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

Abstract

Absorption spectra (250–800 nm) of yellow substance were measured in 1993 and 1994 along five onshore–offshore transects in the Southern Bight 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 1 mFl unit to an absorption coefficient at 380 nm of 0.056 m −1, and using the exponential relation a ys( λ)= a ys( λ ref)exp(− S( λ− λ ref)) for extrapolation to UV–Vis spectral absorption.