Changes In Diatom Assemblages Research Articles

Diatom-based indications of an environmental regime shift and droughts associated with seasonal monsoons during the Holocene in Biển Hồ maar lake, the Central Highlands, Vietnam

The interactions of the different monsoon systems across Southeast Asia create extreme climate phenomena. Central Vietnam, located near the centre of this transitional region, has encountered numerous effects. As a result, its sediments from lakes or speleothems are valuable archives for interpreting past climate variability. However, there is still a lack of high-resolution paleoenvironmental and palaeoclimatic reconstructions during the Holocene in Vietnam. Our study presents a paleoenvironmental diatom-based record of sediment cores collected from Biển Hồ maar lake (14°03′N, 108°00′E) in the Central Highlands of Vietnam covering nearly the entire Holocene. Based on changes in diatom assemblages in the sediment sequence, we identified two periods of the Early Holocene (~11,700–7800 cal BP) and the Mid- to Late-Holocene (~7800–360 cal BP), which mark a remarkable shift in the environment around Biển Hồ. Alternations of key diatom species during the Early Holocene indicate intensity variations between water-mixing and thermal stratification mechanisms in meso-eutrophic conditions. During the Mid- to Late-Holocene, the complete dominance of Aulacoseira granulata var. granulata implies year-round destratification and intense mixing of the lake water column in a permanently eutrophic environment. Its morphological variability reveals intervals of dry environmental conditions driven by pronounced droughts across the Asian continent.

Differential dissolution of biogenic silica significantly affects the utility of sediment diatoms as paleoceanographic proxies

AbstractDiatoms and biogenic silica (biogenic Si) in sediments are commonly analyzed as paleoceanographic environmental indicators. However, the correspondence between these sedimentary components and their counterparts in the water column above can vary over time and space. This study, undertaken in the northern South China Sea, compares diatoms and biogenic Si in sinking particles from the water column with those in surface sediments below. The results indicate substantial losses of diatoms and biogenic Si during sinking and burial, along with changes in diatom assemblages. About 6% of diatoms sinking from 1000 m depth are estimated to be preserved in sediments: 32% are recycled in the water column and 62% are lost at the sediment–water interface. Approximately, 12% of biogenic Si is preserved in sediments: 6% is remineralized in the water column and 82% is lost at the sediment–water interface. The differences between diatom and biogenic Si preservation and remineralization are likely due to differences in dissolution resistance among the various diatom species and between diatoms and other silicifiers (e.g., radiolarians), along with effects of lateral transport. This variability introduces uncertainties into paleoceanographic inferences made from these two proxies from the sediments. Thalassionema nitzschioides was found to dominate sediment assemblages, which we attribute to its great resistance to dissolution, and to be negatively correlated with diatom sinking fluxes. These properties suggest that in the northern South China Sea, T. nitzschioides may potentially be useful as an indicator of low paleoproductivity (e.g., as seen during strong El Niño events).

Before and after mink fur farming: water chemistry and sedimentary diatom assemblages in lakes from southwestern Nova Scotia, Canada

Libera N, Rühland KM, Kurek J, Smol JP. 2024. Before and after mink fur farming: water chemistry and sedimentary diatom assemblages in lakes from southwestern Nova Scotia, Canada. Lake Reserv Manage. 40:18–35. Since the 1930s, farmed mink pelts have been an important contributor to the economy of rural Nova Scotia (NS, Canada). However, these farms are a potential source of pollutants to nearby ecosystems. To determine how regional lakes have been affected by fur farms, we compared modern water chemistry and sedimentary diatom assemblages in 14 lakes in southwestern Nova Scotia. We categorized lakes into 3 groups: (a) 5 lakes with fur farms in the catchment or near inlets; (b) 4 lakes downstream from or near fur farms; and (c) 5 reference lakes without hydrological connection to fur farms. To assess whether lake conditions have changed since the establishment of fur farms, we conducted a “before-and-after” paleolimnological analysis comparing surface sediments (representing modern environments) to dated sediments deposited prior to fur farm operations. Diatom assemblages registered distinct responses to eutrophication in several hypereutrophic lakes with farms within their catchment boundaries. In lakes ∼25 km downstream from fur farms, diatom assemblage changes were more characteristic of climate warming and acidification than of eutrophication, despite surface water nutrient concentrations well above levels in reference lakes. We found no association between the presence of fur farms and surface water trace metals that could potentially bioaccumulate and/or biomagnify. Our data indicate that decades of nutrient inputs from fur farms have caused eutrophication at several lakes where fur farms are close to the shoreline or to inlet streams.

Comparison of advanced methodologies for diatom identification within dynamic coastal communities

AbstractDiatom community composition has a critical influence on global ocean health and ecological processes. Developing accurate and efficient methods for diatom identification under dynamic environmental conditions is essential to understanding the implications of diatom community changes. Two developing methods for identifying and enumerating phytoplankton, cell imaging and molecular sequencing, are experiencing rapid advancements. This study aims to compare diatom taxonomic composition results within natural assemblages derived from rapidly advancing methods, FlowCam imaging and metabarcoding of the V4 region of the 18S rRNA gene, with traditional light microscopy cell counting techniques. All three methods were implemented in tandem to analyze changes in dynamic diatom assemblages within simulated upwelling experiments conducted in the California upwelling zone. The results of this study indicate that, summed across all samples, DNA sequencing detected four times as many genera as morphology‐based methods, thus supporting previous findings that DNA sequencing is the most powerful method for analyzing species richness. Results indicate that all three methods returned comparable relative abundance for the most abundant genera. However, the three methods did not return comparable absolute abundance, primarily due to barriers in deriving quantities in equal units. Overall, this study indicates that at the semi‐quantitative level of relative abundance measurements, FlowCam imaging and metabarcoding of the V4 region of the 18S rRNA gene yield comparable results with light microscopy but at the qualitative and quantitative levels, enumeration metrics diverge, and thus method selection and cross‐method comparison should be performed with caution.

High-Resolution Lacustrine Records of the Late Holocene Hydroclimate of the Sikhote-Alin Mountains, Russian Far East.

There is little information about moisture changes in different altitudinal belts in mountainous regions of the southern Russian Far East. We present ecological and taxonomic compositions of the diatom flora and identify the botanical composition of peat in small mountain lake/mire complexes located in the Central Sikhote-Alin Mountains, within large landslides on the paleovolcanic slopes. Frequent changes in diatom assemblages and peat-forming plants indicate unstable hydroclimatic conditions with varying degrees of wet and dry conditions up to the overgrowth of the lakes. Frequent change in sphagnum mosses with different trophic preferences was identified. The chronology is based on 11 radiocarbon dates. Accumulation rates reached 1.7-1.9 mm/year, and the temporal resolution for the reconstructions was up to 30-40 yr. The tendencies of lake evolution depended on different scale hydroclimatic changes over the last 4400 yr. The most detailed data for the last 2600 yr were obtained from the Nizhnee Lake sequence, which is more sensitive to climatic changes. The main reason for the change in the hydrological regime of the lakes was variations in precipitation during short-term climatic changes. The sediment record of moisture fluctuations is relatively well correlated with regional patterns reflecting summer monsoon intensity and cyclogenesis activity.

Impacts of anthropogenic pressures on underwater light conditions and diatom functional group distributions in mountain lakes

Underwater light availability and exposure of ultraviolet radiation (UV) in mountain lakes is mainly controlled by dissolved organic matter and ice cover. However, both of these factors are affected by climate warming and other anthropogenic pressures. Still, little is known of the impacts of long-term fluctuations in underwater light conditions onto functional distribution of diatoms, species sensitive to changes in climate and UV penetration in mountain lakes. Two mountain lakes in the Italian Alps were analysed using paleolimnological approaches to investigate impacts of anthropogenic pressures (e.g., climate warming, acidification) on underwater light availability, exposure to UV and diatom functional group distributions, focusing on the post-industrial era. Contemporary diatom communities were collected from the main habitats in the shallow, high irradiance littoral zones of these lakes and geochemical proxies describing the development of lake-water transparency and carbon dynamics were analyzed from the sediment cores covering the time period from ca. 1400 CE to present. The geochemical data indicate that cultural acidification decreased lake-water organic carbon concentrations in both lakes, which suggests increased light availability and UV exposure during the past century. The responses of the studied lakes to anthropogenic pressures varied, as the lake situated at a lower altitude with a larger catchment showed only a few biotic changes indicating higher resilience, whereas the lake situated at higher altitude showed distinct changes in its ecological status. In this alpine lake, almost 30% of the diatom functional groups shifted from benthic to planktic during the post-industrial era. The role of increased light availability and UV exposure as a driver of diatom functional group distributions could not be unambiguously separated, and such diatom assemblage changes have been shown in many regions and most closely linked to a warming climate. However, low guild functional groups, prevalent also in the contemporary samples from the shallow littoral zone, dominated the diatom communities throughout the studied period, suggesting high tolerance to UV radiation.

Volcanic, tectonic and climate controls on lacustrine sedimentary supplies over the last millenia in NE Chilean Patagonia (Lake Esponja, Aysen, 45°S)

The environmental variability of Northern Chilean Patagonia during the last millennia is evaluated using a multi-proxy analysis of sediment cores from Lake Esponja (45°S 72°W) to decipher if the sediment deposition is controlled by volcanic eruptions, landslides induced by earthquake or heavy rainfall. The lake is located in a glacio-tectonic valley in Patagonia. The organic-rich clayey silt sediment with low biogenic silica content was analysed for grain size, magnetic susceptibility, organic matter, biogenic silica content and diatom assemblages, mineralogy (X-ray diffraction), organic (IRMS C and N analyses) and inorganic (XRF core-scanner) geochemistry and glass shard major composition (Microprobe, SEM). The combination of 210Pb, 137Cs, 14C and tephrochronology indicates an averaged accumulation rate of 0.4 mm/year, leading to a record of ~3.5 kyr within 154 cm. The sedimentary geochemistry records changes in volcanic supplies, diatom productivity and detrital inputs. The sediments were interrupted by millimetric to centimetric layers corresponding to tephra deposition related to explosive eruptions of nearby volcanoes Macá, Melimoyu and Hudson. Concerning the diatoms, the dominant planktonic species (80–150 cm) are replaced by benthic species in a transition interval (55–80 cm) and then by Surirella spp. in the upper core. This last genus indicates a closure of the basin ~2 ka ago, probably related to an uplift linked to a rejuvenation of the Mañihuales fault. This local change could reflect regional tectonic instability. Indeed, a partial earthquake rupture occurred around ~AD100 along the southern part of the Valdivia segment, recorded as a mass transport deposit in Aysén fjord sedimentation. The fine detrital input varies over time with more variable Si/Al values in the lower part of the LEs14 core than in the upper 80 cm. The higher values may reflect wetter conditions, leading to an higher lake level and more turbid conditions in agreement with changes in diatom assemblages.

A century of diatom monitoring in acidified and warmed Dutch moorland pools

Diatoms are well-known indicators for acidification and recovery of lakes, as mainly with paleolimnological methods. However, from the last few decades, such studies are scarce, especially in Europe, and there is a poor insight into the interaction between the impact of the recovery from acidification and drought periods on diatoms. The surface water chemistry of Dutch moorland pools has shown strong signs of chemical recovery as a response to declines in sulphur and nitrogen deposition between 1978 and 2018. We expected that changes in water chemistry were associated with changes in diatom assemblages and that the diatom assemblages at the end of the monitoring period would be dissimilar to pre-acidification diatom assemblages due to the changed biogeochemical processes in the pools. To monitor the recovery from acidification, diatoms from plants and surficial sediments were sampled in 11 Dutch moorland pools from 1978 to 2018. As far as we know, this is the longest series of semi-annual diatom records in stagnant shallow waters recovering from acidification. Early 20th-century samples were retrieved from collections. Changes in the diatom assemblages were assessed by analyses of traits (including pH-preference) and ecological groups. Direct correspondence analysis revealed high correlations between the species composition and environmental variables (pH, sulphate, ammonium, dissolved organic carbon, calcium, and aluminium). Between 1916 and 1978, the greatest species composition changes occurred in pools with relatively flat banks exposed to the air in extremely dry summers. After the dry summer of 1921, the changes were insignificant, but after the extremely dry summer of 1976, the changes were dramatic due to acidification by oxidation of the reduced sulphur and nitrogen compounds stored in the moorland pool bottom. After 1980, the changes were consistent with chemical recovery from acidification. However, there was a significant interaction with climatic factors: drought episodes in the short term and temperature increase in the long term. Some species in the samples from the last decades indicate an acid, but eutrophic environment, due to internal eutrophication. The species composition did not return to the historical composition (1920 s), as the chemical dynamics due to the large stock of sulphur and nitrogen compounds are now entirely different from those a century ago.

Fossil diatom assemblage changes due to paleoenvironment change, tsunami, and typhoon in southern Japan

AbstractTracking paleoenvironmental change and past event deposits is very important to evaluate the natural hazard spatially. This paper presents how the environment changes and implies the event deposit depending on the diatom assemblage change. To investigate paleoenvironmental change and identify the difference between tsunami and typhoon deposits, we analyze diatoms from the sediments in two coastal lakes in southern Japan where flood deposits have been linked to historical typhoon and tsunami events (Lakes Kawahara and Ryuo). The sediment cores extend from B.C.E. 500 to approximately C.E. 1000 and the downcore variation in diatom assemblages indicates a series of transitions from saline to fresher conditions in both Lake Kawahara and Lake Ryuo between approximately C.E. 500 and 1700. We observe an obvious deviation in diatom assemblages in event deposits previously identified to be either of tsunami or typhoon in origin. For the most prominent event deposit preserved in Lake Ryuo by the Hoei tsunami of C.E. 1707, the deposition of marine diatoms serves as evidence of marine flooding, while the subsequent deposition of soil and freshwater diatoms indicates the mobilization of terrigenous sediment during returning seaward flows. In contrast, the most prominent event deposit in Lake Kawahara is associated with freshwater flooding by the Kamikaze typhoon of C.E. 1281 and contains very low diatom abundances and a peak of freshwater taxa, followed by a peak in diatom counts potentially due to greater biological activity induced by a resultant influx of nutrients and re‐oxygenation during the event.

Abrupt diatom responses to recent climate and land use changes in the Cantabrian Mountains (NW Spain)

The multi-proxy study of sediment cores from Lake Isoba (43° 02′ N, 5° 18′ W; 1400 m a.s.l.) allows a detailed assessment of the past hydrological and environmental dynamics in north-western Iberia resulting from the interplay between climate variability and anthropogenic impact. The combination of diatom stratigraphy, sedimentology and high-resolution elemental geochemistry along with a robust chronological framework (established by 210Pb, 137Cs and 14C dating) provides a detailed environmental reconstruction for the past ~ 500 years. Abrupt changes in the fossil diatom assemblages indicate a high sensitivity of this small lake to past environmental change and allow identifying four major stages related to the main climate fluctuations of the Little Ice Age (LIA) and recent warming. High lake levels, enhanced runoff and higher productivity characterised the middle phase of the LIA (~ 1550 to 1630 CE), indicating an overall wet climate. Conversely, shallow lake levels, decreased runoff and relatively low productivity prevailed during the last phase of the LIA and the onset of the Industrial Era (~ 1630 to 1925 CE), likely due to colder and drier conditions. High lake levels and higher carbonate input occurred after ~ 1925 CE until the 1980s CE, when our data show an abrupt drop in lake levels probably caused by a regional negative rainfall anomaly related to climate warming during the past decades. Finally, since ~ 1997 CE a remarkable and abrupt increase in the lake nutrient load and turbidity is detected, probably associated with the replacement of transhumant sheep flocks with staying cattle. The main environmental changes reconstructed at Lake Isoba mostly agree with other palaeoclimatic records from northern Spain. However, the hydrological patterns reconstructed are opposed to those observed on the northern slopes of the Cantabrian Mountains. The recent and strong impact of land-use changes on the lake, causing more ecological disruptions than previous climate changes, is noteworthy and demonstrates the high sensitivity of mountain lakes to human activities in a global change context.

Long-term response of diatoms in high-elevation streams influenced by rock glaciers

In this study, we monitored diatom assemblages at various sites along a stream-lake network draining a glaciated alpine cirque landscape (2600 m asl) in the Swiss National Park over 19 years (2001–2020). The primary goal of this study was to examine the long-term changes in diatom assemblages in the running waters at high-elevation influenced by rock glaciers, and particularly the use of diatoms as indicators of environmental change. Because streams in the catchment are interconnected with lakes, another objective was to compare diatom assemblages between lake inlets and lake outlets as well as along the chain of lakes. The Macun cirque comprises two main drainage basins with one influenced by waters from rock glaciers. Lakes are interconnected by stream segments in both basins before merging at an outflow lake. We expected changes in diatom assemblages would reflect changes in long-term physico-chemistry of surface waters between basins, between lake inlets and outlets, and along lake chains. The novelty of this work is that covers a unique heterogeneous aquatic environment in the Alps affected by climate change, an area where the temperature has increased over twofold more than other regions in the northern hemisphere. This long-term study provides the opportunity of forecasting how other alpine freshwater ecosystems globally may change in the future. We combined a classical taxonomic approach and scanning electron microscopy (SEM) along with physico-chemical data across a broad spatio-temporal scale. Diatom assemblages showed distinct spatial differences between basins, reflecting basin-wide differences in water physico-chemistry. Diatoms in both basins displayed a major temporal shift in composition between samples collected pre-2010 with those collected post-2010 that also followed temporal changes in physico-chemistry, suggesting a tipping point occurred in the system. There were subtle differences in diatom assemblages between lake inlets and outlets, most evident in the precipitation/groundwater fed north basin. Lastly, a clear lake order effect was detected in both basins with upper basin sites differing from lower basin sites at similar altitude. We conclude that diatoms were effective in documenting the environmental changes that occurred in these alpine waters, specifically reflecting the abrupt change in physico-chemistry (concurrent tipping point in diatom assemblages) and the concomitant increase in surface water homogeneity and similarity of diatom assemblages over the study period.

Climate change and nutrient enrichment altering sedimentary diatom assemblages since pre-industrial time: evidence from Canada’s most populated ecozone

Lakes worldwide are under threat by a myriad of environmental stressors that have been increasing in number and magnitude. These stressors can be regional such as climate change, or local such as nutrient-rich runoff, invasive species, and road salt contamination, to name but a few. To protect lake ecosystems from further deterioration, we need long-term data to define pre-disturbance baselines and to identify stressors that are causing the greatest ecological changes. Paleolimnology is an effective approach to reconstruct limnological history, providing an important window into past changes. Here, we applied paleolimnological tools to explore the pre-industrial and contemporary diatom assemblage changes of 27 lakes located in the most populated ecozone in Canada, the Mixedwood Plains. We also examined a full sediment core for Lac des Chicots (Southern Québec), aiming to disentangle the impacts of natural versus anthropogenic interactions and to assess their relative effects on the lake’s biotic structure. Our ordination analysis suggests that the Mixedwood Plains lakes have experienced varying lake-specific ecological changes over the past ~150 years, with two major trends across most study lakes: (1) a prevalent increase in planktic species, and (2) a rise in mesotrophic/eutrophic taxa in lakes receiving high human impacts. Our case study of Lac des Chicots identifies ecological impacts from both historical natural events and recent human activities, such as cultural eutrophication and climate warming. Overall, our study demonstrates that lakes in the Mixedwood Plains ecozone have experienced marked ecological changes that are mainly associated with human impacts.

Pervasive changes in algal indicators since pre-industrial times: A paleolimnological study of changes in primary production and diatom assemblages from ~200 Canadian lakes

Anthropogenic stressors affect lakes around the world, ranging in scale from catchment-specific pollutants to the global impacts of climate change. Canada has a large number and diversity of lakes, yet it is not well understood how, where, and when human impacts have affected these lakes at a national scale. The NSERC Canadian Lake Pulse Network sought to create the first nationwide database of Canadian lake health, undertaking a multi-year survey of 664 lakes spanning 12 ecozones across Canada. A key objective of the network is to determine where, by how much, and why have Canadian lakes changed during the Anthropocene. To address this objective, we compared sedimentary chlorophyll a and diatoms from modern and pre-industrial sediment intervals of ~200 lakes. The lakes spanned a range of sizes, ecozones, and degrees of within-catchment land use change. We inferred the quantity of chlorophyll a, its isomers and main diagenetic products using visible reflectance spectroscopy. We found widespread increases in primary production since pre-industrial times. Primary production increased, on average, across all ecozones, human impact classes, and stratification classes. Likewise, an increase in planktonic diatom taxa over time was detected in the majority of sampled lakes, likely due to recent climate warming. However, regional factors (ecozones) explained the most variation in modern diatom species assemblages as well as their temporal turnover. Furthermore, lakes with high human impact (i.e., higher weighted proportions of human land use in the catchment) exhibited greater taxonomic turnover than lakes with a low human impact class. The greatest diatom turnover was found in the agriculture-rich Prairies and the lowest in the sparsely populated Boreal Shield and Taiga Cordillera ecozones. Overall, our study highlights that drivers operating at different geographic scales (i.e., climatic and land-use changes) have led to significant alterations in algal indicators since pre-industrial times across the country.

Using diatoms to track road-salt seepage into small, shallow, softwater Ontario lakes

Since the 1950s, the widespread application of road salt for winter road maintenance and safety in cold regions has led to increased conductivity levels in many freshwater systems. Salting practices have adversely affected freshwater biota; however, the magnitude of ecological impacts may vary by species and ecosystem. Here, we examine diatom assemblage changes during the past ∼200 years from the sedimentary records of five impacted lakes (measured specific conductance values of 149–350 µS·cm−1) and a reference lake (18 µS·cm−1) located in the Muskoka River Watershed, south-central Ontario, Canada. Diatom compositional changes in the road-salt-impacted sites were consistent with increasing conductivity and increased diatom-inferred (DI)-conductivity was evident during the latter half of the 20th century in the impacted lakes, concurrent with known road-salt application. The strongest predictor of DI-conductivity changes among the six lakes was the kilometre equivalents of roads within the watershed (i.e., kilometres of road × number of lanes). Similar to changes observed in a previous study focusing on cladoceran assemblages, we conclude that even modest applications of road salt can affect diatom assemblages in softwater lakes.

Diatom assemblage responses to multiple environmental stressors in a deep brackish plateau lake, SW China.

Eutrophication, climate change, water level fall, fish introduction, and salinity have been widely recorded to impact lake ecosystems around the world. However, the combined responding pattern in the lake ecosystem to the above multiple environmental stressors is not well understood. Here, we present diatom assemblage and geochemical indicators (total organic carbon (TOC), total nitrogen (TN), and total phosphorous (TP)) in lake sediment to investigate the long-term trends in the aquatic ecosystem in response to multiple environmental stressors in Lake Chenghai, a deep brackish lake located on Yunnan-Guizhou Plateau, SW China, during the past 80years. We identified 8 genera and 15 species of diatom reaching a relative abundance of ≥ 2% in at least one sample, with the dominant taxa such as Cyclotella rhomboideo-elliptica and Aulacoseira alpigena through the sediment core. There was a clear shift in the diatom community from oligotrophic species of C. rhomboideo-elliptica to eutrophic species such as Cyclotella meneghiniana and Cyclostephanos dubius becoming dominant since ca. 1998 CE. In addition, the changes in the fish introduction, water level, temperature, and salinity also resulted in the variation in abundance of planktonic and benthic diatoms. The increase in the abundance of diatoms of C. rhomboideo-elliptica since ca. 1986 CE may be related to the decline in water level and increasing fish production. For one thing, the decline in water level reduces the concentrations of benthic diatoms but increases the relative biomass of planktonic diatoms. For another, the increasing fish production results in the decrease in the zooplankton biomass, thus reducing the predation pressure on planktonic algae. Besides, some specific species such as C. meneghiniana may respond to the increased nutrient release and increased salinity since ca. 1998 CE. Combined, our findings demonstrate that trophic level is the main driver of diatom assemblage changes, and other environmental variables including water level, fish introduction, and climate warming also contribute to diatom community variation in this brackish plateau lake during the last 80years.

Multiple drivers of ecological change in Arctic lakes and ponds.

Griffiths et al. (2017) analyzed several ponds and lakes from the Cape Herschel region of Ellesmere Island in order to “…explicitly examine the role of ice cover as the dominant driver of diatom assemblage change…”. I reanalyze their data and suggest that their classification scheme, that they propose is due to differences in ice cover seasonality (“warm”, “cool”, “cold”, and “oasis”), is confounded with other morphological and chemical variables that better explain the differences between the groups. The “cold” sites are the deepest (lakes) and differ from the small, shallow ponds that occasionally dry, which would therefore have different diatom assemblages and histories. The “oasis” sites are nutrient enriched and probably have more stable water supplies, thereby enabling an aquatic flora providing habitats for diatoms. A key part of their interpretation is that “warm” sites have responded more rapidly to recent climate change than “cold” or “cool” sites, but their chronologies do not allow for such a conclusion. There is no clear difference between “cool” and “warm” sites, and problems in dating the sequences means inferences about their histories are not supported by data. Their results, which are restricted to the past century, are contradicted by a Holocene sequence from the region.

Diatom sedimentary assemblages and Holocene pH reconstruction from the Canadian Arctic Archipelago’s largest lake

ABSTRACT The Arctic has warmed significantly over the past decades. However, the evolution of Arctic climate during the Holocene remains to be clarified in more detail, and regional factors controlling aquatic ecosystem evolution need to be better defined to grasp the sensitivity of lakes to rapid environmental change. Nettilling Lake was studied for changes in sedimentary diatom assemblages over the last 5 000 years. Lake water pH was reconstructed by applying a diatom-based lake water pH inference model. We hypothesized that the changes in diatom assemblages were driven by variations in lake water transparency and attendant water turbidity associated with the input of fine suspended solids from glacial meltwaters. Reduced underwater light resulted in greater abundance of planktonic over benthic taxa from ca. 5 000 to 3 000 yrs. cal. BP, followed by less turbid conditions and proliferation of benthic taxa during regional cooling. The lake water was slightly alkaline throughout the Holocene, ranging between pH 7.1 and 7.7. Our results support the notion that hydrological processes, dependent on climate variations, have a first-order influence on the regulation of the lake water pH through glacial meltwater inputs, which will likely continue to control the lake’s long-term chemical and biological evolution.

Diatom and Macroinvertebrate assemblages to inform management of Brazilian savanna’s watersheds

Human activities are increasingly affecting freshwater ecosystems and biodiversity, especially in neotropical regions like the Brazilian savanna. Limited research and data availability have inhibited the development and implementation of systematic bioassessment programs and management guidelines. Identifying drivers of biological assemblages’ composition and ecological thresholds along human disturbance gradients is an important step to protect and recover freshwater ecosystems while avoiding threats to biodiversity, goods and services of value to humans. The objectives of this study were to: 1) assess changes in the composition and density of periphytic diatom and macroinvertebrate assemblages relative to natural and human disturbance gradients in Brazilian savanna’s streams, and 2) identify ecological thresholds for direct and indirect human disturbances and potential indicator taxa that could inform the development of biomonitoring programs in Brazil and other neotropical countries. Samplings were carried out in 52 stretches of streams located in central Brazil during two campaigns throughout 2018. Ordination analyses (NMDS) were applied to identify the main drivers of biological assemblages' composition and analyses of taxa distribution in disturbance gradients (TITAN) were carried out to detect possible thresholds and potential bioindicator taxa. Our results pointed out that the scale of land use in the catchment, treated sewage input, and water quality variables (nitrate, phosphate, and conductivity) were the main drivers of diatom and macroinvertebrate assemblages. Taxa such as Eunotia (diatoms) and some families of the orders Ephemeroptera, Plecoptera, and Trichoptera (EPT, macroinvertebrates) were associated with natural conditions, while Nitzschia palea and Gomphonema (diatoms) and Oligochaeta and Hirudinea (macroinvertebrates) were related to environments with a higher degree of disturbance. Although ecological thresholds along disturbance gradients varied among taxa and biotic groups, our results revealed that relatively minor increases in land use in the riparian zone (>0%) and in the upstream catchment (0–33%) were sufficient to trigger significant changes in macroinvertebrate and diatom assemblages. The limits of tolerance for conductivity, nitrate and phosphate were also low, varying between 7-163 µS.cm−1, 0.3–1.0 mg.L−1 and 0.03–1.5 mg.L−1, respectively. In general, the values we found were more restrictive than those provided by Brazilian government guidelines, suggesting that the latter would be insufficient to maintain the integrity of biological assemblages in Brazilian savanna’s watersheds. We provided valuable knowledge about the sensitivities and tolerances of diatom and macroinvertebrate’s taxa/assemblages that can be especially useful for the proper freshwater and watersheds management.

Sensitivity of tidal marshes as recorders of major megathrust earthquakes: constraints from the 25 December 2016 Mw 7.6 Chiloé earthquake, Chile

ABSTRACTWe present evidence of land‐level change resulting from the 2016 Mw 7.6 Chiloé earthquake from tidal wetlands along the southern coastline of Isla de Chiloé, Chile, to test criteria for the detection of low‐level, <0.1 m, coseismic land‐level change. In order to record coseismic land‐level change in tidal wetland sediments, both the creation and preservation thresholds must be exceeded. High‐resolution diatom analyses of sediment blocks at two tidal marshes reveal that the 2016 earthquake exceeded the creation threshold and a statistically significant change in diatom assemblage is recorded. In contrast, the preservation threshold was not exceeded and the record of coseismic land‐level motion is not preserved at any location visited. After nine months, interseismic and coseismic changes are statistically indistinguishable. The most sensitive part of the tidal wetland is not consistent between research locations, possibly as a result of changes in sedimentation after the earthquake. We compare records of change from great earthquakes in Alaska with the record from the Chiloé earthquake to explore the detection limit. We propose that coastal palaeoseismological records are highly likely to underestimate the frequency of major (Mw 7–8) earthquakes, with important implications for recurrence intervals and assessment of future seismic hazards.

Evolutionary inferences from the sedimentary deposits of Lake LH73, Larsemann Hills, East Antarctica

Lakes from the coastal oasis of Larsemann Hills respond to changes in regional climate. They are mainly influenced by the ice-sheet dynamics and regional eustatic variation based on their proximity to either of them or both. The sedimentary sequences of the ice-free regions of Antarctica offer unique climate records (lake sediment deposits). Here, we present the evolution of a lake reconstructed from its sedimentary sequence. A 60 cm long radiocarbon-dated sediment core spanning the last 11.8 kyr from Lake LH73 located in Broknes Peninsula of Larsemann Hills is studied for grain size fractions, magnetic susceptibility, and diatom abundance. Based on the diatom and grain fraction data, it is inferred that the lake persisted as a proglacial lake during the early-Holocene (11.8 to 7.2 ky BP) dammed by ice-sheet towards the south. The lake level was at least 3 m higher than the present level (4 m) as inferred from the diatom transfer function owing to the presence of ice dam. The retreat of the ice-sheet and the collapse of the ice dam between 7.2 and 7 ky BP resulted in the lake transitioning to an isolated lake with the lake level attaining current level (4 m). As a consequence, the catchment area increased and the lake received meltwater from the snowbanks hence largely modulating the sedimentary process. The absence of glacial clay from 5 ky BP suggests that the lake was cut-off from the glacial input. The appearance of diatom at ~ 8.6 ky BP indicates the establishment of optimal condition for productivity in the lake ecosystem. Stauroforma inermis and Plannothidium quadripunctatum (Psammothidium abundans) dominates between 7 ky BP till present (11.8 to 7 ky BP), responding to lake level variation. Ice-sheet retrieval and ablation led to transition of proglacial-to-isolated lake leading to shallowing of the lake as reflected in diatom assemblage changes.