Search

The centric diatom species Aulacoseira konstantinovii was described from Lake Tonlé Sap (Cambodia) but was erroneously assigned to the genus Aulacoseira. Subsequently, based on a unique set of morphological characters, the genus Praecyclostephanos with the single species P. boengchmariensis was established based on the material from this lake. Examination of published data has allowed us to compare the morphology of A. konstantinovii and P. boengchmariensis and confirm their conspecificity. A new combination, Praecyclostephanos konstantinovii comb. nov., is therefore proposed. [ABSTRACT FROM AUTHOR]

Paleolimnological records from temperate South-Central Chilean lakes revealed a recent and marked increase in the relative abundances of the key-planktonic diatom species Asterionella formosa and Aulacoseira distans. To better understand these changes, we studied the ecological niches of planktonic diatoms in 42 temperate Chilean lakes (36–43°S). We used morphological analysis of spring and early summer diatom communities, as well as sub-fossil sedimentary assemblages along gradients in mixing regime, light climate, and water chemistry. For Asterionella formosa and the species belonging to the genus Aulacoseira, morphology based enumerations were combined with molecular phylogenies using the 18S rDNA and rbcL genes to determine their taxonomic position and explore potential hidden diversity. Analysis of extant and sub-fossil communities revealed similar patterns, with small Discostella species (D. mascarenica and D. pseudostelligera) and Urosolenia eriensis being characteristic for large, well-mixed lakes, whereas Aulacoseira pusilla (previously erroneously identified as A. distans in Chile) is more dominant in smaller, warmer, and stratified water bodies. This suggests that recent increases in the latter taxon might be related to changes in the physical lake properties in the region, rather than a recent increase in nutrient concentrations. By contrast, Asterionella formosa occurred in lakes with a high Si concentration. Phylogenetic data of the most common species belonging to the genera Aulacoseira and Asterionella showed little evidence for hidden diversity in most of the taxa, except for the Aulacoseira granulata complex. This indicates that the Chilean strains are closely related to populations in the Northern Hemisphere. The results of this study will allow for more accurate diatom-based reconstructions of limnological changes in South-Central Chilean lakes. This is crucial to better understand climate dynamics in the region, as these lakes are known to be sensitive to alterations in windiness and precipitation in response to changes in the intensity and latitudinal position of the Southern Hemisphere Westerlies and the El Niño-Southern Oscillation. [ABSTRACT FROM AUTHOR]

A new species of Aulacoseira Thwaites is described from sediment deposits of Surprise Lake, an isolated lake located in the New Jersey Highlands. The species is characterized by (1) a low mantle, (2) unevenly distributed mantle areolae of variable size and shape, and (3) a valve face covered by rounded areolae. The new species is most similar to Aulacoseira biseriata (Grunow in Van Heurk) Houk, R. Klee & H. Tanaka. Both taxa possess frustules with mantles that are low, with irregular pervalvar rows or scattered areolae. However, the two species differ in the shape and pattern of the areolae, the shape and structure of the mantle wall, the shape and depth of the ringleist, and other morphological features. In this study, we present light and scanning electron microscopic images of the new species and compare it to the most similar taxa. [ABSTRACT FROM AUTHOR]

An overview of the diatom flora of the last interglacial (Eemian) lacustrine gyttja deposit at Sokli, northeastern Finland is presented together with descriptions including photomicrographs of resting spores similar to the Aulacoseira islandica-skvortzowii group, not found in recent material from Europe. Comparisons are made with literature data on similar taxa. The morphology of the resting spores (e.g., the number of striae and areolae in 10 µm) links them to the A. islandica-skvortzowii group, but the lack of vegetative cells in the Sokli record makes the determination of true synonymy difficult. The presence of spores similar/related to the A. islandica-skvortzowii group in the Sokli Eemian palaeolake may reflect a more widespread distribution of this group in the past. [ABSTRACT FROM AUTHOR]

Aulacoseira is a freshwater diatom genus found today in numerous lakes, ponds and rivers worldwide, and inhabiting a wide range of environmental conditions. The genus is one of the oldest diatom lineages known to colonize freshwater environments, dating to the late Cretaceous and radiating over much of the Cenozoic. The purpose of this paper is to describe a new species, Aulacoseira chockii, from an early Eocene locality situated near the Arctic Circle in northern Canada. The exquisitely preserved specimens have allowed for a detailed examination of frustule morphology of this early Cenozoic taxon. Three characteristics clearly separate A. chockii from all other known fossil and modern species in the genus. First, the spines of A. chockii are formed by coalescence of extensions from three to five mantle costae and multiple ribs that originate on the valve face. This type of spine design has never been reported and represents a potentially ancient trait in the Aulacoseiraceae lineage. Second, the valve has multiple sessile rimoportulae that are each connected to the end of a tube or canal that runs parallel to, and inside, the mantle wall. Rimoportulae with this structure are rare, and found primarily on extinct species of Aulacoseira. Third, the mantle striae are sinistrorse, and represent another rarely observed character within the genus. Because frustules of A. chockii possessed only separation spines, existed largely as single cells and not in long filaments, and were found in associated with remains of numerous heliozoans as well as testate euglyphids and sponges, it is believed this species grew in the littoral zone of a shallow waterbody. [ABSTRACT FROM AUTHOR]

Background and aims – Diatoms began to inhabit freshwater by at least the Late Cretaceous, becoming well established by the early to middle Eocene. Aulacoseira, an important diatom in numerous ponds, lakes and rivers today, was one of the earliest known genera to colonize freshwater ecosystems. Members of this genus with characteristics familiar to those found on modern species became increasingly more abundant by the Eocene, and continued to thrive throughout the Miocene to the present. We describe a new species of Aulacoseira from an early to middle Eocene site near the Arctic Circle in northern Canada.Methods – Twelve samples taken from the Giraffe Pipe core were analysed in this study. Light and scanning electron microscopy were used to document morphological characters. Morphometric measurements were made from 200 specimens per sample (n = 1200), and used to investigate changes in valve size over time.Key results – The new species, Aulacoseira giraffensis, has valves with a length:width ratio close to 1, a hyaline valve face, straight mantle striae, a shallow ringleiste, branched linking spines, concave-convex complementarity on adjacent valve faces, and rimoportulae with simple papillae-like structure. The suite of characters, especially the highly branched spines, concave-convex valves and simple rimoportulae, is unique for this species. Large numbers of A. giraffensis specimens were found over a ten-metre section of the core, representing thousands of years. These high concentrations are indicative of abundant, bloom-like, growth.Conclusions – The locality represents one of the earliest known records of Aulacoseira dominating a freshwater community. Findings confirm that the morphological body plan for the genus was well established by the Eocene. Although findings indicate evolutionary stasis in morphological structure for A. giraffensis over a time scale of thousands of years, oscillations in valve morphometrics could potentially be used to trace changes in the environment of this ancient Arctic waterbody.

Millennial-scale climate change history in eastern Siberia and relationships between diatom diversity, paleoclimate, and sediment-geochemical lake system trajectories are still poorly understood. This study investigates multi-proxy time series reaching back to the Late Pleistocene derived from radiocarbon dated Lake Bolshoe Toko sediment cores, southeastern Yakutia, Russia. We analyzed diatoms, elements (XRF), minerals (XRD), grain-size, organic carbon, and included chironomid analyses and published pollen-data for quantitative paleoclimate reconstruction. Changes in diatom species abundances reveal repeated episodes of thermal stratification indicated by shifts from euplanktonic Aulacoseira to Cyclotella species. Chironomid and pollen-inferred temperature reconstruction reveal that the main shift between these diatom species is related to the onset of Holocene Thermal Maximum (HTM) at 7.1 cal ka BP. Comparison to other paleoclimate records along a north-south transect through Yakutia shows that the HTM was delayed as far south as the Stanovoy mountains. Relationships between sediment-geochemistry, paleoclimate variability and diatom species richness (alpha diversity) was tested in a moving temporal offset approach to detect lead-lag relationships. Sediment-geochemical data, mainly uniform during the Holocene, revealed strongest positive or negative correlations ahead of species richness changes. Mean July air temperature (TJuly) reconstructions correlate with both Hill numbers and relative assemblage changes indicated by sample scores of multidimensional scaling analysis (MDS) over the entire time series. We found that sediment organic carbon revealed distinct positive correlations, i.e., centennial-scale delay to increases in diatom effective richness (Hill numbers N0 and N2). We conclude that a lag of deposited organic carbon concentrations behind changes in diatom alpha diversity reveals that species richness can augment the production and thus sequestration of organic matter in comparable lake systems.

When diatoms undergo vegetative cell division the new siliceous wall components are slightly smaller than those of the parent because they are produced within the confines of the parent wall. Thus, with continued growth the mean size of cells in a population declines. Given this unique feature of diatom cell division, if the growth of a species in a lake increases (decreases) under more (less) favorable conditions, then the mean size of the resulting population will decline (increase). Numerous paleolimnological investigations rely on shifts in the relative abundances of diatom species over time to infer lake conditions. Although relative abundance data yield information about the dominance of species in the community, they do not necessarily provide evidence about growth of a given species. For instance, a species could have increased in growth, but simply to a lesser extent than other taxa, resulting in a decline in relative abundance. In a similar fashion, relative abundance values can be misleading when used to infer environmental change, such as trophic status change in lakes. We propose that including data on mean size of diatom valves can yield greater insight into changes in growth and improve observations and conclusions based on relative abundance data. To test this concept, we examined changes in the mean diameter of Aulacoseira ambigua (Grunow) Simonsen valves relative to known shifts in lake trophic status in a core from Bantam Lake, Connecticut, representing ~ 130 years of sediment accumulation. The mean valve diameter of A. ambigua declined from 9.7 to 7.6 µm, with the largest declines clearly tracking significant increases in trophic status. We conclude that changes in the mean size of diatom frustules over time can provide valuable information for understanding long-term environmental changes. [ABSTRACT FROM AUTHOR]

Hanabanilla Reservoir, south-central Cuba, is used for electric power generation, potable water supply and tourism. We examined stratigraphic shifts in algal assemblages that accumulated in the reservoir sediments from the time of construction in 1960 through 2012, and inferred the environmental drivers of such biological changes. Historical fluctuations in algal assemblages were driven by scouring of the reservoir bottom, changing water level, and input of organic matter and nitrogen to the water body. Stage records, in conjunction with algal counts, confirm the importance of the pennate/centric diatom ratio for reconstructing past water-level changes. Although nutrient and organic matter inputs to the reservoir also influenced algal abundance and community composition, our findings suggest that regulating reservoir hydrology could be an effective strategy for preventing future cyanobacterial blooms. Shifts in the relative abundances of algal taxa, and dominance of R strategists associated with extreme fluctuations in water level, showed the strong influence of hydrology as a determinant of primary producer assemblage structure. [ABSTRACT FROM AUTHOR]

The River Ganga is reeling from pressures of rapid urbanization and resulting anthropogenic forcings. In this study, phytoplankton community assemblages were deduced from the Dakshineswar site located in the lower stretch of River Ganga to quantify and understand the health status of this river. Surface water samples were collected from six pre-defined stations of Dakshineswar spanning across monsoon and post-monsoon seasons of 2019 and 2020. Stations were categorized into point source and surface water based on proximity to municipal discharges. Measurement of in situ environmental parameters showed significant differences in values for dissolved oxygen, total dissolved solids, electrical conductivity and suspended particulate matter between the two seasons during the study period. In particular, concentrations of dissolved nitrate and silicate were found to be higher in point source stations compared to surface water stations. The concentration of Chlorophyll-a (Chl-a) was found to be higher in post-monsoon compared to monsoon seasons. Phytoplankton communities consisted of 23 diatom taxa and 14 green algal taxa and they showed distinct seasonal and spatial variations in the study site. Phytoplankton communities were dominated by diatom taxa namely Aulacoseira , Bacillaria, Coscinodiscus , and green algal taxa such as Ulothrix, Chlorella, and Scenedesmus . There was a dramatic increase in cell abundance of Aulacoseira spp. in post-monsoon seasons indicating a bloom-like scenario. Moreover, the rapid increase in cell abundance of Aulacoseira spp. also coincided with an increase in Chl-a and a sharp fall in the concentration of dissolved silicate. Some of the encountered phytoplankton taxa such as Tetraedron , Cosmarium, Nitzschia and Scenedesmus showed strong co-occurrence patterns indicating possible association at ecological scales. Four distinct clusters were formed in nMDS ordination plot based on the influences of environmental variables on encountered phytoplankton taxa. Network analysis revealed evidence of co-occurrence patterns between several diatoms and green algal taxa.

Background and aims - Diatoms began to inhabit freshwater by at least the Late Cretaceous, becoming well established by the early to middle Eocene. Aulacoseira, an important diatom in numerous ponds, lakes and rivers today, was one of the earliest known genera to colonize freshwater ecosystems. Members of this genus with characteristics familiar to those found on modern species became increasingly more abundant by the Eocene, and continued to thrive throughout the Miocene to the present. We describe a new species of Aulacoseira from an early to middle Eocene site near the Arctic Circle in northern Canada. Methods - Twelve samples taken from the Giraffe Pipe core were analysed in this study. Light and scanning electron microscopy were used to document morphological characters. Morphometric measurements were made from 200 specimens per sample (n = 1200), and used to investigate changes in valve size over time. Key results - The new species, Aulacoseira giraffensis, has valves with a length:width ratio close to 1, a hyaline valve face, straight mantle striae, a shallow ringleiste, branched linking spines, concave-convex complementarity on adjacent valve faces, and rimoportulae with simple papillae-like structure. The suite of characters, especially the highly branched spines, concave-convex valves and simple rimoportulae, is unique for this species. Large numbers of A. giraffensis specimens were found over a ten-metre section of the core, representing thousands of years. These high concentrations are indicative of abundant, bloomlike, growth. Conclusions - The locality represents one of the earliest known records of Aulacoseira dominating a freshwater community. Findings confirm that the morphological body plan for the genus was well established by the Eocene. Although findings indicate evolutionary stasis in morphological structure for A. giraffensis over a time scale of thousands of years, oscillations in valve morphometrics could potentially be used to trace changes in the environment of this ancient Arctic waterbody. [ABSTRACT FROM AUTHOR]

Fragilarioid diatom taxa are often deemed ubiquitous in shallow lake systems. Their presence has been described as contributing to statistical noise in paleolimnological studies of cold-temperate lakes. In shallow, warm-temperate lakes of Florida, long-term transitions from assemblages dominated by Aulacoseira spp. to fragilarioid taxa, particularly Pseudostaurosira brevistriata, Staurosira construens var. venter, and Staurosirella pinnata, often occur. Distinctly higher limnetic nutrient optima are demonstrated by these fragilarioid taxa than by planktonic Aulacoseira spp. Community successions occur during eutrophication, and progressive replacement of Aulacoseira spp. and other planktonic taxa by fragilarioid taxa is concurrent with and apparently related to the onset of cyanobacterial dominance. We examine successions from Aulacoseira-dominated to fragilarioid-dominated assemblages in sediment cores from subtropical Florida lakes that have undergone eutrophication. Diatom profiles are compared with sedimented pigments, nitrogen stable isotopes of organic matter, and with silica accumulation rates. These study lakes have little if any macrophyte presence. Their light-extinction depths are extremely shallow, yet diatom communities are dominated by bottom-dwelling rather than planktonic taxa. Frequent wind-generated mixing, sometimes to lake bottoms, is sufficient to sustain the light needs of benthic and tychoplanktonic taxa. We conclude that assemblage changes generally are not caused by reduced water depths, silica limitation, nor increased incipient stratification, but that cyanobacteria are responsible for reducing planktonic Aulacoseira in favor of fragilarioid taxa. Cyanobacteria blooms persist over a wide seasonal range because of warm climate and high limnetic nutrient concentrations in Florida lakes. Cyanobacteria progressively displace and outcompete Aulacoseira and other planktonic taxa as eutrophication proceeds. Reduced light availability, changes in mineral/nutrient availability, and other aspects of competitive exclusion, such as cyanobacterial allelotoxins, might contribute to observed changes. Climate warming is not likely to account for Aulacoseira reduction as in colder regions because it is less pronounced in this subtropical district. Lakes with low nutrient levels and less cyanobacteria still sustain large Aulacoseira populations, and decreases in limnetic nutrients sometimes lead to the return of planktonic Aulacoseira. Rather than simply representing statistical noise for paleolimnological reconstructions, shifts to certain fragilarioid taxa indicate when subtropical Florida lakes progressed to hypereutrophic conditions that were marked by cyanobacterial proliferation. [ABSTRACT FROM AUTHOR]

Post iron ore mining reservoirs in Łęczyca (Poland) can be considered extreme habitats because they are characterized by high pH, conductivity, and sulfate, sodium and potassium concentrations. We observed high abundances in these water bodies of several centric diatom species that are otherwise known from less extreme environments. These included Thalassiosira duostra, Cyclostephanos dubius, Stephanodiscus binatus, Stephanodiscus invisitatus, Stephanodiscus parvus, Stephanodiscus hantzschii, Lindavia aff. balatonis, Discostella woltereckii, Discostella pseudostelligera, and Cyclotella meneghiniana, revealing the eurytopic nature of these species. Morphological differences between populations from extreme habitats and their counterparts from less extreme environments may indicate phenotypic responses of eurytopic taxa to the differing environmental regimes, or the presence of distinct, more stenotopic taxa. In both cases, this could be relevant for biomonitoring. For this reason, we carefully observed the morphological variability of our target taxa in the Łęczyca reservoir samples compared with literature accounts from less extreme habitats, and in some cases, with type material of the species concerned. Specimens similar to Aulacoseira muzzanensis α-chains were observed. Following comparison with the type material of Melosira muzzanensis Meister (≡A. muzzanensis), we concluded that the Łęczyca populations are sufficiently different to be recognized as a separate species, here described as Aulacoseira pseudomuzzanensis sp. nov. The importance of identifying centric diatoms to species level for water management surveys is emphasized, even for those taxa that are difficult to identify using only LM. [ABSTRACT FROM AUTHOR]

A new centric diatom belonging to the genus Aulacoseira was found during a survey of the diatom community from peatbogs of Tierra del Fuego (southern Argentina). The present paper describes Aulacoseira frenguelliana sp. nov. based on detailed light and scanning electron microscopy and compares its morphological features with similar species. Aulacoseira frenguelliana is characterized by short chains joined by spatulate spines, one row of marginal areolae, a very narrow ringleist and only one rimoportula. The type material of Melosira laevis var. fuegiana Frenguelli, a species described from Tierra del Fuego showing some morphological resemblance to the new species, was studied using light microscopy to clarify the taxonomic position of this species. A transfer to Aulacoseira is proposed as A. fuegiana (Frenguelli) comb. et stat nov. [ABSTRACT FROM AUTHOR]

Fossil freshwater diatoms from tuffaceous–terrigenous deposits of the Yamato Rise, the Sea of Japan are described. The diatom flora includes 133 species and intraspecific taxa belonging to 44 genera. Species ofAulacoseira(19 taxa) andTetracyclus(17 taxa) are highly diversified, and species ofAulacoseiraandActinocyclusoccur very abundantly.Aulacoseirais mainly represented by extinct taxa, including those with elliptic valves that are characteristic of the Early Miocene (23–16 Ma). Such findings of numerous fossil freshwater diatoms in the Yamato Rise deposits strongly suggest that, during the Early Miocene, the Yamato Rise contained a lake basin within a terrestrial environment. Fifty-seven diatom taxa are illustrated. [ABSTRACT FROM PUBLISHER]

Two new centric diatom species were found in samples collected from Vietnam and Cambodia, Aulacoseira konstantinovii sp. nov. and Aulacoseira krylovii sp. nov. The former differs from other members of the genus mainly by a characteristic strong undulation in the central area of the valve face. The latter is close to A. konstantinovii sp. nov. but differs in its valve face morphology and having fewer rimoportulae. The new species are illustrated by light microscope and scanning electron microscope images. [ABSTRACT FROM AUTHOR]

We applied a diatom-based thermal stratification index to sediment cores from three lakes in the northeast USA to evaluate the influence of lake morphometry and within-lake processes on diatom responses to climate change. The three lakes all had surface area >5 km and experienced the same regional declines in wind speed and progressively earlier ice-out dates, but differed in morphometry, particularly surface area and mean depth. We coupled this paleolimnological approach with contemporary ecological measurements to validate the use of two indicator species ( Aulacoseira subarctica (O. Müller) Haworth and Discostella stelligera (Cleve & Grunow) Houk & Klee) in the stratification index. D. stelligera was abundant during stratified conditions in Tunk Lake and Sebago Lake, but not in Lobster Lake. Diatom-inferred stratification shifted to shallower and/or longer duration in Tunk Lake starting in the late nineteenth century, and continued to become shallower over much of the twentieth century. A shift to shallower and/or longer duration was also apparent in Sebago Lake starting around 1850, after which the index suggested little change, even though ice-out occurred 30 days earlier in 2000 than in 1807 and wind speed began to decline in the 1960s. This lake has very slow sedimentation rates, experiences regular seiches, and has D. stelligera present during circulation and stratified periods. These factors may smooth stratification-driven diatom responses. Contemporary ecological measurements did not support the application of the stratification index in Lobster Lake because D. stelligera bloomed only during spring turnover. In Lobster Lake, the relative abundances of D. stelligera in the sediment record showed some variation over time, were generally lower in periods with earlier ice-out in the region, and have been lower since the 1980s. Our results highlight the different responses recorded in the sediments of three large lakes in the same region to climate-driven changes and support the use of indicator species in reconstructing lake thermal stratification patterns when paired with site-specific morphometric and ecological data. [ABSTRACT FROM AUTHOR]

The abundance of Aulacoseira granulata (Ehrenburg) Simonsen and Gloeocystis planctonica (West & G.S.West) Lemmermann was assessed during the summers of 2005 and 2010 in the eutrophic Fox River, Wisconsin, USA. In both years, a mid-summer bloom of G. planctonica was followed by the rapid growth of A. granulata. Laboratory experiments in which A. granulata was grown in cell-free filtrate of a G. planctonica culture revealed that the growth of A. granulata was stimulated in the G. planctonica-treated medium relative to controls. This effect was detected when dormant A. granulata cells were used as the source culture for the experiment but not when actively growing cells were used. Dormant A. granulata also grew more rapidly in river water collected after the 2010 G. planctonica bloom relative to river water collected before the bloom. These results suggest that the summer bloom of A. granulata in the river was stimulated by G. planctonica. This relationship can be described as stimulated rejuvenation, an interaction where the transition of an algal resting stage into active growth is triggered by exposure to another species. [ABSTRACT FROM AUTHOR]

The Plio-Pleistocene Tilatá Formation records a tectonic paleo-lake system in the Colombian Andes. This unit exhibits recurrent episodes of volcanism and diatomite deposits. Outcrop-based facies analysis of eight stratigraphic sections varying in thickness from 4 to 40 m indicates variations in the sediment supply, accommodation space, climatic conditions, water depth, and organic activity. Fourteen sedimentary facies are grouped into three facies associations (FAs) representative of alluvial, fluvial, and lacustrine settings. FA1 (alluvial fan deposits) corresponds to conglomerates, sandstones, and subordinated mudstones. FA2 (mixed fluvial system) represents sandy conglomerates, cross-bedded/massive sandstones, organic-rich mudstones, lignites, and diatomaceous mudstones. FA3 (lacustrine) comprises mainly organic-rich mudstones, diatomites, laminated sandstones, and tephra layers. The Upper Tilatá Formation reflects the evolution of an overfilled to a balanced-fill lake with a progradational-retrogrational-progradational stacking pattern. The organic facies revealed variations in the origin of the Amorphous Organic Matter (AOM) from aquatic to terrestrial, indicating episodes of shallowing when terrestrial conditions dominated. Seven families, eight genera, and eleven diatom species were identified in diatomites and diatomaceous mudstones, mainly in the lacustrine facies. The most abundant species was Aulacoseira ambigua , suggesting moderate energy in the water column (turbulence), solar light availability for photosynthesis, and mesotrophic to eutrophic conditions. The sedimentological record implies that bedrock, fluvial, pluvial, and volcanic input played a decisive role in nutrient supply for the proliferation of diatoms in the paleo-lake. This scenario is concordant with similar Quaternary depositional systems in northern South America. • Plio-Pleistocene diatomites are recorded in a lake system in the Colombian Andes. • Eleven diatom species were identified with an abundance of Aulacoseira ambigua. • Tectonics, pluviosity, and drainage damming caused the development of an intermontane paleo-lake. [ABSTRACT FROM AUTHOR]

Most diatoms undergo a cyclical process of size reduction and size restoration, which is used to time their life cycle. However, the effect of this on population dynamics of natural populations is poorly understood. Therefore, their interaction was investigated over several life cycles of a planktonic diatom (Aulacoseira baicalensis) in Lake Baikal as part of a multidisciplinary study lasting 10 years. Changes in abundance and diameter were affected in a complex way by a number of interacting factors that included silica availability, the number of divisions in a growing season, diameter-dependent sedimentation losses, grazing, limited frustule lifetime and faster size decline than predicted in stressed cells. There was also a density-dependent effect on size restoration and the recruitment of large cells into the population, which interacted with the other factors to cause large changes in annual cell abundance observed in the lake over the last 60 years. [ABSTRACT FROM PUBLISHER]

Diatom preservation can be a major taphonomic issue in many lakes but is often unrecognised and its impacts on qualitative and quantitative inferences (such as productivity and biodiversity estimates) from sedimentary archives are seldom explored. Here two palaeolimnological case studies of 20th-century anthropogenic eutrophication of freshwater lakes in Northern Ireland (Lough Neagh and Lough Augher) are re-visited and new data presented on diatom preservation. Assessing problems of taphonomy challenges previous interpretations of silica dynamics and diatom productivity at these sites. Diatom preservation was assessed in both sediment trap material and sediment cores from Lough Neagh, and in sediment cores from Lough Augher. Preservation data, combined with geochemical analysis (Si, Fe), provide an insight into silica cycling and diatom accumulation over a range of temporal scales from these lakes. Diatom preservation was generally good for the Lough Neagh material, although differential (better) preservation of the smaller Aulacoseira subarctica compared to the larger Stephanodiscus neoastraea sensu lato valves was clear, especially in sediments. Porewater silica showed a complex seasonal pattern in the upper sediment, against expectations of steady-state. The Lough Augher material was generally poorly preserved, although preservation (dissolution) was significantly (and positively) correlated to bulk sedimentation rate, and was found to be a major control on (net) diatom accumulation rate across the basin. Past seasonal and severe anoxia at Lough Augher did not improve diatom preservation, contrary to some previous studies, which may be due to extreme changes in sedimentary redox conditions. Finally, using published experimental relationships between dissolution and diatom valve loss, correction factors were applied to previously published profiles of diatom accumulation over the last ~150 years (biovolume from Lough Neagh and frustule accumulation rate from Lough Augher), which suggest that diatom productivity estimates from sedimentary records are underestimated by a factor of 2-4 due to dissolution effects alone. The results clearly have implications for the reliability and accuracy of diatom-based inferences made from sediment records, both qualitative and quantitative, especially for those that employ diatoms as direct measures of productivity or biodiversity. [ABSTRACT FROM AUTHOR]

Centric diatoms of 107 different Hungarian running waters were investigated. Among them the largest was the River Danube, from which more than one hundred plankton samples were analysed by scanning electron microscopy (SEM). Only one sample was analysed from creeks, which were the smallest running waters analysed in this study. There were also channels with slow currents flowing out of rivers or connecting different rivers. In total, 41 centric taxa belonging to 11 genera were found during this study. The average number of taxa found in a single watercourse was 7, the maximum 40 and the minimum 1. Cyclotella meneghiniana was the most frequently encountered species (present in 60% of sites). Twelve taxa were found in more than 20% of sites, 7 taxa between 5-10% and 6 taxa only in one site. [ABSTRACT FROM AUTHOR]

The spatial distribution of surface sediment diatoms in Chaohu Lake (southeast China), and their relationships with environmental and spatial variables were analyzed in this study. The diatom assemblages were dominated by planktonic species. Three dominant species Cyclostephanos dubius, Aulacoseira granulata and Aulacoseira alpigena are unevenly distributed across the lake. The distribution of surface sediment diatoms must be subject to trophic status, hydrodynamics and other spatial variables in the lake. [ABSTRACT FROM AUTHOR]

Recent observations have confirmed the presence of dense accumulations of the diatom Aulacoseira islandica within surface lake ice on Lake Erie. MERIS (Medium Resolution Imaging Spectrometer) full-resolution satellite imagery is shown to clearly distinguish the optical signature of these phytoplankton blooms. The MERIS Level 1 maximum chlorophyll index is applied to Lake Erie under winter, partially (>50%) ice-covered conditions, demonstrating for the first time the potential for satellite detection of blooms within, or surrounding, surface lake ice. [ABSTRACT FROM PUBLISHER]

A desktop incubator with temperature control over the range 1-20°C (±0.5°C) was designed to hold two microtitre plates. Illumination of individual wells in the plate was by a matrix of 96 light-emitting diodes, whose intensity, period and pulsation could be controlled individually in each of 12 rows of eight chambers. The incubator was used to test how the length of light period affected cell length of the planktonic diatom Aulacoseira baicalensis, which only grows below 4°C. Short cells (mean length 26 µm) were formed under a 8-h : 16-h light-dark cycle, intermediate cells (mean length 35 µm) under a 4-h : 20-h light-dark cycle and longer cells (mean 38 µm) under a 2-h : 22-h light-dark cycle. These were similar to changes in mean cell length from 28 to 49 µm found in Lake Baikal during a decrease in light period caused by increased convective mixing from 15 m under-ice to 94 m after ice break-up. The laboratory experiments confirmed that decreasing light period was the environmental cue that initiated the production of long cells that then developed into resting stages. [ABSTRACT FROM AUTHOR]

Diatom assemblages preserved in lake sediment records can provide proxy data of past environmental changes in biological conditions. In order to investigate past changes in the environment of north-central Michigan, diatom assemblages were studied in sediment cores retrieved from 24 lakes. Diatoms were analyzed from the 'top' and 'bottom' of each core to reconstruct land-use changes in this area. Aulacoseira taxa were identified and evaluated with light and scanning electron microscopy. Results of these observations showed the presence of some variability of the morphological features within North American species populations. Diatom species composition in surface sediments and differences between tops and bottoms corresponded to changes in land use surrounding the lakes, ranging from predominantly forest and rangeland to urban and agriculturally impacted. Diatom-inferred past conditions revealed that the observed morphotypes probably represent taxa with different ecological preferences. The main factors influencing the variability of these morphotypes are changes in the trophic status of the lakes. [ABSTRACT FROM AUTHOR]

In recent decades, the planktonic diatom community in central Lake Victoria changed from dominance of Aulacoseira species (mainly African endemic A. agassizii, A. nyassensis) to dominance of long needle shaped Nitzschia (often referred to as N. acicularis). This shift was referred to be caused by eutrophication, resulting in increased growth of diatom taxa with reduced requirement of silica in open water. In contrast, in the large shallow Nyanza Gulf A. granulata and A. granulata var. angustissima remained abundant, probably because of increased turbidity and nutrient availability. In this study, from July 2017 to July 2018, the planktonic diatom community composition was investigated monthly at three sampling stations in Nyanza Gulf, i.e. West Gulf (WG, located close to the open basin via Mbita channel), Mid Gulf (MG, central part of the Gulf) and East Gulf (EG, located most closely to Kisumu bay). A significant gradient in water quality conditions was observed along distance of 60 km from the main basin, i.e. the water transparency decreased from 0.8–1.2 m to 0.2–0.5 m while suspended solids increased from 3.3–9.8 mg/L to 12–63.8 mg/L. In total 19 common diatom morphospecies were documented and related to the gradient in trophic state. At WG, A. granulata and A. granulata var. angustissima formed diatom blooms or co-occurred with long needle shaped Nitzschia species such as N. kavirondoensis, N. lacustris , N. nyassensis , N. rusingae, Synedra cunningtonii , as well as Cyclostephanos malawiensis , C. damasii and Urosolenia victoriae indicating more oligotrophic conditions closer to the main basin. In the inner Gulf A. granulata (var. angustissima) and N. cf. fonticola var. pelagica possibly living epiphytically on the colony-forming cyanobacterium Microcystis aeruginosa dominated. In contrast to N. cf. fonticola var. pelagica , however, Aulacoseira spp. biovolume was found negatively related to Microcystis biovolume implying its suppression under Microcystis bloom conditions. [ABSTRACT FROM AUTHOR]

The survival and subsequent growth potential of Anabaena spp. and other filamentous cyanobacteria and the cells of Aulacoseira spp. (diatom) and Ceratium hirundinella (dinoflagellate) following passage through the Multi Level Inlet Tower (MLIT) and offtake works at Chaffey Reservoir in New South Wales, Australia was investigated in late summer. The study aimed to test whether the phytoplankton cells were destroyed or otherwise rendered less viable during passage through the outlet works. The reservoir was strongly thermally stratified with a shallow surface mixed layer, which contributed to considerable temporal variability in the numbers of phytoplankton cells present immediately opposite the intake portal of the outlet works. To compensate, considerable replicate sampling was undertaken both upstream and downstream of the MLIT. Results indicate limited destruction of cyanobacteria, with fewer cells present immediately downstream compared to upstream. Greater destruction of cells was indicated at lower mean daily discharge rates compared to higher discharge rates. Filament lengths of both cyanobacteria and Aulacoseira were also reduced during passage. There was no apparent reduction in Ceratium cell number. Laboratory incubation studies on surviving cells collected downstream indicated no impairment on the viability of any taxa. Calculations of rates-of-strain likely to be experienced by the phytoplankton as they transited through the offtake revealed very high stress being applied to the filaments and cells at the valve, and within the spillway sections of the works. These were several orders of magnitude greater than published values shown to disrupt cells and filaments, and to impair viability for subsequent growth in laboratory studies. However, exposure times to the high rates-of-strain at Chaffey Reservoir were brief, which may reduce the impacts of the high turbulence. The conclusions were that unless cyanobacterial cell destruction during passage through an outlet works can be shown to be more effective at larger reservoirs, the withdrawal of warm, cyanobacterial infested waters from close to the surface is unlikely to provide an acceptable management action for the prevention of cold water pollution downstream. [ABSTRACT FROM AUTHOR]

The phylogeny of 67 populations representing 45 species of Aulacoseira Thwaites was estimated by maximum parsimony methods using a combination of nucleotide sequence data and qualitative and quantitative morphological characteristics of the silica cell wall gathered primarily from original observation by LM and SEM. A new type of character using continuous quantitative variables that describe the ontogenetic-allometric trajectories of cell wall characteristics over the life cycle (size range) of diatoms is introduced. In addition to the 45 Aulacoseira species, the phylogeny also incorporated one Miosira Krammer, Lange-Bertalot, and Schiller species and two outgroup species ( Melosira varians Agardh and Stephanopyxis nipponica Gran & Yendo). Fifteen species, represented by 24 populations, also contained molecular data from the nuclear genome (18S rDNA), and 11 of these species (18 populations) contained data from the chloroplast genome ( rbcL) as well, which were sequenced or downloaded from GenBank. The phylogeny of Aulacoseira is composed of five major clades: 1) an A. crenulata (Ehrenburg) Thwaites and A. italica (Ehrenburg) Simonsen clade, which is the most basal; 2) an A. granulata (Ehrenburg) Simonsen complex clade; 3) an A. ambigua (Grunow) Simonsen clade; 4) an A. subarctica (O. Müller) Haworth and A. distans (Ehrenburg) Simonsen clade; and 5) an A. islandica (O. Müller) Simonsen clade that also contained endemic species from Lake Baikal, Siberia and many extinct Aulacoseira taxa. Monophyly of Aulacoseira can only be achieved if Miosira is no longer given separate generic status. [ABSTRACT FROM AUTHOR]

The Lampang Basin is the second largest Tertiary graben in northern Thailand and was formed in response to the Himalayan orogeny. Deposition started with Miocene sandstone, lignite, mudstone, shale and oil shale of the Mae Sot Formation, which gave way to Pliocene diatomite, diatomaceous clay and silty clay of the Ko Kha Formation, with coarse clastics being restricted to the basin margins. Secondary iron-staining and iron-rich nodules are also common in the latter formation. Diatoms only occur in the Ko Kha Formation, where they are abundant and well-preserved. Five assemblages, indicative of fresh to moderately alkaline water, are present. These floras are variously dominated by Aulacoseira granulata, A. granulata var. valida, A. agassizi, A goetzeana, A. ambigua, A. italica var. bacilligera and A. italica var. tenuissima. Pennate species are present, but infrequent. During the Pliocene, in the Ban Pa Muang area, non-diatomaceous lacustrine sediments were initially laid down. These gave way to the deposition of diatomaceous clays and diatomites dominated by A. granulata and A. agassizi, which flourished in the deeper and shallower sectors (respectively) of a fresh water body. The palaeolake then became mildly alkaline, with a flora dominated by A. granulata var. valida. Shallow, fresh conditions followed, characterised by varied Aulacoseira diatoms, before the lake again became deeper. This latter phase may be related to increased river recharge. [ABSTRACT FROM AUTHOR]

• Curved Aulacoseira filaments were good bioindicators to water environments. • High curve degree was a good indicator to high water eutrophication levels. • A standard to define the curved filaments as bioindicators might be that curve degree ≥ 0.1. Morphological traits of diatoms have been paid more and more attention recently, since new ecological indicators are desired under current rapidly changing environments. The shape features (including curved filaments and ending spines) of Aulacoseira filaments have been proved to be good indicators to spatio-temporal patterns of aquatic environments in the downstream of the Pearl River. Furthermore, curved Aulacoseira filaments with a wide variation range in curve degree have attracted our attention. Here, we hypothesized that these curved filaments were also good indicators to water environments. The major difficulties are how to quantify the values of curve degree and define its indicating functions to water environments. To test this hypothesis, morphological traits of these curved Aulacoseira filaments, including traditional traits (cell and filament size) and novel traits (filament ending spines and curve degree), were collected and analyzed. The different curve degrees were quantified and artificially divided into four curve degree ranges, thereby more effective ranges to water environments could be confirmed. Based on ecological modelling, the results showed that clusters mainly exhibited spatially variations based on cell size, and clusters mainly exhibited temporally alternations based on occurrence proportion of filament ending spines. The high curve degree was a potentially good indicator to high eutrophication levels, because the highest curve degree generally occurred in clusters that mainly originated from the city center. The occurrence proportion of filament ending spines was a good indicator to seasonal changes, since it showed lower values in flood seasons and higher values in low flow seasons. A more ideal standard to define the curved filaments might be that curve degree ≥0.1, based on the effective indicating function. This is the first study to quantify the different curve degrees of Aulacoseira filaments and use them as ecological indicators, which has been proved based on comparing with traditional morphological traits. This study enhances the role of shape features as bioindicators to environments. The conclusions are more applicable in aquatic environments dominated by filamentous diatoms. [ABSTRACT FROM AUTHOR]