Evaluating sedimentary DNA for tracing changes in cyanobacteria dynamics from sediments spanning the last 350 years of Lake Tiefer See, NE Germany

Since the beginning of the Anthropocene, lacustrine biodiversity has been influenced by climate change and human activities. These factors advance the spread of harmful cyanobacteria in lakes around the world, which affects water quality and impairs the aquatic food chain. In this study, we assessed changes in cyanobacterial community dynamics via sedimentary DNA (sedaDNA) from well-dated lake sediments of Lake Tiefer See, which is part of the Klocksin Lake Chain spanning the last 350 years. Our diversity and community analysis revealed that cyanobacterial communities form clusters according to the presence or absence of varves. Based on distance-based redundancy and variation partitioning analyses (dbRDA and VPA) we identified that intensified lake circulation inferred from vegetation openness reconstructions, δ13C data (a proxy for varve preservation) and total nitrogen content were abiotic factors that significantly explained the variation in the reconstructed cyanobacterial community from Lake Tiefer See sediments. Operational taxonomic units (OTUs) assigned to Microcystis sp. and Aphanizomenon sp. were identified as potential eutrophication-driven taxa of growing importance since circa common era (ca. CE) 1920 till present. This result is corroborated by a cyanobacteria lipid biomarker analysis. Furthermore, we suggest that stronger lake circulation as indicated by non-varved sediments favoured the deposition of the non-photosynthetic cyanobacteria sister clade Sericytochromatia, whereas lake bottom anoxia as indicated by subrecent- and recent varves favoured the Melainabacteria in sediments. Our findings highlight the potential of high-resolution amplicon sequencing in investigating the dynamics of past cyanobacterial communities in lake sediments and show that lake circulation, anoxic conditions, and human-induced eutrophication are main factors explaining variations in the cyanobacteria community in Lake Tiefer See during the last 350 years.

Long term subfossil Cladocera record from the partly varved sediment of Lake Tiefer See (NE Germany)

<p>The partly varved and well-dated sediment record of Lake Tiefer See (NE Germany) allowed the high resolution paleolimnological reconstruction of the lake evolution during the whole Holocene. This paper presents results of subfossil Cladocera analysis. During the Holocene, the fauna of subfossil cladoceran was represented by 36 species belonging to 6 families. Cladocera were dominated by typical open-water species, belonging especially of the Bosminidae family. The sediment record of Lake Tiefer See exhibited distinct decadal- to centennial-scale alternations of well- and non-varved intervals, which were related to changes in the thermal circulation of the lake water column. In general, well varved sediments were deposited during periods of reduced lake circulation, and were characterised by maximum abundance of Cladocera, whereas non-varved sedimentation phases occurred during periods of increased lake circulation and showed a lower number of Cladocera specimens. The most suitable conditions for the development of cladoceran fauna occurred during the early Holocene and from ~ 2055 – 725 yr cal BP. On the basis of the increasing number of species associated with high lake productivity, eight stages of increasing trophy were inferred. The first two were attributed to climate warming, while the next six to human impact. Higher human driven trophic conditions of Lake Tiefer See occurred in the periods 5750-5500 and 4500-4100 yr cal BP, and four times from 1000 to50 yr cal BP. During the last 750 years and in the period from 6000 to 2500 yr cal BP, the species of <em>Eubosmina</em> produced extreme morphs. The cyclomorphosis of <em>Eubosmina </em>was likely connected to more pronounced changes in the lake environmental conditions.</p>

Varve microfacies and varve preservation record of climate change and human impact for the last 6000 years at Lake Tiefer See (NE Germany)

The Holocene sediment record of Lake Tiefer See exhibits striking alternations between well-varved and non-varved intervals. Here, we present a high-resolution multi-proxy record for the past ~6000 years and discuss possible causes for the observed sediment variability. This approach comprises microfacies, geochemical and microfossil analyses and a multiple dating concept including varve counting, tephrochronology and radiocarbon dating. Four periods of predominantly well-varved sediment were identified at 6000–3950, 3100–2850 and 2100–750 cal. a BP and AD 1924–present. Except of sub-recent varve formation, these periods are considered to reflect reduced lake circulation and consequently, stronger anoxic bottom water conditions. In contrast, intercalated intervals of poor varve preservation or even extensively mixed non-varved sediments indicate strengthened lake circulation. Sub-recent varve formation since AD 1924 is, in addition to natural forcing, influenced by enhanced lake productivity due to modern anthropogenic eutrophication. The general increase in periods of intensified lake circulation in Lake Tiefer See since ~4000 cal. a BP presumably is caused by gradual changes in the northern hemisphere orbital forcing, leading to cooler and windier conditions in Central Europe. Superimposed decadal- to centennial-scale variability of the lake circulation regime is likely the result of additional human-induced changes of the catchment vegetation. The coincidence of major non-varved periods at Lake Tiefer See and intervals of bioturbated sediments in the Baltic Sea implies a broader regional significance of our findings.

Dynamic diatom response to changing climate 0–1.2 Ma at Lake El'gygytgyn, Far East Russian Arctic

The Lake El'gygytgyn sediment record contains an abundant diatom flora through most intervals of the lake's history, providing a means to create and test hypotheses concerning the lake's response to changing climates. The 0–1.2 Ma core interval is characterized by shifts in the dominant planktonic genera and events of exceptional concentration and diversity. Warm interglacial intervals have enhanced concentration and diversity of the plankton. This response is most extreme during exceptional events corresponding to marine isotope stages (MIS) 11 and 31. Diatom concentration and diversity also increase during some cold intervals (e.g., MIS 2), suggesting conditions of lake circulation and nutrient cycling promoting diatom production during these events. Short intervals of low plankton concentration accompanied by shifts in the dominant genus of the lake suggest conditions during certain cold events generate a severe impact on plankton production. The absence of these events during extended intervals of low summer insolation variability suggests a muted cold-event response of the lake system linked to regional climate.

Dynamic diatom response to changing climate 0–1.2 Ma at Lake El'gygytgyn, far east Russian Arctic

The Lake El'gygytgyn sediment record contains an abundant diatom flora through most intervals of the lake's history providing a means to create and test hypotheses concerning the lake's response to changing climates. The 0–1.2 Ma core interval is characterized by shifts in the dominant planktonic genera and events of exceptional concentration and diversity. Warm interglacial intervals have enhanced concentration and diversity of the plankton. This response is most extreme during exceptional events corresponding to MIS 11 and 31. Diatom concentration and diversity also increases during some cold intervals (e.g. MIS 2), suggesting conditions of lake circulation and nutrient cycling promoting diatom production during these events. Short intervals of low plankton concentration accompanied by shifts in the dominant genus of the lake suggest conditions during certain cold events generate a severe impact on plankton production. The absence of these events during extended intervals of low summer insolation variability suggests a muted cold-event response of the lake system linked to regional climate.