Estimating the diffuse attenuation coefficient from optically active constituents in UK marine waters

Abstract

We report a study of the attenuation of submarine Photosynthetically Active Radiation (PAR) in relation to the concentrations of Optically Active Constituents (OACs) in a range of water types around the United Kingdom. 408 locations were visited between August 2004 and December 2005. The diffuse attenuation coefficient ( K d) was estimated from profiles of downwelling PAR. Concentrations of Suspended Particulate Matter (SPM) were measured gravimetrically and concentrations of phytoplankton chlorophyll (chl) were measured by fluorometrically. Chromophoric Dissolved Organic Matter (CDOM) was measured either by fluorescence or as its proxy, salinity. Several empirical models for K d as a function of SPM, chlorophyll and CDOM were fitted to the data set. It was found that including all three explanatory variables (CDOM, chlorophyll and SPM) gave a slightly better fit for coastal and offshore waters, whereas a fit based only on SPM and chlorophyll worked well for transitional (estuarine) waters. The use of SPM as a single predictor of K d in all water types resulted in only 3% loss of accuracy. The effect of seasonal variations in the light climate and the OACs was investigated with high frequency data from moorings in the Thames embayment and Liverpool Bay. K d was estimated from data recorded from pairs of vertically separated PAR sensors. Using the empirical models to estimate K d from these OACs showed that reliable estimates of attenuation could be made throughout the year, with some scatter of estimated K d about observed K d during the growing season. The reliability of these findings was validated by non-linearly fitting of a mechanistic model, based on semi-intrinsic optical parameters, to the spatial data set. Estimated values of absorption cross-section and scattering cross-section were in good agreement with the literature, and help to justify parameter values obtained from the empirical models.