Although multi-proxy paleoclimatic data from Middle Pleistocene archeological sites are important to understand patterns of climate variability as well as prehistoric human adaption, only few well-preserved records exist in Central Europe. The Lower Paleolithic sites in the open-cast mine of Schöningen, Northern Germany, have become internationally renown because of their worldwide oldest wooden hunting weapons associated with exceptional preservation of remains of a diverse flora and fauna. The Middle Pleistocene paleolake sediments of the locally named Reinsdorf Interglacial, deposited between the Elsterian and Saalian glaciations and correlated at archaeological site Schöningen 13 II with MIS 9, provide a valuable environmental archive for investigating an interglacial transition into an early glacial phase. Diatoms and ostracods were analyzed for more detailed information about the characteristics of the paleolake development and thus smaller-scale climate fluctuations. The lake was characterized by brackish water within the oligo- to mesohaline range. Taxa preferences show deposition near the lakeshore, often associated with abundant aquatic plants. Calcium content was elevated with at least 18 mg Ca L-1. Distinct changes in assemblages, concentrations and taxa dominances point to highly variable lake conditions throughout the terminating interglacial. Presence and absence of shallow water ostracods and abundances of benthic diatom taxa suggest that the paleolake was characterized by strongly fluctuating lake levels with repeating shallowing-upward cycles, probably climatically induced. High abundances of tychoplanktonic diatoms and ostracod taxa associated with streaming water and less brackish conditions in certain levels are indicative of a variable freshwater inflow from springs nearby. Average paleo-temperatures were reconstructed using the Mutual Ostracod Temperature Range method. Mean values varied between approx. +16 and +20 °C for July and approx. -11 and +1 °C for January. Highest numbers and best preservation of ostracod shells were found not in sediments representing full interglacial conditions but during a later phase. This climate oscillation at the transition into the following glacial period lead again to more favorable conditions for aquatic organisms. The youngest part of the sequence was void of ostracods hinting at desiccation of the site and proceeding climate deterioration during the early glacial. Our findings highlight the relevance of Schöningen for understanding Central European interglacial climate dynamics.