Diatom community responses to environmental change in Lake Ohrid (Balkan Peninsula) during the mid-Pleistocene Transition

The Quaternary is characterized by a series of glacial-interglacial cycles and a shift in the Earth's climate state known as the mid-Pleistocene Transition (MPT, ca. 1200 ka to 720 ka). The progressive increase in glacial cycle length from approximately 41 ka to on average 100 ka affected marine and terrestrial life, but how it affected freshwater communities is far less known. Here, diatom and paleoenvironmental data between 1050 ka and 815 ka from a sediment succession of ancient Lake Ohrid are used to understand how the freshwater communities responded to environmental changes during key-periods of glacial intensification associated with the MPT. Based on stratigraphically constrained incremental sum of squares cluster (CONISS), a distinct diatom community turnover in Lake Ohrid was identified during the study period. The first community existed between 1050 and 955 ka MIS (Marine Isotope Stages) 30–25 and was mostly influenced by environmental parameters related to nutrient supply, mixing processes and primary productivity in the lake. The second community existed between 955 and 815 ka (MIS 25–21) and was largely driven by ontogenetic processes related to the ongoing deepening of the lake, changes in bottom water redox conditions and changes in mixing processes in the lake. The increased severity and duration of cold, glacial stages during the MPT shows only a weak impact on the diatom community in Lake Ohrid. Shifts in species phenotypes, abundance and composition are found in both communities, likely as adaptation in response to the changing environmental conditions. This study shows that the diatom communities were only weakly affected by the progressive intensification of the glacial intensity during the MPT, emphasizing the role of local environmental changes in regulating freshwater communities.