A time-resolved fluorescence study of dissolved organic matter in a riverine to marine transition zone

Abstract

Time-resolved and steady-state fluorescence measurements were used to characterize dissolved organic matter (DOM) in bulk water samples from a fresh to marine transition zone. The region studied was the Shark River and Florida Bay in Everglades National Park in Southwestern Florida. Samples were taken from the fresh waters at the head of the Shark River, the mouth of the river, and in the fresh-water river plume as it mixed into the saline waters of Florida Bay. The salinity varied from 0.47 at the head of the river to 36.0 for this series of 11 samples. Steady-state fluorescence intensity decreased with increasing salinity from the head of the Shark River into Florida Bay. This is due to dilution by mixing, as changes in the fluorescence intensity also correlated with changes in the levels of total organic carbon (TOC). Fluorescence lifetime measurements were performed on the five most concentrated samples (salinity from 0.47 to 30.5). The decay of the time-resolved fluorescence could be reasonably fit to a multi-exponential function, with three lifetime components on the order of 0.5–0.8, 2–3, and 6–9 ns. These ranges are in agreement with previous literature results for humic acids, fulvic acids, and marine CDOM concentrated by ultrafiltration. This is the first study of CDOM fluorescence lifetimes in coastal waters in a fresh to marine transition zone. Although photochemical and biological transformations of CDOM occur in these zones, no statistically significant difference in lifetimes was observed with increasing salinity. However, irradiating fresh water samples from the head of the Shark River at 280 and 334 nm resulted in a significant decrease in the two shorter lifetime components (by a factor of ∼4), but only a slight decrease in the longest lifetime component.