[1] In March and early April 2000, spectral irradiance measurements were conducted on 10 occasions above and beneath the landfast sea ice in Santala Bay near the entrance to the Gulf of Finland. The measurements included the spectral albedo and transmittance of the sea ice and the downwelling and upwelling spectral irradiance at different depths in the water column. Spectral albedos integrated over 400-700 nm photosynthetically active radiation (PAR) were in the range of 0.16-0.58, with most common values between 0.33 and 0.42. Maximum transmittances for the ice are found close to 570 nm, decreasing toward shorter and longer wavelengths. Correspondingly, diffuse attenuation coefficients for both seawater and sea ice agree well with earlier measurements in the Baltic Sea and have a PAR attenuation of 0.4-0.7 and 3.1-4.7 m À1 , respectively. During the measurement period the ice reached its maximum thickness of 28 cm and thereafter started to decay. The sea ice was snow free; however, the formation of high-scattering melt/freeze layers above the freeboard largely increased the surface reflectance. Discharged meltwater was retained beneath the sea ice, forming a low-salinity layer which only disappeared as the ice ablated in April. Compared with many parts of the Arctic, the sea ice and seawater in Santala Bay contain higher amounts of dissolved and particulate matter, which are indicated by high absorption at wavelengths below 700 nm and thus potentially increase the melt rate of the sea ice.
Light transmission through ice and light conditions beneath ice have been investigated in the mild winter of the year 2000 in eight Estonian lakes and in one braclush water basin, Santala Bay in the Gulf of Finland. A new system designed for optical measurements beneath the ice was successfully tested. In the water body the vertical profiles of photosynthetically active radiation (PAR), temperature and oxygen were mapped. The concentrations of optically active substances (dissolved organic matter, chlorophyll a, particles) were estimated for water samples and meltwater of ice samples. The PAR band albedo was 0.28-0.76 and transmittance was 1-52% for the icelsnow cover. The light field below ice was much more diffuse than in open water conditions. Euphotic depth was 0.1-5.5 m. The amount of yellow substance in lake ice is very small in comparison with the lake water; lake ice may contain a lot of particles, but their source is atmospheric fallout rather than the water body. In some lakes a depletion of oxygen was observed. There were considerable differences between the fresh and brackish water ice (structure, stratigraphy, amount of impurities), which influenced the underwater light field.
A field programme on light conditions in ice-covered lakes and optical properties of lake ice was performed in seven lakes of Finland and Estonia in February–April 2009. On the basis of irradiance measurements above and below ice, spectral reflectance and transmittance were determined for the ice sheet; time evolution of photosynthetically active radiation (PAR) transmittance was examined from irradiance recordings at several levels inside the ice sheet. Snow cover was the dominant factor for transmission of PAR into the lake water body. Reflectance was 0.74–0.92 in winter, going down to 0.18–0.22 in the melting season. The bulk attenuation coefficient of dry snow was 14–25 m–1; the level decreased as the spring was coming. The reflectance and bulk attenuation coefficient of snow-free ice were 0.1–0.4 and 1–5 m–1. Both were considerably smaller than those of snow cover. Seasonal evolution of light transmission was mainly due to snow melting. Snow and ice cover not only depress the PAR level in a lake but also influence the spectral and directional distribution of light
The properties of wave fields induced by high-speed ferries and recently introduced conventional ferries with increased cruise speeds are analysed for a site in Tallinn Bay, the Gulf of Finland, the Baltic Sea, located about 3 km from the sailing line and up to 8 km from the wave production area. The analysis is based on high-resolution profiling of the water surface for about 650 wakes from fast ferries, measured during 4 weeks in June-July 2008. The new large conventional ferries with cruise speeds of 25-30 knots (~45-55 km/h) sail at near-critical speeds along extensive sections of eastern Tallinn Bay, and excite wakes equivalent to those of high-speed ferries. The peak periods of these wakes are between 10 and 13 s. The typical daily highest ship wave is approximately 1.2 m, measured prior to wake breaking. The largest recorded ship wave in calm conditions had a height of 1.5 m and in the presence of some wind wave background 1.7 m. The cumulative impact of ship wakes results in a gradual increase in the suspended matter concentration in near-bottom water over the course of a day. The largest and longest ship waves produce considerable wave runup at the coast and prevent several coastal sections from achieving an equilibrium state. The largest ship waves have an asymmetric shape both in terms of the water surface elevation above and below the mean level and in terms of the shape of the wave front and back. The overall intensity of anthropogenic waves has remained at the same level as it was in the year 2002, although the ships that produced the highest waves in the past are no longer in service
The main objective of the present study is to test various methods for describing the absorption spectra of coloured dissolved organic matter (CDOM) and to determine the numerical values of some optical parameters of CDOM in lakes with diverse water quality. First, the parameters of an exponential model in different spectral intervals were determined. In addition, the suitability of some other models for the approximation of CDOM spectra was estimated. Specific absorption coefficients of CDOM were calculated from the absorption coefficients and dissolved organic carbon (DOC) concentrations. The experimental initial data were differences between spectral attenuation coefficients of filtered and distilled water. Two datasets were used: 1) for 13 Estonian and 7 Finnish lakes (altogether 404 spectra between 350 and 700 nm) measured by the Estonian Marine Institute (EMI); 2) for 10 Finnish lakes (73 spectra) measured by the Finnish Environment Institute (FEI). The spectra of CDOM absorption coefficients (aCDOM) were calculated from experimental data taking into account the correction due to scattering properties of colloids in the filtered water. The total content of CDOM in natural waters of Estonian and Finnish lakes was expressed by means of aCDOM at the wavelength of 380 nm. It varied significantly, from 0.71 to 19.5 m−1, the mean value (of all the investigated lakes) being around 6.6 m−1. Slopes of the exponential approximation varied widely, from 0.006 to 0.03 nm−1. Averaged over all lakes values of slope for the interval 380-500 nm obtained from the EMI dataset are close to those obtained from the FEI dataset: from 0.014 nm−1 (without correction) to 0.016-0.017 nm-1 (with different types of correction). These results are in good correspondence with most published data. Attempts to describe the spectra in the region of 350-700 nm by means of hyperexponential functions (∽ exp(-αλη)) show that: (1) η < 1 (in the case of traditional exponential approximation η = 1); (2) a promising idea is to seek the best fit only for wavelengths λ > λ1, where λ1 will be chosen taking into account the real shape of aCDOM spectra. The mean value of the specific absorption coefficient (a*CDOM) at the wavelength 380 nm obtained in this study (0.44 L mg−1 m−1) is close to the values published in the literature, if we assume that a*CDOM (380) is calculated using the data of dissolved organic matter (DOM). The optically non-active fraction of DOM in our study was high and therefore a*CDOM (380) was considerably higher (1.01 L mg−1 m−1) than a*CDOM (380). The results of the present work could be used in the modeling of underwater light field as well as in the interpretation of radiation measurements and optical remote sensing results.
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