Regional and seasonal differences in light absorption by yellow substance in the Southern Bight of the North Sea

Absorption spectra (250-800 nm) of yellow substance were measured in 1993 and 1994 along five onshore-offshore transects in the Southern Eight of the North Sea in different seasons. All spectra had a common shape, so the amount of yellow substance could be established using one reference wavelength as a proxy variable. The exponential slope parameter at 380 nm (S) of the absorption spectra ranged from -0.016 to -0.023 per nm when regression was based upon the 250-650 nm wavelength range; values were 25% lower when fitting was performed over the more restricted waveband 360-540 nm, partly due to a minor shoulder at 260-275 nm in all spectra. The concentration of yellow substance, expressed as the absorption coefficient at 380 nm (a(380); m(-1)), varied over an order of magnitude, from low values down to 0.17 m(-1) in the English Channel, up to 1.00-1.25 m(-1) near the Rhine and Scheldt outflow, to maxima of up to 1.75 m(-1) in the neritic-estuarine waters at the entrance to the shallow Wadden Sea. This regional distribution pattern was in agreement with the known water-mass circulation and with the location of sources of yellow substance: highest concentrations close to the shore under the influence of freshwater inflow, lower values with increasing distance from the coast, where Atlantic Ocean water is the major water-mass component. No significant seasonal variation in yellow-substance concentration was found anywhere when a correction was applied for salinity. Extrapolation to zero salinity yielded an absorption coefficient at 380 nm of 5.38 m(-1) for the freshwater input at both the Texel and Walcheren transects, but of 3.29 m(-1) at the Noordwijk transect, reflecting differences in yellow-substance concentration between the Rhine, the Meuse, and Lake IJssel, as previously noted in fluorescence measurements. Yellow-substance concentration was not only correlated with salinity but also, albeit much less, with chlorophyll concentrations; the contribution of phytoplankton to the yellow-substance pool was marginal but significant. A relation appeared to exist between yellow-substance absorbance at 380 nm and yellow-substance fluorescence intensity; fluorescence measurements in the southern North Sea can be translated to absorption (more appropriate for ocean colour detection by remote sensing) by equating I mFl unit to an absorption coefficient at 380 nm of 0.056 m(-1), and using the exponential relation a(ys)(lambda) = a(ys)(lambda(ref)) exp(-S(lambda - lambda(ref))) for extrapolation to UV-Vis spectral absorption. (C) 1999 Elsevier Science B.V. All rights reserved.