Using a compact airborne spectrographic imager to monitor phytoplankton biomass in a series of lakes in north Wales

Three lakes in Snowdonia and two on the island of Anglesey were surveyed using a Compact Airborne Spectrographic Imager (casi). Water samples for chlorophyll measurements and phytoplankton counts were collected at the time of the overflight and the optical characteristics of the lakes recorded by a scanning spectroradiometer. Spectral measurements in the five lakes showed that their optical characteristics were primarily determined by the concentration of phytoplankton in suspension. All the lakes were relatively clear, but the more productive waters contained small amounts of dissolved organic carbon of autochthonous origin. The airborne imaging spectrometer was programmed to measure upwelling radiance in eight narrow bands and linear regressions used to compare the performance of a selected range of chlorophyll retrieval algorithms. The results showed that the most effective algorithms were those based on measurements in the blue and green portions of the spectrum. The best single hand algorithm was centred on 440 nm and explained 75% of the measured variation in the concentration of chlorophyll. The best multi-band algorithm was based on measurements taken at 560 and 440 nm and accounted for 94% of the recorded variation. Chlorophyll maps produced by this algorithm showed that the phytoplankton in most of the lakes was homogeneously distributed but surface patches of cyanobact- eria were detected at one location.