Seasonal Export of Dissolved Organic Matter from a New England Salt Marsh

The seasonality of dissolved organic matter (DOM) export from a New England salt marsh was investigated using multiple estuary transects along with laboratory plant leaching experiments. From 2001 through 2013, 35 transects of the Neponset Estuary in Boston, Massachusetts, were conducted to measure high-resolution in situ chromophoric dissolved organic matter (CDOM) fluorescence, along with other standard oceanographic parameters, via sensors incorporated into an undulating depressor wing known as the mini-shuttle. Water samples were collected during each cruise for discrete dissolved organic carbon (DOC) and CDOM fluorescence analyses. Seasonal salt marsh DOC export rates ranged from 1.1 x 10(7) to 15 x 10(7) mol C season(-1) (10-140 mol C m(-2) season(-1)). The total export was found to be 23 +/- 17 x 10(7) mol C y(-1) (210 mol C m(-2) y(-1)) with river discharge, temperature, and day length significantly affecting this rate. Results from seasonal salt marsh vegetation (Spartina alterniflora, Spartina patens, and Phragmites australis) incubation experiments showed that the leaching of DOC and CDOM from plant biomass was a rapid process occurring over several days. Sunlight exposure, biomass type (above-versus belowground biomass), and microbial activity had significant effects on DOC leaching rates from plant matter. These incubations, in conjunction with the cruise data, support the fall dump hypothesis, in which a pulse of DOC leaches from plants during the fall as marsh plants begin to senesce for winter. The long-term trend in salt marsh DOM contributions to the estuary suggested a decrease in the in situ inputs from the marsh to the estuary during the length of this study.