Changes In Diatom Communities Research Articles

Refining the history of extreme coastal events in southern Newfoundland, NW Atlantic, with lake sediment archives

Extreme events, including tsunamis, storm surges and hurricanes, affect the western North Atlantic coast, threatening communities and impacting ecosystems. Although these extreme coastal events (ECEs) are currently thoroughly monitored in the area, older ECEs are known only from short historical evidence. Therefore, establishing a reliable ECE history that extends more than several decades into the past requires data from geological records. This study aims to refine the existing record of ECEs on the southern Burin Peninsula (Newfoundland, Canada) that extends back to the mid-18th century by providing the first lake-sediment-based reconstruction of the event history of the Canadian province of Newfoundland and Labrador. We investigated short sediment cores collected from four shallow coastal lakes. The identification of ECEs relied mainly on lithostratigraphic evidence and peak detection of sand grain counts. Diatoms, dry bulk density, and loss-on-ignition were also analyzed. The age-depth models of the cores were based on 210Pb and 137Cs dating. The correlation of our records with major historical events confirmed that statistically significant sand count peaks, sand layers, and redeposited peat clasts are indicative of major ECEs. We found that the type and characteristics of ECE records depended mainly on the availability of sediment, distance from the seashore, and magnitude of the event. Although some of our ECE markers are associated with marine inundations, only slight diatom community changes were observed, contrary to what is commonly expected from the disturbance of brackish and freshwater ecosystems by major saltwater intrusions. Combined, our records indicate that at least eight ECEs occurred since the late 17th century, three of which might correlate with historically documented events, namely the 1755 CE Lisbon tsunami, the 1775 CE Great Independence Hurricane, and the 1929 CE Newfoundland tsunami. Although further investigations are required to better constrain the type and magnitude of particular ECEs, our study confirms the suitability of geological archives for improving hazard assessment in coastal regions.

Comparison of advanced methodologies for diatom identification within dynamic coastal communities

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

Permafrost-thaw lake development in Central Yakutia: sedimentary ancient DNA and element analyses from a Holocene sediment record

In Central Yakutia (Siberia) livelihoods of local communities depend on alaas (thermokarst depression) landscapes and the lakes within. Development and dynamics of these alaas lakes are closely connected to climate change, permafrost thawing, catchment conditions, and land use. To reconstruct lake development throughout the Holocene we analyze sedimentary ancient DNA (sedaDNA) and biogeochemistry from a sediment core from Lake Satagay, spanning the last c. 10,800 calibrated years before present (cal yrs BP). SedaDNA of diatoms and macrophytes and microfossil diatom analysis reveal lake formation earlier than 10,700 cal yrs BP. The sedaDNA approach detected 42 amplicon sequence variants (ASVs) of diatom taxa, one ASV of Eustigmatophyceae (Nannochloropsis), and 12 ASVs of macrophytes. We relate diatom and macrophyte community changes to climate-driven shifts in water level and mineral and organic input, which result in variable water conductivity, in-lake productivity, and sediment deposition. We detect a higher lake level and water conductivity in the Early Holocene (c. 10,700–7000 cal yrs BP) compared to other periods, supported by the dominance of Stephanodiscus sp. and Stuckenia pectinata. Further climate warming towards the Mid-Holocene (7000–4700 cal yrs BP) led to a shallowing of Lake Satagay, an increase of the submerged macrophyte Ceratophyllum, and a decline of planktonic diatoms. In the Late Holocene (c. 4700 cal yrs BP–present) stable shallow water conditions are confirmed by small fragilarioid and staurosiroid diatoms dominating the lake. Lake Satagay has not yet reached the final stage of alaas development, but satellite imagery shows an intensification of anthropogenic land use, which in combination with future warming will likely result in a rapid desiccation of the lake.

A palaeoecological study investigating the impacts of multiple tephra depositions on a lacustrine ecosystem in Northeast China, using diatoms as environmental indicators

Tephra layers are common in lake sediments and although they have often been used as chronological controls, few studies have investigated the impacts of past tephra depositions on lake ecosystems (Tephropalaeoecology). For the first time we systematically assess how different types of tephra layers vary in their ecological impact on the same lacustrine system. We use a diatom-based tephropalaeoecological approach to infer the impacts of five tephra deposits on Lake Sihailongwan, a well-studied volcanic lake in Northeast China, over the past 30,000 years. The five tephra layers (including two micro-tephras) have varying thicknesses and were deposited in time periods with different climatic conditions. Changes in diatom communities and chrysophyte-cyst concentrations between pre- and post-tephra samples were used to infer changes in lake conditions and highlight the importance of lake background conditions in mediating the impact of tephra. While the two micro-tephra layers did not cause observable changes, the three thicker tephras induced pronounced changes in lake conditions and thus diatom communities. The two thick tephras deposited in more eutrophic and warmer lake conditions caused larger responses from diatoms. We argue that water-column phosphorus decreased due to reduced sediment–water-phosphorus loading as thick tephra layers formed an impermeable layer at the lake bottom. This is supported by a decrease in total diatom concentration and a decline in high phosphorus-requiring taxa such as Discostella stelligeroides and Stephanodiscus minutulus as well as modern limnological observations which showed that groundwater influxes from the lake bottom are the main source of nutrients to the lake. By contrast, the thick tephra deposited in more oligotrophic and colder lake conditions caused less conspicuous changes. When the lake was already low in phosphorus, diatoms did not respond to a further decline in phosphorus but rather responded to the minor increase in silica from the dissolution of tephra particles in the water column. This was inferred from the slight increases in overall diatom concentration and opportunistic taxa such as Pantocsekiella comensis f. minima. Diatom analysis of the post-tephra sediments above the three thick tephras showed that the aquatic ecosystem did not completely recover, indicating the long-lasting effects of these thick tephras and shifts to new lake-ecosystem equilibria.

Distribution of marine benthic diatoms on the coasts of the sea of Marmara and their responses to environmental variables

The Sea of Marmara, located in northwest Turkey, comprises challenging environmental conditions and is an important bridge between the Black Sea and the Aegean Sea. In this study, we assessed the trophic situation of the coasts of the Sea of Marmara using the benthic diatom composition and their relation to the environmental variables in the spring and autumn of 2019 and 2020. A total of 120 samples were collected and 452 species were observed; Navicula Bory and Nitzschia Hassal were represented with the highest numbers of taxa. Analysis of Similarity (ANOSIM) revealed that diatom communities differed significantly between the spring-autumn periods and years. Canonical Correspondence Analysis (CCA) showed that diatoms grouped under water, temperature, salinity, dissolved oxygen, and TIN influence. TRIX observations revealed a highly mesotrophic-eutrophic status along the coasts. Our study contributes to the knowledge of diatom diversity, distribution and community changes in spring and autumn on the coasts Sea of Marmara. Our findings suggest that marine benthic diatoms could be used as eutrophication indicators in the coastal waters for long-term monitoring with the support of environmental parameters.

Multiproxy paleolimnological records provide evidence for a shift to a new ecosystem state in the Northern Great Plains, USA

AbstractWetlands in the Prairie Pothole Region of the North American Northern Great Plains perform multiple ecosystem services and are biodiversity hotspots. However, climatological changes can result in sudden shifts in these important ecosystems. For example, marked increases in precipitation in the last few decades have resulted in a widespread shift in wetlands across the Prairie Pothole Region to a new ecohydrological state. We used multiproxy analyses (diatom community composition and invertebrate stable isotopes) of 210Pb‐dated sediment cores from two adjacent, but morphologically and hydrologically different, prairie‐pothole wetlands to assess the effects of hydroclimatic variability on these wetland ecosystems. Our results provide evidence that the recent ecohydrological shift in the region's wetlands is unprecedented over the past ca. 178 yr. Oxygen stable isotopes in chironomid head capsules provide a record of paleohydrology changes. The most recent sediments (i.e., those deposited after the state shift) from both wetlands revealed novel changes in diatom communities that differed greatly from earlier community compositions. In addition, a depleted signal in deuterium and 13C carbon stable isotopes observed in chironomid head capsules and Daphnia ephippia, respectively, after 1993 is likely related to an increase in methane production in these wetlands. Our study highlights the importance of considering basin morphometry including whether a wetland has an overflow point, and multiple biological indicators to study climate‐change influences on freshwater ecosystems. Research using these techniques can lead to an improved understanding of recent ecosystem shifts, an understanding that will be essential for future climate‐change adaptation and mitigation in this ecologically important region.

Human-induced marine degradation in anoxic coastal sediments of Beppu Bay, Japan, as an Anthropocene marker in East Asia

The timing and magnitude of coastal marine degradation in East Asia during a possible transition from the Holocene to the proposed Anthropocene have still not been quantified with well-dated proxy records over centennial-to-millennial timescales. This study uses multi-proxy sedimentary records to address this issue and to document biological and environmental changes in an anoxic coastal marine basin, Beppu Bay, Japan. Analysis of sedimentary diatom and pigment records revealed a notable change in diatom and phytoplankton communities and an abrupt increase in their productivity beginning 1960 s CE. Biogeochemical indices, including total organic carbon, total nitrogen, biogenic opal, bromine, and nickel as well as total sulfur, showed a notable increase in values, reflecting the enhanced primary productivity in the water column due to eutrophication, and reduced oxygen-levels in bottom conditions. Biological changes seen in diatom concentrations and communities were unprecedented over the last 1300 and ~670 years, respectively. The eutrophication-associated proxy records demonstrate that anthropogenic degradation of the coastal marine environment occurred in the 1960 s, a time which was associated with the exponential, world-wide spread of coastal marine degradation during the Anthropocene, proposed to commence in the mid-20th century. Beppu Bay sediments have a wealth of excellent anthropogenic proxy records and, therefore, could be a representative archive of coastal Holocene-Anthropocene transitions in East Asia.

Long-term trends in diatom diversity and palaeoproductivity: a 16 000-year multidecadal record from Lake Baikal, southern Siberia

Abstract. Biological diversity is inextricably linked to community stability and ecosystem functioning, but our understanding of these relationships in freshwater ecosystems is largely based on short-term observational, experimental, and modelling approaches. Using a multidecadal diatom record for the past ca. 16 000 years from Lake Baikal, we investigate how diversity and palaeoproductivity have responded to climate change during periods of both rapid climate fluctuation and relative climate stability. We show dynamic changes in diatom communities during the past 16 000 years, with decadal shifts in species dominance punctuating millennial-scale seasonal trends. We describe for the first time in Lake Baikal a gradual shift from spring to autumnal diatom communities that started during the Younger Dryas and peaked during the Late Holocene, which likely represents orbitally driven ecosystem responses to long-term changes in seasonality. Using a multivariate classification tree, we show that trends in planktonic and tychoplanktonic diatoms broadly reflect both long-term climatic changes associated with the demise of Northern Hemisphere ice sheets and abrupt climatic changes associated with, for example, the Younger Dryas stadial. Indeed, diatom communities are most different before and after the boundary between the Early and Middle Holocene periods of ca. 8.2 cal kyr BP, associated with the presence and demise of Northern Hemisphere ice sheets respectively. Diatom richness and diversity, estimated using Hill's species numbers, are also shown to be very responsive to periods characterized by abrupt climate change, and using knowledge of diatom autecologies in Lake Baikal, diversity trends are interpreted in terms of resource availability. Using diatom biovolume accumulation rates (BVARs; µm3 cm−2 yr−1), we show that spring diatom crops dominate palaeoproductivity for nearly all of our record, apart from a short period during the Late Holocene, when autumnal productivity dominated between 1.8–1.4 cal kyr BP. Palaeoproductivity was especially unstable during the Younger Dryas, reaching peak rates of 18.3 × 103 µm3 cm−2 yr−1 at ca. 12.3 cal kyr BP. Generalized additive models (GAMs), which explore productivity–diversity relationships (PDRs) during pre-defined climate periods, reveal complex relationships. The strongest statistical evidence for GAMs were found during the Younger Dryas, the Early Holocene, and the Late Holocene, i.e. periods of rapid climate change. We account for these differences in terms of climate-mediated resource availability, and the ability of endemic diatom species in Lake Baikal to adapt to extreme forms of living in this unique ecosystem. Our analyses offer insight into how productivity–diversity relationships may develop in the future under a warming climate.

The effect of altered flow regimes on aquatic primary producer communities: Diatoms and macrophytes

AbstractRivers are intensively managed worldwide through unprecedented flow regime alterations on a global scale. This has led to an increasing interest in the development of quantitative tools to assess the ecological response of organisms to flow alteration. To date, studies reflect a large diversity of responses that make the intensity and prevalence of effects difficult to generalize. The present study analysed how flow alterations caused by dams affect the structure, composition and traits of diatoms and macrophytes in three Spanish river basins (Cantabric, Ebro and Duero). By using a control‐impact design based mainly in hydrological similarity, our results showed consistent patterns of change in diatom and macrophyte communities. Our study showed a shift from resistant traits such as non‐colonial diatom forms and crust‐forming algae, which are adapted to the extreme events and natural dynamism of unregulated rivers, towards planktonic diatoms, free‐floating algae and the mass development of aquatic plants in hydrological altered sites. Both communities shared common thresholds of response to altered hydrological attributes that could be considered in a flow management context. The hydrological indices that impacted diatom and macrophyte communities the most were related with the magnitude and duration of minimum flows. However, our results also show that it is important to consider the interrelationships between the different hydrological attributes such as the seasonal variability of monthly flows, the magnitude of maximum annual extremes, the timing of flow events, and the frequency and rate of flow changes.

The link between climate change and biodiversity of lacustrine inhabitants and terrestrial plant communities of the Uvs Nuur Basin (Mongolia) during the last three millennia

The paper is focused on changes in biodiversity, the environment, and human activity in the Uvs Nuur Basin during the last three millennia based on biological and geochemical proxies from the lake Bayan Nuur. Regions with high biodiversity and relatively low anthropogenic pressures are typically the most vulnerable to both climate change and human activities. One such area is the Uvs Nuur Basin located on the north of the Great Lake Depression of Mongolia. The main objective of this study is to assess changes in the past biodiversity of the lake’s microflora and microfauna, and surrounding vegetation biodiversity in the Uvs Nuur Basin, and to determine the main drivers of diversity change. Based on the analysis of pollen and chironomids we conclude that the most humid and afforested phase was between 1400 and 1800 CE. We assume that the Little Ice Age in the Uvs Nuur Basin was humid with mean annual precipitation ca. 305 mm/year and mean July temperature about 13°C. Conversely, the warmest and most arid period was between 650 and 1350 CE with mean annual precipitation ca. 280 mm/year and mean July temperature of about 16°C, attributed to the Medieval Warm Period. The biodiversity of terrestrial plants, chironomids, and Cladocera positive react to changes in annual precipitation and July temperature, whereas diatoms do not correlate directly to the climatic factors. The diversity and the evenness of plants are strongly correlated with the change in the leading biomes. The calculated species turnover suggests no significant changes in plant and Cladocera taxa composition, but significant changes in diatom and chironomid communities. This may be explained by the instability of lake ecology due to the fluctuation of the salinity and acidity of the water. An additional aim was to assess if dung fungi in lacustrine sediments reflect changes in human population density around the lake. We found that neither historical sources of human presence nor the influx of coprophilous fungi are correlated with the inferred climate changes. Coprophilous fungi can be used as individual or additional sources of assessment for the peopling and human-related herbivore density including overgrazing of the studied area.

Effects of thorium on bacterial, microalgal and micromeiofaunal community structures in a periphytic biofilm

Few ecotoxicity studies are available on thorium (Th) which hinders the ability to evaluate its ecotoxicological risk. Its release in the environment is often associated with the extraction of rare earth elements and uranium, as well as the field applications of phosphate fertilizers. This study investigates the effects of Th on microbial communities of periphytic biofilms. Ceramic plates were left to colonize for one month in the laboratory with a biofilm sampled from Cap Rouge river (QC, Canada). Plates were randomly placed in channels containing culture media representing three different conditions: a control condition (C0; background Th concentrations of 0.004 ± 0.002 nM), a low Th concentration condition (C1; 0.18 ± 0.09 nM Th) and a moderately high Th condition (C10; 8.7 ± 3.4 nM) for up to 4 weeks. The presence of Th modified the diatom community by changing its taxonomic structure, reducing diversity and increasing cell density. The taxonomic structure of the bacterial community, followed by 16S metabarcoding analysis, was affected with a significant decrease in Pseudanabaena and Shingopyxis genera in the two Th exposed conditions. No direct toxic effect of Th was observed on counted micromeiofauna but the changes in diatom and bacterial communities could explain the higher number of individual diatoms and micromeiofauna observed in Th-exposed conditions. This work shows that low concentrations of Th can modify biofilm structure, which, in turn, could disturb its ecologically key functions.

Diatom Response to Global Warming in Douhu Lake, Southeast China

AbstractA large number of lacustrine sedimentary records indicate that global warming is the main factor leading to significant changes in diatom communities in lakes of the northern hemisphere. However, due to the intensification of human activities since 1850, some scholars have emphasized that the increasing lake trophic level may be the main reason for the changes in diatom communities. The debate is ongoing. In order to avoid falling into the complex relationship between diatom changes and the seasonal cycle that characterizes lakes in mid and high latitudes, we chose a lake located at a low latitude, where the relationship between diatoms and temperature is not indirect but direct. The diatom record spans the past ca. 100 years and reveals that the abundance of Aulacoseira granulata increased from 1900 until 1985, replacing the previously dominant Aulacoseira ambigua. These changes are in agreement with the increasing trend in global temperature. Since 1985, the percentages of the small‐celled Discostella stelligera and the benthic diatom Navicula heimansioides have increased, while Aulacoseira granulata has decreased. This latest shift is caused by further global warming. We conclude that warming is the main factor leading to changing diatom communities in Douhu Lake.

Effects of climate change on a subtropical montane peatland over the last two centuries: Evidence from diatom records

Climate variability can induce rapid changes in peatland ecosystems, affecting both carbon cycling and vegetation succession. Diatoms are an important group of ubiquitous and diverse algae in peatlands. Until now, the responses of diatom communities to climate variability have rarely been explored in peatlands, especially in subtropical regions. In this study, monitoring and paleolimnological datasets were combined to decipher environmental changes of a subtropical montane peatland (central China) over the last two centuries. Seasonal monitoring data revealed that diatom communities were closely correlated with precipitation, depth to the water table, conductivity, nitrate and temperature. Sedimentary records revealed that temporal changes in diatom assemblages and geochemical elements displayed similar trends in two peat cores after the 1950s. The first gradient in diatom composition represented a shift from Pinnularia species to taxa preferring less-acidic habitats, which was closely linked to climate warming and the enrichment of inorganic elements (e.g. sodium and calcium) since the early 20th century. Meanwhile, changes in diatom communities were further related to precipitation variability, atmospheric deposition and local hydrogeomophic setting. Taken together, the succession of diatom communities was closely linked to climate-regulated availability of nutrients and moisture in this subtropical peatland over the last two centuries. In order to achieve sustainable management of these scarce peatlands, further biological monitoring and paleoecological studies are needed to improve our knowledge of peatland ecosystem evolution in response to future climate change.

EDNA reveals estuarine benthic community response to nutrient enrichment – evidence from an in-situ experiment

Nutrient loading is a major threat to estuaries and coastal environments worldwide, therefore, it is critical that we have good monitoring tools to detect early signs of degradation in these ecologically important and vulnerable ecosystems. We carried out a seven-month manipulative experiment in two estuaries to assess the effects of nutrient loading on benthic communities. Environmental DNA metabarcoding was used to examine the response of eukaryotic (18S rRNA), diatom (rbcL), and bacterial (16S rRNA) communities to two levels of nutrient enrichment (150 and 600 g N m-2). Multivariate analyses demonstrated consistent changes in eukaryotic, diatom, and bacterial communities in response to enrichment, despite differing environmental conditions between sites (Fig. 1). These patterns aligned with changes in macrofaunal communities identified using traditional morphological techniques, confirming concordance between disturbance indicators detected by eDNA and current monitoring approaches. Clear shifts in eukaryotic and bacterial indicator taxa were seen in response to nutrient loading while changes in diatom communities were more subtle. Community changes were discernable between nutrient levels, suggesting that estuary health assessment tools could be developed to detect early signs of degradation. Existing eDNA-based biotic indices (microgAMBI and mtMBI) were able to detect these community shifts, suggesting transferability of these indices to other regions and systems. This work represents a first step towards the development of molecular-based estuary monitoring tools, which could provide a more holistic and sensitive approach to ecosystem health assessment with faster turn-around times and lower costs.

Controls on diatom biogeography on South Carolina (USA) barrier island beaches

Benthic microalgae are of great significance to coastal productivity and nutrient cycling, yet determinants of their biogeography and community assembly remain unclear. We identified and compared spatial and environmental mechanisms that shape beach diatom biogeography and diversity at regional scales. We sampled at various spatial scales (from 10 cm to 120 km) from 4 barrier island beaches in South Carolina, USA, and characterized diatom assemblages using DNA metabarcoding. Twelve environmental variables from sediments, adjacent waters, and atmosphere were recorded. Despite attempts to sample from similar habitat types, environmental factors varied among islands. Correspondingly, biomass and α-diversity metrics likewise varied and were positively related to sediment nutrient levels. Community composition also varied, with each island dominated by different diatom taxa. Redundancy analysis revealed that spatial influences, in addition to environmental factors (water temperature and porewater nutrients), were significantly related to community dissimilarity (β-diversity). Variation partitioning attributed only ~5% of community variation to geographic distance alone, whereas 22% was explained by combined spatial/environmental factors. A high proportion (73%) of community dissimilarity remained unexplained, which likely implicates stochastic processes, although unmeasured physico-chemical or biological factors cannot be discounted. Weak spatial effects suggest that dispersal limitation was relatively unimportant, likely due to extensive sediment transport between beaches. Instead, spatially structured environmental variables appeared to condition beach diatom biogeography. These findings broaden our understanding of microbial biogeography to less well-studied microorganisms and marine habitats. More practically, results suggest some degree of predictability in diatom community changes in the face of anthropogenic challenges.

Spatial changes in the summer diatom community of the northern Bering Sea in 2017 and 2018

In recent years, the northern Bering Sea has experienced changes in the timing of sea-ice retreat and in hydrographic conditions during the summer. The influence of these environmental changes on the diatom community has not been examined. In this study, we investigated the spatial changes in the diatom community of the northern Bering Sea during the summers of 2017 and 2018, and evaluated the effects of environmental variability on these communities. We found that the diatom cell density and diversity varied with water masses. A cluster analysis based on cell density revealed that the diatom communities were separated into four groups, and that the distributions of three of these groups were different spatially between 2017 and 2018. In the Bering Strait and the Chirikov Basin regions, the diatom communities differed between 2017 and 2018. In 2017, these diatom communities were dominated by cold-water species such as Chaetoceros gelidus and Chaetoceros spp. (subgenus Hyalochaetae), while in 2018, the community was dominated by cosmopolitan species such as Thalassionema nitzschioides and Chaetoceros spp. (subgenus Phaeoceros). NMDS and multiple regression analysis indicated that the timing of the sea-ice retreat was the most important contributor to the differences in the diatom community. In contrast, there was no year-to-year difference south of St. Lawrence Island, possibly because nutrients were depleted and phytoplankton types other than diatoms were dominant.

Environmental DNA metabarcoding reveals estuarine benthic community response to nutrient enrichment – Evidence from an in-situ experiment

Nutrient loading is a major threat to estuaries and coastal environments worldwide, therefore, it is critical that we have good monitoring tools to detect early signs of degradation in these ecologically important and vulnerable ecosystems. Traditionally, bottom-dwelling macroinvertebrates have been used for ecological health assessment but recent advances in environmental genomics mean we can now characterize less visible forms of biodiversity, offering a more holistic view of the ecosystem and potentially providing early warning signals of disturbance. We carried out a manipulative nutrient enrichment experiment (0, 150 and 600 g N fertilizer m−2) in two estuaries in New Zealand to assess the effects of nutrient loading on benthic communities. After seven months of enrichment, environmental DNA (eDNA) metabarcoding was used to examine the response of eukaryotic (18S rRNA), diatom only (rbcL) and bacterial (16S rRNA) communities. Multivariate analyses demonstrated changes in eukaryotic, diatom and bacterial communities in response to nutrient enrichment at both sites, despite differing environmental conditions. These patterns aligned with changes in macrofaunal communities identified using traditional morphological techniques, confirming concordance between disturbance indicators detected by eDNA and current monitoring approaches. Clear shifts in eukaryotic and bacterial indicator taxa were seen in response to nutrient loading while changes in diatom only communities were more subtle. Community changes were discernible between 0 and 150 g N m−2 treatments, suggesting that estuary health assessment tools could be developed to detect early signs of degradation. Increasing variation in community structure associated with nutrient loading could also be used as an indicator of stress or approaching tipping points. This work represents a first step towards the development of molecular-based estuary monitoring tools, which could provide a more holistic and standardized approach to ecosystem health assessment with faster turn-around times and lower costs.

Genetic and morphologic determination of diatom community composition in surface sediments from glacial and thermokarst lakes in the Siberian Arctic

Lakes cover large parts of the climatically sensitive Arctic landscape and respond rapidly to environmental change. Arctic lakes have different origins and include the predominant thermokarst lakes, which are small, young and highly dynamic, as well as large, old and stable glacial lakes. Freshwater diatoms dominate the primary producer community in these lakes and can be used to detect biotic responses to climate and environmental change. We used specific diatom metabarcoding on sedimentary DNA, combined with next-generation sequencing and diatom morphology, to assess diatom diversity in five glacial and 15 thermokarst lakes within the easternmost expanse of the Siberian treeline ecotone in Chukotka, Russia. We obtained 163 verified diatom sequence types and identified 176 diatom species morphologically. Although there were large differences in taxonomic assignment using the two approaches, they showed similar high abundances and diversity of Fragilariceae and Aulacoseiraceae. In particular, the genetic approach detected hidden within-lake variations of fragilarioids in glacial lakes and dominance of centric Aulacoseira species, whereas Lindavia ocellata was predominant using morphology. In thermokarst lakes, sequence types and valve counts also detected high diversity of Fragilariaceae, which followed the vegetation gradient along the treeline. Ordination analyses of the genetic data from glacial and thermokarst lakes suggest that concentrations of sulfate (SO42−), an indicator of the activity of sulfate-reducing microbes under anoxic conditions, and bicarbonate (HCO3−), which relates to surrounding vegetation, have a significant influence on diatom community composition. For thermokarst lakes, we also identified lake depth as an important variable, but SO42− best explains diatom diversity derived from genetic data, whereas HCO3− best explains the data from valve counts. Higher diatom diversity was detected in glacial lakes, most likely related to greater lake age and different edaphic settings, which gave rise to diversification and endemism. In contrast, small, dynamic thermokarst lakes are inhabited by stress-tolerant fragilarioids and are related to different vegetation types along the treeline ecotone. Our study demonstrated that genetic investigations of lake sediments can be used to interpret climate and environmental responses of diatoms. It also showed how lake type affects diatom diversity, and that such genetic analyses can be used to track diatom community changes under ongoing warming in the Arctic.

Discrepancy in the responses of diatom diversity to indirect and direct human activities in lakes of the southeastern Tibetan Plateau, China

Abstract Both global warming and human impacts are inimical to the maintenance of biodiversity across the planet. Although there is some consensus regarding the probable responses of biodiversity changes under different global change scenarios, the factors controlling biodiversity on regional scales are unclear. Using surveys of lake-bed surface sediments and an analysis of diatoms from six lake cores across 116 lakes in the southeastern Tibetan Plateau of China, we investigated (i) changes in diatom richness and (ii) β diversity and its components at various spatial and temporal scales. β diversity partitioning revealed that the most important factor affecting changes in diatom communities was species turnover across both spatial and temporal scales. This finding indicates that conservation planning requires a regional approach that focuses on multiple lake systems. Temporal changes in the trend of β diversity were shown to result from changes in nestedness components. The results showed that climate warming and indirect human impacts caused the decline of species nestedness, thereby increasing biodiversity, while the intensification of direct human activities caused species nestedness to rise, resulting in concomitant loss of biodiversity. Since nestedness is often related to ordered species gain/loss dynamics, the results suggest that species gain slows down in lakes affected indirectly by human activities, while species loss accelerates in lakes subjected to direct human impacts. Thus, we predict that species richness will continue to decline across this vast region in the future.

Changes in summer diatom composition and water quality in urban lakes within a metropolitan area in central China

AbstractUrbanization is altering water quality and biotic communities of lake ecosystems worldwide, especially in developing regions. In this study, the diatom community structure and water quality of the epilimnion were analyzed in 38 lakes (40 sampling sites) in Wuhan City (central China) to assess the impacts of urbanization on lake ecosystems. Diatom communities displayed clear variation along the urban‐to‐rural gradient, with blooms of eutrophic species found in urban lakes in contrast to higher abundances of mesotrophic or oligotrophic taxa in rural lakes. Redundancy analyses revealed that the diatom community recognition to species, genus, and ecological guild level were all significantly correlated with Chl a, Si, and secchi depth, indicating that the changes in diatom communities were mainly explained by differences in nutrient concentrations and light condition. Considering the rapid urbanization and population expansion, it is important to carry out effective measures for the protection of these lake ecosystems, especially by reducing sewage discharge. The high consistency found between diatom community classification and water quality status highlights the applicability of diatoms for bioindication of shallow urban lakes, especially in rapidly developing cities. Even the taxonomic determination at the level of genera or ecological guilds can enable rapid water‐quality assessment.