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In Lake Baikal, frontal zones are a factor forming the pelagic structure, including area limits of pelagic communities. The spacial distribution of Baikal micro-organisms and diatom algae is known to be influenced by the vertical circulation of Lake Baikal deep waters during the spring thermal bar. In June 2001, we studied the distribution of plankton invertebrates and ways in which it depends on physico-chemical and biological parameters of water in the vicinity of spring thermal bars. Samples were taken through June 11-13th along three transects, across the middle basin of Lake Baikal and the Selenga shallow waters, namely at the Boldakovo River, Cape Sukhinskiy and the Kukuy branches of the Selenga River. As a consequence of the lake possibly being warmed later than usual, and compare with a previous study in 1993, the upper layers with temperatures close to 4°C, were located near the shore where water depth was 14-100 m, from north to south. The littoral waters, warmed above 4°C and reaching 12.7°C near Cape Sukhinskiy, are known to differ from the open lake in hydrochemical indexes, raised number of bacterioplankton and the structure of algae coenoses (in particular, the presence of algae which indicate the spread of the Selenga River waters). An unusual vertical distribution of autotrophic picoplankton in terms of raised concentrations at the bottom, 60 m deep, occurred at the Cape Sukhinskiy transect within a thermal bar zone. This indicated the descent of upper waters rich in picoplankton. The zooplankton structure in the littoral also differed from that in more open waters where temperatures were below 4°C, mainly by its richer diversity increased by littoral inhabitants. The pelagic amphipod Macrohectopus branickii preferred cold waters less than 4°C. Crustaceans and rotifers were separated into different groups according to spacial distribution.

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An illustrated description of Diacyclops zhimulevi Sheveleva et Timoshkin, sp.n. (Crustacea, Cyclopoida: Cyclopidae) is provided. The new species is widely distributed along the southwestern coast of Lake Baikal, inhabiting sandy bottoms of the shallow littoral zone. A comparison is made with the previously studied and closely related species Diacyclops eulitoralis Alekseev et Arov, 1986 from the sandy interstitial environment of Lake Baikal. The new species differs from its most closely related species D. eulitoralis by a combination of characters: shorter, nearly square caudal furca (furcal index 1:1 in contrast to 1:3.5 in D. eulitoralis), a larger number of antennular segments, and a different segment pattern of the swimming legs: