Attenuation versus Transparency

The differential response of the two commonly used measures of light penetration in water, Secchi disc transparency (SD,m) and the attenuation coefficient for downwelling irradiance (Kd,m-1), to the light attenuating processes of absorption and scattering is described. It is demonstrated that the commonly invoked constancy in the product Kd⋅SD should only be expected when the relative contributions of scattering and absorption to Kd remains uniform. A number of scenarios and documented phenomena are presented for which preferential effects on scattering or absorption, and therefore variable values of Kd⋅SD, are expected. Measurement errors also contribute to variability in Kd⋅SD, particularly at low (<1.5m)SD values. Paired observations of Kd and SD presented from four different systems, representing a wide range of Kd and SD values, demonstrate the variability of the magnitude of Kd⋅SD within individual systems. Secchi disc transparency is probably not a reliable indicator of Kd in many systems. It is recommended that the uncertainty associated with invoking the constancy of Kd⋅SD in various ecological modeling efforts, in which Kd measurements are missing or are temporally limited, be accommodated.