ALBERT

Notes: SUMMARY 1. Climate warming is now widely recognised as a major factor influencing ecological processes in terrestrial, marine and freshwater habitats. Here, we investigated how a recent period of warm springs and summers has affected the population dynamics of various cyclopoid copepods in a central European lake. We compared (i) the duration of the period when the species were present in the water column, and (ii) their annual peak density in a period dominated by cool summers (1980–91) and one dominated by warm summers (1992–99).2. The copepods under investigation were (i) Thermocyclops oithonoides, (ii) Mesocyclops leuckarti and (iii) Acanthocyclops robustus. These species differ in their thermal demand and seasonal phenology. Therefore, we hypothesised that enhanced summer warming would produce species-specific responses.3. The active phase of the copepods was usually prolonged both in spring and autumn. The earlier emergence of T. oithonoides (May in the warm years, July in the cool years) was probably related to high water temperature in late spring. The later onset of winter diapause in all species may have been coupled to raised temperature in late summer and autumn.4. The annual peak abundance of the two thermophiles M. leuckarti and T. oithonoides increased significantly in the warm period. In the latter case, the increase was probably because of the early start to population growth. In contrast, M. leuckarti probably responded primarily to mid-summer heat waves, in that its development time was likely to be short. We speculate that the increase in population size of both species resulted from the development of an additional generation (three instead of two cohorts per year). In contrast to these thermophiles, the coexisting A. robustus, which is adapted to a broader temperature range, did not respond noticeably to the warming trend.5. In general, the nature of these responses to summer warming varied substantially among species, and depended on the detailed seasonal patterning of the warming. Our findings thus support the hypotheses that single species are sensitive indicators of climate change, and that the seasonal timing of warming is crucial in the context of climate–ecosystem relationships.6. Moreover, our results add to the body of evidence that climate warming produces shifts in the seasonal phenology of aquatic and terrestrial organisms.