Temporal variability of fluorescent dissolved organic matter at a brackish, tidal marsh-estuary interface

Marshes are both terrestrial and aqueous, sitting as an intermediate between land and water. Studies over constrained numbers of tidal cycles have demonstrated that these marshes are net exporters of dissolved organic carbon (DOC) to adjoining estuaries, however, there is need for continuous monitoring to better understand the temporal variability of this flux: tidally, seasonally, and during episodic rain events. Through use of a YSI EXO2 sonde, an in situ optical sensor at the interface of the brackish, tidal Kirkpatrick Marsh and Rhode River sub-estuary in Edgewater, Maryland (located on the northwestern shore of the Chesapeake Bay), we were able to continuously monitor fluorescent dissolved organic matter (FDOM). Corrections for attenuation by particle scattering and absorbance were developed for this sensor and site, and evaluated. We assessed the capability of retrieving DOC concentrations (a biogeochemical variable) based on in situ FDOM and additional water quality parameters. The EXO2 variables that produced the best relationship to estimate DOC concentrations were: raw FDOM, temperature, pH, and dissolved oxygen. We were able to achieve an R2 value of 0.732 between estimated DOC and measured DOC, encompassing 207 data points from all seasons and tidal stages. Differences between DOC fluxes based on measured versus estimated DOC ranged from 6% to 29%, and these DOC fluxes consistently indicated that the Kirkpatrick Marsh was a source of DOC to the Rhode River sub-estuary. Accurately estimating DOC from in situ optical parameters will improve the temporal variability of DOC measurements available, which will allow for improvements in coastal carbon cycle modeling and advances in coastal DOC remote sensing satellite retrievals. In addition to carbon analysis, we were able to analyze particulate and physicochemical exchanges at the marsh-estuary interface between low and high tide water, and spatially within the Rhode River sub-estuary using two end members. This allowed for characterizations to be made about marsh-estuary exchanges, inputs, and transformations