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For estimations of the ecological state of a lake and its future trends, data on seasonal and long-term variations of primary production are most necessary. The methods of in situ measurements of production are time consuming, rather complicated, and very expensive. Bio-optical model calculations provide a good alternative here. A semi-empirical model for estimating phytoplankton primary production (Arst et al., 2008, Aquatic Biology, Vol. 3, No. 1, pp. 19-30) allows calculating the vertical profiles and areal (integrated over water column) values of primary production using chlorophyll a concentration, incident irradiance, and light attenuation coefficient in the water. In the present study this model was developed further by elaborating its automated version. It enables performing rapid and greatly replicated estimations of the circumstantial variability of phytoplankton primary production at hourly intervals from morning to evening and as daily and monthly sums based on a table of initial parameters and depths. For demonstrating the practical application of the model we calculated primary production in two large eutrophic North-European lakes (Võrtsjärv and Peipsi), using a database collected during four warm months in 2009 (123 days in both lakes). [ABSTRACT FROM AUTHOR]

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Diurnal variation of the underwater quantum irradiance, the corresponding primary production profiles, and the integral primary production values were estimated by model calculations in three Estonian lakes (Peipsi, Võrtsjärv, and Harku) for 12 days in the summers of 2007 and 2008. In parallel, in situ measurements of primary production were carried out and the main bio-optical parameters of the water were determined. The concordance of the measured and modelled production profiles was satisfactory. Our results show that a model in which the initial data are daily variation of incoming irradiance in the PAR region combined with episodic measurements of chlorophyll a concentration and diffuse attenuation coefficient in the water is suitable for estimating variation of primary production in lakes. [ABSTRACT FROM AUTHOR]

Spatial and temporal variations in coloured dissolved organic matter (CDOM) were studied in two large, shallow and eutrophic Estonian lakes (Peipsi and Võrtsjärv), and in the CDOM-rich Pärnu Bay (in the Gulf of Riga, Baltic Sea). The concentration of CDOM, determined by its absorption coefficient at wavelength λ = 380 nm, ranged from 4.17 to 22.3 m-1 in Peipsi, from 3.96 to 15.7 m-1 in Võrtsjärv and from 2.24 to 32.9 m-1 in Pärnu Bay. The amount of CDOM was spatially variable in all investigated waterbodies. It was highest in the coastal/onshore areas and in the estuaries of the main rivers. It usually decreased from spring towards autumn, and the seasonal patterns were most distinct in the onshore areas. In standing water bodies, the short-term dynamics of discharges had a more significant effect on CDOM concentration in onshore areas than in offshore areas, where the influence of the discharges became visible over a longer period. The results suggest that in large and shallow water bodies the share of the allochthonous component in the CDOM pool decreases towards offshore areas. The impact of CDOM discharged into the inflowing rivers reaches further from shores if the ratio of the catchment area to the volume of the standing water body (C/V) is larger. The influence of precipitation on CDOM absorption proved to be insignificant. [ABSTRACT FROM AUTHOR]

A ten-year (1995-2005) research programme SUVI (Suomi-Viro) on the optics of Finnish and Estonian lakes has been completed. The objectives were to examine the light conditions, to map optically active substances (OAS), and to develop remote sensing methods. Altogether 14 Estonian and 7 Finnish lakes representing different types of water, from oligotrophic to hypertrophic and dystrophic were included. We performed extensive analyses of concentrations of three main OAS, light attenuation and Secchi depth. They varied among lakes and with seasons, but no systematic temporal change could be detected during the 10-year period. We studied the underwater light field using a spectrometer and two PAR quantum sensors, and elaborated three versions of optical classification of lake waters based on: (1) apparent optical properties and the amonnt of OAS, (2) irradiance reflectance spectra, and (3) light attenuation coefficient and the predominant OAS. We developed two models for determining the diffuse attenuation spectra of light, using data for (1) one wavelength and (2) three wavelengths, and elaborated a semiempirical model for qnantitative description of the underwater light regime. We developed a bin-optical model for interpretation of remote sensing data that allows simulation of the reflectance spectra based on the concentrations of OAS. Winter expeditions were undertaken to examine the optical properties of ice and snow and the light conditions in the water beneath the ice cover. Gas pockets are the main optically active impurity in snow and ice. The transparency of ice is similar to that of lake water. The ice cover lowers the light level and makes the light more diffuse in the near surface liquid water layer. [ABSTRACT FROM AUTHOR]

The structure and optics of ice and snow overlying bodies of water were studied in the years 2000-2003. The data were collected in the northern temperate region (nine Estonian and Finnish lakes and one brackish water site, Santala Bay, in the Gulf of Finland). In the present paper we describe the results concerning the radiative characteristics of the system "snow + ice cover on the water": albedo, attenuation of light, and planar and scalar irradiances through the ice. The basic data consist of irradiance measurements above and below ice cover for the PAR band of the solar spectrum (400-700 nm). Albedo varied across wide limits (0.20-0.70 for ice, 0.63-0.94 for snow), depending on the optical and physical properties of ice/snow and weather conditions. The vertically averaged light attenuation coefficient of the ice layer in the brackish waters of Santala Bay was higher than that in the lakes. The ratio of irradiance beneath the ice to incident irradiance increased 2.5-20 times after removing the snow, depending on the albedo and the thickness of ice and snow as well as on their optical properties. In the upper layer of water beneath the ice the ratio of planar to scalar quantum irradiances increased with depth (according to our earlier results obtained in summer this ratio decreased with increasing depth). [ABSTRACT FROM AUTHOR]

The reliability of the models elaborated for recreating the spectra of the light attenuation coefficient, Kd,λ, on the basis of its measured values at 1.3 wavelengths was studied. The coefficients of the one-point. (490 nm) model, elaborated earlier by A. Reinart and A. Herlevi, were tested and approved using 43 new spectra of underwater irradiance measured in 1999-2000. A new version (named C-model) allowing recreation of the spectrum of Kd,λ on the basis on its measured values at three wavelengths was elaborated and tested by statistical analysis. The C-model was found to enable to reconstruct the spectra of Kd,λ with high reliability. Both models gave truthful values of Kd,PAR (mean value of Kd,λ in the region 400.700 nm). For the estimation of the reliability of the experimental initial data, the values of Kd,λ derived from in situ measurements by an underwater spectrometer LI-1800 UW and an instrument ac-9 were compared. The results showed that Kd,λ (ac-9) is mostly lower than Kd,λ (LI-1800UW), the differences increasing with wavelength. [ABSTRACT FROM AUTHOR]

A comparison of values of photosynthetically absorbed radiation calculated with spectral and broad-band models was performed. Special attention was paid to turbid lake waters (Secchi depth ranging from 0.3 to 3 m). Ten measurement series of bio-optical characteristics were carried out in June and August 2003 at three Estonian lakes, Peipsi, Võrtsjärv and Harku. Photosynthetically absorbed radiation was calculated frommeasured quantumirradiance and a radiation model. The results obtained showed that in case of moderately clear waters (Secchi depth about 3 m) it is especially important to use a correct spectral model, as the errors of the broad-band model can be more than 40%-50%. For very turbid waters these errors usually do not exceed 20%. In our measurement results there was a rather remarkable variability of the bio-optical characteristics of the lakes. The relationships between depth-averaged assimilation number and primary production and between depth-integrated primary production and chlorophyll concentration could be described by polynomial functions with intercept = 0. [ABSTRACT FROM AUTHOR]

Possibilities of describing the spatial variation and properties of the ice cover in water bodies by simultaneous in situ and satellite measurements were studied. In 2000-03 a field programme on the structure and optics of ice and snow was performed in six Estonian and three Finnish lakes and in coastal waters of the Baltic Sea. Additionally properties and development of the ice cover in the Baltic Sea during winter 2002/03 were estimated relying on the data of the Moderate Resolution Imaging Spectroradiometer (MODIS), installed on the satellites Terra and Aqua. The surface albedo of the water bodies varied from about 95% for fresh snow to 19-22% for slush or the thin water layer on the ice. The vertically averaged diffuse attenuation coefficient of ice for brackish water Santala Bay exceeded considerably that for lakes. It was demonstrated that MODIS images give rather detailed information on ice conditions, which makes it possible to distinguish snow covered fast ice from new and melting ice, and also to detect the relative changes of surface albedo. Key words: wintertime water bodies, optical properties of ice cover, albedo, remote sensing of ice. [ABSTRACT FROM AUTHOR]