Species diversity and spatial distribution of the summer rotifer assemblages in Lake Ladoga

The rotifer assemblages of Lake Ladoga were studied in samples of the 0–10 m water layer taken at 60 sampling stations in different parts of the lake in August 1994 and 1995. In general, the species composition of the pelagial assemblages was rather similar throughout the lake. However, the dominant species differed markedly between the northern part, the central area and the Volkhov Bay. The highest species diversity (Shannon index H ́ = 3.03) and lowest wet weight biomass of rotifers (0.015 mg WW l) were recorded in the Volkhov Bay, maximum biomass (2.53 mg WW l) in the northern pelagic region, and lowest species diversity (H ́ = 0.25) in the northern archipelago. The filter-feeding microphagous rotifers Conochilus unicornis and Keratella cochlearis dominated most of the microzooplankton communities in the northern parts of the lake, while grasping phytophagous species of the genus Polyarthra predominated in the central and south-eastern parts. Species diversity, spatial distribution and the role of rotifers in the zooplankton community structure are discussed in relation to the ecological zoning of Lake Ladoga.