Basin-specific records of lake oligotrophication during the middle-to-late Holocene in boreal northeast Ontario, Canada

Descriptions of regional climate expression require data from multiple lakes, yet little is known of how variation in records within morphometrically complex lakes may affect interpretations. In northeast Ontario (Canada), this issue was addressed using records of pollen, pigments, and diatoms in three sediment cores from two small boreal lakes spanning the last ~6000 years. Pollen analysis suggested warm conditions between ~6000 and ~4000 cal yr BP, coherent with previous assessments from boreal eastern Ontario and western Quebec. Analysis of phototrophic communities from fossil pigments and diatom valves suggested relatively eutrophic conditions with lower lake-levels during this interval. Generalized additive model trends identified significant regional changes in pollen assemblages and declines in pigment concentrations after ~4000 cal yr BP consistent with cooler and wetter climate conditions that resulted in regional lake oligotrophication and increased lake levels during the late-Holocene. Despite contemporaneous changes in pollen and pigment biomarkers across lakes, cores collected from adjacent basins of the same lake (Green Lake) did not show similar trends in fossil pigments likely reflecting preferential deposition of clay-rich allochthonous material in the deeper central basin and suggesting that regional signals in climate may be complicated by lake- or basin-specific catchment processes.

Drivers of change and ecosystem status in a temperate lake over the last Post-Glacial period from 14.5 kyr

Understanding the long-term dynamics of ecological communities on the centuries-to-millennia scale is important for explaining present-day biodiversity patterns. Placing these patterns in a historical context could yield reliable tools for predicting possible future scenarios. Paleoarchives of macro+ and micro-fossil remains, and most importantly biomarkers such as fossil pigments and ancient DNA present in various sedimentary deposits, allow long term changes in ecological communities to be analysed. We use recent compilations of data including fossil pigments, metabarcoding of sedimentary ancient DNA and microfossils together with data analysis to understand the impact of gradual versus abrupt climate changes on a lake`s ecosystem status over the last 14.5 kyr. We give examples of hypotheses that need long-term data, which can be addressed using well-established paleoproxy variables. These variables describe the climate, together with vegetation change and the appearance of anthropogenic forcing, either as a gradual change or an abrupt event. We were able to detect abrupt changes in the lake ecosystem during the stable period of the Holocene Thermal Maximum and we highlight the increased frequency and degree of perturbation in lakes due to non-immediate human activity over a larger region. Both observations demonstrate an impaired relationship between a gradual external driver and ecosystem response and apply to future scenarios of climate warming and increased human impact in north-eastern Europe.

A 260 000-year reconstruction of diatom community dynamics and photosynthetic pigments in Lake Chala, a tropical crater lake

<p>Lake Chala is a c. 90 m deep meromictic, oligotrophic crater lake near Mt. Kilimanjaro in equatorial East Africa. This sub-humid tropical region experiences two rainy seasons separated by a long dry season in June-August, when deep mixing fuels the epilimnion with nutrients resulting in increased phytoplankton primary production. Within the ICDP DeepCHALLA project, a 215-m long, continuous sediment sequence was obtained, which provides a unique opportunity to study long-term climate dynamics and aquatic ecosystem response during the past c. 260 000 years. Here we analyzed fossil pigments and diatom assemblages to reconstruct temporal dynamics in the lake’s phytoplankton community structure with millennial-scale resolution. Fossil pigments were analyzed using high-performance liquid chromatography, and a minimum of 400 valves were counted and identified with best-possible taxonomic discrimination from sediment samples taken at c. 800-yr intervals throughout the last glacial cycle (back to c. 160 kyr BP) and at c. 1600-yr intervals throughout earlier lake history. The most abundant pigments were zeaxanthin and lutein, reflecting the presence of cyanobacteria and green algae. Despite the high diatom content of the sediments, the diatom marker pigment fucoxanthin was almost absent, which we attribute to its labile nature. A small cyclotelloid diatom resembling the tychoplanktonic species <em>Discostella stelligera</em> at the base of the sequence probably reflects open-water conditions with the proximity of littoral habitats during the early filling stages of lake ontogeny. High proportions (20-50%) of an <em>Encyonema</em> species at c. 240-230 kyr BP indicate increased availability of benthic habitats, possibly because of a marked low-stand at the time of early sediment infilling when the central ash cone on the basin floor was still exposed. A phase of abundant needle-like <em>Nitzschia</em> and generally higher, yet fluctuating, pigment concentrations suggest a relatively moist environment with deep water and a stable stratification at c. 220-140 kyr BP. After c. 140 kyr BP, <em>Afrocymbella barkeri</em> appears for the first time, and from then onwards until the modern times the diatom community is composed of fluctuating abundances of <em>Afrocymbella</em> and needle-like <em>Nitzschia</em> taxa. The highest diatom biovolumes yet pronounced low carotenoid concentrations occur during the <em>Afrocymbella</em>-dominated (up to 100 %) intervals between c. 110 and 90 kyr BP and between c. 22 and 17 kyr BP, which broadly coincide with the MIS5 African megadrought and the Last Glacial Maximum, respectively. This suggests that during those time periods, the lake experienced pronounced dry and windy climate conditions, which triggered relatively deep mixing. This probably enhanced internal nutrient cycling and the injecting of oxygen to the bottom waters, which facilitated diatom growth in the epilimnion and resulted in pigment degradation in the hypolimnion. Superimposed on these long-term patterns, we found many short-term fluctuations in the appearance of different <em>Nitzschia</em> taxa, which may reflect stochastic colonization and extinction events, rather than actual climate-driven changes in the abiotic environment of Lake Chala.</p>

Recording of climate and diagenesis through sedimentary DNA and fossil pigments at Laguna Potrok Aike, Argentina

Aquatic sediments record past climatic conditions while providing a wide range of ecological niches for microorganisms. In theory, benthic microbial community composition should depend on environmental features and geochemical conditions of surrounding sediments, as well as ontogeny of the subsurface environment as sediment degraded. In principle, DNA in sediments should be composed of ancient and extant microbial elements persisting at different degrees of preservation, although to date few studies have quantified the relative influence of each factor in regulating final composition of total sedimentary DNA assemblage. Here geomicrobiological and phylogenetic analyses of a Patagonian maar lake were used to indicate that the different sedimentary microbial assemblages derive from specific lacustrine regimes during defined climatic periods. Two climatic intervals (Mid-Holocene, 5 ka BP; Last Glacial Maximum, 25 ka BP) whose sediments harbored active microbial populations were sampled for a comparative environmental study based on fossil pigments and 16S rRNA gene sequences. The genetic assemblage recovered from the Holocene record revealed a microbial community displaying metabolic complementarities that allowed prolonged degradation of organic matter to methane. The series of Archaea identified throughout the Holocene record indicated an age-related stratification of these populations brought on by environmental selection during early diagenesis. These characteristics were associated with sediments resulting from endorheic lake conditions and stable pelagic regime, high evaporative stress and concomitant high algal productivity. In contrast, sulphate-reducing bacteria and lithotrophic Archaea were predominant in sediments dated from the Last Glacial Maximum, in which pelagic clays alternated with fine volcanic material characteristic of a lake level highstand and freshwater conditions, but reduced water column productivity. Comparison of sedimentary DNA composition with that of fossil pigments suggested that post-depositional diagenesis resulted in a rapid change in the initial nucleic acid composition and overprint of phototrophic communities by heterotrophic assemblages with preserved pigment compositions. Long DNA sequences (1400–900 bp) appeared to derive from intact bacterial cells, whereas short fragments (290–150 bp) reflected extracellular DNA accumulation in ancient sediments. We conclude that sedimentary DNA obtained from lacustrine deposits provides essential genetic information to complement paleoenvironmental indicators and trace post-depositional diagenetic processes over tens of millennia. However, it remains difficult to estimate the time lag between original deposition of lacustrine sediments and establishment of the final composition of the sedimentary DNA assemblage.