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Theoretical pelagic primary production of phytoplankton and benthic primary production of periphyton were modelled for two small lakes in Estonia (Northeast Europe). Although located only 500 m apart, the water colour and light attenuation of these two lakes differed markedly. The Secchi depth (SD) in the clear‐water lake was 4.5 m and only 0.47 m in the dark‐water lake. The total phosphorus (TP) concentrations were, respectively, 15 μg/l and 28 μg/l. An empirical model whose inputs were morphometric, light conditions and dissolved organic carbon parameters obtained from in situ measurements was employed for the present study. The model calculated primary production with a time‐step of 10 min, and a spatial resolution of 10 cm, from sunrise to sunset and from lake surface to lake bottom. The primary production of periphyton and phytoplankton was almost equal in the clear lake, whereas only phytoplankton contributed to whole‐lake primary production in the dark lake because of the stronger light attenuation in the water column. The results of the present study indicated the depth‐distribution profiles differed dramatically between the two lakes. The clear lake had a deep, U‐shaped curve, with the productive layer reaching considerable depth soon after sunrise and maintaining a similar profile throughout the light hours. In contrast, the dark lake production declined rapidly with increasing depth, whereas the profile changed over the day reaching the greatest depth at noon. [ABSTRACT FROM AUTHOR]

Lake ice cover is described by its thickness, temperature, stratigraphy and overlying snow layer. When the ratio of ice thickness to lake size is above ∼10−5, the ice cover is stable; otherwise, mechanical forcing breaks the ice cover, and ice drifting takes place with lead-opening and ridging. This transition enables a convenient distinction to be made between small and large lakes. The evolution of the ice cover on small lakes is solved by a wholly thermodynamic model, but a coupled mechanical–thermodynamic model is needed for large lakes. The latter indicates a wide distribution of ice thickness, and frazil ice may be formed in openings. Ecological conditions in large lakes differ markedly from those in small lakes because vertical mixing and oxygen renewal may take place during the ice season, and the euphotic zone penetrates well into the water column in thin ice regions. Mesoscale sea ice models are applicable to large lakes with only minor tuning of the key parameters. These model systems are presented and analysed using Lake Peipsi as an example. As the climate changes, the transition size between small and large lake ice cover will change. [ABSTRACT FROM AUTHOR]

During a yearlong observation, settling fluxes of seston in the epilimnion and hypolimnion of Lake Viitna Linajärv varied from 230 to 1400 mg m.2 d.1. Most of the seston consisted of particulate organic matter (POM). The POM content in the dry matter of surface sediments sampled from the entire lake floor was on average over 50%. Seasonal changes in the distribution of chlorophyll a in the lake and the content of POM reveal the important role of fresh planktonic particles in the deposition. A strong correlation between POM and particulate inorganic matter in the settling matter during stratification indicates flocculent settling of the mineral particles. Resuspension from the littoral zone results in the focusing of sediment into the deep basin of the lake. [ABSTRACT FROM AUTHOR]

In small lakes (<10 ha) the process of sedimentation is not influenced very strongly by wind and wave activity and spatio-temporal variations in the composition of sediments are related with the topography of the current basin. Investigations on surface sediments (LOl, grain-size composition) in two small Estonian lakes - Lake Viitna Linajärv and Lake Väike Juusa - showed that up to 50% of the lithological composition of sediment is determined by the slope inclination, distance to the shore, and water depth. A composite dimensionless parameter (CP) developed enables to use the same approach to the description of the location of the site in lakes of different size, bottom topography, and hypsographic curve. Using the CP value it is possible to estimate the location of the sampling site in either an accumulation area (A area) or erosion-transportation area (E + T area) with A < CP = 2.5 ± 0.5 < E + T. [ABSTRACT FROM AUTHOR]