Winter has traditionally been considered as an ecologically insignificant season and, together with technical difficulties, this has led winter limnology to lag behind summer limnology. Recently, rapidly expanding interest in climate warming has increased water research in winter. It has also become clear that neither winter conditions of lakes nor underice communities are as static as often supposed. Although interannual differences in water temperature are small, close to the maximum density temperature, they may have profound effect on under-ice hydrodynamics. Thus, stochastic variations in weather, particularly those preceding the time of freezing and ice melting, may have important consequences for hydrodynamics which then affect the distributions and conditions of microorganisms and probably further to higher trophic levels. Even fish distributions can be dictated by under-ice conditions and their activities as well as behavior can sometimes approach those in summer. Life in freshwater ice is one of the least studied aspects of winter limnology and recent studies suggest that a thorough evaluation is needed. Altogether there are strengthening signs that winter should be considered as an integral part in the functioning and dynamics of lakes affecting quantitative and qualitative characteristics of aquatic communities in summer. There are great prospects that more thorough understanding of the prevailing limnological conditions in winter will improve our understanding of lake ecosystems in their entirety, and there is no doubt that such an approach requires multidisciplinary and long-term studies at different spatial scales.

The Eurasian spiny waterflea (Bythotrephes longimanus) is a predacious zooplankter that has increased its range in Europe and is rapidly invading inland water-bodies throughout North America's Great Lakes region. To examine the genetics of these invasions, we isolated five microsatellite DNA loci with between 5 and 19 alleles per locus. We sampled three populations where B. longimanus has been historically present (Switzerland, Italy, and Finland) as well as an introduced European population (the Netherlands) and three North American populations (Lakes Erie, Superior, Shebandowan). Consistent with a bottleneck during colonization (i.e. founder effect), average heterozygosities of the four European populations ranged from 0.310 to 0.599, and were higher than that of three North American populations (0.151-0.220). Pairwise F(ST) estimates among North American populations (0.002-0.063) were not significantly different from zero and were much lower than among European populations (0.208-0.474). This is consistent with a scenario of high gene flow among North American populations relative to that of European ones. Contrary to an invasion bottleneck, however, Erie and Superior populations contained similar numbers of rare alleles as European populations. Assignment tests identified several migrant genotypes in all introduced populations (the Netherlands, Erie, Superior, Shebandowan), but rarely in native ones (Switzerland, Italy and Finland). A large number of genotypes from North America were assigned to our Italian population suggesting a second, previously unidentified, invasion source somewhere in the region of northern Italy. Together, our results support an invasion bottleneck for North American populations that has been largely offset by gene flow from multiple native sources, as well as gene flow among introduced populations.

Bythotrephes Leydig is a predatory, onychopod cladoceran native to Eurasia that typically inhabits oligo-and mesotrophic water bodies of the Palaearctic region. It recently invaded 70 North American lakes, prompting a re-evaluation of the taxonomic status, global distribution, and determinants of local occurrence and abundance.

Bycatch mitigation is currently the most important issue for conservation of the Critically Endangered Saimaa ringed seal Phoca hispida saimensis found exclusively in Lake Saimaa, Finland. Two types of fishing restrictions have been established in order to reduce mortality: a ban on gillnet fishing during the most critical season (spring) to enhance pup survival and a yearround ban on the types of fishing gear most likely to cause seal mortality (e.g. certain specified fish traps, hooks baited with fish, strong-mesh gillnets, multifilament nets and trammel nets). The spatial and temporal coverage of these restrictions has not always been based entirely on the ecology and movements of the seals. The current distribution area of the population is not fully known, and this hampers the assessment of conservation needs. In this study, adult Saimaa ringed seals were tracked with GPS phone tags to estimate home ranges and movements. In addition, pup movement data from VHF telemetry studies were used to determine current breeding areas and nursing sites. The mean home ranges of both adults and pups were around 90 km 2. The current distribution area of the seals covers about 70% of the surface area of Lake Saimaa (4400 km 2), and the breeding area covers about 51%. The current springtime fishing restriction covers 63% of the estimated distribution area, and the restriction on fishing gear types covers 55%. This new information on the spatial ecology of Saimaa ringed seals should be used to improve efforts to conserve the species.

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