Seasonal variation in molecular mass and optical properties of chromophoric dissolved organic material in coastal waters of southwest Florida

One of the most important natural sunlight absorbing substances in water is the chromophoric dissolved organic material (CDOM). Its influence on optical properties has been studied for many years, but questions of how its structural and optical characteristics change in the environment still remain. CDOM water samples were collected from coastal waters of southwest Florida during three cruises in the year 2001: one in the dry season (June) and two in the rainy season (September and November). Analyses of molecular mass (MM) distribution and optical characteristics were done using the flow field-flow fractionation (FlFFF) separation technique with absorbance and fluorescence detectors. On the basis of the results, mixing, source variability, degradation, and excited state quenching processes are important determinants of CDOM composition and optical properties in this region. During the dry season, the MM distribution did not change significantly spatially. In September and November, higher MM compounds and an increase of CDOM fluorescence were observed, found inside the rivers and in near-coastal samples, consistent with the rivers being a significant CDOM source. Results suggest that chromophores were broken down to smaller MM compounds faster than fluorophores, indicating that the fluorescent moieties are more resistant to degrading/removal processes than the non-fluorescing compounds, even though the fluorophores were always centered at lower MM. On the other hand, the fluorescence quantum yield decreased faster than absorbance coefficients, suggesting that fluorophores were quenched by complexing with some metals. For all months, the chromophoresʹ MM for offshore samples remained the same. The carbon concentration ratio between chromophores and fluorophores changed from lower to higher salinities, suggesting that the composition of the waters changed toward offshore. The differences in the optical characteristics, MM distributions, and carbon concentration observed suggest that the CDOM sources, physical, and photochemical degradation processes change seasonally.