Diaphanosoma mongolianum Ueno, 1938 (Cladocera: Sididae) in Lakes of Yaroslavl Oblast (Russia)

Findings of the cladoceran Diaphanosoma mongolianum have been recorded in lakes Pleshcheevo (since 2004) and Nero (since 2007) in Yaroslavl oblast at the northern boundary of its range. Similar species D. brachyurum, previously described in both lakes, is found in insignificant numbers. In 2004–2015, the seasonal abundance, spatial distribution, and diel vertical migration of D. mongolianum were studied in deep mesotrophic Lake Pleshcheevo. The species was found in the plankton from early June until October, with the maximum abundance in July–August. The late appearance of males and gamogenetic females (September) is characteristic of its population. D. mongolianum was the most abundant in the epilimnion of the pelagic zone, where it performed diel vertical migrations, concentrating in the surface layer (0–2 m) at night and at a depth of 2–6 m during the day.