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Compared to the well-studied open water of the "growing" season, under-ice conditions in lakes are characterized by low and rather constant temperature, slow water movements, limited light availability, and reduced exchange with the surrounding landscape. These conditions interact with ice-cover duration to shape microbial processes in temperate lakes and ultimately influence the phenology of community and ecosystem processes. We review the current knowledge on microorganisms in seasonally frozen lakes. Specifically, we highlight how under-ice conditions alter lake physics and the ways that this can affect the distribution and metabolism of auto- and heterotrophic microorganisms. We identify functional traits that we hypothesize are important for understanding under-ice dynamics and discuss how these traits influence species interactions. As ice coverage duration has already been seen to reduce as air temperatures have warmed, the dynamics of the under-ice microbiome are important for understanding and predicting the dynamics and functioning of seasonally frozen lakes in the near future. [ABSTRACT FROM AUTHOR]

The aim was to disclose the seasonality in the internal seiches during summer stratification in 2010 within the small, 3 km long, ellipse-shaped, temperate Lake Arendsee (Germany) with simple morphometry. We used observations of temperatures from two thermistor chains (09 April to 24 September), conductivity and temperature profiles, and wind speeds and directions (January to December). To analyze the seasonality in the periods, appearance and modal structure of the seiches, and the wind forcing, we applied spectral analysis on monthly time series of isotherms and wind speeds, and wavelet analysis on 6 month long time series of integrated potential energy and wind speeds. To determine the effect of earth's rotation on seiches, we applied two analytical models based on three-layer density stratification. Earth's rotation affected internal seiches significantly even in a small lake. That was shown by the models and the Burger number being < 1 during stratification. Vertical mode 1 Kelvin-type seiches dominated the internal seiche weather in Lake Arendsee, with their periods changing because of seasonal variations in stratification. Weak, free seiches of vertical mode 2 were amplified during three episodes: (1) in mid-summer, when their periods coincided with the inertial period at the lake's latitude, (2) in late spring, and (3) in autumn, both when their periods approached 24 h. The amplifications were caused by resonance between free seiches and wind-forced oscillations at diurnal and inertial periods. Such short-term resonance events, associated with enhanced mixing, are inherent for the seasonal stratification cycle in the most temperate lakes. [ABSTRACT FROM AUTHOR]

A field and theoretical study of the mean circulation patterns, gravity currents, and heat fluxes in a mid-latitude, ice-covered lake is presented. Thermistor chains captured the lake-wide pressure gradients and thermal regime, acoustic instruments measured bottom and interior velocities, and a sediment temperature probe measured sediment temperatures and heat fluxes. Typical Rossby numbers showed that the mean circulation was geostrophic, in the form of either an anti-cyclonic or a cyclonic basin-scale gyre. Each gyre reached a steady state after ~ 10-15 d, circulating at a maximum velocity of ~ 1 mm s-1. The gyres persisted throughout most of the winter and generally decayed with time. The cyclonic gyre was generated by the lateral temperature gradients created by sediment heat release. It was not possible to determine the driver of the anti-cyclonic gyre. Because most ice-covered lakes are controlled by the Earth's rotation, anti-cyclonic and cyclonic circulation is postulated as being the most common mean interior flow pattern. The horizontal heat fluxes were of the same order as the vertical heat fluxes, which is consistent with the findings of previous research. The sediment heat flux decayed with time, stabilizing after ~ 60 d. A scaling analysis of down-sloping advection with dynamic, seasonal-scale temperatures showed that a warmer lake and an increase in the length-to-depth geometric aspect ratio (both within a given lake) and an increase in lake size all increased advection time scales, and therefore circulation time scales, near the boundaries. [ABSTRACT FROM AUTHOR]

Using the carcass sinking rate and density determined in laboratory for several freshwater zooplankton species, we developed a model of zooplankton carcass sinking as affected by turbulence and stratification. The model was subsequently used to estimate the residence time of zooplankton carcasses in the water column of Lake Stechlin, a typical temperate dimictic lake in northeastern Germany. The residence time varied among the different species and was strongly affected by thermal stratification. At the peak of summer stratification, the carcasses stayed up to 5d in the 70m-deep water column before reaching the lake bottom. Residence time was long enough that zooplankton carcasses could serve as an important matter and energy source for bacteria in the lake's pelagic zone and hence have the potential to significantly affect aquatic carbon and nutrient cycling. The proposed model of sinking rates, based on physically sound relationships, can be easily applied to other passively sinking particles, and be integrated into large ecosystem models. [ABSTRACT FROM AUTHOR]