No abstract

Shallow lakes are susceptible to catastrophic regime shifts characterised by the presence or absence or macrophytes. However, the long-term controls on macrophyte succession in shallow lakes are incompletely understood. To investigate this, we analysed multiple sediment proxies in Lake Rotcze (Eastern Poland), a small, shallow and densely macrophyte-covered lake to (1) reconstruct the 'reference conditions' (sensu WFD) and development of the lake in recent centuries, (2) compare historical evidence with the sedimentary record, and (3) identify the natural and anthropogenic drivers of macrophyte succession. Before the twentieth century, conditions in the lake may be referred to as 'reference conditions'. Subsequently forest clearance in the catchment resulted in lower water transparency, but concurrent catchment drainage lowered water levels and increased macrophyte development. Since 1950 elevated nutrient supply and climatically driven increases in water levels led to the deterioration of water transparency and partial macrophyte withdrawal. At the end of the twentieth century lake-level drawdown led to low phytoplankton biomass and clear water creating a novel ecosystem where macrophytes invade the whole lake. These patterns suggest that both natural and anthropogenically induced water level fluctuations have been critical drivers of macrophyte development.

In spite of the existence of a number of papers applying the biomarker approach using pigments or akinetes to discuss the impact of cyanobacteria on the functioning of lakes in the palaeoenvironmental context, their sedimentary imprint in the form of microcystins (MCs) has never been taken into consideration. Our objective was to reconstruct 200 years of the development of chironomid assemblages in a shallow Polish lake with notoriously blooming toxic cyanobacteria. A 50-cm long sediment core sampled from the central part of the lake was sliced every 1 cm, dated, and analysed for the subfossil remains of Chironomidae and concentrations of MCs. The fauna underwent four distinct stages in its recent history: three periods when the assemblages were typical of hypertrophic conditions (the first one comprising the entire nineteenth century, and the second and third one including approximately the last 30 years of the twentieth century), separated by a ''eutrophic'' assemblage during the first 70 years of the twentieth century. The typical features of the ''eutrophic'' midge assemblage were high total densities and high values of taxonomic richness and diversity. This period coincided with relatively low concentrations of MCs in the sediments. Sediments from the hypertrophic periods were distinguished by taxonomic impoverishment of chironomids, low Shannon diversity index, a strong decrease in chironomid numbers, and high concentrations of MCs. Significant negative correlations with MCs along the sediment profile were observed for eurytopic chironomids. MC concentrations were also negatively but not significantly correlated with total density of chironomids, their benthic and epiphytic assemblages, species richness, and diversity index.

Archived data and sedimentary macrofossil records of vegetation and invertebrates deposited in a 60-cm long sediment core were analysed to examine if macrophyte dominance was a permanent feature of the recent history of lowland Lake Kleszczów. For the last several centuries, the lake has not been dominated by phytoplankton but by floating-leaved vegetation at strongly reduced water level. Starting from the midnineteenth century, probably as a result of climate fluctuations, vegetation switched at first into submerged angiosperms, and then, in the second half of twentieth century, towards charophytes. Within charophytes there were switches between Chara globularis and C. vulgaris communities, depending on lake productivity or hydrological stress. No symptoms were detected of a switch to a turbid regime as a result of potential internal supply of phosphorus from sediments covered with a dense carpet of charophytes. Our study shows that within a longer period with clear water, the community of macro-vegetation can be highly dynamic. It can be represented by various types of vegetation as a response to different productivity levels and/or hydrological stress, largely determining the composition of other hydrobionts and course of various processes, and as a consequence, the functioning of the ecosystem and its resilience.