Surface Sediment Diatom Research Articles

Spatial characteristics of surface-deposited diatoms in Weishan Lake and their relationship with the water environment (Shandong Province, China)

The Weishan Lake serves as a crucial reservoir and regulating lake in the Eastern Route of the South-to-North Water Diversion Project and the ecological health of its lake ecosystem directly impacts the water quality for water diversion. Diatoms are particularly sensitive to changes in water environments, and the composition of diatom communities serves as a significant indicator of the health of aquatic ecosystems. To understand the composition of diatom communities in Weishan Lake and their relationship with environmental factors, thereby providing scientific grounds for the protection of the lake’s ecological environment, surface sediment diatoms, water samples, and sediments were collected and analyzed. The study revealed that there are 64 species from 19 genera of diatoms in Weishan Lake, predominantly comprising epiphytic and planktonic species. Dominant species include Fragilaria brevistriata, Achnanthes minutissima, and Stephanodiscus parvus, indicating an overall meso-eutrophic status. According to the diatom composition and biodiversity index, the diatom community was divided into three combination zones. Redundancy analysis (RDA) results indicated that the water environmental factors influencing the composition and distribution of diatom communities in Weishan Lake are TP, TOC, DO, NO2 −–N, and Ca2+, collectively explaining 35.60% of the variation in diatom community composition. Due to the influence of agricultural and industrial wastewater runoff, domestic sewage discharge, and cage aquaculture, Weishan Lake is in a mesotrophic eutrophication state. Therefore, it is recommended that strict control over inappropriate human activities in the lake area be implemented in the future to ensure the safety of water quality for water diversion.

Influence of hydrological changes on the composition of phytoplankton and sediment diatoms in the Rio de la Plata estuary

ABSTRACT ENSO phases produce changes in precipitation and river flow in Eastern South America. We aimed to evaluate phytoplankton and surface sediment diatoms' response to ENSO within the Río de la Plata estuary. A 180-km-long transect off the Uruguayan coast was undertaken in May 2009 (La Niña) and May 2010 (El Niño), where physical, chemical and biological variables were measured. During El Niño we registered lower temperatures/salinities throughout the transect, and the riverine zone was specifically more homogeneous in terms of temperature, salinity, turbidity and chlorophyll-a than during La Niña. We observed changes in the species composition of both planktonic and sediment diatoms. Concomitantly, a significant depletion in planktonic diatom abundance and richness in both planktonic and benthic domain during El Niño was observed. In addition, there was a higher proportion of dinoflagellates than during La Niña, which was explained by a higher water column stability. The dominance of the chain-forming and meso-eutrophic diatom Aulacoseira was registered during El Niño in both domains, mainly in the fluvial zone, related to a high river discharge. Our findings are important to anticipate changes in microalgae composition related to oceanographic variability during potential ENSO events.

Surface sediment diatoms in the northern and middle Okinawa Trough and their correlation with environmental variables

Determining the relationship between diatoms in surface sediments and the local sedimentary environment is important for paleoenvironmental reconstruction, including in marine settings. In this study, we used canonical correspondence analysis of 41 surface sediment samples from the northern and middle Okinawa Trough to analyze the relationship between diatom species and environmental variables. The principal results are as follows. (i) The diatom assemblages are closely correlated with the environmental variables, specifically, July sea surface salinity (SSS-Jul), February sea surface temperature (SST-Feb), and June sea surface temperature (SST-Jun). (ii) Three diatom assemblages are distinguished, representing environments with different hydrological characteristics. The high-temperature diatom assemblage may indicate a warm-water mass (the Kuroshio Current), and the polyhaline diatom assemblage can be used as an indicator of a high salinity water mass. The mixed diatom assemblage is associated with the warm-water masses of the Kuroshio Current, and shelf water masses with a low salinity and low temperature. (iii) The high-temperature and mixed diatom assemblages occur on the boundary of the continental shelf and Okinawa Trough, and are temperature sensitive; the high-temperature assemblage is located further upstream of the Kuroshio Current, indicating higher temperatures. Changes in the distribution of the polyhaline diatom assemblage within sediment cores can be regarded as an indicator of changes in water masses, and of possible salinity variations, in this area. These three assemblages are potentially important for future palaeoceanographic studies.

Spatial distribution of surface-sediment diatom assemblages from 45 Tibetan Plateau lakes and the development of a salinity transfer function

The Tibetan Plateau (TP) is often referred to as the “Asian Water Tower”, as it plays a crucial role in supplying water for ∼1.9 billion people. Lakes in this arid mountain region have a wide salinity gradient and are sensitive to the complex interactions between climate and the water cycle, affecting aquatic ecosystems and species distributions. Here, we analyzed relationships between measured environmental variables and the spatial distribution of surface sediment diatom assemblages from 45 lakes located throughout the TP with the intention of developing a diatom-inference model for salinity. Diatom assemblage composition and diversity differed among lakes from the western (W), southern (S), northern (N), and southeastern (E) regions of the TP. The most common taxa observed in the lake set, Pantocsekiella ocellata, Nitzschia palea, and Staurosira venter, tended to have widespread distributions across the TP. In general, diatom diversity and richness tended to be highest in lakes with salinity concentrations between 0.1 and 10 g/L and were lowest in high salinity lakes (>20 g/L) of the N region. Canonical correspondence analysis (CCA) identified lake water salinity and maximum lake depth as explaining a statistically significant portion of diatom assemblage variation. Weighted-averaging (WA) was used to develop a diatom-based salinity inference model (R2jack = 0.71, RMSEPjack = 0.28) based on diatom optima from freshwater to meso-hyposaline lakes of the TP. Diatom-salinity reconstructions and changes in diatom community composition can potentially be used to track long-term patterns in climate and environmental conditions across the highly complex mountain landscape of the TP.

Surface sediment diatom assemblages from four alpine lakes in the Zelengora Mountains (Bosnia and Herzegovina): A pilot study

This pilot study presents surface sediment diatom assemblages in four alpine lakes in the Zelengora Mountains. The four lakes are distributed across an alpine treeline, spanning temperate mixed forest and treeless alpine tundra zones, thus offering an excellent opportunity to study the interaction of physicochemical properties and biotic communities in different alpine lake settings. A total of 52 taxa from 28 genera were identified, with only one taxon (Staurosirella neopinnata) present in every lake. In the Orlovacko, Donje Bare and Crno lakes, the diatom assemblages were mainly comprised of benthic and tychoplanktonic species, as opposed to the Kotlanicko Lake where the planktonic Lindavia radiosa was the most abundant species. Our results identified water turbidity, macrophytes and pH to be the prevailing environmental factors which influence the diatom assemblage composition in the studied lakes. The results of this study also represent a starting point in establishing biomonitoring programmes for the sustainable management of these alpine aquatic ecosystems in a rapidly changing environment.

Diatom response to environmental gradients in the high mountain lakes of the Colombia\u2019s Eastern Range

A survey of 60 high mountain lakes of Colombia’s Eastern Range was performed to evaluate the response of surface-sediment diatoms to environmental variables. In each one of these lakes, water samples were taken for physical and chemical characterization, and diatoms were collected from the superficial bottom sediment at the deepest part. Multivariate statistical analyses were made to determine the relationships between environmental and biological data, specifically which environmental variables explain the diatom distribution. For each of these significant environmental variables, optima and ecological tolerances were calculated using the weighted-average method, which allowed for the classification of the species according to their environmental preferences. The lakes showed a wide range of environmental gradients in variables such as pH, alkalinity, and nutrients. In addition, the depth of the lakes was a direct determinant of the light environment of the water column. A total of 339 diatom taxa were identified belonging mainly to the genera Eunotia and Pinnularia. Variables related to pH-alkalinity gradient, trophic condition (nitrates and phosphorus), and physical factors (radiation at the bottom) had a significant effect on diatom composition. Despite the fact that the total organic carbon environmental range was high, the effect of this variable on diatom species composition was not significant. In conclusion, the diatoms of the studied lakes showed a significant ecological relationship with environmental variables which are potentially important in environmental reconstruction. Diatoms in the study sites can provide useful and independent quantitative information to investigate the recent impacts of global change on tropical high mountain ecosystems.

Long-Term Studies Of Surface-Sediment Diatom Assemblages In Assessing The Ecological State Of Lake Ladoga, The Largest European Lake

The study continues a series of observations started in the late 1950s, aimed at inferring changes in the Lake Ladoga ecosystem state recorded in the surface-sediment diatom assemblages. At the pre-anthropogenic stage (prior to the 1960s), the composition of the surface-sediment diatom assemblages indicated an oligotrophic state of Lake Ladoga. With the increased P load to the lake (late 1960s–1980s), the transition to a mesotrophic state was recorded via increased proportions of eutrophic species and decreased abundances of the taxa typical of the pre-anthropogenic stage. In the early 1990s, the composition of the surface-sediment diatom assemblages still indicated a mesotrophic state despite a decreased external P load. At the present de-eutrophication stage of Lake Ladoga (the 2000s), the abundances of eutrophic taxa steadily decrease while some taxa typical of the pre-anthropogenic period return to their dominating position in the surface-sediment diatom assemblages. However, despite the decreased P concentrations, the Lake Ladoga ecosystem has not returned to its pre-anthropogenic state as indicated by the present-day composition of the surface-sediment diatom assemblages. This suggests a delayed ecosystem response to the decreased anthropogenic pressure, and possibly some irreversible changes resulting from the eutrophication. At present, de-eutrophication processes and ecosystem recovery are superimposed upon the recent climatic changes that govern the onset and duration of the vegetative seasons for the phytoplankton communities in Lake Ladoga. The diatom-inferred changes in the ecological state of Lake Ladoga are in agreement with the results of longterm hydrochemical and hydrobiological studies.

Surface Sediment Diatom as A Water Quality Indicator: Case Study: Cilalay and Cibuntu Ponds, Cibinong

Diatom is classified as algae within the Division of Bacillariophyta. They are unicellular eukaryotic organisms characterized by siliceous cell walls that can be long preserved in sediments. Therefore, diatom analysis in sediment records is a potential water quality indicator for present or paleo studies. The current knowledge on the distribution and diversity of diatoms in the sediment in the urban pond is poorly known. This study aimed to identify the distribution and diversity of diatoms from the sediments of the pond. We expect to obtain a primary database of a variety of diatoms. The sediment samples were taken from Cibuntu and Cilalay Ponds in Cibinong Botanical Garden. Sediments were digested using HCL and H2O2. The resulting diatom solution was dried and transferred onto glass coverslip, which subsequently mounted onto microscope slides using Naphrax (Refraction index 1.7). Diatom identification was examined using a light microscope at 1,000x magnification. Diatom communities in Cibuntu Pond were dominated by species Aulacoseria ambigua, Eunotia bilunaris, Cymbopleura sp, Discostella stelligera, and Rossithidium sp with diversity index of 2.4 and species evenness of 0.8. Whereas, species Fragilaria sp, Eunotia monodon, Navicymbula pusilla, Eunotia bilunaris, and Pinnularia viridis were predominant in Cilalay Pond with diversity index of 1.6 and species evenness of 0.5. Based on the diatom community, Cibuntu Pond is eutrophic indicated by the occurrence of Aulacoseria ambigua, whereas Cilalay Pond is meso-eutrophic indicated by the dominance of Fragillaria. This exploratory survey provides the first inventory of diatom assemblage in Cibuntu and Cilalay Ponds for roughly inferring the environmental changes in a shallow lake ecosystem.

Surface Sediment Diatom Assemblages Response to Water Environment in Dongping Lake, North China

The relationship between the diatom taxa preserved in surface lake sediments and environmental variables in Dongping Lake was explored using multivariate statistical methods. The statistical analysis showed that the lake was eutrophicated in all seasons. Transparency, chlorophyll a (Chla) and total phosphorus (TP) were the dominant environmental factors in spring and summer, and NH4+-N and chemical oxygen demand (COD) were the dominant environmental factors in autumn and winter. Sixteen genera and 43 species of diatom were found in the surface sediments, and the dominant diatom genera were Aulacoseira, Ulnaria, Cyclotella, Navicula and Fragilaria. A redundancy analysis (RDA) and Monte Carlo permutation 20 test revealed that COD, pH, TP, conductivity and transparency were significant factors influencing diatom assemblage change, meaning that the distribution of the diatom assemblages were mostly influenced by nutrient composition, light intensity and ion concentrations.

Ice retreat in the eastern Antarctic Peninsula region: the application of diatoms for understanding climate change

Abstract Climate-driven retreat of ice shelves on the eastern side of the Antarctic Peninsula has opened up new coastal marine environments, enabling the development of primary productivity. Diatom assemblages in surface sediment samples from the eastern side of the Antarctic Peninsula were analyzed to investigate the use of diatoms as paleoenvironmental indicators of changes in glacial and sea ice extent. Here we present surface sediment diatom assemblage data from samples taken on 5 cruises between 2000-2012 from the northeastern-most tip of the Antarctic Peninsula, Prince Gustav Channel, Larsen A and Larsen B embayments, and a single sample from just east of Larsen C Ice Shelf. Absolute diatom abundance increases to the northeast reflecting greater contribution of diatom valves originating from surface waters that are ice free for a longer period of time. This trend is mirrored by the relative abundance of Chaetoceros subg. Hyalochaete, a spring bloom indicator group. The genus Fragilariopsis is dominated by sea ice-associated species, including Fragilariopsis curta, Fragilariopsis cylindrus, Fragilariopsis obliquecostata, Fragilariopsis sublinearis and Fragilariopsis vanheurckii. Fragilariopsis spp. relative abundance varies inversely with Chaetoceros subg. Hyalochaete. Distributions of Thalassiosira antarctica T1 and Thalassiosira antarctica T2, interpreted as a cooler and warmer forms, respectively, while generally inverse to one another with the T. antarctica T2 more common to the northeast, are not straightforward, with a pocket of high relative abundance in Larsen B embayment. These diatom data and their comparison to modern oceanography of the eastern Antarctic Peninsula provide a basis for further paleoceanographic work in the region.

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.

Modeling cyanobacteria biomass by surface sediment diatoms in lakes: problems and suggestions

Cyanobacteria dominance threats ecological integrity and diminishes ecological service of lakes throughout the world. Variety kinds of chemical and physical environmental factors have been used to model cyanobacteria condition in lakes except biological environmental factors. Essential resources competition plays a central role in shaping algae community in lakes. Cyanobacteria and diatoms are assumed to have conserved species’ functional traits and use resources and habitats in similar ways. Therefore, diatom metric, inferred-total phosphorus based on diatoms (DI-TP), could be a valuable predictor for cyanobacteria biomass in lakes. With data sets from the 2007 National Lakes Assessment (NLA) of the US Environmental Protection Agency, we compared the performance of DI-TP with TP and other predictors on predicting cyanobacteria biomass (CBB) by boost regression tree analysis in around 1,000 lakes. In light of effects of lake types and diverse cyanobacteria functional groups on model performance, we did a priori classification on lakes based on lakes types (deep/shallow, natural/man-made lakes) and cyanobacteria functional groups (bloom-forming, potential toxigenic, heterocyst-producing and potential N2-fixing cyanobacteria). Our results showed: (1) DI-TP was informative for modeling CBB in different lake types or various cyanobacteria functional groups, but its importance was not as significant as traditional TP; (2) Performance of DI-TP on modeling CBB was better in deep man-made lakes than in shallow natural lakes; (3) DI-TP performed better on modeling the biomass of potential N2-fixing cyanobacteria than other functional groups of cyanobacteria. The lower importance of DI-TP could be caused by different sampling locations of diatom and cyanobacteria from a same site. The uneven distribution of number of shallow lakes along TP gradient could contribute to a better performance of DI-TP on predicting CBB in deep lakes than in shallow lakes. In man-made lakes, a shorter water residence helped diatoms coexist with cyanobacteria and contributed to a better performance of DI-TP on predicting CBB. We conclude that DI-TP performed better on modeling CBB in deep man-made lakes and potential N2-fixing cyanobacteria biomass. Further studies are needed to thoroughly assess the valuableness of DI-TP on predicting CBB in lakes.

The silicification value: a novel diatom-based indicator to assess climate change in freshwater habitats

Formation of diatom frustules depends on the process of silicification. In many planktonic species of algae, ambient temperature and nutrients are already known to affect growth and morphology, making them suitable bioindicators. In this study, a similar indicator function of benthic diatoms is proposed based on the principle that cell wall thickness is inversely proportional to temperature and nutrients. Therefore, higher turnover rates at warmer temperatures are hypothesized to favour smaller and lighter diatoms due to their shorter regeneration time. Accordingly, benthic diatom species were classified into seven categories according to their intensity of silicification, referred to as ‘Silicification Value’. For each diatom assemblage, an abundance-weighted average silicification value was calculated and tested for its applicability to assess global change impact on mountain lakes in the northern calcareous Alps of Europe. Surface sediment diatom samples and morphometric and ecological parameters of 41 lakes from a wide altitudinal range in the Bavarian and Tyrolian Alps were collected and analysed. Statistical tests revealed that temperature conditions in a lake and its altitude were the most decisive proxies related to the ‘Silicification Value’. Moreover, trophic status and the presence of macrophytes had some influence. This supported the hypothesis that lakes, which are warmer and richer in nutrients, show a weaker average silicification in diatom assemblages. A dependence of silicification on valve size could be shown but appears to be negligible in climatological issues. In particular, the correlation with temperature makes the silicification value an applicable palaeoecological proxy to assess climate change impact in freshwater lakes.

Multiple environmental variables influence diatom assemblages across an arsenic gradient in 33 subarctic lakes near abandoned gold mines

We examined surface sediment diatom assemblages from 33 subarctic lakes around the historic gold mines in Yellowknife, Northwest Territories (Canada), where lake-water As concentrations ([As]) still range between 1.5 and 2,780 µg/l, even though the roasting of the arsenopyrite-bearing gold ores ceased in 1999. Water chemistry variables related to gold mining pollution (arsenic, antimony, and sulfate) declined with increasing distance from the mines. The diatom assemblages varied substantially in species composition across our study lakes with the planktonic Discostella stelligera/pseudostelligera complex dominating in deep lakes where [As] were less than 5 µg/l. However, in lakes that exceeded the guideline for the protection of aquatic life ([As] > 5 µg/l), opportunistic benthic diatoms (small fragilarioid taxa and Achnanthidium minutissimum) were generally present in higher abundances. The subtle differences in diatom species richness and diversity among lakes with varying [As] suggest these indices may not be affected strongly by long-term arsenic pollution. Furthermore, canonical correspondence analysis of the diatom assemblage and environmental data indicated that nutrient and ionic compounds explained most of the variation in the assemblages, while mining-related variables played a limited role. Our results indicate that multiple limnological variables (nutrients, ions, and contaminants) and subarctic climatic conditions are important factors shaping the diatom assemblage composition in lakes impacted by mining activities and land-use changes near Yellowknife.

Environmental and spatial drivers of diatom assemblages in the water column and surface sediment of tropical reservoirs

The relative contribution of environmental and spatial drivers of community structure has been the focus of many ecological studies. In the context of metacommunity theory, diatoms have been associated mainly with local environmental factors, but in some cases spatial factors have also been identified as important drivers. This study aimed to determine the relative importance of environmental and spatial drivers as predictors of surface sediment and water column diatom assemblages in tropical reservoirs. Sampling was carried out in 33 reservoirs in southeast Brazil, which range in trophic state from oligotrophic to hypereutrophic, and span five hydrological basins. We performed a partial RDA between the predictor matrices (environmental and spatial predictors) and the response matrix (either surface sediment or water column diatom assemblages) to identify the main environmental drivers, and used variation partitioning to assess the relative contribution of environmental and spatial predictors of diatom assemblages. In general, diatom assemblages were structured by a combination of environmental and spatial drivers. Comparisons of results between surface sediment and water column ordinations demonstrated that the assemblages were structured by similar environmental variables (productivity-related variables such as total phosphorus, water transparency and pH) and by a similar set of diatom species, mainly centric diatoms like Aulacoseira granulata var. granulata, A. tenella, Discostella stelligera and Spicaticribra kingstonii. Overall, surface sediment diatoms were more strongly associated with environmental factors than were water-column assemblages, probably because they integrate information in space and time, confirming that surface sediment assemblages are robust indicators of environmental conditions.

Local habitat heterogeneity determines the differences in benthic diatom metacommunities between different urban river types

Benthic diatoms are useful indicators of the ecological state of river systems. To understand the factors determining benthic diatom metacommunity composition in urban rivers, we studied in situ surface sediment diatom communities from 23 rivers in Shanghai City. Based on our study results on the metacommunity structure of benthic diatoms and hierarchical cluster analysis (HCA), we found substantial differences between restored (G1) and unrestored rivers (G2–G4) in taxa richness, relative abundances, and dominant and indicator taxa of benthic diatoms. The epiphytic diatoms Cocconeis placentula and Amphora libyca var. baltica were representative of the restored rivers (G1), where aquatic macrophytes were more abundant and the water was clearer. The motile epipelic diatoms Navicula recens and Navicula germainii dominated the moderately polluted rivers (G2). The eutrophic taxa Cyclotella meneghiniana, Aulacoseira granulata, and Cyclostephanos tholiformis dominated in G3, which comprised relatively heavily polluted rivers with low organic matter sediment and high disturbance. The polysaprobic taxon Nitzschia palea and the halophilous taxon Fallacia pygmaea represented relatively heavily polluted rivers with a comparatively higher sedimentary salinity (SSal) (G4). Redundancy analysis (RDA) revealed that total phosphorus (TP), dissolved organic carbon (DOC), silicon dioxide (SiO2), dissolved oxygen (DO), Secchi depth (SD), SSal, and the ratio of carbon to nitrogen (C/N) in the sediment were important environmental factors explaining variation among benthic diatom metacommunity composition. Partial RDA (pRDA) implied that the relative importance of environmental factors in structuring benthic diatom metacommunity was much higher than spatial factors. Classification and regression trees (CART) further indicated that DOC, the sediment C/N ratio, and SSal were the key local environmental factors affecting grouping patterns of benthic diatom metacommunities. Our study proposes that benthic diatom metacommunities respond to the complex characteristics of local environment in urban rivers and provides useful knowledge for consideration in the ecological monitoring of urban river systems.

Climate drivers of diatom distribution in shallow subarctic lakes

Abstract Global warming can induce profound changes to the functioning of northern freshwater ecosystems. Diatom (Bacillariophyceae) communities often provide early warning signs of associated ecological regime shifts, responding sensitively to alterations in underwater light climate, nutrient regimes, habitat availability and lake water acid–base balance. The underlying mechanisms are manifold and may be mediated via direct climate impact on the physical and chemical properties of lakes or via changes in the terrestrial environment and catchment‐lake coupling. To address catchment‐mediated climate effects on diatom community composition, spatial diatom distribution in the surface sediments of 31 subarctic treeline lakes displaying a broad gradient in terrestrial dissolved organic matter (tDOM) was contrasted with limnological indices of light climate, nutrient availability and lake water pH. To evaluate direct and indirect climate impacts on the long‐term development of benthic phototrophic communities at the subarctic treeline, fossil diatom assemblages in the sediments of a shallow oligotrophic lake were examined against established temperature variability and inferences of terrestrial influence over the past 600 years. The regional lake set was used to test local calibration models for reconstructing dissolved organic carbon as well as lake water pH that is a fundamental environmental determinant for diatom distribution and may echo temperature variability in dilute lakes. Across the treeline, lake water pH imposed primary control over the benthic‐dominated surface sediment diatom communities. The pH influence was connected to catchment geomorphology, soils and vegetation cover and, together with habitat controls, largely superseded tDOM impact on underwater light attenuation and nutrient levels. Similarly, temporal changes in diatom distribution in the sediment core appeared to be relatively little affected by tDOM variability. The species shifts were subtle yet occurred in distinct synchrony with centennial temperature fluctuations, attributed to changing length of the ice cover period and associated effects on lake water chemistry, nutrient regimes and physical habitats. Our results suggest that diatom flora in shallow lakes at the subarctic Fennoscandian treeline may be comparatively resilient towards climate‐driven changes in terrestrial carbon and nutrient fluxes. Diatom communities in poorly buffered lakes may, however, be susceptible to catchment greening and changes in hydrology through effects on lake water acid–base balance. While diatom responses in the sediment sequence were subtle, the palaeolimnological record indicates that periphytic diatom communities in shallow oligotrophic subarctic lakes may be sensitive to the effects of global warming.

Factors controlling recent diatom assemblages across a steep local nutrient gradient in central-eastern Finland

The Iisalmi Route, a chain of lakes in central-eastern Finland, is more eutrophic than its surroundings. We used multivariate analyses to study the influence of selected environmental factors (water quality, basin characteristics, and subcatchment surficial geology) on the recent diatom assemblages across this steep local nutrient gradient. In addition to the spatial analysis of surface sediment diatom assemblages from 51 sampling sites (48 lakes), temporal changes in the total phosphorus (TP) concentrations of one Iisalmi Route lake (Lake Kirmanjarvi) were analyzed using weighted averaging partial least squares regression and 27 fossil diatom samples. Both TP and electrical conductivity (EC) showed statistically significant independent signals in the modern diatom data. The TP gradient was related to till grain size variation suggesting that geological factors affect the spatial TP variation directly or indirectly through differences in land use. Based on the temporal study, the direct effect of geology is most likely behind the steep nutrient gradient in the area because Lake Kirmanjarvi was found to be naturally eutrophic. EC, on the other hand, seems related to anthropogenic disturbance. Our study highlights the importance of taking the local geology into account when assessing past or present water quality or planning for lake management.

Surface–sediment diatom assemblages of the urban ponds of St. Petersburg, Russia

The diatom assemblages of the surface sediments have been studied in 53 urban ponds and lakes of St. Petersburg for the first time. In total, 350 species and infraspecific taxa have been registered; the species of Achnanthidium, Cocconeis, Cyclostephanos, Cyclotella, Gomphonema, Lemnicola, Navicula, Nitzschia, and Stephanodiscus genera were the most common. The wide distribution of small centric planktonic taxa, Cyclostephanos dubius, C. invisitatus, Cyclotella pseudostelligera, Stephanodiscus hantzschii, and S. minutulus, is preconditioned by the depletion of dissolved silica, which is a result of the high phosphorus load. The high abundance of macrophytes in the shallowest sites favors the dominance of epiphytic Fragilaria, Staurosira, and Staurosirella taxa. Bottom-living diatoms in the shallow eutrophic and hypereutrophic urban ponds are light-limited; this is a result of either macrophytes or phytoplankton development. Alkaliphilous and alkalibiontic species prevail among the pH-indicative species, while eutraphentic and hypereutraphentic diatoms, as well as those thriving in a wide range of trophic conditions, dominate over other trophic groups. The β- and α-mesosaprobic species are the most common saprobity-indicative taxa.

HISTORIA AMBIENTAL DE UN LAGO ALPINO EN EL CENTRO DE MÉXICO (1230-2010)

High altitude lakes, with a low mineralization, alkalinity and nutrient levels, are sensitive ecosystems to natural or anthropogenic disturbances and currently are in risk due to global warming. There are only two high altitude lakes (> 3,800 m asl) in Mexico, La Luna and El Sol, in the Nevado de Toluca crater. Chemical parameters and surface sediment diatom assemblages show differences between both lakes, La Luna has lower pH and mineralization, and in spite of their geographic proximity, each lake has a characteristic diatom assemblage, with lower diatom concentration and species richness in La Luna. In this lake Encyonema perpusillum and Psammothidium helveticum are dominant while in El Sol are Cavinula pseudoscutiformis, Psammothidium levanderi and Aulacoseira distans. This ecological distribution is the basis for the interpretation of the diatom record from La Luna, from a 57cm long core dated with 210Pb y 14C for which cladocera and magnetic susceptibility data are also available. These proxies allowed identifying the beginning of a trend towards colder and dryer climates around 1350-1510. This period correlates with the Little Ice Age (LIA) in which the coldest and driest conditions were from 1660 to 1760, during the Maunder solar minimum. Cooler and drier conditions are recoded until the begining of the 20th century. Correlation with other paleoenvironmental records show that there was a regional trend to drier climates during the LIA in central Mexico.