Diatom Assemblage Composition Research Articles

Seasonal variations of diatoms in a low-latitude mountain lake: a case study from Douhu lake-Southeast China

In the scientific literature on diatom ecology whether temperature drives changes in diatom assemblage directly or indirectly remains a subject of debate. In order to clarify the relationship between temperature and diatom assemblage composition, we focused on a region with significant seasonal changes but without a freezing period. From March 2016 to March 2018, sediment trap samples were collected monthly to monitor the seasonal succession of diatoms in Douhu Lake, in subtropical southeast China. The results show that in 2016 the variations in abundance of Aulacoseira granulata and Aulacoseira ambigua followed an obvious seasonal pattern, corresponding to the seasonal variation in water and air temperature. In 2016, the thermophilic taxon A. granulata was dominant in the warm season, and in the cold season its abundance declined, while that of A. ambigua, a taxon better adapted to cooler conditions, increased relatively. In 2017, however, there was no such typical seasonal variation. In the cold season of 2017, A. granulata did not decline and there was no significant increase in the cold-preferring A. ambigua. This was related to the large drop in the lake level associated with human activity (substantial water extraction) that occurred during the winter of 2017, which disturbed the natural seasonal fluctuation in water temperature. Thus, in 2017, while air temperature followed the regular seasonal cycle, lake water temperature did not decrease significantly in winter, which allowed A. granulata to maintain a relatively high abundance in the cold season. From these results and an extensive survey of the literature, we conclude that water temperature, more than the seasonal succession, directly drives the fluctuation in abundance of these two Aulacoseira species.

Response of Diatoms to the Changing Water Quality in the Myristica Swamps of the Western Ghats, India

Myristica swamps are one of the rarest wetland ecosystems within the sub-tropical evergreen forests of the Western Ghats, India. As their name indicates, they harbor trees belonging to the ancient family Myristicaceae. Due to the waterlogged conditions and high humic decomposition, these swamps are acidic, harbor rare and endemic biotas, and provide ecosystem services to humans. Monitoring this rare ecosystem is crucial because the swamps that once formed a large hydrological network across the Western Ghats are now confined to isolated patches due to human disturbance such as agricultural interventions, roads, and dam construction. Due to the change in land use, there is also a drastic change in water chemistry and associated biodiversity. Biomonitoring is more precise than physical and chemical monitoring. So, the current study aimed to undertake a comprehensive analysis of the physical, chemical, and biological assessment of these swamps. The diatom assemblages are strongly affected by water chemistry and serve as a powerful indicator of environmental changes in the freshwater aquatic systems. However, there is no information on diatom assemblages in these swamps, and the present study aimed to determine the diatom assemblage structure in the Myristica swamps and their response to changing water quality. Diatom samples were taken at 17 different swamps across the central Western Ghats, and a set of environmental parameters was evaluated. Analysis revealed a total of 91 species of diatoms belonging to 27 genera across the 17 sites, from which 44 diatom species showed restricted distribution to this unique environment. Overall, the dominant diatom genera inside the swamps included, Navicula (19.8%), Gomphonema (16%), Eunotia (13.3%), Ulnaria (9.4%), Achnanthidium (8%), Frustulia (6.2%), Planothidium (5.2%), and Brachysira (2.8%). High diatom species richness was observed in the swamps having less anthropogenic disturbance, and diatom assemblage composition was primarily determined by dissolved oxygen, pH, and conductivity. The significant number of geographically restricted taxa in this study points towards our limited understanding of this tropical biome and calls for a dire need for more studies from here, not only to improve our knowledge concerning the diversity, ecology, and biogeography of these diatoms but to further encourage their use in applied (paleo) environmental sciences. Our results indicate that diatoms can prove useful environmental indicators even in harsh environments like swamps and can be a potential tool for assessing ecological and climatic change.

Benthic diatoms and macroinvertebrates status with relevant to sediment quality of islands shores in the Nile River, Egypt

Sediments are utilized as a marker for events that endure long enough to manifest their environmental impacts and determine the contamination levels. The purpose of the present study was to highlight the current sediment quality of four Nile islands shores by utilizing a variety of physical, chemical, and biological aspects and indices. In addition, the status of benthic diatoms and macroinvertebrates, as well as their responses to sediment variables were investigated. The metals followed a decreasing concentration order: Al > Fe > Mn > Zn > Ni ≈ Cr > Cu > Co > Pb > Cd. The outcomes of metal pollution indices revealed some localized contaminated sites, by mostly Ni and Cd. In addition, a moderate toxic risk from metals was observed for El-Keratten shores, while the sediments of El-Warraq, El-Zamalek, and El-Manial islands shores were at low toxic risk. A total of 112 diatom species belonging to 24 genera were identified in the total sample set, where most of the diatoms encountered were cosmopolitan. Metals had a remarkable impact on the composition, distribution, and dominance of diatom assemblages, but had little effect on diatom diversity, while there was a noticeable effect of grain size on diatom distribution and diversity. Thirty-four species of benthic invertebrates were identified in the collected samples from the islands shores. The dominant taxa and groups of benthic invertebrates at different islands were affected by various sediment parameters, where the levels and types of such variables differed from one species to another.Graphical abstract

Quantifying spatial and temporal relationships between diatoms and nutrients in streams strengthens evidence of nutrient effects from monitoring data.

Observational data are frequently used to better understand the effects of changes in P and N on stream biota, but nutrient gradients in streams are usually associated with gradients in other environmental factors, a phenomenon that complicates efforts to accurately estimate the effects of nutrients. Here, we propose a new approach for analyzing observational data in which we compare the effects of changes in nutrient concentrations in time within individual sites and in space among many sites. Covarying relationships between other, potentially confounding environmental factors and nutrient concentrations are unlikely to be the same in both time and space, and, therefore, estimated effects of nutrients that are similar in time and space are more likely to be accurate. We applied this approach to diatom rbcL metabarcoding data collected from streams in the East Fork of the Little Miami River watershed, Ohio, USA. Changes in diatom assemblage composition were consistently associated with changes in the concentration of total reactive P in both time and space. In contrast, despite being associated with spatial differences in ammonia and urea concentrations, diatom assemblage composition was not associated with temporal changes in these nitrogen species. We suggest that the results of this analysis provide evidence of a causal effect of increased P on diatom assemblage composition. We further analyzed the effects of temporal variability in measurements of total reactive P and found that averaging periods greater than ~1 wk prior to sampling best represented the effects of P on the diatom assemblage. Comparisons of biological responses in space and time can sharpen insights beyond those that are based on analyses conducted on only 1 of the 2 dimensions.

The evolution of the ecosystem of the Unitskaya Bay (Lake Onega) in the late- and postglacial times as inferred from the siliceous microalgae study

Abstract. The study of the siliceous microalgae (diatoms and chrysophytes) enabled reconstructing the evolution of the ecosystem of the Unitskaya Bay, the northern bay of Lake Onega. Changes in the composition of diatom assemblages, floristic diversity and abundances of siliceous microalgae indicated past productivity changes, as well as shifts in duration of hydrological and biological seasons.

Composition and age of the Neogene freshwater diatom assemblages from the Satomi Formation and Okukamisato lignite bearing Formation in the Kamisato area, Tsubetsu Town, Abashiri County, Hokkaido, Japan

Composition and age of the Neogene freshwater diatom assemblages from the Satomi Formation and Okukamisato lignite bearing Formation in the Kamisato area, Tsubetsu Town, Abashiri County, Hokkaido, Japan

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.

Drivers of the diversity of diatoms in an oligotrophic Andean stream

An in situ experiment determined that the addition of ammonia N-NH4+, nitrate N-NO3– and phosphate P-PO43– to an oligotrophic Andean stream significantly affected diatom composition and diversity. Over a nine-month period, fertilized and non-fertilized sections of the stream were studied for their environmental characteristics and diatom composition. Nutrient concentrations were increased three times in a downstream section with respect to those occurring naturally upstream. Alpha diversity (Shannon-Wiener [H”] and Simpson [S] indices) were determined monthly. In the non-fertilized control section, diatom diversity was significantly (p ≤ 0.05) related to water flow and P and N concentrations. In the fertilized section diversity was associated to the enhanced ammonia and phosphorus. A redundancy analysis (RDA) showed that conductivity, water flow and nutrient concentrations contributed to the diatom assemblage composition (83% along the first two axes). Achnanthidium minutissimum, Cocconeis placentula, and Epithemia sorex occurrence was related to low phosphorus concentration. Epithemia adnata var. minor, Nitzschia dissipata, and Reimeria sinuata were associated with high P concentrations, high water flow and low conductivity Navicula rynchocephala, Gomphonema parvulum, Navicula capitatoradiata, and Melosira varians were associated with high P.

Variability of diatom community composition and structure in mountain streams

Small rivers support high levels of biodiversity, being especially sensitive to the effects of global change. Temporal records of community composition in minimally impaired streams can be used to explore trends in biodiversity in response to climate change and natural temporal variation. We approached the comparison of two time periods (2003–2008 and 2016–2020) to study whether the composition of diatom assemblages changed over time in twenty-three streams of the mountain range of Picos de Europa (Northern Spain). The stream’s water chemistry indicated significant decreases in N_NO3− and P_PO43− content over time. In these minimally disturbed streams, the specific diatom community was dominated by Achnanthidium pyrenaicum, Achnanthidium minutissimum and Cocconeis euglypta. PERMANOVA analyses did not identify significant changes in diatom assemblage composition between periods or river types. Diatom indices (e.g. IPS, NORTIdiat) indicated high or good ecological status and relatively high alpha diversity values were found in these mountain rivers during the studied years. Although diversity and evenness showed a significant decrease over time, the temporal stability of the river-type diatom reference community between the two periods should be considered as an indicator of biodiversity persistence of high importance when monitoring the ecological status following the reference condition approach.

Impact of Sea‐Ice Dynamics on the Spatial Distribution of Diatom Resting Stages in Sediments of the Pacific Arctic Region

AbstractThe Pacific Arctic region is characterized by seasonal sea‐ice, the spatial extent and duration of which varies considerably. In this region, diatoms are the dominant phytoplankton group during spring and summer. To facilitate survival during periods that are less favorable for growth, many diatom species produce resting stages that settle to the seafloor and can serve as a potential inoculum for subsequent blooms. Since diatom assemblage composition is closely related to sea‐ice dynamics, detailed studies of biophysical interactions are fundamental to understanding the lower trophic levels of ecosystems in the Pacific Arctic. One way to explore this relationship is by comparing the distribution and abundance of diatom resting stages with patterns of sea‐ice coverage. In this study, we quantified viable diatom resting stages in sediments collected during summer and autumn 2018 and explored their relationship to sea‐ice extent during the previous winter and spring. Diatom assemblages were clearly dependent on the variable timing of the sea‐ice retreat and accompanying light conditions. In areas where sea‐ice retreated earlier, open‐water species such as Chaetoceros spp. and Thalassiosira spp. were abundant. In contrast, proportional abundances of Attheya spp. and pennate diatom species that are commonly observed in sea‐ice were higher in areas where diatoms experienced higher light levels and longer day length in/under the sea‐ice. This study demonstrates that sea‐ice dynamics are an important determinant of diatom species composition and distribution in the Pacific Arctic region.

Are recent protection strategies sufficient for maintaining diverse freshwater benthic diatom assemblages?

Even though microscopic algae play pivotal role in the healthy functioning of freshwater ecosystems, recent water protection strategies rarely consider them and primarily focus on macroscopic organisms. Here, we studied the effect of protection level and utilization type of lowland standing waters on the composition and diversity of benthic diatom assemblages and on the diatom-based ecological status of waters. We hypothesized that (i) protected waters will sustain more diverse diatom assemblages and better ecological quality than not protected ones. We also hypothesized that (ii) the increase in number of utilization will affect negatively on biodiversity and on ecological quality. Clear taxonomic differences were revealed only in protected and not utilized waters while trait composition in protected waters was independent from the utilization type. Neither biodiversity nor ecological status of waters were influenced by protection level. The increase in number of utilization types, however, significantly decreased functional richness. Although high biodiversity of algae can effectively buffer the negative effects of climate change and anthropogenic impact, recent protection strategies are insufficient to support it.

Response of epilithic diatom assemblages to weir construction on the Nakdong River, Republic of Korea

In South Korea, sixteen multifunctional weirs were constructed on four major rivers (Han, Nakdong, Geum, Yeongsan) from 2009 to 2012. Hydrological changes resulting from weir construction altered the physical, chemical, and biological characteristics of natural large rivers. We aimed to investigate the variation in epilithic assemblages responding to changes in physicochemical, meteorological, and hydrological variables in the eight weir (lentic) sections of the Nakdong River, which is the longest river in South Korea. Sampling was conducted to determine the influence of environmental parameters on community composition, and to monitor the seasonality of environmental and biological patterns.A total of 173 taxa including 26 major epilithic diatom taxa were identified. Among them, nutrient-tolerant diatom species were characterized by Cyclostephanos dubius, Navicula gregaria, Fragilaria crotonensis, Aulacoseira granulata, Melosira varians, Navicula minima, Nitzschia amphibia, Amphora pediculus, and Nitzschia inconspicua. These species were primarily dominant in the midstream and downstream weir sections of the river adjacent to urban-industrial land use zones relating to nutrient enrichment. Moreover, preference for longer hydraulic retention time (HRT) was identified in C. dubius, N. gregaria, and F. crotonensis, in addition to its tolerance for high nutrition. Redundancy analysis based on the relative abundance of diatom taxa demonstrated that the diatom assemblage composition was closely related to total nitrogen (TN), total phosphorus (TP), phosphate phosphorus (PO4-P), electrical conductivity, precipitation, water discharge, and HRT. Variation partitioning analysis further revealed that hydrological-meteorological variables play a synergistic role in enhancing the function of physical and chemical variables in structuring diatom assemblage composition soon after weir construction. A generalized linear model using the Akaike’s information criterion also suggests that the trophic diatom index (TDI) could indicate the water quality status in the weir sections, as the TDI was positively correlated (p < 0.05) with nutrients (TN and PO4-P). Our findings inform the applicability of using the TDI for efficiently monitoring and managing the water quality of consecutive weir sections of the Nakdong River and the weir systems of other rivers in temperate regions.

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.

Effects of anthropogenic pressures on diatom communities using morphological and molecular approaches

The relevance of molecular composition of diatom assemblages to detect river impairment caused by different intensive land uses (industrial, agricultural, and urbanization) was tested in this study with data from two rivers (Ferreira and Sousa rivers) and 21 sampling sites located in the north of Portugal. The Water Framework Directive (WFD) gives the legal basis for the use of this ecological indicator for water quality assessment (Vasselon et al. 2017 ). However, the morphological identification and count of diatoms using the light microscope requires a high level of expertise, is time-consuming and costly (Valentin et al. 2019). DNA metabarcoding combined with high-throughput sequencing techniques (HTS), offers a promising alternative to classic methodologies, reducing time and costs (Mortágua et al. 2019 ). Thus, here we compared the response of the two approaches in terms of ecological assessments (IPS Ecological Quality Ratios) to the different types of pressures felt in the 21 sites. Diatoms were sampled at 21 sites located in the North of Portugal in autumn of 2019 (Fig. 1). Samples were submitted in parallel to the molecular and morphological analyses. The eDNA was extracted, PCR amplified (312 bp rbcL DNA barcode), and finally sequenced (Illumina MiSeq). The Mothur software was used to obtain the Operational Taxonomic Units (OTUs), which were then taxonomically assigned to the species through the R-Syst::diatom version 7.1 (Rimet et al. 2018) reference library. EQR values indicated a good correlation between morphological and molecular methods (Fig. 2). PCA analysis (Fig. 3 ) revealed that in urban, agricultural, and industrial areas there is a greater concentration of nutrients (phosphorus and nitrogen), organic matter, and heavy metals due to the discharge of urban/industrial effluents, while in places considered natural (without any type of anthropogenic pressure) we find low levels of these pollutants and high concentrations of dissolved oxygen (DO). The BEST (BIO-ENV) analysis (Tables 1, 2) shows in the case of the morphological approach, the combination of 4 environmental variables (NO₃⁻, Li, K, and Cu) is highly correlated with the biological patterns, and in the molecular approach the combination of only 2 of the environmental variables (Li and K), explains the distribution of diatom communities composition and has a slightly higher correlation. The morphological methodology seems to demonstrate a better response to urban pressures, mainly to effluent discharges, while the molecular one demonstrates a more diffuse response with special emphasis on good correlation with variables such as zinc and nitrate, which may also be related to effluent discharges and use of fertilizers in agriculture. However, it is necessary to improve the reference library so that there is a better response of the molecular methodology to the existing pressures.

Duration of water flow interruption drives the structure and functional diversity of stream benthic diatoms

Flow cessation affects river ecosystems submitted to low precipitation and increased water demand, and creates unfavourable conditions to aquatic biological communities. Diatoms are amongst the most sensitive biological groups to hydric stress, making them good indicators of preceding hydrological conditions. We here analyse the response of diatom assemblages to the duration and frequency of non-flow periods in 23 Mediterranean temporary streams. All of them experienced a strong decrease in water flow during summer, leading to a period of flow cessation. In addition, other ten permanent streams (zero dry days during the study period) were included in the study for comparative purposes. Temporary and permanent streams showed similar diatom species richness, evenness, and alpha diversity. However, beta diversity was higher in temporary than permanent streams, regardless rare taxa (<1% occurrence) had similar numbers in the two types of streams. Alpha diversity, richness, and evenness of diatom assemblages in the temporary streams changed with the duration of the non-flow period. Durations of 50–100 days were associated to higher alpha diversity, richness, and evenness, but longer non-flow periods caused their decrease. Diatoms thriving under the most extreme conditions were mostly aerophilic, pioneer, and motile taxa. The proportion of aerophilic diatom taxa increased beyond 100 dry days, particularly in those sites receiving more intense solar radiation. Overall, the taxonomical and functional composition of diatom assemblages mostly responded to the duration of the non-flow period, irrespectively of these being consecutive or not. This study shows that diatom assemblages from temporary streams contain taxa adapted to non-flow conditions, but remain vulnerable to further reductions of water flow associated to climate or global change. ‘Unimpacted’ watercourses in the Mediterranean region, independently of their flow regime, should be urgently protected and used as taxonomical and functional early indicators of climate change.

Diatom Assemblages From Surface Sediments of Two Coastal Lagoons, the Central Part of the Red Sea, Saudi Arabia and Their Associated Environmental Variables

The composition and distribution pattern of diatom assemblages in the surface sediments of Sharm Al-Kharrar and Al-Shuaiba lagoons, situated on the central part of the Red Sea coastline of Saudi Arabia, were analyzed. A total of 191 diatom species from 53 genera were identified. A highly diverse diatom flora dominated by the genera Mastogolia, Amphora sensu lato, Lyrella, and Navicula sensu lato species was observed. Diatom species composition showed the dominance of the benthic forms over the epiphytic and planktonic ones, which can be attributed to the shallowness of the environments. The relationship between the recognized diatom taxa and measured environmental variables was explored using multivariate statistical techniques. Hierarchical ascending clustering and detrended correspondence analyses differentiated six representative diatom assemblages in each Lagoon. Canonical correspondence analysis revealed that grain size, organic matter content, and water depth appeared to be significant environmental factors affecting the distribution and structure of the documented diatom assemblages, while pH, salinity, and temperature were less influential. The majority of species recorded were polyhalobous benthic diatoms, accompanied by a limited abundance of mesohalobous and substantial amounts of oligohalobous forms, which are probably of allochthonous origin and resulting from freshwater input of near seasonal wadi streams and human activities. A comparison between the recognized diatom assemblages and the previously recorded taxa in surrounding areas was made.

Effects of spatial heterogeneity on the estimation of diatom assemblage composition: an example of Lake Imbradas (NE Lithuania)

High spatial resolution diatom and loss-on-ignition analyses were carried out on the surface sediments of a shallow, medium-sized lake in north-eastern Lithuania to examine the degree of spatial heterogeneity of diatom assemblages in the lake, as well as the influence of water depth on diatom compositions. The compositional properties of sediments and diatom assemblages show a strong relationship with water depth; some less prominent changes were attributed to the tributary catchment. Diatom assemblage compositions are rather homogenous throughout the lake (Morisita-Horn similarity indices make &gt;0.9 in most locations of the lake, in relation to others), especially in its deeper parts. Our case shows that the most representative point does not lie in the centre of the lake, as it is assumed in most of fossil studies. Studies of spatial heterogeneity of modern diatoms can facilitate site selection and fossil diatom data interpretation.

A diatom-based Holocene record of sedimentary and oceanographic environmental changes within the Beibu Gulf, NW South China Sea

A sediment core from a nearshore setting (10.8 m in water depth) of the inner Beibu Gulf, northwestern South China Sea, was analyzed in terms of diatom assemblage composition, grain size distribution and radiocarbon dating, with aim to describe sedimentary and oceanographic environmental changes during the Holocene. The fossil diatom record and changes in granulometry reveal that sea-level fluctuations had a strong influence on the evolution of hydrographic system, which changed significantly with rapid sea-level rise in the early Holocene ca. 10,000–7800 cal. yr. BP. These fluctuations continued in a series of stages dated to the middle Holocene ca. 7800–6200 cal. yr. BP, with sea-level increase, followed by a prolonged sea-level retreat during ca. 6200 to 3300 cal. yr. BP, and finally a relatively high sea level from ca. 3300 cal. yr. BP to the present. The resultant sedimentary facies could be divided into seven stages inferred by the diatom distribution and grain size characteristics: a river channel/beach environment at ca. 10000–9500 cal. yr. BP, a lagoonal environment at ca. 9500–7800 cal. yr. BP, a relatively broad embayment at ca. 7800–6200 cal. yr. BP, a bay at ca. 6200–3300 cal. yr. BP and a coastal environment setting up to present day. The coastal environmental setting has been subject to some shifts in climate and sea-level beginning with strong terrestrial influx at ca. 3300–1400 cal. yr. BP, which was then replaced by strong hydrological circulation at ca. 1400–800 cal. yr. BP, and a relative stable sedimentary environment approximate to the coast from ca. 800 cal. yr. BP to present.A very obvious increase in diatom abundance in the early and late Holocene reflected terrestrial influx transporting nutrients, driven by precipitation changes influenced by the Asian monsoon circulation pattern, particularly in the early Holocene. Increased anthropogenic impact, i.e. an enhanced nutrient condition, was inferred from higher abundances and reasonably high diatom diversity in this local record from 1400 cal. yr. BP, especially in the last 800 years. Rapid shifts in the diatom composition and grain size distribution at about 1400 and 1000 cal. yr. BP were observed that could be interpreted as overlapping influences of a global scale paleo-climate anomaly and local human activities. This study demonstrates the effectiveness of using diatom for understanding regional paleoenvironment change driven by both global climate changes and anthropogenic impact in coastal area of the Beibu Gulf, northwestern South China Sea.

The diatom response to Postglacial environments in the White Sea, the European Arctic

The White Sea is a small semi-enclosed Arctic sea influenced by Atlantic waters. It was completely covered by the Scandinavian Ice Sheet in the Late Pleistocene and plays a key role in the reconstruction of the degradation of the Scandinavian Ice Sheet and Atlantic Water advection into the Arctic Ocean. We examined the inflow of Atlantic water into the White Sea Basin as the leading feature of postglacial sedimentation processes. Palaeoreconstructions were conducted through detailed studies of diatom assemblages in two sediment cores from the central area (PS-6042) and outer part (PS-6050) of Dvina Bay. Stratigraphic division of the core sediments was performed on the basis of palynostratigraphy and accelerator mass spectrometry 14C dates. The composition of diatom assemblages in postglacial sediments from Dvina Bay indicate a change in the environment from a freshwater sea bay to fully marine conditions over the past 12 cal kyr. Based on the obtained results, we established that the Atlantic water started to flow into the central Dvina Bay in the first half of the Preboreal (11.7–11.3 cal ka BP), and the greatest increase in water surface temperatures in central Dvina Bay occurred during the period from the end of the Atlantic to the first half of the Subboreal. We confirmed these conclusions through comparison with previously obtained results of aquatic palynomorphs, pollen and spores, grain size analysis, and measurements of total organic carbon and biogenic silica contents, as well as through correlation with published palaeoreconstructions of the White Sea and the Arctic Ocean.

Development of a novel metric for evaluating diatom assemblages in rivers using DNA metabarcoding

Fundamental differences in the nature of diatom assemblage composition data generated using light microscopy and molecular barcoding create problems when applying current paradigms and metrics developed for ecological assessment. We therefore describe the development of a new metric designed specifically for diatom rbcL barcode data gathered using high throughput sequencing (HTS). Although the structure of datasets collected using HTS is similar to that collected using light microscopy (LM), differences in the proportions of key species between the two methods mean that the use of metrics designed for LM on HTS data gives biased results. We therefore recalibrated the Trophic Diatom Index in order to produce a version that is sensitive to nutrient pressures in rivers but that can be used with HTS data. Correlation between the LM and HTS metrics is good (r = 0.86 on a cross-validated model); however, 30% of sites will change class when the current Water Framework Directive classification approach is applied. Although less than 15% of diatom taxa recorded from UK and Ireland are included in the rbcL barcode reference database, gaps in this database are not a major source of variation between the HTS and LM models. We argue that use of metrics calibrated using HTS data is a more realistic option than applying correction factors to enable HTS data to be used with existing indices. We also stress the importance of starting the process of integrating HTS into ecological assessments with relatively conservative approaches. This enables the data collected by HTS to be related to those generated by established approaches, both now and during long-term monitoring, making it possible for scientists, regulators and stakeholders to have an informed conversation about the benefits and challenges of HTS. Overall, the study demonstrates that it is possible to translate the legal requirements of an ecological assessment framework from LM to HTS, though differences in these two approaches mean that there is unlikely to be perfect agreement between their outputs.