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.

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.

During the study period

The influence of two winter periods with a different duration of the ice cover on Lake Peipsi (Estonia) on plankton and nutrient content was analysed. The winters of 2005 and 2006 were cold with ice duration of 140 days, whereas the winters of 2007 and 2008 were mild with about 50 and 15 days of ice duration, respectively. Total phosphorus (TP) concentration was lower, while silicon content and the nitrogen-phosphorus ratio (TN : TP) were markedly higher in the springs after the short winters of 2007 and 2008. The high Si concentration and TN : TP ratio persisted throughout the growing season of those years. Unicellular centric diatoms showed a sharp increase in April 2008, while the large filiform diatom Aulacoseira islandica dominated in the cool winters and after ice break-up. The high spring peak of diatoms was followed by their low biomass in summer. In May 2008, total zooplankton biomass, cladoceran biomass, and rotifer biomass were two times and that of copepods three times as high as in the Mays after the long-lasting ice cover. In the Junes after the mild winters the biomasses of total zooplankton and both crustacean groups (Cladocera, Copepoda) were about two times as high as the corresponding indicators for the Junes after the cool winters. The biomass of rotifers, on the contrary, was two times lower in the Junes after the warm winters because the numerous cold stenotherms Polyarthra dolichoptera and Synchaeta verrucosa had totally disappeared from zooplankton. The influence of ice duration on phytoplankton is most likely indirect, acting through nutrients, and on zooplankton direct, acting through water temperatures. The springs after warm winters related positively to zooplankters' mean weight, zooplankton-phytoplankton biomass ratio, and the timing of the clear water period.

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