Relationships between suspended mineral concentrations and red-waveband reflectances in moderately turbid shelf seas

This paper considers the uncertainties that arise in estimating the concentration of suspended minerals by optical remote sensing in waters which contain unknown concentrations of other optically significant constituents. Relationships between suspended mineral concentrations and remote sensing reflectance were calculated by radiative transfer modelling using representative specific inherent optical properties (SIOPs) for phytoplankton (CHL), suspended mineral particles of terrigenous origin (MSSter) and coloured dissolved organic matter (CDOM) that were derived from measurements at 173 stations in UK shelf seas. When only suspended minerals were present, remote sensing reflectance (Rrs) was related to MSSter by a family of saturation curves whose shape depended strongly on wavelength. However the addition of CHL and CDOM made this relationship considerably more complex. Polynomial expressions were therefore derived for the maximum and minimum values of MMSter consistent with a given Rrs667 in the presence of independently varying concentrations of CHL and CDOM. For CHL ranging from 0 to 10 mg m−3 and CDOM from 0 to 1 m−1, for example, an Rrs667 observation 0.01 sr−1 could corresponded to MSSter values between 7 and 12 g m−3. The presence of biogenic minerals in the form of diatom frustules, MSSdia had little influence on the accuracy of MSSter retrieval. The degree of variability in the relationship between MSSter and Rrs667 predicted by the model was confirmed by measurements of radiometric profiles and mineral concentrations at 110 Irish Sea stations. Uncertainties in the remote sensing of MSSter in coastal waters are more appropriately indicated by upper and lower limits set according to the likely ranges of other optically significant constituents than by percentage errors. Moreover, the influence of these constituents should be eliminated before variations in the relationship between MSSter and Rrs are attributed to qualitative changes in mineral particle characteristics.