Occurrence and Interannual Abundance Variation of Glacial Relict Calanoids Limnocalanus macrurus and Eurytemora lacustris in Lithuanian Lakes

In Lithuania, permanent populations of glacial relict calanoids Limnocalanus macrurus and Eurytemora lacustris inhabit lakes of 25–48 and 38–60 m maximal depth, correspondingly. There are 45 lakes, which due to their morphometric characteristics may be considered to be potentially suitable for these calanoid species. For these lakes, there are historical records of the presence of L. macrurus in 13 lakes and that of E. lacustris in seven lakes. In the current survey, the glacial relict calanoid L. macrurus was found in 12 lakes, including one new locality, while another calanoid E. lacustris was present in five lakes. The habitat-specific interannual abundance variation in populations of L. macrurus was higher than that in E. lacustris suggesting a greater sensitivity of the former species to oxygen conditions in the lake hypolimnion. Although L. macrurus is more widespread throughout Lithuanian lakes in comparison to E. lacustris, the probability of its local population’s extinction in lakes is higher.