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

Abstract

Abstract. The Secchi depth and its relationships to other properties of the sea water in the Oslofjord-Skagerrak area have been investigated. White and black disks of different sizes have been applied, and the Secchi depth has been observed with the naked eye, through colour filters and with a water telescope. Spectral luminances and illuminances have been calculated from recordings of radiance and irradiance, and attenuation coefficients have been determined. A theoretical expression for the Secchi depth based on luminances has been tested against field observations, and it is found that the field results for the product of Secchi depth and attenuation coefficients are on average only 4 % less than the predicted value for the white disk. For the Secchi depths observed through colour filters or for the black disk, the average field results are more than 30 % smaller than the theoretical estimates. The reduction in the disk diameter from 30 to 10 cm should theoretically reduce the Secchi depths by 13-22 %, while the field observations show an average reduction of 10-20 %. Similarly we find from theory that the removal of sun glitter should increase the Secchi depth by 12 %, while the observed increase is 14 % on average for the white disk. Our overall conclusion is that the theoretical expression works well for the white disk, but less so for the colour filter observations and the black disk.Statistical relationships between Secchi depths and attenuation coefficients have been determined, and it is found that the root-mean-square errors relative 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 quanta irradiance (PAR) can be estimated with a relative root-mean-square error of 23 % from observations of the white Secchi depth. Similar estimates of chlorophyll a and total suspended material will have rms errors in the range 40-90 %. Our conclusion becomes that the Secchi depth observation is a very useful tool for checking the value and order of magnitude of other related quantities in the OslofjordSkagerrak area.

Resolving the long‐standing puzzles about the observed Secchi depth relationships

“…The decision of sighting a Secchi disk in water by the human eye-brain system is based on information in water's transparent window (at the wavelength of maximum transmittance) (Megard and Berman 1989;Aas et al 2014;Lee et al 2015a), not the full visible spectrum as depicted in the classical theory or at a fixed wavelength (He et al 2017). This is further supported with experiments in blue and green waters (Aas et al 2014;Lee et al 2017). 3.…”

“…with parameter C reported in a range of 5-10 ( Gordon and Wouters 1978;Preisendorfer 1986). This model was subsequently echoed in Tyler (1968), Preisendorfer (1986), Zaneveld and Pegau (2003), Levin and Radomyslskaya (2012), and Aas et al (2014) and has been followed by the community in the past 601 years to theoretically interpret Z SD data. A few later models (e.g., Preisendorfer 1986; Levin and Radomyslskaya 2012) included a term to explicitly consider the air-sea surface effects, but that term was generally imbedded in the parameter C in most of such models.…”

“…3. The decision of sighting a Secchi disk in water by the human eye–brain system is based on information in water's transparent window (at the wavelength of maximum transmittance) (Megard and Berman ; Aas et al ; Lee et al a ), not the full visible spectrum as depicted in the classical theory or at a fixed wavelength (He et al ). This is further supported with experiments in blue and green waters (Aas et al ; Lee et al ). The decision on detection of human eyes is based on absolute difference in radiance or reflectance (Blackwell ; Bartleson and Breneman ) rather than on relative difference (the ratio of ( R t – R w ) to R w ) as employed in the classical theory (Duntley ; Preisendorfer ). As discussed detailed in Lee et al ( a ), the evaluation of relative difference is a good measure of the sharpness of an object, but it does not match the concept of “brightness constancy” in visual detection (Bartleson and Breneman ; Freeman ).…”

Spatial Distribution of Diffuse Attenuation of Photosynthetic Active Radiation and Its Main Regulating Factors in Inland Waters of Northeast China

“…SDD is correlated with K d (PAR) as they are both influenced by the absorption and scatter characteristics of the OACs; in effect, both measurements represent the penetration of light in the water [56]. The difference between them is the varied contributions of the extent of spectral bands as SDD is related to the visible domain (410-665 nm) [57,58] and K d (PAR) is 400-700 nm [5,33].…”

Estimation of upward radiances and reflectances at the surface of the sea from above-surface measurements

“…The bio-optical properties of this area have been presented by Aas et al (2005), Høkedal et al (2005) and Sørensen et al (2003Sørensen et al ( , 2004Sørensen et al ( , 2007. While the annual range of the Secchi disk depth at this location stretches from 2 m during vernal algal bloom to 12 m under winter conditions (Aas et al, 2014), the Secchi disk depths on the 4 days in Table 1 were in the range of 5.0-6.5 m. The content of yellow substance or CDOM (Coloured Dissolved Organic Material) can be quantified by its absorption coefficient at 442 nm. The mean value ± the standard deviation of the coefficient at this wavelength, based on data mainly from the Skagerrak and the Oslofjord is 0.62 ± 0.60 m −1 , according to Sørensen et al (2007).…”