Eemian environmental changes recorded in lake deposits from Rzecino (NW Poland): Cladocera, isotopic and selected geochemical data

Analysis of Cladocera and organic matter from the sediments of the Rzecino paleolake are presented. These are probably among the first detailed published data from the Eemian Interglacial (MIS 5e) organic matter. The Rzecino paleolake is located in NW Poland in an area covered by the last glaciation. Approximately 5.8 m of sediment was analyzed. Previous palynological data indicate a continuous accumulation from the final phase of the Wartanian to the early Vistulian Glaciation. Data on the Cladocera correlated with the results of the analyses of organic matter, which enabled the reconstruction of varying environmental conditions and geneses of organic matter. In the final phase of the Wartanian and at the beginning of the Eemian Interglacial, low frequencies of Cladocera specimens and species were identified, which suggests unfavorable conditions for the development of zooplankton. Organic matter (OM) was characterized by a small amount of organic carbon and nitrogen, a high value of C/N (18–27) and a δ13C of approximately −26 ‰, which suggests a higher plant source and a predominantly terrestrial origin for OM. An improvement in environmental conditions occurred during the pollen zone E3. The number of Cladocera species and individuals increased. An increase in TOC and TN suggests an enhancement of primary productivity and the development of macrophytes in the lake. A lower C/N ratio indicates a primary role of algae in the production of organic matter. During the Eemian optimum (pollen zones E4–E5), mostly Cladocera species preferring warm water occurred. Algae remained a significant component of OM. The early Eemian post-optimum (pollen zone E6) was characterized as a period of maximal abundance of zooplankton (20 species). During this period, rapid decreases in TOC, TN and the C/N ratio and an increase in δ13C values suggest the reduced abundance of higher plants in the lake. In pollen zone E7, the frequency and diversity of Cladocera rapidly decreased. Species that tolerate cold water were dominant, which reflects unfavorable conditions for the development of zooplankton but a high water level. The low content of organic carbon and nitrogen, a C/N ratio in the range of 13–17 and δ13C values in the range of −26.2 to −28 ‰ suggest the inhibition of terrestrial plant delivery. During the Early Vistulian (pollen zone EV1), rare faunal remains and the features of OM (the lowest amounts of TOC and TN, and the highest value of C/N) reflect the decline of lake conditions and a lack of primary production and aquatic vegetation in the lake.