A Holocene high‐resolution record of aquatic productivity, seasonal anoxia and meromixis from varved sediments of Lake Łazduny, North‐Eastern Poland: insight from a novel multi‐proxy approach

Abstract

ABSTRACTAnthropogenic eutrophication and spreading anoxia in freshwater systems is a global concern. Little is known about anoxia in earlier historic times under weaker human impact, or under prehistoric natural conditions with different trophic, land cover and climatic regimes. We use a novel approach that combines high‐resolution hyperspectral imaging with µ‐XRF and HPLC‐pigment data, which allows us to assess chloropigments (productivity) and bacteriopigments (anoxia) at seasonal subvarve‐scale resolution. Our ~9700 cal a bp varved sediment record from NE Poland suggests that productivity increased stepwise from oligotrophic Early Holocene conditions (until ~9200 cal a bp) to mesotrophic conditions in the Mid‐ and Late Holocene. Natural eutrophication was mainly a function of progressing landscape evolution with intense weathering under dense forest and warm‐moist climatic conditions. Generally, anoxia increased with increasing productivity. Seasonal anoxia and some multi‐decadal periods of meromixis were the common mixing patterns throughout the Holocene except for a period of persisting meromixis between ~5200 and 2000 cal a bp. Anthropogenic deforestation around 400 cal a bp resulted in substantially better lake oxygenation despite high productivity. In this small lake, aquatic productivity and lakeshore forest cover (wind shield) were more important factors controlling oxic/anoxic conditions than Holocene temperature variability.