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.

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Abstract:The chronostratigraphy of the sedimentary record of Limnopolar Lake, located on Byers Peninsula (Livingston Island, South Shetland Islands, Maritime Antarctica), is described based on radionuclides and radiocarbon age dating. The oldest moss macrofossil age was 6700 ± 50 yr BP (7510 ± 80 cal yr BP) from which the age/depth model estimates a basal age for the sedimentary record of c. 8300 cal yr BP, suggesting an earlier deglaciation of Byers Peninsula than reported in previous studies. Lithological units and other stratigraphic zones are described throughout the sediment core, showing different mineralogical composition and a fine alternation of clays and silty clays and moss layers of Drepanocladus longifolius. Based on magnetic susceptibility analyses, a number of probable primary and reworked tephra layers were identified, seven of them confirmed by SEM observations, and most of them in agreement with the regional tephrachronology stratigraphy for the north-west Antarctic Peninsula. Sedimentation rates showed no significant changes during the last 5000 years with the exception of an abrupt event that took place around 5400 cal yr BP, which implied the sedimentation of c. 30 cm of clays in a very short time, probably related to a period of glacial re-advance that caused abrupt changes in geomorphological processes in the catchment.

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Lake Cimera (Lat. 40°15'50'' N; Long. 05°18'15" W, Zmax = 9.4m, A = 4.5 ha, 2140 m a.s.l,) is one of the most remote and unpolluted high mountain lakes on the Sierra de Gredos (Central Spain). Intrannual and interannual variability in maximum water temperature and winter oxygen depletion can be related to climate variability (mainly air temperature), through controlling ice cover length. The extent of the oxygen depletion during ice cover period, which is related to this ice cover length, is a key factor controlling the relative abundance of chironomid (Diptera: Insecta) taxa, especially the low oxygen content adapted Chironomus sp. In this way, we have found a high negative correlation between the relative abundance of Chironomus head capsules in the sediment and the reconstructed air temperature in the last 200 years (n = 20, r = -0.75, p <0.001). The interpretation of such relationship throughout the fossil chironomid assemblage points to a recent warming (since ca mid 1980s) in Lake Cimera. The ecological interpretation of other taxa also supports this view. When applying to fossil chironomids of Lake Cimera the transfer functions developed to reconstruct summer past temperatures in the Alps, it is also well correlated with reconstructed air temperatures (n = 20, r = 0.45, p <0.01), especially when only the most accurate dating levels (top of the core, ca 75 years) are taken into account (n = 13, r = 0.75, p <0.01). However, 1) the linear regressions of both models show significantly different slopes, and 2) chironomid reconstruction underestimates in ca. 3 ºC air reconstruction. The later is probably because the fossil chironomid model has been developed for a different geographical region. Nevertheless, both models provide an independent line of evidence of a recent warming (since ca mid 1980s) in Lake Cimera. Our data also supports the use of chironomids head capsules as an effective tool to infer past temperatures