Diversity, abundance, and life histories of littoral chydorids (Cladocera: Chydoridae) in a subarctic European lake

Littoral chydorids were sampled with a bottom sledge in Takvatn, a 15 km2 north Norwegian oligotrophic lake with poor vegetation. Three out of eight recorded species of chydorids were common, with abundance minima in late summer and maxima in autumn. Eurycercus lamellatus (O.F. Müller, 1776) and Acroperus harpae (Baird, 1835) were monocyclic, whereas Chydorus sphaericus (O.F. Müller, 1776) was seemingly acyclic. Females of E. lamellatus appeared in early June and grew to maximum sizes of 2–2.5 mm in early August. Parthenogenesis started in late July and two summer generations could be distinguished by size distributions. Gamogenesis took place in September. Acroperus harpae females appeared in early June and grew to maximum sizes of around 0.7 mm in early August. Parthenogenesis started in early July and gamogenesis took place in September and October. Parthenogenetic females of C. sphaericus appeared already at ice-break and had constant sizes of around 0.4 mm through the entire season. Males or ephippial females were not observed. The acyclic life history may be an adaptation to the challenging environment of this subarctic lake. Summer generations could not be distinguished in the two small-sized species, but both populations reached high abundances in autumn. The abundance of E. lamellatus increased from August, but then decreased, possibly as a result of fish predation. We argue that the present results are typical for large oligotrophic lakes in subarctic Europe