Secchi depth in the Oslofjord–Skagerrak area: theory, experiments and relationships to other quantities

[摘要] The Secchi depth and itsrelationships to other properties of the sea water in theOslofjord–Skagerrak area have been investigated. White and black disks ofdifferent sizes have been applied, and the Secchi depth has been observedwith the naked eye, through colour filters and with a water telescope.Spectral luminances and illuminances have been calculated from recordings ofradiance and irradiance, and attenuation coefficients have been determined. Atheoretical expression for the Secchi depth based on luminances has beentested against field observations, and it is found that the field results forthe product of Secchi depth and attenuation coefficients are on average only4% less than the predicted value for the white disk. For the Secchi depthsobserved through colour filters or for the black disk, the average fieldresults are more than 30% smaller than the theoretical estimates. Thereduction in the disk diameter from 30 to 10 cm should theoretically reducethe Secchi depths by 13–22%, while the field observations show an averagereduction of 10–20%. Similarly we find from theory that the removal ofsun glitter should increase the Secchi depth by 12%, while the observedincrease is 14% on average for the white disk. Our overall conclusion isthat the theoretical expression works well for the white disk, but less sofor the colour filter observations and the black disk.

Statistical relationships between Secchi depths and attenuation coefficientshave been determined, and it is found that the root-mean-square errorsrelative to the mean value are smaller for the beam attenuation coefficients(12–24%, white disk) than for the vertical attenuation coefficients(16–65%, white disk). The depth of the 1% level of surface quantairradiance (PAR) can be estimated with a relative root-mean-square error of23% from observations of the white Secchi depth. Similar estimates ofchlorophyll a and total suspended material will have rms errors in therange 40–90%. Our conclusion becomes that the Secchi depth observation isa very useful tool for checking the value and order of magnitude of otherrelated quantities in the Oslofjord–Skagerrak area.

[学科分类] 海洋学与技术