Long-term changes in light scattering in Chesapeake Bay inferred from Secchi depth, light attenuation, and remote sensing measurements

Abstract

The relationship between the Secchi depth (Z(SD)) and the diffuse attenuation coefficient for photosynthetically active radiation (K(d)(PAR)), and in particular the product of the two, Z(SD)center dot K(d)(PAR), is governed primarily by the ratio of light scattering to absorption. We analyzed measurements of Z(SD) and K(d)(PAR) at main stem stations in Chesapeake Bay and found that the Z(SD)center dot K(d)(PAR) product has declined at rates varying from 0.020 to 0.033 yr(-1) over the 17 to 25 years of measurement, implying that there has been a long-term increase in the scattering-to-absorption ratio. Remote sensing reflectance at the green wavelength most relevant to Z(SD) and K(d)(PAR) in these waters, R(rs)(555), did not exhibit an increasing trend over the 10 years of available measurements. To reconcile the observations we constructed a bio-optical model to calculate Z(SD), K(d)(PAR), Z(SD)center dot K(d)(PAR), and R(rs)(555) as a function of light attenuating substances and their mass-specific absorption and scattering coefficients. When simulations were based exclusively on changes in concentrations of light attenuating substances, a declining trend in Z(SD)center dot K(d) entailed an increasing trend in R(rs)(555), contrary to observations. To simulate both decreasing Z(SD center dot)K(d)(PAR) and stationary R(rs)(555), it was necessary to allow for a declining trend in the ratio of backscattering to total scattering. Within our simulations, this was accomplished by increasing the relative proportion of organic detritus with high mass-specific scattering and low backscattering ratio. An alternative explanation not explicitly modeled is an increasing tendency for the particulate matter to occur in large aggregates. Data to discriminate between these alternatives are not available.