Interactions between microbial degradation of sedimentary organic matter and lake hydrodynamics in shallow water bodies: insights from Lake Sarbsko (northern Poland)

In this paper we demonstrate spatial and seasonal changes in the pathways of microbial decomposition of organic matter within the surface sediments of Lake Sarbsko, a coastal water body located on the middle Polish-Baltic coast. We studied lake waters and bottom sediments at 11 sampling stations throughout the basin and in different seasons between November 2007 and September 2008. It was established that, in this very productive and shallow lake, microbial activity increases in warmer seasons and ceases during winter. In spring, bacterial activity is fuelled by increased influx of highly reactive planktonic organic matter, which is decomposed via methanogenesis, reduction of NO3 -, SO4 2-, and Fe and Mn oxides. On the other hand, during summer, oxidation processes (mainly oxidation of CH4) tend to predominate. The change from reduction to oxidation is attributed to wind-induced vertical mixing of Lake Sarbsko waters and resuspension of bottom deposits. Degradation of sedimentary organic matter in Lake Sarbsko results in appreciable changes in the pH and the concentrations of red-ox sensitive ions in pore waters, but it has little effect on the chemistry of bottom and surface waters. However, release of PO4 3- from the sediments might be a source of this nutrient in the lake. Internal loading of phosphates in Lake Sarbsko occurs under both oxic/mildly reducing and anoxic conditions

Fires, vegetation, and human - The history of critical transitions during the last 1000 years in Northeastern Mongolia

Fires are natural phenomena that impact human behaviors, vegetation, and landscape functions. However, the long-term history of fire, especially in the permafrost marginal zone of Central Asia (Mongolia), is poorly understood. This paper presents the results of radiocarbon and short-lived radionuclides (210Pb and 137Cs) dating, pollen, geochemical, charcoal, and statistical analyses (Kohonen's artificial neural network) of sediment core obtained from Northern Mongolia (the Khentii Mountains region). Therefore, we present the first high-resolution fire history from Northern Mongolia covering the last 1000 years, based on a multiproxy analysis of peat archive data. The results revealed that most of the fires in the region were likely initiated by natural factors, which were probably related to heatwaves causing prolonged droughts. We have demonstrated the link between enhanced fires and “dzud”, a local climatic phenomenon. The number of livestock, which has been increasing for several decades, and the observed climatic changes are superimposed to cause “dzud”, a deadly combination of droughts and snowy winter, which affects fire intensity. We observed that the study area has a sensitive ecosystem that reacts quickly to climate change. In terms of changes in the vegetation, the reconstruction reflected climate variations during the last millennium, the degradation of permafrost and occurrence of fires. However, more sites with good chronologies are needed to thoroughly understand the spatial relationships between changing climate, permafrost degradation, and vegetation change, which ultimately affect the nomadic societies in the region of Central and Northern Mongolia.National Science Centre, Poland (Grant no.: 2017/01/X/ST10/01216 and 2018/31/B/ST10/02498); Polish-Mongolian Joint Research Project – “Environmental changes in the Northern Mongolia under recent and past climate variability”. Russian Science Foundation (Grant No. 20-17-00110

Paleolimnologiczna interpretacja krzemionki biogenicznej - dyskusja na przykładzie wybranych jezior Niżu Polskiego

The paper outlines some problems in the interpretation of biogenic silica content in lacustrine sediments in the context of reconstruction of past changes in lake ecosystems throughout the Holocene. The discussion is centered on four lakes located on the Polish Lowland (Lake Sarbsko, Lake Ostrowite, Lake Skrzynka and Lake Gąsak), which display different lithology and chemical composition of bottom deposits. From the data collected it emerges that SiO2biog cannot be regarded as an unambiguous indicator of lake trophic state. Instead, paleolimnological interpretations of biogenic silica should include dilution by carbonates, SiO2ter and/or organic matter, H4SiO4-limited diatom productivity and lake water dynamics. The understanding of SiO2biog benefits from supplementation with other geochemical and paleoecological proxies