Colonization, Succession and Environmental Conditions of the Macrozoobenthos in a Regulated, Polyhumic Reservoir, Western Finland

The colonization and succession of the macrozoobenthos in a polyhumic, winter‐regulated reservoir, Lake Kyrkösjärvi in Western Finland (62° 45′N, 22°48′E, A = 6.4 km2 at summer HW, zmean = 2.5 m and zmax = 7 m) was studied from its filling in 1981 until 1989 (in 1981–83, 1986 and 1989). The zoobenthos was sampled over the whole reservoir bottom using qualitative and quantitative methods during three open‐water seasons and winter samples were taken in the regulated and submerged areas during the maximum draw down (2 m) in March‐April. Colonization during the first summer revealed two phases. The first phase featured dominance of actively swimming and adult‐dispersing taxa. The second phase was characterized by a mass occurence (high abundance and biomass) of chironomids (Chironomus, Glyptotendipes) and the isopod Asellus aquaticus. Asellus originated in a small bog pond in the northern outlet basin, and it was the dominant zoobenthic species during the study period. The species of Chironomus, most of them pool inhabitants of natural waters, succeeded each other. In the later study years, when the zoobenthos at the sites was determined more by habitat (spatially) than by the succession (temporally), the reservoir bottom could be divided into three areas: a) deep areas, ≧3 m, with an increasing occurrence of lacustrine species (eutrophic or dystrophic), b) shallower organic bottoms, ≦ 2 m, with many pool‐inhabiting and eurytopic species, and c) some eroded areas in the shallow littoral, ≦ 0.5 m, where lacustrine species became common. Water‐level regulation in winter killed only a small proportion of the fauna in the organic‐rich peatland and forest bottom areas representing the majority of the regulated zone. Hence, the regulated areas had almost everywhere high abundance and biomass values throughout the year, deviating from the results found by other authors in strongly regulated lakes, which have maximum values just below the draw down limit.