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 suspend ed sediment load in shelf seas and estuaries. Even in moderate concent rations, mineral suspended solids affect algorithms for retrieving chl orophyll concentrations from remotely-sensed colour data. Brown and Si mpson (1990) showed empirically that the slope of a log-log graph of c hlorophyll against the blue-green reflectance ratio increased as the c oncentration of mineral suspended solids increased. In an attempt to e xplain this, we have measured the absorption spectrum of over 100 samp les of mineral suspended solids collected from the Menai Strait in the Irish Sea. The absorption spectra of the mineral suspended solids par ticles on filters consistently show an exponential decrease with incre asing wavelength across the visible spectrum. The absorption coefficie nt at a single wavelength is proportional to the concentration of mine ral suspended solids. This spectrum has been incorporated into a simpl e model of the blue-green ratio in the presence of both chlorophyll an d mineral suspended solids. The results are in qualitative agreement w ith Brown and Simpson's observations and can be made to be in quantita tive agreement by correcting the absorption of mineral suspended solid s on filters to in situ absorption using a pathlength correction facto r beta=4. The main effect of mineral suspended solids is to decrease t he sensitivity of the blue-green ratio to changes in chlorophyll in sh elf seas and we conclude that the blue-green ratio may not be the best remote sensing tool for measuring chlorophyll in case 2 waters.