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.
Polar lakes respond quickly to climate-induced environmental changes. We studied the chemical limnological variability in 127 lakes and ponds from eight ice-free regions along the East Antarctic coastline, and compared repeat specific conductance measurements from lakes in the Larsemann Hills and Skarvsnes covering the periods 1987-2009 and 1997-2008, respectively. Specific conductance, the concentration of the major ions, pH and the concentration of the major nutrients underlie the variation in limnology between and within the regions. This limnological variability is probably related to differences in the time of deglaciation, lake origin and evolution, geology and geomorphology of the lake basins and their catchment areas, sub-regional climate patterns, the distance of the lakes and the lake districts to the ice sheet and the Southern Ocean, and the presence of particular biota in the lakes and their catchment areas. In regions where repeat surveys were available, inter-annual and inter-decadal variability in specific conductance was relatively large and most pronounced in the non-dilute lakes with a low lake depth to surface area ratio. We conclude that long-term specific conductance measurements in these lakes are complementary to snow accumulation data from ice cores, inexpensive, easy to obtain, and should thus be part of long-term limnological and biological monitoring programmes.
Abstract:This study aimed to use nutrients in lake inflows as proxies for assessing human impact and separating this from natural transformations of material in the soil active layer. Nutrients, conductivity and δ 18 O were monitored in surface and subsurface (using ceramic tipped piezometers) lake inflows during summer in near natural and human impacted catchments. The nutrient levels were highly variable but generally higher during the last weeks of the flow, in both subsurface waters and in human impacted catchments. Up to 2000 µgN l -1 subsurface dissolved inorganic nitrogen (DIN) was measured in human impacted catchments but only 315 µg N l -1 in natural catchments. Subsurface levels of dissolved reactive phosphorus (DRP) were up to 310 µgP l -1 in natural catchments and up to 108 µgP l -1 in human impacted catchments. The maximum levels of both DIN and DRP in surface inflows were much higher in human impacted than in natural catchments. Conductivity and δ 18 O data showed general enrichment of snowbank meltwater presumably through evaporation from the active layer. This combined with fluctuating nutrient levels in catchment waters indicated that soil brines and decaying organic matter of natural and human origin were possible sources for nutrients and other salts. Marked salinization and substantially increased DIN levels near the research stations indicated that lake waters were receiving nutrients generated by humans.
Abstract-The Ilumetsa impact craters were discovered in 1938 in the course of geological mapping.In the crater field area, the Middle Devonian bedrock consists of light-yellow weakly cemented siltstones and sandstones of the Givetian Burtnieki Regional Stage, which are overlain by a 1-2 m thick layer of reddish-brown loamy till. Pdrguhaud, the biggest crater, has a diameter of 75-80 m at the top of the uplifted rim and is 12.5 m deep. The zone of authochtonous breccias below the apparent crater extends to 30 m deep. The crater is partly filled with a thin layer of gyttja and peat up to 2 m thick. Radiocarbon ages of 6030 ? 100 (TA-3 10) and 5910 ? 100 (TA-725) years B.P. from the lowermost organic layer and palynological evidence suggest that the age of the impact was -6000 14C years B.P. The Sugavhaud crater has a diameter of 50 m at the top of the rim and is 4.5 m deep. Organic matter on the bottom of the crater is absent. As precise age determination of the Ilumetsa craters by direct dating methods has proved inconclusive, we proposed a method of geological correlation which is based on the occurrence of impact spherules in lake and bog sediments around the crater field. Radiocarbon dating of samples from a peat layer with glassy spherules of impact origin in the Meenikunno Bog, 6 km southwest of the Ilumetsa crater field, yielded the ages of 6542 2 50 (Tln-22 14) for the depth interval 5.6-5.7 m and 6697 ? 50 (Tln-23 16) years B.P. for the depth interval 5.7-5.8 m. These dates suggest that the Ilumetsa craters were formed -6600 years ago.
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