Optical measurements in the North Sea-Baltic Sea transition zone. III. Statistical analysis of bio-optical data from the Eastern North Sea, the Skagerrak and the Kattegat

Abstract

Bio-optical measurements (downward and upward spectral irradiance, beam attenuation, yellow substance absorption, Chi- a, CTD and nutrients) were gathered in the Eastern North Sea, the Skagerrak and the Kattegat during nine cruises over the years 1990–1992. The light data were used to describe the spatial and seasonal variations of the underwater light climate in the region and to develop ocean color models for estimation of water quality parameters. Empirical relations were established between the vertical attenuation coefficients K d( λ) and K d(488) which showed good agreement with the results obtained by Austin and Petzold ( Optical Engineering, 25, 471–479, 1986). Local ocean color algorithms for estimating z q(10%), the depth of the 10% downward quanta irradiance level and K d(488) were derived and showed fair agreement with previously established expressions from the literature. irradiance reflectance measurements were subjected to principal component analysis that showed that more than 95% of the variance in the reflectance spectra for Danish coastal waters was explained by the first two eigenvectors. The model by Kirk ( Limnology & Oceanography, 29, 350–356, 1984) was used to determine the inherent optical properties a (light absorption) and b (light scattering) from irradiance and reflectance measurements. The results were in accordance with the few historic measurements that exist from the area though the scattering estimates were slightly higher than previous measurements. Concentrations of Chl- a and suspended matter and yellow substance absorption were estimated by the inverse method of Jain and Miller ( Applied Optics, 15, 886–890, 1976). The results showed fair agreement with in situ measurements and the model tended to overestimate the concentrations of suspended matter and yellow substance absorption.