Absorption spectra of inorganic particles in the Irish Sea and their relevance to remote sensing of chlorophyll

Mineral suspended solids contribute significantly to the total suspended sediment load in shelf seas and estuaries. Even in moderate concentrations, mineral suspended solids affect algorithms for retrieving chlorophyll concentrations from remotely-sensed colour data. Brown and Simpson (1990) showed empirically that the slope of a log-log graph of chlorophyll against the blue-green reflectance ratio increased as the concentration of mineral suspended solids increased. In an attempt to explain this, we have measured the absorption spectrum of over 100 samples of mineral suspended solids collected from the Menai Strait in the Irish Sea. The absorption spectra of the mineral suspended solids particles on filters consistently show an exponential decrease with increasing wavelength across the visible spectrum. The absorption coefficient at a single wavelength is proportional to the concentration of mineral suspended solids. This spectrum has been incorporated into a simple model of the blue-green ratio in the presence of both chlorophyll and mineral suspended solids. The results are in qualitative agreement with Brown and Simpson's observations and can be made to be in quantitative agreement by correcting the absorption of mineral suspended solids on filters to in situ absorption using a pathlength correction factor beta=4. The main effect of mineral suspended solids is to decrease the sensitivity of the blue-green ratio to changes in chlorophyll in shelf seas and we conclude that the blue-green ratio may not be the best remote sensing tool for measuring chlorophyll in case 2 waters.