Optical water quality of a blackwater river estuary: the Lower St. Johns River, Florida, USA

This paper reports measurements of absorption and scattering coefficients in relation to standard water quality measurements in the St. Johns River (Florida, USA), a blackwater river in which phytoplankton chlorophyll and non-algal particulates as well as colored dissolved organic matter (CDOM) contribute substantially to the inherent optical properties of the water. Extremely high concentrations of CDOM in this river present special problems for the measurement of inherent optical properties, such as the presence of very fine particulate matter that passes through most glass fiber filters. Empirical relationships are presented for estimating true dissolved absorption at very high CDOM concentrations. Specific-absorption and -scattering coefficients of suspended particulate matter varied widely, but appeared to decline steadily with salinity at salinities above 5, consistent with increasing influence of large-sized, unconsolidated mineral particulates with increasing tidal energy near the estuary mouth. Relationships are given for prediction of inherent optical properties from water quality concentrations for use in radiative transfer modeling, and changes in water quality measurements are recommended that can avoid the need for empirical corrections.