Galley Proof Production of resting stages is a common strat- egy of genome protection against strong selec- tive forces used by many organisms inhabiting environments that suffer cyclic deterioration (WILLIAMS 1996). In freshwater habitats of the temperate zone, production of resting stages is commonly associated with drastic changes in abiotic (e.g. over-freezing) or biotic (e.g. short- age of resources) conditions of the environment during winter. In some habitats however, dia- pause is also observed in summer. While “sum- mer diapause” of freshwater organisms inhabit- ing ephemeral habitats may have a clear-cut ex- planation of survival during temporal deteriora- tion of the physical conditions of the environ- ment (WILLIAMS 1996), the ultimate reasons for this seasonal phenomenon in permanent lakes appear more ambiguous. The few studies re- porting summer diapause of pelagic cladocer- ans indicate its possible association with the clear water phase (THRELKELD 1979, JANKOWS- KI 2003, CÁCERES & TESSIER 2004 a, b), a sea- sonal phenomenon occurring in late spring or early summer in many eutrophic lakes (SOM- MER et al. 1986). The clear water phase is a relatively short pe- riod (days to weeks) of high water transparency and low phytoplankton biomass resulting from high grazing rate of fast growing populations of large-body cladoceran filterers (most common- ly of the genus Daphnia). High grazing pres- sure of zooplankton may reduce standing crop of edible algae and promote development of inedible phytoplankton (SOMMER et al. 1986), which in turn may limit reproduction of herbiv- orous zooplankton (HULSMANN 2003). On the other hand, strong pressure of invertebrate predators (DE BERNARDI 1974, HOVENKAMP 1989) or of young of the year (YOY) planktiv- orous fish (MILLS & FORNEY 1983) that hatch synchronously in spring, when planktonic cladocerans have their peak of density, may greatly impact planktonic grazers. Fry of most freshwater fish feed readily on zooplankton in their early ontogenetic stages before they switch to some other food sources at older stages of development. In turn, dominant popu- lations of large-bodied cladocerans may be dec- imated or exterminated and further replaced by smaller, less vulnerable planktonic herbivores (SOMMER et al. 1986). Regardless of which, if any, of the mentioned reasons (lack of edible phytoplankton or high predation pressure) plays the key role in shap- ing this seasonal event, consequences for large bodied cladocerans are repetitively terminal. Consequently, large bodied cladocerans either disappear completely from the water column or remain there in low densities throughout sum- mer. From an individual perspective, it should be more profitable to produce resting eggs in advance or during the terminal season than count on unlikely survival in an active stage. Surprisingly, records of the “clear water phase diapause” of planktonic cladocerans in perma- nent lakes are much less common than evi- dence of their seasonal decline in abundance. Existing reports of summer diapause of Daph- nia in permanent lakes of the temperate zone concern D. pulicaria in North America (THRELKELD 1979, CÁCERES & TESSIER 2004 a, b), D. magna (LAMPERT 1991) and D. galeata (JANKOWSKI 2003) in Europe. However, ulti- mate reasons for this summer diapause were not the focus of these studies. Hereby, we report main results of a study for which the primary objective was to reveal the ultimate causes of the clear water phase dia- pause of Daphnia pulicaria observed in Lake Brome located in southwest Quebec, Canada. Verh. Internat. Verein. Limnol. 29 0000 – 0000 Stuttgart, February 2006 Ultimate reasons for summer diapause of Daphnia in a permanent lake Mirek S ´ lusarczyk, Bernadette Pinel-Alloul and Malorie Gelinas 0368-0770/06/0029-1449 $ 1.00 ©2006 E. Schweizerbart’sche Verlagsbuchhandlung, D-70176 Stuttgart