Since estuaries are unique environments, it is not surprising that processes unique to this boundary between freshwater and seawater impact the retention and release of sediment-and particle-bound compounds. One of these processes is the flocculation of DOM, which occurs with the dramatic increase in ionic strength from freshwater (^0.002 M) to seawater (~0.7 M). The flocculation includes both humic and fulvic acids, which remained dissolved due in part to charged functional groups that interact with polar water molecules in freshwater. However, as DOM transitions from freshwater to seawater the increase in ionic strength and major cation (Ca2+, Mg2+, Na+, and K+) concentrations neutralizes the charges responsible for keeping the DOM dissolved and in solution, and thus the organic material flocculates and precipitates out.
Colloids, very small particles (0.001-1 mm) that are not truly dissolved but are kept in solution by electrostatic forces and turbulent fluid flow, also flocculate due to the increase in ionic strength in estuarine waters. At low ionic strength these small particles with similar surface charges tend to repel one another, preventing the close physical interaction necessary to form large particles; but at higher ionic strengths these electrostatic forces are destabilized. The freshwater-seawater interface is also generally accompanied by an increase in pH and a reduction in flow velocity, which further contribute to colloidal desta-bilization, flocculation, and precipitation at this interface.
Flocculation and sedimentation of organic matter and colloidal particles are responsible for a substantial decrease not only in the concentration ofDOM in the water column, but also in the total water concentration of trace metals and other compounds which are commonly particle bound or complexed by riverine organic matter. This often causes a concurrent increase in sediment organic matter and trace metal levels in estuarine sediments, which may exceed those of freshwater sediments upstream or marine sediments downstream. Consequently, estuarine sediments often act as a trap for nutrients and contaminants, which then tend to be recycled between sediments and overlying waters within estuaries.
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