Reflectance properties of hydrographically and optically stratified fjords (Scottish sea lochs) during the Spring diatom bloom

Reflectance spectra of Scottish sea lochs were measured using a specially constructed floating spectroradiometer with a bandpass of 400-800 nm and 4 nm resolution over a period of 10 weeks during the development of a Spring diatom bloom. Most stations showed marked salinity stratification with high surface gelbstoff concentrations and chlorophyll levels up to 80 mug l(-1). Characteristic features of the reflectance spectra included very low values in the blue, a maximum around 600 nm and distinct solar stimulated fluorescence peaks at 685 nm. The height of the fluorescence peak (scaled by dividing by the maximum reflectance value) was well correlated with near-surface chlorophyll concentrations. The ratio of reflectances at 400 and 650 nm could be used as an indicator of surface gelbstoff concentration. However the commonly observed correlation between the blue/green reflectance ratio (443 nm:555 nm) and surface chlorophyll concentration held only at stations where a surface layer of low-salinity, high gelbstoff water was not present. Calculations of the diffuse attenuation coefficient at 440 nm showed that layers with the highest chlorophyll concentrations were frequently located below one optical depth in the water column, implying that much of the phytoplankton biomass in these waters would not be detectable by optical remote sensing.