechanism in which inorganic sediments can inhibit S. alterniflora photosynthesis. Further work is needed to quantify the magnitude of the effect in terms of biomass production to determine whether this is a concern for marsh accretion.  If climate change and sea level rise enhance epiphytic coating development or residence time through, for example, creek bank erosion, sediment mobilization, or by extending rain-free periods, then this process may need to be incorporated in marsh elevation models.