Intensity of autumnal gamogenesis in chydorid (Cladocera,Chydoridae) communities in southern Finland,with a focus on Alonella nana (Baird)

The aim of this study was to examine whether there was variation in the intensity of gamogenesis (sexual reproduction) in communities of chydorid cladocerans during the autumnal sexual reproduction period. The proportions of gamogenetic individuals (i.e., intensity) in the chydorid communities of seven lakes in southern Finland were determined in weekly samples throughout the autumn of 2005. The period of gamogenetic reproduction began very synchronously in the lakes as a response to climatic forcing and proportions of gamogenetic individuals progressively increased towards winter. However, wide variation was found in intensity among the communities. The high intensity probably was a response to some environmental stressors (e.g., invertebrate predation, crowding, competition, or changes in water chemistry) to ensure genetic variability and future populations. One common species, Alonella nana showed exceptional, dualistic, gamogenetic behavior, since in some communities it reproduced with high and in others with extremely low gamogenetic intensity. It is possible that in the former it responded to environmental stressors by exhibiting high intensity of gamogenesis, thus renewing its genotypes, while in the latter it succeeded primarily by parthenogenetic (asexual) reproduction, and was possibly perennial. The high gamogenetic intensity in A. nana was related to dystrophic and mesotrophic conditions, but it correlated positively only with water conductivity.