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We singled out spring (season with a water temperature of 4–15 °C) and autumn (15–4 °C), i.e. the seasons with the most variable meteorological conditions, in order to study the development of zooplankton as well as concurrent meteorological conditions (air and water temperature, ice conditions) and their relationships in these seasons. The aims of this study were (1) to review the spring zooplankton versus autumn zooplankton of a shallow polymictic lake; (2) to assess how much the zooplankton of the transition seasons, i.e. spring and autumn, is influenced by water temperature; (3) to clarify what factors are the main drivers of water temperature in such type of lakes. Proceeding from these aims, a long-term (1965–2014) study was conducted in the shallow (mean depth 2.8 m) polymictic Lake Võrtsjärv (Estonia). The main drivers of water temperature were air temperature and ice conditions. The water of the lake warmed up from 4 °C to 15 °C within 48 ± 2 days in spring and it cooled down from 15 °C to 4 °C during 57 ± 1.5 days in autumn. Both the air temperature in the territory of the studied lake and the water temperature in the lake increased while the duration of an ice cover on the lake decreased during the study period. The abundance of zooplankton in the seasons with the highly variable water temperature was also variable and largely dependent on the water temperature. In spring the effect of water temperature was greater than in autumn. Statistical analysis showed that when water temperature rose one degree in spring, the abundance of zooplankton increased by 27%, and when water temperature fell one degree in autumn, zooplankton abundance decreased by 9%. Zooplankton abundance was almost three times as high in spring (922 ind L) as in autumn (325 ind L) and was dominated by rotifers, small-bodied cladocerans, and juveniles of cyclopoid copepods. The domination of rotifers was more pronounced in spring (92%) than in autumn (70%). The share of cladocerans was negligible in the spring zooplankton (2%) but appreciable (24%) in the autumn zooplankton. The share of copepods in the total zooplankton abundance (6%) was modest and similar in both seasons. A shift (i.e. a marked increase in zooplankton abundance, switch from cold-water to warm-water species) in the abundance of spring zooplankton occurred in spring at a water temperature of about 10 °C (critical time window). A comparable but less conspicuous change (decrease in abundance, switch from warmwater species to cold-water species) was found at the same water temperature in autumn. During the 50 study years, the period with a mean water temperature of 10 °C shifted by 7 days to an earlier date in spring and by 6 days to a later date in autumn.

How are spring zooplankton and autumn zooplankton influenced by water temperature in a polymictic lake?