Timing of sexual reproduction in chydorid cladocerans (Anomopoda, Chydoridae) from nine lakes in southern Finland

INTRODUCTION Cladocerans (Crustacea) have two strategies of reproduction. During most of their active period, which occurs in the ice-free season in northern temperate and subarctic climates, they reproduce asexually (parthenogenetically). Sexual reproduction, or gamogenesis, usually begins towards the end of the active period and leads to hatching of males and gamogenetic females, which results in fertilized resting eggs. Resting eggs are very tolerant of extreme environmental conditions, such as freezing and drying, and ensure the survival of cladoceran populations during the winter, from which new asexually reproducing females hatch in spring. Much of the research on cladoceran sexual reproduction has focused on the genus Daphnia (e.g. Stross & Hill, 1965, 1968; Kleiven et al., 1992; Pijanowska & Stolpe, 1996; Slusarczyk, 1995, 2001), while the patterns of sexual reproduction in the family Chydoridae (chydorids) have not been widely examined. It is generally known that sexual reproduction in chydorids also occurs in autumn in central and northern Europe (e.g. Poulsen, 1940; Maemets, 1961; Flossner, 1964; Green, 1966; Frey, 1982; Koksvik, 1995), probably triggered by declining temperatures and shortening photoperiods (Shan, 1974; Frey, 1982). Nevertheless, the mechanisms that induce gamogenetic reproduction in chydorids have not yet been entirely clarified. Frey (1982) generalized the life cycles of chydorids as predominantly monocyclic with autumnal gamogenetic periods, with some species having variable and insignificant earlier periods of gamogenesis. Berg (1929) and Flossner (1964) stated that some chydorid communities on the European mainland also have sexual individuals present during the summer months; i.e. they are dicyclic. Some chydorid species are dicyclic as far north as in Denmark (Raen, 1995) and Estonia (Maemets, 1961). There is also one record of summer gamogenesis from southern Finland. Jarnefelt (1956) recorded gamogenetic females ofAlonella excisa (Fischer) from Lake Tuusulanjarvi in early June during a single year. The aim of the present study was to examine the patterns of sexual reproduction in chydorids from several lakes in southern Finland. Our focus was on the timing of sexual reproduction among chydorid populations, i.e. does summer gamogenesis occur in most species and are induction and incidence of autumnal gamogenesis synchronous among populations? Nine lakes in southern Finland were monitored weekly for living chydorids and their sexual reproduction during the open-water season of 2005. Most of the lakes chosen for the study differed in their physical, chemical, and ecological conditions, but were situated in the same climate region. SITES The nine study lakes are located in southern Finland, near Helsinki (Fig. 1). The limnological variables of the lakes measured during three seasons are presented in Table 1. The mean annual air temperature in the area is approximately 4.5 [degrees]C, the mean July air temperature 17[degrees]C, and the mean January air temperature -6[degrees]C (Helminen, 1987). The open-water season lasts about 7 months, from early May to late November (Kuusisto, 1986), depending on yearly variation in spring and autumn temperatures. Lakes Hauklampi (60[degrees] 18' N, 24[degrees]36' E, 76.7 m above sea level, area 2.7 ha), Iso Majaslampi (60[degrees]19' N, 24[degrees]36' E, 92.7 m a.s.l., area 6.3 ha), and Pieni Majaslampi (60[degrees]19.3' N, 24[degrees]35.7' E, 97.3 m a.s.l., area about 1 ha) are located close to one another. They are acidic and oligotrophic lakes whose catchments are small and characterized by patches of mire, bedrock outcrops, and pine (Pines) forests. The shores are inhabited by sedges and Sphagnum mosses, and the aquatic macrophyte zone consists only of European white waterlily (Nymphaea alba). The lakes were severely acidified during the 1980s as a result of acid deposition, but experienced a recovery process during the 1990s. …