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
This study aims to identify reference conditions (nutrient status and diatom assemblages) as required by the European Water Framework Directive (WFD) for stratified, carbonate-rich lowland lakes with a large watershed area (watershed area to lake volume ratio (WV) [ 1.5 km 2 10 -6 m -3 ) and a retention time (RT) from 0.1 to 10 years (Central Baltic Lake-Type 1, German Lake-Type 10) in European ecoregion 14. Diatoms, pollen and geochemistry were analysed from sediment cores of six lakes from northern Germany representing different subtypes of Lake-Type 10 (varying WV and RT) and covering the past 290-1,750 years. Historic total phosphorus levels were inferred using diatom-based transfer functions selected from a merged European data set and from optimised data sets identified with the moving-window approach. Pollen and geochemical proxies were used to identify occurrence and intensity of anthropogenic catchment usage. Lake trophic state reference conditions and associated diatom assemblages were identified for three of the six study lakes. In contrast, according to fossil pollen assemblages, two lakes were already strongly impacted by intensive catchment usage when the oldest investigated sediments were laid down. Thus, reference conditions of these already eutrophic lakes could not be identified. Similarly, the lowermost samples of a core from the sixth lake showed signs of impact, and it remains unclear whether the identified dystrophic conditions occurred naturally or if they were due to the drainage of wetlands in Medieval times. Lakes with a relatively small WV (1.5-5.0 km 2 10 -6 m -3 ) and RT [ 1 year were naturally oligotrophic to low mesotrophic and a typical, representative diatom assemblage was identified. In contrast, typical reference conditions or diatom assemblages for lakes with higher WV (5-18.6 km 2 10 -6 m -3 ) and RT \ 1 year could not be identified as chemical precipitation and upstream lakes (nutrient sinks or sources) additionally influenced natural nutrient levels. Therefore, the reference situation of both trophic state and diatom assemblages in a lake may be strongly influenced by other modifying, limnological processes in addition to WV and RT. Overall, this study helps to implement the WFD by identifying reference conditions and by discussing the level of differentiation of lake types required to set reference conditions.
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