ON THE VARIABILITY OF YELLOW SUBSTANCE IN THE SKAGERRAK AND THE KATTEGAT

The variability of ''yellow substance'' in waters of the Skagerrak and the Kattegat was studied during the international Skagex programme in 1990 and 1991. Data on attenuation of light and fluorescence of disso lved organic matter (DOM) in sea water were collected with a spectral transmissiometer and a submersible fluorometer within a depth range of 0-250 m at several stations. More than fifty-fold variations of fluor escence intensity over space and time were due to the combined action of different sources of DOM in the straits. The surface maximum of flu orescing DOM, related to river outflow, was spreading from Oslo fjord to the central Skagerrak in late spring. This maximum may be ecologica lly important because it diminishes shortwave PAR. In the autumn the b rightest DOM fluorescence was found in the Kattegat and along the Norw egian coast in accordance with earlier studies. The local minima of DO M fluorescence are associated with the Jutland current. Both the insta bility of the current and a ten-fold difference in DOM content between deep layers of the Skagerrak and Kattegat resulted in strong synoptic variability of fluorescence in the transitional area between the stra its. Seasonal and synoptic variabilities of ''yellow substance'' were minimal in the southern Kattegat. The vertical gradients of DOM fluore scence were mostly negative at the depth of the thermocline of the ope n Skagerrak in spring, reaching values of 40-50 % m-1. Fine structure and complex shape were inherent to fluorescence profiles from areas of intensive water mixing. DOM fluorescence closely covaried with attenu ation of UV light: the correlation coefficient was + 0.9 for wavelengt h w <= 350 nm regardless of season. The inverse dependence between wav elength derivative and magnitude of UV attenuation in the straits did not change with season. DOM fluorescence and water salinity were negat ively correlated at depths of 25-50 m but they varied independently in the upper 10 m. No significant correlation between fluorescence and n itrates has been found. Estimates of correlation with phosphates range d from 0.7 to 0.8. In general, the changes of fluorescence were due to variations of DOM content in sea water but not to variability of DOM composition or state. The properties of DOM fluorescence in the Skager rak-Kattegat area make it a useful natural tracer of water movements s uitable for active remote sensing with airborne lidar.