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.

With the implementation of the EU Water Framework Directive (WFD), the member states have to classify the ecological status of surface waters following standardised procedures. It was a matter of some surprise to lake ecologists that zooplankton were not included as a biological quality element (BQE) despite their being considered to be an important and integrated component of the pelagic food web. To the best of our knowledge, the decision of omitting zooplankton is not wise, and it has resulted in the withdrawal of zooplankton from many so-far-solid monitoring programmes. Using examples from particularly Danish, Estonian, and the UK lakes, we show that zooplankton (sampled from the water and the sediment) have a strong indicator value, which cannot be covered by sampling fish and phytoplankton without a very comprehensive and costly effort. When selecting the right metrics, zooplankton are cost-efficient indicators of the trophic state and ecological quality of lakes. Moreover, they are important indicators of the success/ failure of measures taken to bring the lakes to at least good ecological status. Therefore, we strongly recommend the EU to include zooplankton as a central BQE in the WFD assessments, and undertake similar regional calibration exercises to obtain relevant and robust metrics also for zooplankton as is being done at present in the cases of fish, phytoplankton, macrophytes and benthic invertebrates.

On the basis of long-term (1964-2011)

In the eutrophic Lake Võrtsjärv (Central Estonia, area 270 km 2 , mean depth 2.8 m) rotifers form ca. 90% of total abundance and 80% of biomass in winter zooplankton community. The winter rotifer assemblage was dominated by Polyarthra dolichoptera, both in abundance and in biomass. Synchaeta verrucosa and Keratella quadrata were the subdominants. Thus, winter rotifer community had low diversity and high dominance of a few species. This pattern probably refers to the period of extreme environmental conditions where the rotifer assemblage is composed of few well-adapted species, and the low diversity here was not indicating instability of community structure, but the scarcity of suitable niches. These community structure indices indicate that the winter rotifer assemblage of L. Võrtsjärv was very similar to autumn assemblage, but very different from the spring one. In winter, small raptors were the most important functional group. The second place is occupied by larger raptors. Marginal role of fine particle sedimentators, absence of suckers and high proportion of large raptors were contrasting features of the winter trophic structure in comparison with the other seasons. Changes have taken place in the winter rotifer assemblage in L. Võrtsjärv in 1990Võrtsjärv in -2007 Against the background of diminishing rotifer abundance, the dominant species has become even more prevalent, and the diversity of the winter rotifer assemblage has decreased. Shifts in the community trophic structure were also observed.

The grazing impact of different sized zooplankton on 'edible' and total phytoplankton biomass and primary production was measured in L. Võ rtsjä rv during a seasonal study in 1998 and 2000. The organisms of 48-100 lm size class, composed of ciliates and rotifers, contributed significantly to the total grazing of zooplankton community throughout the study period (average 68%). The average daily filtering and grazing rate of the whole zooplankton community (micro-and macro-zooplankton) remained low, corresponding to a filtration of 44% of the water volume, 4% of the total phytoplankton biomass and 29% of primary production. However, a strong grazing pressure on small-sized phytoplankton (<30 lm) was estimated in most of the study period (average 44% d -1 ). Among size classes of 'edible' phytoplankton, the size range 5-15 lm was the most important algal food for the dominant zooplankton grazers (herbivorous ciliates, Polyarthra spp., Chydorus sphaericus and Daphnia cucullata) in L. Võ rtsjä rv.