Mixing Behavior of Colored Dissolved Organic Matter and Its Potential Ecological Implication in the Caloosahatchee River Estuary, Florida

Colored dissolved organic matter (CDOM) is one of the most important water quality constituents impacting light attenuation in estuaries; its concentration and distribution influence light quality and quantity available to phytoplankton and submerged aquatic vegetation. By combining field surveys (March 2009–January 2011) and laboratory studies, we examined the estuarine mixing behavior of CDOM and potential loss processes affecting mixing behavior in the Caloosahatchee River Estuary (CRE), Florida. The CDOM absorption coefficient at 355 nm (a CDOM(355), m⁻¹) varied from 0.5 to 64 m⁻¹, with higher values in the upper estuary and lower values downstream, and increased with increasing freshwater inflow. CDOM exhibited three apparent mixing patterns with respect to hypothetical conservative mixing, with (1) conservative behavior or (2) addition at lower inflow and (3) loss at higher inflow. Laboratory studies indicated that flocculation was not a major loss process and that CDOM was susceptible to photolysis. The concentration of CDOM declined as a function of cumulative solar irradiation with a rate of ∼0.003 m² mol⁻¹, suggesting a photobleaching half-life for CDOM of about 1 w. Apparent nonconservative mixing of CDOM increased or decreased light attenuation by 15–30 %, depending on freshwater inflow and location in the estuary. Light attenuation in the CRE was controlled primarily by CDOM in the upper estuary and by turbidity in the lower estuary, with the average contribution of CDOM to total light attenuation of 55 % (2–92 %) and turbidity of 23 % (3–79 %). The contribution of chlorophyll a (Chl a) to light attenuation was less than both CDOM and turbidity, accounting for about 12 % on average (2–24 %), regardless of location. These results suggest that any nutrient management scenario aimed at improving water clarity through reduction in Chl a concentration should consider the contributions of color and turbidity as well.