Niels K. Højerslev

Optical measurements in the North Sea-Baltic Sea transition zone. I. On the origin of the deep water in the Kattegat

Abstract In the North Sea-Baltic Sea region, several studies have shown that yellow substance can be treated as a quasi-conservative parameter that is negatively correlated with salinity. A review of more than 5000 historic and recently gathered yellow substance absorption and salinity measurements from the transition zone between the North Sea and the Baltic Sea have been analyzed for the purpose of water mass identification. Salinity-yellow substance scatter plots show that three water masses can be identified in the area: (1) North Sea water (high salinity, low yellow substance content); (2) Baltic Sea water (low salinity, intermediate-high yellow substance content); and (3) German Bight/Southern North Sea water (intermediate-high salinity, high yellow substance content). Based on the volume flow estimates used in a two-layer box-model of the Kattegat (Jorgensen, Continental Shelf Research , 12 , 103–114, 1992) and conservation of yellow substance, it is argued that the long-term average composition of the bottom layer inflow to the Kattegat from the Skagerrak consists of about 90% North Sea/Atlantic water and 10% German Bight/Southern North Sea water. The German Bight content estimate is substantially lower than the ones given in earlier studies (50–67%) and implies that the Jutland Coastal Current only has a small impact on the water quality of the Kattegat.

Yellow Substance in the Sea

The break-down processes of plankton and detritus in the marine environment cause no appreciable in situ production of yellow substance. The amount of yellow substance is small and approximately the same in oligotrophic and eutrophic open oceanic areas.

Analysis of underwater radiance observations: Apparent optical properties and analytic functions describing the angular radiance distribution

A primary data set consisting of 70 series of angular radiance distributions observed in clear blue western Mediterranean water and a secondary set of 12 series from the more green and turbid Lake Pend Oreille, Idaho, have been analyzed. The results demonstrate that the main variation of the shape of the downward radiance distribution occurs within the Snell cone. Outside the cone the variation of the shape decreases with increasing zenith angle. The most important shape changes of the upward radiance appear within the zenith angle range 90°–130°. The variation in shape reaches its minimum around nadir, where an almost constant upward radiance distribution implies that a flat sea surface acts like a Lambert emitter within ±8% in the zenith angle interval 140°–180° in air. The ratio Q of upward irradiance and nadir radiance, as well as the average cosines μd and μu for downward and upward radiance, respectively, have rather small standard deviations, ≤10%, within the local water type. In contrast, the irradiance reflectance R has been observed to change up to 400% with depth in the western Mediterranean, while the maximum observed change of Q with depth is only 40%. The dependence of Q on the solar elevation for blue light at 5 m depth in the Mediterranean coincides with observations from the central Atlantic as well as with model computations. The corresponding dependence of μd shows that diffuse light may have a significant influence on its value. Two simple functions describing the observed angular radiance distributions are proposed, and both functions can be determined by two field observations as input parameters. The e function approximates the azimuthal means of downward radiance with an average error ≤7% and of upward radiance with an error of ∼1%. The α function describes the zenith angle dependence of the azimuthal means of upward radiance with an average error ≤7% in clear ocean water, increasing to ≤20% in turbid lake water. The a function suggests that the range of variation for μu falls between 0 and 1/2, and for Q it is between π and 2π. The limits of both ranges are confirmed by observations. By combining the e and α functions, a complete angular description of the upward radiance field is achieved.

Visibility Of The Sea With Special Reference To The Secchi Disc

A general visibility theory valid in inhomogeneous hydrosols is presented and experimentally verified for some special cases. The field measurements used in the experimental verification were obtained from both oligotrophic and eutrophic regions in the sea and comprise of a fairly large Danish and French data set consisting of spectral downward and upward irradiances, downward quanta irradiance, spectral light transmission, spectral radiance distributions and finally, spectral Secchi disc measurements.

Optical measurements in the North Sea-Baltic Sea transition zone. II. Water mass classification along the Jutland west coast from salinity and spectral irradiance measurements

Abstract Spectral irradiance measurements were gathered along three fixed sections outside Thyboron, Hanstholm and Hirtshals on the north west coast of Denmark during seven cruises with R. V. Svanic in the years 1990–1992. It is argued that the irradiance attenuation coefficient Kd (410) can be taken as a good indicator of yellow substance in the study region. Salinity - Kd (410) scatter plots show that three water types can be identified in the area: Atlantic/Central North Sea water, German Bight water and Baltic water. Estimates of the mean width of the part of the Jutland Coastal Current that primarily is influenced by the freshwater inflow to the German Bight are given for each of the three sections. A salinity- K d (410) water mass analysis was carried out based on long-term mean characteristic values of salinity and K d (410) for the three core water types and the measurements from this study. The average contents of Atlantic/Central North Sea water, German Bight water and Baltic water was estimated for the surface layer at each of the three sections. Based on the volume flow estimates (provided as a function of salinity) from the three sections as listed in Rydberg (Report No. 53, Department of Oceanography, University of Gothenburg, Sweden, 1993) an estimate of the mean volume flow of water with German Bight origin entering the Skagerrak is given.

Water color and its relation to primary production

It is shown both experimentally and theoretically that the depth of the euphotic zone is related to the color of the sea defined as a color index equal to the ratio of the upwelling blue (450 nm) and green (525 nm) nadir daylight just below the sea surface. The relationship is valid for most sea waters where the albedo from the bottom is negligible near the sea surface and where the composition of suspended and dissolved matter is not largely determined either by water drainage from land or by turbidity currents. The standing stock of phytoplankton and its primary productivity within the euphotic zone can approximately be given in terms of the color of the sea. The accuracy in determining each of the above two biological quantities from color measurements at the sea surface is not satisfactory at present. This holds particularly true when the color of the sea is measured remotely by present satellite techniques.

Analytic remote-sensing optical algorithms requiring simple and practical field parameter inputs

Spectral in-water measurements of downward irradiance (E(d)), upward irradiance (E(u)), and nadir radiance (L(u)) are sufficient to calculate the scalar irradiances E(0), E(0d), and E(0u), the average cosines mu, mu(d), and mu(u), the light absorption coefficient a, the backscattering coefficient b(b), and the so-called f factor that relates to R, a, and b(b). The solar elevation of 42 degrees is a special case in which mu(d) is independent of all variables except solar elevation. The algorithms are valid for solar elevations between 12 degrees and 81 degrees for horizontally stratified clear and turbid deep waters.

A RELATIONSHIP FOR THE PENETRATION OF ULTRAVIOLET B RADIATION INTO THE NORWEGIAN SEA

A linear relation between the vertical attenuation coefficients of UV-B and blue irradiances is obtained for the Norwegian Sea. The Atlantic waters in this region are on average characterized by a deeper 1% depth of UV-B irradiance than the surrounding waters, the depth being situated between 23 and 33 m.

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 Kd(λ) and Kd(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 zq(10%), the depth of the 10% downward quanta irradiance level and Kd(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.

Assessment of Some Suggested Algorithms on Sea Colour and Surface Chlorophyll

The feasibility for converting intrinsic colour measurements of the sea into concentration of surface chlorophyll a, as well as into depths of the euphotic zone, is outlined.