New records of the rare glacial relict Eurytemora lacustris (Poppe 1887) (Copepoda; Calanoida) in atypical lake habitats of northern Germany

During monitoring investigations of lakes in Schleswig-Holstein (northern Germany) in 2000 and 2008, the calanoid copepod Eurytemora lacustris (POPPE 1887) was found in three lakes of the Ratzeburger Lake complex: the Kleine Küchensee, the Große Küchensee and the Große Ratzeburger See. The species has a broad geographic distribution but has become rare and endangered by eutrophication and global change. The lakes are mesotrophic (Gr. Ratzeburger See) and eutrophic (Kl. Küchensee, Gr. Küchensee) with cool (<10 °C) hypolimnia; they have surface areas of 12.6, 0.2 and 1.8 km2 and a maximum depth of 24.4, 12.8 and 14.7 m, respectively. All three lakes are shallower than lakes reported as typical habitats for E. lacustris. Oxygen conditions in the hypolimnion were different in the two years of investigation. In 2000, the oxygen concentration in summer (July) was zero from 13 m downwards in the Gr. Ratzeburger See and zero from 7 m downwards in the Kl. and Gr. Küchensee. In the year 2008 the hypolimnia of both lakes were well oxygenated in summer (>1 mg O2 L-1). Food and physical conditions seem to be favourable enough to permit survival and reproduction of the species at least in spring. It remains unclear, however, whether the populations in the studied lakes are autochthonous. Eurytemora lacustris possibly invaded the lakes or was transported via a canal from the Schaalsee upstream, where a viable population exists.