Modern Diatom Assemblages Research Articles

Modern diatom assemblages and their association with meteorological conditions in two lakes in northeastern Poland

The population dynamics of diatoms are affected by a variety of environmental variables. Due to their short generation times and high sensitivity to changes in physicochemical conditions, diatoms are considered good environmental indicators. The main goal of our study was to find and explain the relationships between changes in meteorological conditions and diatom fluxes and taxonomic composition based on the example of two small lakes: Łazduny and Rzęśniki. Using meteorological data, sediment traps, and regular measurements of limnological and hydrochemical properties of the water column, we collected a three-year-long, high-resolution series of observations. The results show that total diatom fluxes are indirectly influenced by changes in meteorological conditions, acting through changes in the mixing regimes that determine the nutrient and light availability in lakes. Statistical analyses showed that the variability of the diatom data is correlated with air temperature and wind speed. Nevertheless, their influence on diatom assemblages is most likely the surrogate for the complex changes in the physical structure of the investigated lakes. Despite many similarities between the studied lakes such as mixing regime patterns, dominant diatom taxa, and seasonal dynamics of diatom fluxes, we recorded differences in both the seasonal succession of specific diatom taxa and the occurrence of the peaks of total fluxes, and differences in taxonomic composition referring to less dominant taxa. We attribute these dissimilarities to the local conditions, such as the hydrological types of the lakes, the extent of the littoral zone, and exposure to the sunlight connected to the position in the catchment.

Ecological distribution of modern diatom in peatlands in the northern Greater Khingan Mountains and its environmental implications.

Relationships between diatom assemblages and environmental variables in peatlands of the northern Greater Khingan Mountains are helpful for understanding the indicative significance of diatoms to environment changes, and potentially provide a reference for environmental monitoring and paleoenvironment reconstruction in the edge of monsoon region. In this study, we analyzed modern diatom assemblages and their relationships with environment factors in 30 surface samples from shrubby-herbaceous and herbaceous peatlands based on ordination analysis. Benthic and epiphytic Pennatae diatoms are mainly ecological types, whereas planktonic Centricae diatoms are relatively fewer. The most diverse genera are Pinnularia and Eunotia. Eunotia paludosa and Achnanthidium minutissimum dominated in shrubby-herbaceous peatlands, while Navicula minima and Fragilaria capucina dominated in herbaceous peatlands. The diatom community structures are different in different vegetation types and the diatom species diversity in herbaceous peatlands is higher than that in shrubby-herbaceous peatlands. CODMn and pH are the most important environmental factors affecting diatom species composition and diversity. Eunotia bilunaris, Eunotia mucophila, and Eunotia paludosa can be used as indicators of acidic water environments. Caloneis silicula, Fragilaria capucina, Hantzschia amphioxys, and Navicula radiosa can be applied to indicate the weak alkaline conditions. Eunotia bilunaris and Eunotia paludosa can indicate low conductivity, while Sellaphora pupula indicates the medium-high conductivity. Fragilaria capucina and Navicula radiosa can indicate oligotrophic habitats.

A diatom-based predictive model for inferring past conductivity in Chadian Sahara lakes

For decades, diatoms have been recognized as powerful bio-indicators of modern water quality. They have also been utilized in the design of transfer functions, which can be applied to diatom assemblages in lake sediment cores to infer aspects of past lake hydrochemistry and estimate variables that can be incorporated into paleohydrology models. The Ounianga lakes, in the heart of the Chadian Sahara, possess unique and well-preserved sediment records that extend back beyond the middle Holocene. Today, the lakes display a range of hydrochemical conditions, from fresh to hypersaline. Mainly fed by fossil groundwater that originates in the Nubian Sandstone Aquifer System, measured conductivity across these lakes varies from 217 to 352,000 µS cm−1, values that are influenced by factors such as hydrology, local geomorphology (e.g., depth and area), and aquatic vegetation. Although these lakes have been on the UNESCO World Heritage List since 2012, they have never been studied in detail because they are located on the fringes of the Chadian Sahara. The distribution of diatom taxa in the lakes today is closely linked to water-column physical and chemical conditions, especially conductivity. Whereas each lake has particular features that influence its diatom flora, diatoms across a conductivity gradient enabled identification of three distinct waterbody types, freshwater lakes, meso-saline to hyper-saline lakes, and freshwater springs. Relationships between diatom species distributions and environmental variables were examined using multivariate analysis, which revealed that conductivity is the variable that explains most of the variance in the diatom flora. We used modern diatom assemblages from the lakes to develop a predictive model (transfer function) for conductivity, using the weighted averaging method. Our conductivity prediction model is strong, with a coefficient of determination (R2) of 0.89 between estimated and measured values, and a value of 0.78 using jackknife estimates of prediction. This study better constrained conductivity optima and tolerance values for diatom species found in the Ounianga lakes, thereby enabling development of a model that will yield better inferences for past conductivity, using diatoms from lake sediment records in the region.

Staurophora ouniangaensis sp. nov. (Bacillariophyceae, Anomoeoneidaceae), a new diatom from the Ounianga Lakes in the Sahara, Chad

Modern diatom assemblages were investigated in the Ounianga Lakes located in the driest part of the Chadian Sahara. These lakes still exist today due to groundwater seepage. They contain a variety of diatom species, one of which could not be identified. Here we describe a new species of Staurophora, S. ouniangaensis sp. nov. based on detailed light and scanning electron microscopy observations. The new taxon presents all morphological characters attributed to this genus and differs from other Staurophora species mainly by its specific valve outline and the slightly twisted frustules along the transapical axis. We compared this new species with the morphologically resembling Cymbella obtusa which was reported from Chad. The latter taxon has a much coarser stria density than our newly described species. Staurophora ouniangaensis sp. nov. supports that Staurophora species prefer inland saline environments. Indeed, in the Ounianga lakes, the new species was mainly observed in mesosaline to hypersaline waters.

Diatom-based paleoenvironmental reconstruction from the coast of Northern Patagonia, Argentina

Estuaries and deltas are common along modern coasts and are well represented in the geologic record because of the high preservation of their deposits. Worldwide, estuaries are more abundant than deltas because of the relatively short time available for Holocene coastal sedimentation due to sea level variations. The evolution of estuaries and deltas during the recent geological past can be reliably reconstructed. Among others, paleostudies in estuaries aim to reconstruct sea level and freshwater input variations, and the timing and frequency of extreme events (e.g., tsunamis). A significant number of studies have been published in the past 20 years, which used preserved remains of microorganisms accumulated during long intervals in sediments. The preserved record of fossil remains evidences the response of microorganisms to environmental changes at the time of their deposition. Among the proxies preserved in sediments, diatoms are one of the most reliable bioindicators to trace environmental changes. Due to rapid reproduction rate under favourable conditions and their siliceous frustule, they are abundant in recent and older sediments from estuaries and deltas. In this manuscript, results from previous studies were reviewed, integrated and statistically analyzed with the aim to reconstruct Holocene coastal changes in relation to sea level curves. These studies were conducted along a latitudinal transect on four estuarine mesotidal systems from Northern Patagonia, Argentina: Colorado River delta (39°40′S; 62°08′W), San Blas Bay (40°33′S; 62°13′W), Negro River (40°57′S; 62°49′W) and Chubut River estuaries (43°22′S; 65°03′W). The focus is on the preserved diatom assemblages and the paleoecological information they represent. Seven coastal sedimentary cores were correlated. Based on multivariate analysis (NMDS), fossil and modern diatom assemblages were integrated and compared. These estuary systems are dominated by littoral drift, where the plains became flooded by the sea about 7000 14C years BP ago. When the sea transgression occupied the area, Colorado delta and San Blas Bay evolved from tidal channels flooded to tidal flats, sand flats and marshes during the late Holocene regression. At Colorado River, the gradually increase of the fluvial influence is associated with the eastward progradation of the deltaic lobe during the last ca. 7000 14C years BP. At the Negro River estuary, a shallow vegetated brackish/freshwater environment evolved into marine conditions, tidal channels and marshes during the last 2000 14C years BP. A marine coastal environment (estuary) at ca. 4400 14C years BP that evolved into a salt marsh was inferred at the inlet of the Chubut River. Diatom salinity-preferences showed how connections between the wetland and the sea varied over time with sea level changes and gradual infilling. Diatom-inferred paleoenvironments and radiocarbon dates allowed reconstruction of the coastline position to the east in the Early Holocene and to the west during transgressive-regressive events.

Distinguishing reworked diatoms derived from Neogene marine strata in modern coastal assemblages for understanding taphonomic processes and reconstructing Holocene paleoenvironments in the Tokachi coastal area, Hokkaido, Japan

Identification of autochthonous, allochthonous, and reworked (derived fossil) taxa in diatom analyses is of fundamental importance for understanding how taphonomic processes affect diatom assemblages and for estimating paleoenvironments in coastal areas. To characterize modern and Neogene diatom assemblages, we surveyed the distributions of modern diatom assemblages and of Neogene fossil diatom assemblages in the Toberi River area, eastern Hokkaido, Japan. Then, on the basis of these results, we analyzed two cores collected near the Toberi River mouth and examined changes in the fossil diatom assemblages during the late Holocene. The results showed that not only allochthonous and extinct diatom taxa, but also extant taxa that have persisted over an extended geological time period, should be excluded when diatom assemblages are analyzed to reconstruct Holocene paleoenvironments in this area.

An assessment of diatom assemblages in the Sea of Okhotsk as a proxy for sea-ice cover

Abstract. Knowledge of past variations in sea-ice extent is crucial for understanding the relationship between climate change and changes in sea ice. Diatom assemblages could be applied as a proxy for paleo-sea-ice extent; this requires accurate information on the modern species that are indicative of sea ice. Scanning electron microscope observations were performed on modern diatom assemblages in sea ice, sinking particles, and surface sediments in the Sea of Okhotsk. A sea-ice sample was collected in the southwestern Sea of Okhotsk near Hokkaido island in February 2013. Fragilariopsis cylindrus was the dominant diatom species in the sea-ice sample, accounting for 87 % of the total diatom assemblage. Time-series sediment traps were deployed during 1998–2000 at two stations, M4 and M6, off Sakhalin island. Total diatom fluxes ranged from 105 to 108 valves m−2 d−1 with noticeable seasonality. During the sea-ice covering period, the total diatom flux decreased by 1 or 2 orders of magnitude. The highest diatom fluxes were observed in spring and summer. The diatom species composition in sinking particles also showed pronounced seasonal changes. During summer and fall, the Shionodiscus trifultus group and Neodenticula seminae were the major diatom taxa. During the sea-ice covering period, Fragilariopsis cylindrus and Bacterosira bathyomphala resting spores were abundant. Both the sea-ice-related species showed similar flux patterns except for the spring bloom after sea-ice retreat: F. cylindrus fluxes exhibited pronounced spring bloom peaks of 108 valves m−2 d−1; in contrast, the fluxes of Bacterosira bathyomphala resting spores during the spring bloom were 1 order of magnitude lower than those of F. cylindrus. Surface-sediment core XP98-MC4 was obtained near station M6 sediment-trap site off Sakhalin island. The relative abundance of Fragilariopsis cylindrus in the surface-sediment diatom assemblage was only 6.4 %, markedly lower than that in the sediment-trap samples (43.4 %). In the surface sediment, the relative abundances of diatom taxa with heavily silicified valves such as B. bathyomphala resting spores, Shionodiscus variantius, and Thalassionema nitzschioides were greater than their relative abundances in sinking particles.

Assessing the relative impacts of land‐use change and river regulation on Burdekin River (Australia) floodplain wetlands

Abstract The Burdekin River floodplain wetlands are internationally important and act as a sink for sediments and nutrients that would otherwise enter the World Heritage‐listed Great Barrier Reef lagoon. The Burdekin River has the highest natural discharge of any Great Barrier Reef catchment and contributes the greatest mass of sediment to the reef. The river and its catchment have been substantially modified by land clearance, river regulation introduced in the 1960s, the construction of the Burdekin Falls Dam in 1987 and invasive aquatic plants. We hypothesised that the natural variability of Burdekin River discharge would render its wetland ecosystems resistant to human‐induced pressures. Diatoms were analysed from the sediments of two lower Burdekin River floodplain wetlands with contrasting regulation histories. Labatt Lagoon has a long history of flow alteration, whereas Swan's Lagoon has experienced limited regulation. Both wetlands experienced dramatic increases in sedimentation as a result of land clearance, yet the Swan's Lagoon diatom record indicates that the wetland's ecology remains within the range of natural variability. In contrast, Labatt Lagoon is markedly different from its pre‐settlement state. It is now permanent, having been ephemeral for more than 1000 years before European settlement. The conversion to artificial permanence facilitated aquatic plant invasion, resulting in more marked changes than widespread pastoralism. Modern diatom assemblages from 28 lower Burdekin River floodplain wetlands indicate that the contemporary flora of Swan's Lagoon is unusual. Most wetlands have similar assemblages to those in the Labatt Lagoon record in its present, altered state. It is possible, therefore, that the ecological shift registered in Labatt Lagoon is widespread. Diatom records highlight the striking impact that river regulation has had on the trajectory of the Burdekin floodplain wetlands. It is likely that wetland rehabilitation can be facilitated by the re‐introduction of a variable hydrological regime.

Identifying temporal and spatial patterns of diatom community change in the tropical Andes over the last c. 150 years

AbstractAimLakes in the Ecuadorean Andes span different altitudinal and climatic regions, from inter Andean plateau to the high‐elevation páramo, which differ in their historical evolution in the several centuries since the pioneering Humboldt expeditions. Here, we evaluate temporal and spatial patterns of change in diatom assemblages between historical (palaeolimnological) and modern times.LocationEcuadorean AndesMethodsWe compared historical (pre‐1850) and modern (2017) diatom assemblages from 21 lakes and determined the relative role of environmental (water chemistry and climate) and spatial factors (distance‐based Moran's eigenvectors maps) on both assemblages using non‐metric multidimensional scaling (NMDS) with environmental fitting. In addition, we used redundancy analysis (RDA) with variance partitioning to estimate the historical (measured using downcore assemblage composition) effects on modern diatom assemblages and identified diatom species that contributed most to dissimilarity between the two times.ResultsDiatom changes between the two time points were limited across the group of lakes, as indicated by the NMDS ordination. Variance partitioning indicated that modern diatom assemblages were affected by environmental and spatial effects, but with non‐significant effects of past diatom species composition. Ordination results showed that variables related to elevation and water chemistry affected both modern and historical diatom assemblages. Diatom species with the best fit on NMDS axes (i.e. >70%) were influenced by elevation and climatic variables. The most distinctive change between the two time periods was the higher relative abundance of planktic diatom species in top‐core assemblages of some lakes, but in a highly variable fashion across gradients of increased elevation and water depth.Main conclusionsLandscape palaeolimnological analyses of varied Ecuadorean Andean lakes demonstrate both environmental and spatial controls on diatom metacommunities. The multi‐faceted ecological control of the altitudinal gradient on both historic and contemporary diatom assemblages suggests species sorting and dispersal constraints operating at centennial time‐scale. Although a few individual lakes show substantive change between the 1850s and today, the majority of lakes do not, and the analysis suggests the resilience of lakes at a regional scale. We emphasize the potential of diatom palaeolimnological approaches in biogeography to test ecologically relevant hypotheses of the mechanisms driving recent limnological change in high‐elevation tropical lakes.

Modern diatom assemblages from Chilean tidal marshes and their application for quantifying deformation during past great earthquakes

ABSTRACTTidal marsh sediments from south‐central Chile provide evidence for multiple great earthquakes. Diatom transfer functions, statistical models of the relationship between species preserved in the sediment and elevation, provide quantitative estimates of coseismic vertical land‐level change associated with individual earthquakes. However, in south‐central Chile, our ability to quantify land‐level change is currently limited by a lack of understanding of the environmental variables controlling the distribution of diatoms, an essential prerequisite for converting variations in fossil diatom assemblages into quantitative estimates of past elevation changes. We present a new modern diatom dataset for the region and explore the implications of the scale of the dataset used in transfer function models on the reconstructions of land‐level change. Modern training sets containing samples from a regional scale are superior to sub‐regional and local‐scale training sets, providing closer estimates for known deformation during the great 1960 Chilean earthquake, a higher proportion of good modern analogues and uncertainty terms up to 42% smaller than previously published reconstructions. Copyright © 2017 The Authors. Journal of Quaternary Science Published by John Wiley & Sons Ltd.

Evolution of Lake Ailike (northern Xinjiang of China) during past 130 years inferred from diatom data

We have studied diatom assemblages of a 42-cm-long sediment core obtained from Lake Ailike in northern Xinjiang of China to reconstruct the hydrological and ecological variations of the past 130 years. Three environmental parameters of lake water (i.e., diatom-inferred pH, diatom-inferred total phosphorus and diatom-inferred conductivity) were estimated by comparing the fossil diatom assemblages from the Lake Ailike core with the modern diatom assemblages in the European Diatom Database. The reconstruction exhibited two major stages: stage A (1884–1960) was a “natural” stage and stage B (1960–2013) was a “human” stage. The “natural” stage can be further divided into two sub-stages: A-1 (1884–1920) was characterized by an averagely low lake area and A-2 (1920–1960) was a stably high lake-area period. The comparison between the reconstructed lake-area variations from Lake Ailike with the treering-recorded Palmer Drought Severity Index (PDSI) variations from nearby Hutubi shows that the low lake area from 1884 to 1920 was correspondent with a dry period and that the high lake area from 1920 to 1960 with a wet period. The similarity between PDSI from Hutubi and the Atlantic Multidecadal Oscillation (AMO) implies a climatic linkage. The linkage is revealed by the tight in-phase relationship between AMO and SWP (Siberian warm-season precipitation), that is, the AMO-promoted SWP might have extended its influence to northern Xinjiang. The “human” stage can also be divided into two sub-stages: B-1 (1960–2000) was a regressing stage and B-2 (2000–2013) was a transgressing stage. The man-made regression led to dramatic increases in TP of the lake water, TOC and TN of the sediments and also in the resulted decrease in lake water pH. Fortunately, the Irtysh-Karamay Canal started to inject water into the lake in 2001, resulting in a constant expansion of lake-covered area and also in a constant improvement of ecological conditions.

Paleoenvironmental insights into the Quaternary evolution of the southern Brazilian coast based on fossil and modern diatom assemblages

Diatom assemblages provide a strong basis for detailed interpretations of paleoceanography and diatom paleoecology of the southern Brazilian coast. Nine cores obtained in the coastal plain, shelf and continental slope and thirteen surface sediment samples of the Patos Lagoon, provide an excellent opportunity to use a paleoecological approach to study detailed Quaternary environmental changes in the southern coasts. In the interval studied, the basin sedimentation occurred in a marine-dominated environment related to humid periods prior to 43,500yearBP. Abundant diatom taxa are highlighted as excellent indicators of environmental changes, including salinity, sediment composition and transport by incised-valleys, providing additional insights into coastal evolution. Paralia sulcata is dominant in all analyzed cores, occurring continuously from Late Pleistocene to Holocene, but is rare in the modern sediments. Non-marine diatom assemblages occurred continuously, but in very low densities in periods before and after the Last Glacial Maximum. Abrupt changes in sedimentation have been recorded related to shifts from marine-dominated to shallow-estuarine deposition. During the Holocene, sea-level rose rapidly which is responsible for the progressive coastal flooding prior to 8420–7930yearBP. The Holocene barrier system became more continuous and Patos Lagoon developed. Consequently, the secondary former inlets from the Jacuí and Camaquã rivers are closed and only one single inlet (Rio Grande channel) remains active. The spatial distribution of modern assemblages is controlled by salinity gradient, wind action, and climate forcing. The bottom sediments of Patos Lagoon are dominated by freshwater diatom Aulacoseira veraluciae, Aulacoseira sp. 2 and marine-estuarine species of Cyclotella litoralis.

Reconstructing phosphorus levels using models based on the modern diatom assemblages of 55 lakes in southern Quebec

This study shows how modern diatom assemblages, obtained from the surface sediments of 55 deep and shallow lakes in southern Quebec and representing a gradient in trophic state and specific conductivity, can be used to reconstruct the historical water column concentrations of total phosphorus (TP) over time. Inference models were developed with the aid of regression analyses based on weighted means. The TP models demonstrate statistical performances equal to or superior to TP models developed in the past for other regions in Canada and elsewhere in the world. A reconstruction of TP values, based on fossil diatom assemblages of Lac Nairne (Charlevoix, QC), was conducted. Comparing the TP inferred values with the measured values and those estimated using export coefficients, the reliability of the model was proven. In addition, the comparison of the results obtained from reconstructions performed with two inference models (Quebec and Ontario lakes) also revealed that models predicted TP and not a collinear variable.

Diatoms indicate that calcium decline, not acidification, explains recent cladoceran assemblage changes in south-central Ontario softwater lakes

In recent decades, softwater lakes across Canada have experienced a wide array of anthropogenic influences, with acidification and climate warming of particular concern. Here, we compare modern and pre-industrial sedimentary diatom assemblages from 36 softwater lakes located on the Canadian Shield in south-central Ontario to determine whether lake acidification or reduced calcium availability was the main stressor responsible for recent declines in Ca-sensitive cladoceran taxa. Regional surveys of south-central Ontario water chemistry have identified the pH recovery of many formerly acidified lakes, and our fossil diatom-inferred pH analyses indicate that modern lakewater pH in the 36 study lakes is similar to (or higher than) pre-industrial levels, with diatom assemblages from both time periods dominated by taxa with similar pH preferences. In addition, modern diatom assemblages compared to pre-industrial assemblages contained higher relative abundances of planktonic diatom taxa (e.g. Asterionella formosa and the Discostella stelligera complex) and lower relative abundances of heavily silicified diatoms (e.g. Aulacoseira taxa) and benthic fragilarioid taxa. These taxonomic shifts are consistent with warming-induced changes in lake properties including a longer ice-free period, decreased wind speed and/or increased thermal stability. We conclude that recent changes observed within the cladoceran assemblages of these lakes are not a response to acidification, but are likely a consequence of Ca declines. In addition, our data suggest that regional climate warming is now responsible for the diatom changes observed in this region.

Distributions of modern diatom assemblages from surface deposits in Kesennuma Bay and surroundings, northeast Japan

Distributions of modern diatom assemblages from surface deposits in Kesennuma Bay and surroundings, northeast Japan

The significance of modern diatoms as paleoenvironmental indicators along an altitudinal gradient in the Andean piedmont of central Argentina

The objectives of this study were to (a) analyze the distribution pattern of diatom assemblages in freshwater systems from central-western Argentina, (b) investigate the key climatic and limnological factors affecting the species distributions, and (c) evaluate the extent and limitations of diatoms as Quaternary paleoenvironmental indicators in the area. Diatoms were sampled and physical and chemical variables were quantified in a total of 45 lotic and lentic sites. Altitude, annual precipitation, and summer (January) temperature data were obtained from a world climate database. Previously published data from an adjacent region was incorporated into the modern dataset. Environmental variables significantly influencing diatom assemblage composition were identified through partial canonical correspondence analyses (CCAs). In order to assess the degree of analogy between modern and fossil assemblages, the modern analog technique (MAT) was applied. Diatom taxa showed distinct abundance patterns in relation to altitude, with a shift from communities dominated by periphitic and benthic taxa in lower sites to communities dominated by small benthic–tychoplanktonic taxa in high elevated waterbodies. The pH was the strongest variable explaining the variance within the diatom dataset, followed by log-conductivity and summer air temperature. The comparison of modern diatom assemblages with fossil sequences allowed reconstruction of the evolution of shallow, lentic and vegetated habitats. A good analogy between modern and Pleistocene diatom assemblages was found. The strong relationship between diatoms and climate-driven environmental variables, such as pH, conductivity and summer temperature, enhances the importance of the diatom floras as paleoclimatic indicators in the area and encourages future development of quantitative inference models for the region.

Spatio–temporal variability of diatom assemblages in surface sediments of Lake Peipsi

The modern diatom assemblages in surface sediments of large and shallow Lake Peipsi sensu stricto (2611 km 2; maximum depth 12.9 m) in north-eastern Europe (Estonia) were studied. Ordination techniques were used to identify physical environmental parameters (grain size of sediment, water depth and distance from the shore) that explain the distribution of diatoms in the lake. Diatom species distribution and concentration in surface sediment showed large variations (concentrations varied from 3.3 × 10 6 to 149 × 10 6 g −1 dry weight). The texture of surface sediments and concentration of diatoms are highly correlated ( p ≤ 0.05), thus suggesting that the relocation of cohesive particles (silt) due to wind and wave actions affects the spatial variations of diatoms in surface sediments. The diatom data set analysed in the 1970s, and samples taken in 2006 showed some perceptible changes in the composition and concentrations of the diatom assemblages over this time period. However, main dominant diatom taxa have not changed during more than 25 years.

Local physiographic controls on the responses of Arctic lakes to climate warming in Sirmilik National Park, Nunavut, Canada

Twenty high Arctic lakes and ponds were sampled for water chemistry and modern diatom assemblages in two distinct physiographic sectors of Sirmilik National Park, Nunavut, Canada. Sites on southwestern Bylot Island were warmer, more alkaline, less dilute, and had higher concentrations of nutrients, DOC and Chl-a (mesotrophic to oligo-mesotrophic), whereas sites on Qorbignaluk Headland on northern Baffin Island were deeper, very dilute, mostly oligotrophic and had lower pH. Diatom assemblages differed markedly between these two regions as a consequence of limnological differences between them. Paleolimnological records, spanning > 200 years and dated by 210Pb activity, were produced from each region to compare biological responses to recent warming inferred from glaciological studies on Bylot Island and regional syntheses for the Arctic. Diatom assemblages began to change around AD 1900 at both sites. At Qorbignaluk Headland, marked shifts in diatom community composition occurred during the twentieth century, with large increases in the abundance of planktonic diatoms. At Bylot Island, diatom community changes began around the same time, and involved modest decreases in planktonic diatoms and increases in inferred specific conductance, likely because of a decrease in the areal extent of the small lake as a response to warming. The study confirms that responses of freshwater ecosystems to climate warming vary depending on local physiographic factors.

Morphological and phylogenetic comparisons of Neodenticula seminae (Bacillariophyta) populations between the subarctic Pacific and the Gulf of St. Lawrence

Neodenticula seminae is a very important member of modern diatom assemblages in the Bering Sea and at middle to high latitudes of the North Pacific. In the North Atlantic, this species was considered extinct until it was recorded in high abundance in the 2001 spring phytoplankton bloom of the Gulf of St. Lawrence, Eastern Canada. Here, we discuss some of the most distinctive features of its morphology, including variation in some characters between the subarctic Pacific and the Gulf of St. Lawrence specimens. Most importantly, we observed that the deck and basal ridges, and the solid-walled costae (formerly known as primary pseudosepta) characteristic of N. seminae were present in the subarctic Pacific material, but were absent or vestigial in the Gulf of St. Lawrence material and in cultures from both regions. This morphological variation was most likely due to differences in physico–chemical water properties between the subarctic Pacific and the Gulf of St. Lawrence. Phylogenetic analyses of the internal transcribed spacer regions of the nuclear ribosomal DNA showed that the strains of N. seminae collected in the Gulf of St. Lawrence and the subarctic Pacific clustered in two sister clades, but differed from each other only slightly more than the variation among the subarctic Pacific strains. These results confirmed the reappearance of N. seminae in the NW Atlantic after an absence of ∼0.8 Ma. In addition, the phylogenetic analyses based on the large subunit of the nuclear ribosomal DNA positioned N. seminae firmly within the Bacillariaceae (i.e. diatoms with a fibulate raphe system) and proved a close relationship to species of Fragilariopsis.

Modern diatom assemblages in surface sediments from shallow lakes and streams in southern Pampas (Argentina)

Diatom assemblages preserved in continental sedimentary successions of the southern Pampas (Argentina) can provide useful indicators of past environmental changes. Ecological data, however, are scarce. In order to provide modern data for diatom-based paleoenvironmental reconstructions, sediments from shallow lakes and streams of the region were analyzed. A total of 131 diatom species were identified. Canonical correspondence analysis was used to identify the physical and chemical environmental parameters that best explained the distribution of taxa. Cocconeis placentula, Hippodonta hungarica, Navicula veneta, Cyclotella meneghiniana and Nitzschia amphibia were the most widely extended species. The highest percentage variance in diatom data was explained by conductivity and nutrient concentrations. Diatom assemblage composition was more variable in shallow lakes than in streams, which is in agreement with the different hydrological and geomorphological characteristics of these environments. The results obtained in the present work provide useful analogues for paleoenvironmental reconstructions of water bodies in southern Pampas.