Winter conditions are rapidly changing in temperate ecosystems, particularly for those that experience periods of snow and ice cover. Relatively little is known of winter ecology in these systems, due to a historical research focus on summer 'growing seasons'. We executed the first global quantitative synthesis on under-ice lake ecology, including 36 abiotic and biotic variables from 42 research groups and 101 lakes, examining seasonal differences and connections as well as how seasonal differences vary with geophysical factors. Plankton were more abundant under ice than expected; mean winter values were 43.2% of summer values for chlorophyll a, 15.8% of summer phytoplankton biovolume and 25.3% of summer zooplankton density. Dissolved nitrogen concentrations were typically higher during winter, and these differences were exaggerated in smaller lakes. Lake size also influenced winter-summer patterns for dissolved organic carbon (DOC), with higher winter DOC in smaller lakes. At coarse levels of taxonomic aggregation, phytoplankton and zooplankton community composition showed few systematic differences between seasons, although literature suggests that seasonal differences are frequently lake-specific, species-specific, or occur at the level of functional group. Within the subset of lakes that had longer time series, winter influenced the subsequent summer for some nutrient variables and zooplankton biomass.

Remote lakes are usually unaffected by direct human influence, yet they receive inputs of atmospheric pollutants, dust, and other aerosols, both inorganic and organic. In remote, alpine lakes, these atmospheric inputs may influence the pool of dissolved organic matter, a critical constituent for the biogeochemical functioning of aquatic ecosystems. Here, to assess this influence, we evaluate factors related to aerosol deposition, climate, catchment properties, and microbial constituents in a global dataset of 86 alpine and polar lakes. We show significant latitudinal trends in dissolved organic matter quantity and quality, and uncover new evidence that this geographic pattern is influenced by dust deposition, flux of incident ultraviolet radiation, and bacterial processing. Our results suggest that changes in land use and climate that result in increasing dust flux, ultraviolet radiation, and air temperature may act to shift the optical quality of dissolved organic matter in clear, alpine lakes.

The seasonal distributions of phytoplankton biovolume and chlorophyll a content were monitored for 14 months in a deep oligotrophic, high mountain lake (Redó, Pyrenees). An allometric relationship of chlorophyll with biovolume was found throughout the period studied, with a correlation coefficient of 0.66. However, the relationship changed with season and the taxonomic composition of the phytoplankton. Both parameters showed a similar seasonal pattern, but differences in space and time were observed. The chlorophyll maximum was recorded deeper and later than that of phytoplankton biovolume. While the biovolume maximum was related to an improvement in conditions for growth (nutrient input during column mixing periods), and reflected an increase in biomass, the chlorophyll maximum was related to changes in cell pigment content, and to spatial or successional trends in species dominance. Flagellated chrysophytes predominated at the chlorophyll maxima. Chlorophyll content per unit of phytoplankton biovolume fluctuated greatly throughout the year, depending on light intensity, temperature and phytoplankton composition. Of the main groups of phytoplankton in the lake, the dinoflagellates, which dominated the summer epilimnion phytoplankton community, recorded the lowest pigment content per biovolume (which is consistent with their size). Higher chlorophyll contents per biovolume were found in the deep hypolimnion and during the winter cover period associated with small cells such as some species of chlorococcales chlorophytes. When flagellated chrysophytes were predominant, a broad range of chlorophyll values per biovolume was found and there was no significant correlation between the two biomass indices. These findings reaffirm the need to treat phytoplankton biomass estimates with caution, in particular when conducting primary production studies. While our results show that changes in chlorophyll content per cell occur as a photoacclimation response along a vertical profile, they also point out a component of the successional trends which appear in a phytoplankton growth phase in a lake.

We examined the potential limitation of bacterial growth by temperature and nutrients in a eutrophic lake. Dilution cultures from winter and summer were incubated at both high (>20°C) and low (4°C) temperatures and enriched with various combinations of organic carbon (C), inorganic nitrogen (N), and inorganic phosphorus (P). Bacterial abundance, (3)H-thymidine incorporation, and (3)H-leucine incorporation were measured over the growth cycle. For both winter and summer assemblages, low temperature limited growth even when resources (C, N, and P) were added. When temperature was adequate, bacterial growth in dilution cultures was co-limited by C, N, and P Additions of either C, P, or N and P alone provide little or only modest stimulation of growth, suggesting that under in situ conditions both nutrients and organic carbon limit bacterial growth. Our results provide little evidence of seasonal adaptation to low temperatures for bacterial communities in temperate lakes. Instead, bacterial growth appears to be temperature limited during winter and resource limited during summer. We propose that, in general, bacterial growth rates are temperature dependent up to a threshold, but that the patterns of change across temperature gradients are resource dependent, such that temperature has little effect on growth in resource-rich environments but a strong effect in resource-poor environments.

It has recently been shown that a rich community of microorganisms inhabits the slush layers of the winter cover of high mountain lakes. In this study, temporal changes in species assemblages and environmental conditions in the ice and snow cover of Lake Redó in the Pyrenees (Spain) are presented. The winter cover was a highly dynamic environment, in which major changes occurred in physical structure and chemical characteristics. The biomass and species composition of the microbial groups present (autotrophic and heterotrophic flagellates, ciliates, and bacteria) reflected these physicochemical alterations. After an initial phase, during which the ice sheet formed and the first snows accumulated with little or no development of slush layers, there were two stages of microbial assemblages, which coincided with the two main phases in the physical change of the cover: the growth period (from January to mid-April) and the ablation period (from mid-April to ice-out in June). Initially, microbial biomass originated from inputs of phytoplankton-rich (5.9 g chlorophyll a liter Ϫ1 ) surface lake water that flooded the cover as a result of the hydrostatic adjustment induced by the progressive accumulation of snow on top of the ice sheet. This was followed by a differential growth of mixotrophic or heterotrophic flagellated species, such as Ochromonas sp., Cryptomonas spp., Monosiga ovata and Oikomonas termo, which peaked at different times. Species depended primarily on bacterivory for their growth, since during this period, light did not reach the slush layers. By April, the transition from cover growth to ablation was marked by a reduction in biomass and in the number of species. During the ablation period, the rate of change of the cover was greater, and microbial assemblages were characterized by the vertical segregation and by the appearance of new species, some of which were typically nonplanktonic. During that time, the cover was highly influenced by the large amount of water from the melting of the snowpack within the catchment. This water likely provided chemicals and organisms, while increased light availability allowed for the growth of algae (Pteromonas sp. and other volvocales) and of the associated food web in the upper slush. In deeper layers, however, a bacteria-based food web provided prey for large ciliates (Urosoma sp., Dileptus sp., and Lacrymaria sp.), probably carried from littoral or watershed soils.High mountain lakes are covered by ice and snow for several months of the year. Such covers are complex physical structures (Adams and Allan 1987). When the lake freezes, a sheet of clear ice, so-called ''black ice'' forms at the lake surface, and salts are efficiently exsolved, which, in turn, increases salt concentration in the water beneath the ice. Snow subsequently accumulates on top of the ice, and the stress produced causes the lake water to rise and to flood AcknowledgmentsWe thank S. Pla and B. Weitzmann for fieldwork support and M. Ventura and M. Krö␤bacher for assistance in computer an...