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To investigate the extent to which anthropogenic perturbations such as eutrophication and trace metal pollution (i.e., environmental stress sensu Odum 1985) influence the reproductive modes of cladoceran populations, we analyzed the abundance of subfossils of the chydorids Alonella nana (Baird, 1850) and Alona affinis (Leydig, 1860) in sediment cores from three Finnish lakes. Reconstruction of lakes' pollutant history and the biological response of chydorids indicate that in two of the lakes the proportion of individuals reproducing sexually increased with environmental stressors. More specifically, A. nana responded to eutrophication in Lake Hampträsk with greater production of ephippia, while A. affinis responded to aluminum pollution or acidification in Lake Pieni Majaslampi. In contrast, the reference lake, Lake Iso Lehmälampi, showed no radical changes in sexual reproduction over the twomillennium long sediment record. We conclude that chydorids may use sexual reproduction as a strategy for overcoming unexpected environmental stresses.
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