Simultaneous Measurements of Fluorescence and Beam Attenuation: Instrument Characterization and Interpretation of Signals from Stratified Coastal Waters

Hydrographic (conductivity, temperature, depth), optical (fluorescence and beam attenuation) and chemical (suspended particulate material, chlorophyll, gelbstoff) observations were made in sea lochs off the west coast of Scotland. A strong link was observed between hydrographic and optical layering in these waters. In order to assist the interpretation of the optical data, relationships between fluorescence and extracted chlorophyll and dry weight and beam attenuation were determined in the laboratory for five species of cultured phytoplankton and three types of inorganic particles. The inherent variation in these relationships from one class of material to another precluded the development of generally applicable algorithms for retrieving mass concentrations from 2-parameter optical data. However the instrument calibrations were used to partition a bivariate plot of fluorescence against attenuation on which thein situdata formed well defined clusters. This made it possible to deduce distribution profiles of phytoplankton and suspended sediment in the water column. The results indicate that even in the absence of absolute calibrations, multiparameter optical measurements can provide valuable information on fine-scale variations in seawater composition, and enhance the identification and discrimination of water masses in fjords and other highly structured water bodies.