Evidence for the production of marine fluorescent dissolved organic matter in coastal environments and a possible mechanism for formation and dispersion

A positive linear relationship between salinity and fluorescent dissolved organic matter (FDOM) was observed on several occasions along the West Florida shelf at salinities greater than 36.5. This represents a departure from the typical inverse relationship between FDOM and salinity observed in most coastal regions caused by the mixing of riverine FDOM with clear oceanic water. Three-dimensional excitation-emission matrices showed that the high-salinity, high-FDOM water had blue-shifted spectra characteristic of autochthonous, marine FDOM, with peak M concentrations eight times higher than previously reported for seawater. The blue-shifted fluorescence endmember at high salinity was clearly distinguishable from a photobleached FDOM endmember. A high-resolution time series collected in a shallow embayment with significant Gulf of Mexico influence supplemented cruise data and provided a possible mechanism for the formation of high-salinity, high-FDOM water. During a dry period of spring tides, high-salinity, high-FDOM water was exported at ebb tide and lower-salinity, low-FDOM water was imported during flood tide. During neap tide, FDOM and salinity demonstrated no evidence of either export or dilution from incoming seawater. After a significant rain event, a more typical inverse relationship between salinity and FDOM was observed. Production of FDOM-rich water in shallow embayments has not been observed previously. This is likely an important source of organic matter and dispersion of this material may explain observations of a high-salinity, high-FDOM water in adjacent coastal regions.