Long-term, in situ measurements of CDOM spectral-absorption and fluorescence in estuarine and coastal environments

CDOM composition can be inferred from spectral-absorption and fluorescence properties, but few studies have measured both on short sampling periods over extended lengths of time in natural environments. Through a collaborative monitoring effort, long-term in situ measurements of CDOM fluorescence, CDOM spectral-absorption, and water quality parameters were obtained at high frequency from a series of sites with varying degrees of terrestrial/marine influence: the Caloosahatchee River estuary, Sarasota Bay, and a marine site offshore of Sarasota Bay. Results demonstrated significant correlation between CDOM fluorescence (370/460 nm) and absorption (440 nm). CDOM spectral-slope (350 – 400 nm) was inversely related to absorption at 440 nm and was generally higher and more variable at marine sites than terrestrial sites. Trends were evident on short time scales (hourly) due to tidal variations and on longer timescales (daily – weekly) corresponding to changes in river flow and salinity. Interestingly, observed relationships between CDOM absorption, spectral-slope, and salinity provide a means to evaluate changes in CDOM composition (via CDOM slope) at a fixed location in the presence of variable salinities and CDOM concentrations. Over longer timescales, this relationship may be used as an environmental indicator to infer the presence of phytoplankton blooms or land-use and climate change impacts upstream.