Surface Sediment Diatom Assemblages Research Articles

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.

The sub-fossil diatom distribution in the Beibu Gulf (northwest South China Sea) and related environmental interpretation.

Located in northwestern South China Sea (SCS), the Beibu Gulf constitutes an environmentally sensitive region shaped by land-ocean-atmosphere interactions in Asia between the western Pacific and eastern Indian Oceans. This study aims to provide a comprehensive view of the sub-fossil diatom biogeography, distribution pattern and oceanographic environmental controls with support of multivariate methods based on Beibu Gulf core-top samples. Cluster analysis of diatom assemblages divides the distribution pattern into four subclusters. Sea surface salinity (SSS), temperature (SST), trophic state (chlorophyll a concentration in this study) and water depth constrain the diatom distribution pattern through canonical redundancy analysis although only partly support an interpretation of the relationship between these various variables. Chlorophyll a has a strong correlation to diatom distribution, and responds to Paralia sulcata occurrence, while SSS and SST also have significant influence and indicate warm water invasion from the open SCS. Water depth is a subordinate factor in terms of Beibu Gulf diatom distribution. The ca. 25 m water-depth marks the upper extent of Paralia sulcata dominance in the northern Beibu Gulf. A strong mixing area with a complex diatom distribution exists below this water depth in the middle of Beibu Gulf. Coastal currents from north of SCS invade Beibu Gulf through Qiongzhou Strait and south of Hainan Island, as recorded by higher percentages of Paralia sulcata and Cyclotella striata at these sites. Our results provide a selection of evaluation method for a marine ecological red-line definition for sustainable development. This study highlights the perspective relationships between the spatial distribution of sub-fossil diatom assemblages in surface sediments and oceanographic variables, which could serve as a model for paleoenvironmental and paleoclimatic reconstruction in future marginal sea geoscience research for the Beibu Gulf, northwestern SCS.

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.

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 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.

The Canary in the Coal Mine: Testate Amoebae Record Anthropogenic Impacts in Sediments of Oligotrophic Lake George, NY, USA

ABSTRACT Lake George (NY) is surrounded by Forever Wild Forest in the Adirondack Park and has a Class AA Special water quality rating, yet lake monitoring has revealed increasing anthropogenic impacts from salt and nutrient loading over the past 30 years. To reconstruct anthropogenic influence on the lake (e.g., salt loading, eutrophication, climate warming), we characterized modern stable isotopes and testate amoeba and diatom assemblages in surface sediments from 33 lake-wide sites and compared their variability to 36 years of water-quality data. Linear regression analyses support testate amoebae as rapid responders and recorders of environmental change because taxa are strongly correlated with percent change of important water quality parameters. Our assessment indicates that: 1) Netzelia gramen is associated with aquatic plants and filamentous algae, making them a valuable aquatic plant/alga indicator, which is supported by the co-occurrence of the diatom Cocconeis spp.; 2) difflugids are generally good indicators of eutrophication, except for Difflugia protaeiformis; and 3) seasonal differences in water quality trends are reflected in the fossil record on decadal time scales. We show that testate amoebae are highly sensitive to small environmental changes in an oligotrophic lake and exhibit established relationships from eutrophic and mesotrophic lakes as well as new, likely oligotrophic-specific correlations. Correlation coefficients of water quality variables and strains within a species also illustrate gradational relationships, suggesting testate amoebae exhibit ecophenotypic plasticity. Diatom and testate amoeba assemblages categorize modern lakebed sites into four subgroups: 1) benthic macrophyte; 2) high nutrient; 3) high alkalinity; and 4) salt loading assemblages.

Characteristics of Diatom Assemblages in Surface Sediments of the new Yongding River and Haihe River Estuaries, Tianjin

Diatom analysis was conducted on the surface sediments from the estuaries of the new Yongding River and Haihe River in Tianjin. The results showed that the main diatom assemblages in the new Yongding River was Navicula marina-Auliscus caelatus-Coscinodiscus hirosakiensis-Coscinodiscusoculus-iridis and Auliscus caelatus- Coscinodiscusoculus-iridis-Cyclotella meneghiniana-Cyclotella stylorum in the Haihe River. There was saprophilous taxa, Cyclotella meneghiniana, in both estuaries indicating that the two estuaries were polluted to a certain degree.

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.

Distribution of diatoms and geochemical features of holocene sediments from the Kuril Basin (Sea of Okhotsk)

Sediments from the Kuril Basin were collected during the Russian-German deep-sea expedition SokhoBio (Sea of Okhotsk Biodiversity Studies). Concentrations of biogenic and trace elements, quantitative content, and species composition of diatom assemblages in surface sediments and core samples were analyzed. Distribution in the surface sediments depending on different environmental conditions as well as changes in the diatom frustule content, the ratio of dominant diatom species, and the ecological structure of diatom assemblages were studied.The collected sediments were silt and clayey silt, with SiO2am and Corg contents ranging from 15.85% to 36.61% and 0.84–1.92%, respectively. The spatial distribution patterns of the elements studied and the diatoms, as well as vertical profiles of the sediment cores, revealed that the contents of biogenic components and trace elements, such as amorphous silica, organic carbon, barium and phosphorus, changed in parallel with the change in the number of diatoms and depended mainly on the biological productivity of the sea. The change in water mass parameters and in the hydrological regime also affected the chemical composition of the bottom sediments over time.A predominance of oceanic species is observed in the sediments at the bottom of the basin, and the mixed oceanic-neritic composition of diatoms is characteristic of slope deposits. Moreover, the assemblages reflect the influence of Pacific Ocean water masses at stations close to the Kuril Straits, as well as in the warm waters of the Sea of Japan at the station in the Soya Current zone.A nearly synchronous increase in the impact of Okhotsk water masses and the entry of the Sakhalin Current waters into the Kuril Basin as one moved deeper into the core at stations with different climatic conditions was observed.The productivity and species diversity of diatoms and the chemical composition of sediments are reflections of the regional features of climatic processes.

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.

Diatom-based reconstruction of trophic status changes recorded in varved sediments of Lake Żabińskie (northeastern Poland), AD 1888-2010

Abstract We investigated diatom assemblages in surface sediments of 46 lakes in northern Poland and developed a diatom-based transfer function to infer epilimnetic total phosphorus (TP) concentrations. Multivariate ordination techniques (DCA, CCA) were used to identify major environmental gradients and to evaluate the effect of environmental parameters on the distribution of diatoms in the modern diatom dataset. The transfer function was developed using PLS, WA and WA-PLS models, and applied to a varved sediment core from Lake Żabińskie, AD 1888-2010. Annually-resolved quantitative reconstruction of TP concentrations shows that multidecadal changes in the TP level reflect the local settlement history, land-use changes and development of agriculture and tourism. The period of high trophic levels with maximum values of TP was documented until the late 1920s. In the 1930s-1970s period, TP generally decreased and eutraphentic flora was partly replaced by oligotraphentic and oligo-mesotraphentic diatom taxa. The reconstructed TP concentrations have started to increase from the 1970s. After the 1950s, strong short-term fluctuations of TP values were noted and explained by interactions between meteorological conditions, water column mixing and nutrient cycling in the lake.

Diatom Assemblages in Surface Sediments Along Nutrient and Salinity Gradients of Thi Vai Estuary and Can Gio Mangrove Forest, Southern Vietnam

In order to assess the current ecological status of the tidal Thi Vai Estuary and the Can Gio Mangrove Forest in Southern Vietnam, we evaluated the spatial diatom composition and abundance in surface sediments and their relation to physical and chemical parameters of the water. Major changes in the distribution of diatom assemblages were determined by multivariate statistical analyses. Principal component analysis (PCA) was used to ordinate environmental variables of the water, showing a clear gradient along the estuary, with higher nutrient levels (total phosphorus and total nitrogen) and lower dissolved oxygen (DO) concentrations at upstream sites. Canonical correspondence analysis (CCA) was used to determine the species-environmental relationship. Total phosphorus (TP) and DO (p < 0.001) as well as salinity and pH (p < 0.002) were the most significant variables related to diatom composition. The neritic planktonic species Thalassionema nitzschioides (Grunow) Mereschkowsky was the most widespread and abundant species in the data set, presumably also reflecting significant influence of oceanic water. Thalassiosira cedarkeyensis Prasad, a marine benthic species, was the second most abundant species and seems to be related to higher nutrient levels, particularly at upstream sites. High abundances of Cyclotella meneghiniana Kutzing and Cyclotella atomus Hustedt were also recorded upstream sites. Their association to high TP levels and lower DO concentrations suggests deterioration of water quality. This study shows the potential of diatoms as indicators of environmental change even in dynamic systems such as estuaries.

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.

SPATIO-TEMPORAL RESPONSE OF SEDIMENTARY DIATOMS TO WATER LEVEL IN A SHALLOW LAKE

In recent years, human activities have resulted in variations of hydraulic conditions in shallow lakes, thereby affecting interactions between environmental variables and aquatic plants. This feedback of plants on environmental changes is largely recorded in lake sediments. In this study we collected surface sediments in different seasons and a sediment core from a shallow lake Baiyangdian. We investigated the seasonal effects of water level and six physicochemical variables in surface sediments, as well as their impacts on the distribution of diatom assemblages. Therein the response of diatom assemblages to water level and the spatio-temporal effects of water level are highly emphasized. Results indicate that water level significantly changes between seasons, however there is no significant seasonal effects for the com- position of grain size. The lake productivity increases with the vigorous growth of aquatic plants. Water level largely determines the distribution of diatom assemblages in surface sediments and the sediment core. Spatial isolation causes the difference between the habitats and alters the relation between water level and diatom taxa. Human activities also affect the relevance of water level with diatom assemblage. The results can provide the helpful information for the local water resource management.

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.

Spatial distributions and environmental implications of diatom assemblages in surface sediments of Hulun Lake, China

We examined the spatial distributions of diatom assemblages in surface sediments of Hulun Lake and explored the potential relationships between diatom distributions and environmental variables using multivariate analyses. These results may provide a basis for fossil record interpretation. Surface sediment samples were taken at 30 sites throughout the lake to analyse diatom assemblages, and environmental variables potentially influencing diatom distributions were taken concurrently. Principal components analysis (PCA) and redundancy analysis (RDA) were conducted to explore the spatial characteristics of diatom assemblages in the lake surface sediment and their relationship with the main environmental variables. Analyses showed remarkable spatial differentiation such that epiphytic and benthic diatoms dominated surface sediments in the eastern littoral region, while planktonic diatoms were dominant in the central and western profundal region. Additionally, in the southern part of the central and west profundal region, Stephanodiscus neoastraea was more abundant than Aulacoseira perglabra. RDA showed that the total nitrogen of surface sediments (TNs), which acts as a nitrogen source and stays balanced with lake water nitrogen levels, is the primary environmental variable influencing diatom assemblages in the surface sediments. S. neoastraea and A. perglabra in particular may be boosted by high nitrogen content in the sediment. Additionally, correlations of water depth with the distribution of sample sites and the ratio of planktonic to benthic diatoms indicated an inherent relationship between diatom distribution and water depth, where deep water drove a high planktonic/benthic ratio. This result further supports the use of the planktonic/benthic ratio in the fossil record to reconstruct historical change in lake depth.

Composition and distribution of diatom assemblages in the surface sediments of the Bering Sea

Diatoms collected from the top 1 cm of the surface sediment layer at seven Bering Sea stations during the Fourth Chinese National Arctic Research Expedition (4th CHINARE-Arctic) in 2010 were studied. In total, 101 taxa belonging to 38 genera were found. The species were divided into four groups: eurythermal species, boreal-temperate species, polar species, and sea-ice species. The diatom assemblages at the stations in the southwestern basin area were primarily composed of boreal-temperate species, such as Neodenticula seminae , Thalassiosira trifulta , Rhizosolenia hebetata f. hiemalis , and Actinocyclus curvatulus . The northeastern shelf stations were dominated by polar species, including Fragilariopsis oceanica , Thalassiosira antarctica spora, Thalassiosira nordenskioeldii , and Thalassiosira hyalina . The overall abundance was highest at the basin stations with 3.7 ×106 cells·g-1 of wet sediment, whereas the eastern shelf stations had the lowest abundance of 0.7×106 cells·g-1 of wet sediment (excluding the resting spores of Chaetoceros spp.). The relationship between the distribution of the surface sediment diatom assemblages and the environment is discussed.

Assessment of drought over the past two millennia using near-shore sediment cores from a Canadian boreal lake

Lack of long-term studies on drought in the boreal region of northwest Ontario limits our ability to assess the vulnerability of this region to climate change. We investigated changes in diatoms, scaled chrysophytes, and sedimentary pigments in two near-shore cores from Gall Lake to infer limnological and water-level changes over the last two millennia. The two coring locations, at modern water depths of 7.5 and 11.5 m, were selected to contrast inferences for past lake level based on distance from the modern water-depth boundary between predominantly benthic and planktonic diatom assemblages in surface sediments (6.0 m). Diatom-inferred depth inferences were more variable in the core from 7.5-m water depth, consistent with the hypothesis that the greatest changes occurred at water depths closest to the benthic:planktonic boundary. Both sites revealed a pronounced drought from ~AD 950 to 1300, synchronous with the medieval climate anomaly (MCA). This finding suggests a northeast expansion of the arid MCA into northwest Ontario, extending the known spatial extent of this megadrought. Scaled chrysophytes and sedimentary pigments suggest a recent increase in thermal stratification. Our findings indicate this region is more susceptible to changes in moisture than was previously suspected based on the instrumental record for the past century.