A simple bio-optical model, with parameter values derived from measurements of the inherent optical properties (IOPs) and optically active substances that are known to in uence the IOPs, has been developed. A large historical dataset of measurements of the concentration of chlorophyll a and phaeophytine a (Chl), suspended particulate inorganic material (SPIM ) and the absorption coeYcient of Coloured Dissolved Organic Matter (CDOM ), spanning more than 25 years, has been used together with the model to develop algorithms for the retrieval of these water quality parameters, for a site in Lake Mälaren, Sweden. The model takes as input the optically active substances and outputs a re ectance spectrum just above the water surface. From the modelled re ectance, algorithms were derived for Chl, SPIM and CDOM absorption at 420 nm. The algorithms were applied to atmospherically corrected remote sensing data, which were collected by the Compact Airborne Spectrographic Imager, CASI. The radiative transfer code 6S was used for the atmospheric correction of the data. Distribution maps for the three retrieved parameters were constructed and Chl and SPIM were validated by continuous eld measurements of uorescence and beam attenuation. The continuous data were calibrated with water analysis results from nine water samples. The time lag between the image acquisition and the ground data measurements was never more than 3 hours. Even though the model parameter values were collected at diVerent times from that of the CASI over-ight, and from a larger geographic region of Lake Mälaren than that used for the CASI measurements, the independently developed algorithms predicted the concentrations of the optically active substances within a reasonable level of accuracy, allowing spatial variations in the substances to be predicted.

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The dramatic bleaching events on the coral reefs recently have enhanced the need for environmental monitoring. Remote sensing is an important constituent for monitoring of reefs, and an invaluable complement to field observations. This paper discusses the possibilities and limitations of present high resolution satellites for mapping and monitoring coral reefs. The sensors with the best spatial and radiometric resolution available today, e.g. IKONOS, can be useful for mapping and monitoring of reefs, but they are too costly for global surveys. However, our coral bleaching studies indicate that massive bleaching could be detected even from satellites with lower resolution, like Landsat, SPOT, and IRS. They could also be useful for coarser, from a spatial and thematic point of view, global mapping and updating purposes. A more detailed monitoring requires both better spatial resolution and spectral resolution than today's sensors. In the future, it is necessary to construct a more reef specific sensor with a few specially selected narrow bands and a good spatial, radiometric and temporal resolution.

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