Linking diatom community dynamics to changes in terrestrial vegetation: a palaeolimnological case study of Lake Ķūži, Vidzeme Heights (Central Latvia)

Abstract

Diatom and pollen records from the deepest part of Lake Ķūži (Vidzeme Heights, Central Latvia) show the history of the lake and its ecosystem responses to changes in the surrounding vegetation during the Holocene. Principal Component Analysis (PCA) was used to compare the timing of the changes in the diatom and pollen assemblages. We found that major changes in the diatom record were contemporaneous with those in the pollen records. At the beginning of the Early Holocene, when the lake was receiving high inputs of mineral matter, no diatoms were found. Around 11 000 cal. BP, when the upland vegetation became established, periphytic diatom taxa (mostly Fragilaria species) prevailed. The Mid-Holocene period (9000-2000 cal. BP) was characterized by Cyclotella spp. and Tabellaria flocculosa, indicating long ice-free seasons and a rather high water level. Picea was a major tree species around the lake 5300-2500 cal. BP and it facilitated acidification of the lake water via the acidification of the soil, indicated by the increase in the acidophilous diatoms Eunotia spp. and T. flocculosa. The Late Holocene (2000-0 cal. BP) is characterized by anthropogenic impacts on both the upland vegetation and lake ecosystem, depicted by the simultaneous increase in Aulacoseira spp., herbaceous pollen such as Poaceae, Secale, and Rumex, and charcoal fragments. With pollen taxa used as predictor and diatom taxa as response variables, Redundancy Analysis (RDA) provided a statistically significant model that explains the variation in the diatom data. Our results show that the diatoms responded strongly to the catchment-driven changes around Lake Ķūži during the entire Holocene.