Distribution Patterns Of Diatoms Research Articles

Biogeographic Distribution Patterns of Diatoms in Lancang River and Their Key Drivers

As an important primary producer, diatoms play a key role in aquatic ecosystems. However, little is known about the geographical distribution characteristics and driving factors of diatoms in large rivers. In this study, based on a high-throughput sequencing dataset of microeukaryotes, we analyzed the diversities and community compositions of planktonic and sedimentary diatoms in the 1200 km mainstream of Lancang River, a typical large river in southwestern China. The results showed that the diversities of planktonic and sedimentary diatoms in Lancang River were higher in the upstream natural section, and the community compositions of both groups were significantly different among different river sections. Dam construction had a significant effect on the dominant genera. Variance partitioning analysis showed that dispersal limitation was a major driving factor for the distribution pattern of planktonic and sedimentary diatoms, with explanation proportions of 16.7% and 29.8%. Co-occurrence network analyses showed that the interspecific competition relationship and network connectivity of the planktonic diatom network were stronger than the sedimentary ones. The network connectivity of planktonic and sedimentary diatoms in the cascade reservoir section was higher than that of the upstream natural section. This study will help to better understand the biogeographical distribution of diatoms in large rivers and provide useful information for ecological responses of diatoms to dam construction in rivers.

Diatom Taxonomic Composition as a Biological Indicator of the Ecological Health and Status of a River Basin under Agricultural Influence

The Lalin River Basin (LLRB) is a major drainage basin in northeastern China, that has been significantly influenced by agricultural activities. This study focused on exploring diatom taxonomic composition linked to environmental factors at the taxonomic levels of genus and species during ice-covered periods. Nine sampling stations were divided into three groups based on trophic state index (TSI). hierarchical cluster analysis (HCA) and redundancy analysis (RDA) were performed to identify diatom distribution patterns and their relationships to environmental factors. Diatom richness, composition and distribution were analyzed at the levels of genus and species. Our results showed the epipelagic diatom Melosira varians was very abundant at most stations. Benthic diatoms Achnanthidium minutissimum, Encyonema minutum and Gomphonema parvulum were dominant in group-3, which had the highest trophic states. HCA showed the similarity of diatom taxonomic composition spatial distribution patterns between genus and species levels. RDA revealed that the key factors related to genus level distributions are COD, TP and EC, while TP was the key factor in structuring diatom taxonomic composition at the level of species. These results suggest identification of diatoms at genus level can be used as a potential indicator to assess ecological health status of agricultural-influenced rivers during ice cover periods. Further research is necessary to explore the utility of genus level diatom composition as a biological indicator in rivers.

Environmental assessment of Lake Manzala inferred from diatom analysis of the lake surface sediments

The distribution patterns of diatoms in the surface sediments of Manzala Lake have been studied in detail. A total of 110 diatom species belonging to 48 genera have been identified from samples consist mainly of greenish gray mud with relatively some muddy sand sediments. The distribution pattern of the recorded diatom taxa displayed changes in the floristic composition due to changes in the ecological conditions such as salinity, pH, and some chemical elements, which are related to sea-level fluctuations, freshwater influences, waste discharge from the drains and climatic changes. Multivariate statistical techniques including hierarchical ascending clustering and Canonical correspondence analyses were used to identify ecological groups of diatoms and to estimate which environmental variables were important in diatom distribution. Five diatom ecological groups were identified and classified the lake into five distinct ecological zones.

The distribution of phytoplankton in the Baltic Sea assessed by a prokaryotic 16S rRNA gene primer system

Due to the evolutionary relationship between cyanobacteria and chloroplasts of all oxygenic eukaryotic photoautotrophs, both can be amplified by prokaryotic 16S rRNA gene primers. In marine water samples, chloroplast sequences often make up as much as 50% of a 16S rRNA gene library, yet because of the comparatively low phylogenetic resolution within limited chloroplast databases, they are usually removed from further analyses. However, chloroplast 16S rRNA gene databases are constantly improving and our aim was to test if the combined 16S rRNA gene sequences of phototrophic prokaryotes and eukaryotes generated by a prokaryotic primer set could be used to characterize their distribution. Using the phytoREF database, in situ 16S rRNA gene distribution characterization was performed for samples throughout the Baltic Sea (cells >0.2 μm) and compared to microscopic (cells >20 μm) as well as flow-cytometric (cells <7 μm) data. Generally, microscopic and molecular methods revealed similar distribution patterns of diatoms, chlorophytes and filamentous cyanobacteria. Although not directly comparable, flow cytometry delivered semi-quantitative patterns, on a very broad classification level, to the molecular approach. In conclusion, the combination of molecular and non-molecular analyses provided an improved overview of the phototrophic community, demonstrating its usefulness as a tool in monitoring strategies.

Fossil diatom assemblages as paleoecological indicators of paleo-water environmental change in the Ulleung Basin, East Sea, Republic of Korea

The fossil diatom assemblage record from two sediment cores obtained from the Ulleung Basin, East Sea, Republic of Korea, revealed changes in the diatom assemblage zones in PG1 and PD3 core samples. The two sediment cores were δC14 dated and approximately represented the late Pleistocene–Holocene. The analysis of age zones in the PG1 core and PD3 core was assessed based on the frequency of variations, and occurrences of biostratigraphical fossil diatom species. During the Last Glacial Maximum (LGM), the sea level was lower than that at present and the Ulleung Basin became isolated from the Pacific Ocean. As a result, there would have been a limited Tsushima Warm Current (TWC) influence, and salinity would have decreased resulting in increased freshwater and coastal diatoms. The distribution pattern of diatoms presented in the cores was associated with changes in water temperature and salinity and the adding of terrigenous material brought about by the input of freshwater. Changes in the abundance of a tychopelagic diatom, Paralia sulcata, reflected the effect of the water currents. Diatom temperature (Td) values and the ratio of centric/pennate diatoms provided evidence of limited influences of the TWC and freshwater inflow. It is thought that all assemblage zones were influenced by the TWC, which had an important effect on the distribution and composition of fossil diatoms.

Climate is an important driver for stream diatom distributions

AbstractAimThe species distributions of macroorganisms are widely studied, yet microbial distributions at species level remain poorly resolved. We explored the relative contributions of climatic and local environmental factors in explaining the distribution patterns of unicellular diatoms.LocationA geographical gradient of c. 1200 km in Finland (60°–70° N).MethodsWe modelled the distributions of 157 diatom taxa sampled in 227 stream sites using climatic and local environmental predictors with four different modelling techniques: generalized linear models, generalized additive models, boosted regression trees and Random Forest. We used models with three separate sets of predictors: environment only, climate only and the full set of predictors. The model performances were evaluated using the area under the curve of a receiver operating characteristic plot and the true skill statistic values by a four‐fold cross‐validation approach.ResultsWe found that the predictive performance of the full models was highest, indicating the importance of both the local environment and large‐scale climatic factors in diatom distribution patterns. However, climate‐only models outcompeted the environment‐only models in predicting diatom distributions. The explanatory variables had varying importance across species and growing degree days and precipitation had the highest relative importance in the full models. We also found that the predictability of the distributions varied greatly among species, but the differences among families were typically small.Main conclusionsOur results suggest that at a broad geographical scale climate‐related factors are important determinants of diatom distributions and may be stronger drivers than local variables. The inclusion of both climatic and local environmental factors in species distribution modelling facilitates the understanding of the joint effects of these drivers on microorganisms in future conditions. From an applied perspective, our study demonstrated that species distribution models serve as an important tool in explaining and predicting microbial distributions.

The Influx of the Tsushima Current into the Central Ulleung Basin of East Sea (Sea of Japan), Korea

A total of 85 samples, collected from the UBGH1–9 core taken from the Ulleung Basin, East Sea, Korea, were analyzed using diatom assemblages. 111 diatom species belonging to 46 genera were identified, and three diatom assemblage zones were established on the basis of occurrence and distribution pattern of diatoms. Diatom assemblage zone I (134.10–174 m) is characterized by a relatively high abundance of marine species, while the increased number of the brackish species is recorded in diatom assemblage zone II (75–125 m). The assemblage zones IIIa became drastic drop of valve abundances and brackish planktons, whereas it became increase during the IIIb. High Td values which indicate an influence of warm current are recorded both in diatom assemblage zone I and III, and low Td values in diatom assemblage zone II. Analysis of diatom assemblages indicating that the depositional condition moved from oceanic to littoral-neritic environments and that paleotemperature underwent a shift from warm to cold condition at the middle interval, and from cold to warm condition in the upper interval of the UBGH1–9 core. This suggests that the lower (130–162 m) and upper intervals (0–20 m) of the UBGH1–9 core were deposited in the warm current condition (Tsushima Warm Current).

Contribution of microspatial factors to benthic diatom communities

Spatial factors can play important and potentially confounding roles governing the presence, the absence, and the abundance of the diatom species and consequently influence the diatom-based bioassessment. To examine how spatial factors affect diatom community and biomonitoring, diatom community was sampled in three catchments with similar river length and watershed area across a large spatial scale along the Yangtze River, China. Redundancy analysis (RDA) showed that patterns of diatom distribution could be attributed not only to environmental variables (e.g., NO3–N, TN, WT, and COD), but also to spatial factors (e.g., elevation, latitude, and longitude). Partial RDAs partitioned the total variance of diatom data explained (64.7%) into three parts: pure spatial variables contributed the highest proportion (19.8%), followed by pure environmental variables (5.4%), and the shared effects by environmental and spatial sets explained a large proportion of diatom variance (39.5%). Recognizing the potential influences of spatial factors, further accumulations of diatom data within different regions would allow for the development of better diatom-based monitoring methods for watershed diagnosis and management.

New and rare diatom (Bacillariophyta) species from a mountain lake in Eastern Siberia

The examination of a moss sample collected from a lake in a remote mountainous region of Eastern Siberia revealed a diverse diatom flora that included four new species: Eunotia frigida, Brachysira subtile, Encyonopsis vasilievae, and Neidium rugosum. N. rugosum has been previously illustrated from several locations in the arctic and subarctic and is widely distributed across the circumpolar Arctic. The distribution of three other species is likely more limited. Besides these species, a number of rare diatoms were also found in the moss community. These include Neidium boyeri and Stauroneis crassula previously known only from North America; Encyonema sibericum so far only reported from Western Siberia, Encyonema lunatum var. borealis earlier found in Finland, and Eunotia ferefalcata, Pinnularia angustarea, and Naviculadicta mongolica so far reported only from Mongolia. This study contributes to the understanding of biogeographic patterns of diatom distribution.

Distribution of Diatoms Along Environmental Gradients in the Charlotte Harbor, Florida (USA), Estuary and Its Watershed: Implications for Bioassessment of Salinity and Nutrient Concentrations

The relative abundance of diatom species in different habitats can be used as a tool to infer prior environmental conditions and evaluate management decisions that influence habitat quality. Diatom distribution patterns were examined to characterize relationships between assemblage composition and environmental gradients in a subtropical estuarine watershed. We identified environmental correlates of diatom distribution patterns across the Charlotte Harbor, Florida, watershed; evaluated differences among three major river drainages; and determined how accurately local environmental conditions can be predicted using inference models based on diatom assemblages. Sampling locations ranged from freshwater to marine (0.1–37.2 ppt salinity) and spanned broad nutrient concentration gradients. Salinity was the predominant driver of difference among diatom assemblages across the watershed, but other environmental variables had stronger correlations with assemblages within the subregions of the three rivers and harbor. Eighteen indicator taxa were significantly affiliated with subregions. Relationships between diatom taxon distributions and salinity, distance from the harbor, total phosphorus (TP), and total nitrogen (TN) were evaluated to determine the utility of diatom assemblages to predict environmental values using a weighted averaging-regression approach. Diatom-based inferences of these variables were strong (salinity R2 = 0.96; distance R2 = 0.93; TN R2 = 0.83; TP R2 = 0.83). Diatom assemblages provide reliable estimates of environmental parameters on different spatial scales across the watershed. Because many coastal diatom taxa are ubiquitous, the diatom training sets provided here should enable diatom-based environmental reconstructions in subtropical estuaries that are being rapidly altered by land and water use changes and sea level rise.

Epibryic diatoms from ombrotrophic mires: diversity, gradients and indicating options

The present study focuses on comparison of recent diatom diversity within the unique Central European ombrotrophic wetlands in two regions with contrasting levels of past atmospheric depositions (Jizerske hory Mts. and Jeseniky Mts.). Further, two methods of mire diatom sampling (squeezing and mineralization of the entire moss sample) are compared at the same localities to determine whether bryophyte herbarium specimens could be used for studies on the historical development of peat bog areas. There were a total of 47 diatom species at 59 sites within 13 ombrotrophic mires along the CzechPolish-German border areas. The regions did not differ in diatom species richness. The two-group pattern along the Axis 1 in NMDS ordination analysis did not correspond with the regional distribution. The inter-site variation in diatom assemblages was lower than the intra-site variation. The combination of three environmental variables (conductivity, pH, and height above water table – HWT) appears to be able to explain the diatom distribution patterns. No significant differences were found between the composition of diatom species of moss and squeezed samples. However, moss samples can be characterized by significantly greater species richness, most likely because the mineralization of the whole bryophyte minimizes the losses of diatom frustules caused by the squeezing method. These results allow us to reccommend herbarized bryophytes as a source of missing information on the sub-recent diatom diversity of mires.

Growth form analysis of epiphytic diatom communities of Terra Nova Bay (Ross Sea, Antarctica)

Diatoms have been long collected from the Southern Ocean but almost no data exist for epiphytic communities, despite their high ecological significance as an important food source in Antarctic coastal food chains. Here, we present a first growth form analysis of diatoms associated with rhodophyte hosts from Terra Nova Bay, Ross Sea, Antarctica. We performed this study to gather baseline information on the species composition of epiphytic diatom communities, determine the influence of some environmental variables on the diatom distribution patterns, and assess the caveats that must be taken into account in terms of sampling design. Macroalgal material was collected during the Italian Antarctic expeditions between 1990 and 2004. Epiphytic diatoms were studied by means of scanning electron microscopy. In terms of growth forms, there were no significant differences between the diatom communities on the different macroalgal host species. Motile (mainly small-celled Navicula perminuta and other Navicula spp.) and adnate (Cocconeis spp.) diatoms dominated the community throughout the study period. Many of the macroalgal blades examined were also covered by epiphytic animals (calcareous bryozoans, hydroids) over most of their surface, with a significant effect on the associated diatom community structure. Our findings suggest that the bio-physicochemical characteristics of each sampling site affected the epiphytic diatom communities more than the substrate type provided by the macroalgal host or the sampling depth.

The use of the South African Diatom Index as a concervation management tool for Hewitt’s Ghost frog

Distribution patterns of diatoms were investigated to determine the impact ofshading on the distribution of diatomes, which form the main diet of the endangered frogspecies, Heleophryne hewitti.

Distribution of benthic diatoms in Korean rivers and streams in relation to environmental variables

The diatoms are an ecologically important group of algae that have been extensively studied by ecologists and taxonomists. However, the large-scale patterns of diatom distribution and the factors under- lying this distribution are largely unknown. The aims of this study were to identify the large-scale spatial patterns of benthic diatom assemblages in Korean streams and rivers, and to assess the importance of numer- ous environmental factors on diatom distribution. We classified 720 study sites based on diatom flora. Benthic diatoms, water chemistry, altitude, and riparian land cover and use were characterized by multivariate analyses, Monte Carlo permutation tests, and indicator species analysis. In total, we identified 531 diatom taxa. Diatom assemblages were mostly dominated by species of the genera Achnanthes, Navicula, Nitzschia, Cocconeis, Fragilaria (Synedra included), Cymbella, Gomphonema, and Melosira. Cluster analysis partitioned all 720 sites into eight groups based on diatom species composition. Canonical correspondence analysis in- dicated that altitude, land cover and use, current velocity, electrical conductivity, and nutrient levels explained a significant amount of the variation in the composition of assemblages of benthic diatoms. At the national scale, a downstream ecological gradient was apparent, from fast-flowing, mostly oligotrophic highland streams to slow-flowing, mostly eutrophic lowland rivers. Our data suggest that spatial factors explain some of the variation in diatom distribution. The present investigation of the spatial patterns of benthic diatoms, the ecological determinants of diatom occurrence, and the identification of diatom indicator species contributes to development of a program for assessing the biological integrity of lotic ecosystems in Korea.

Distribution of epipelic diatoms in artificial fishponds along environmental and spatial gradients

Although epipelic diatoms play a key role in primary production of many ecosystems, many aspects of their biodiversity, ecology and geographical distribution are poorly understood. The present study is based on sampling of 45 man-made fishponds in the Czech Republic covering an environmental gradient from oligo/dystrophic highland ponds within protected areas to the eutrophic/hypertrophic lowland ponds used for intensive fish production. Diatom distribution patterns assessed using biomass and species composition variables were assessed along environmental and geographical gradients. In total, 185 epipelic diatom taxa were found in the investigated samples. The differences in species composition between sites were correlated with environmental parameters, but not with the geographic distance of the localities. This pattern might suggest that niche-based control, rather than the effect of dispersal limitation, is the main driving force in the species composition of epipelic diatoms in fishponds. The alpha-diversity of sites correlated with altitude, nitrogen and chlorophyll a concentrations but did not correlate with the area of the ponds. The significant relationships between local abundance of species and their regional occupancy were very similar to previous studies of diatoms in boreal streams. In addition, these data concur with patterns known for multicellular organisms suggesting that in this respect diatoms may not differ from groups of organisms with larger body sizes.

Diatom thanatocoenosis in a middle Galician Ría: Spatial patterns and their relationship to the seasonal diatom cycle in the water column and hydrographic conditions

Modern diatom distribution patterns in the surface sediment of the Ferrol Ría and their relationship to the hydrography and diatom patterns in the water column were studied to determine the hydrographic influence on the record of these biogenic components. Diatom abundance in the water column was assessed for different oceanographic periods and compared with the biosiliceous sedimentary record. Very low abundances were found in the water column during the winter, whereas in spring and summer, diatoms proliferated. Chaetoceros spp. formed the bulk of the water column community during spring and summer, followed by Thalassionema nitzschioides and Rhizosolenia spp. Nitzschia longissima represented a significant portion of the winter assemblage, together with Paralia sulcata and benthic taxa . Leptocylindrus danicus, N. longissima and Skeletonema costatum characterized the autumn campaign, when stratification of the waters occurs, with L. danicus being especially abundant in the outer ría. Seasonal hydrographic and associated productivity patterns govern the abundance and assemblage of the diatoms preserved in surface sediments. Samples located in the inner ría area and its margins exhibited the highest abundances of diatoms, and were primarily dominated by benthic species. The freshwater group, crysophycean cysts and phytoliths were present in the landward stations influenced by river runoff. The middle ría was characterized by P. sulcata and Thalassiosira spp., with minor occurrences of the benthic and freshwater group. Chaetoceros R.S., L. danicus R.S. and T. nitzschioides typified the outer ría, an assemblage that corresponds to nutrient-rich coastal areas of high productivity influenced by oceanic waters, demonstrating the impact of oceanic waters flowing into the embayment due to enhanced tidal mixing through the narrow channel. Therefore, sediment diatom assemblages reflect diatom production patterns in the water column of the Ría. However, we must proceed with caution when interpreting the paleorecord in the inner area due to the high contribution of allochthonous taxa, which is indicative of low water depths. This paper contributes to a better understanding of diatom thanatocoenosis in the Galician Rías, where very few studies of this kind have been done to date.

Distribution of moss-inhabiting diatoms along an altitudinal gradient at sub-Antarctic Îles Kerguelen

Altitudinal gradients provide excellent opportunities to study relationships between species distribution and climatic variables. We studied the species composition of 39 samples of moss-inhabiting diatoms, collected at 50 m intervals from 100–650 m above sea level. The samples contained a total of 130 diatom species, of which 51 occurred in 10 or more samples. Altitude appeared to be the most important variable explaining variation in species composition. Of the 51 common species, 33 showed a significant relationship with altitude. Although the majority of the latter declined with increasing altitude, for nine species the probability of occurrence first increased with increasing elevation, but decreased again at higher altitudes, and four species increased systematically with elevation. As a result, expected species richness per sample decreased from an estimated 43 at 100 m to 25 species per sample at 650 m. Diatom distribution patterns proved to be suitable predictors of the altitudinal position of sample sites. Cross-validation yielded a strong relationship between predicted and observed altitudes.

Application of a self-organizing map to select representative species in multivariate analysis: A case study determining diatom distribution patterns across France

Ecological communities consist of a large number of species. Most species are rare or have low abundance, and only a few are abundant and/or frequent. In quantitative community analysis, abundant species are commonly used to interpret patterns of habitat disturbance or ecosystem degradation. Rare species cause many difficulties in quantitative analysis by introducing noises and bulking datasets, which is worsened by the fact that large datasets suffer from difficulties of data handling. In this study we propose a method to reduce the size of large datasets by selecting the most ecologically representative species using a self organizing map (SOM) and structuring index (SI). As an example, we used diatom community data sampled at 836 sites with 941 species throughout the French hydrosystem. Out of the 941 species, 353 were selected. The selected dataset was effectively classified according to the similarities of community assemblages in the SOM map. Compared to the SOM map generated with the original dataset, the community pattern gave a very similar representation of ecological conditions of the sampling sites, displaying clear gradients of environmental factors between different clusters. Our results showed that this computational technique can be applied to preprocessing data in multivariate analysis. It could be useful for ecosystem assessment and management, helping to reduce both the list of species for identification and the size of datasets to be processed for diagnosing the ecological status of water courses.

Relationships between local population persistence, local abundance and regional occupancy of species: distribution patterns of diatoms in boreal streams

AbstractAims We have two aims: (1) to examine the relationship between local population persistence, local abundance and regional occupancy of stream diatoms and (2) to characterize the form of the species–occupancy frequency distribution of stream diatoms.Location Boreal streams in Finland. There were three spatial extents: (1) across ecoregions in Finland, (2) within ecoregions in Finland, and (3) within a single drainage system in southern Finland.Methods Diatoms were sampled from stones (epilithon), sediment (epipelon) and aquatic plants (epiphyton) in streams using standardized sampling methods. To assess population persistence, diatom sampling was conducted monthly at four stream sites from June to October. The relationships between local population persistence, local abundance and regional occupancy were examined using correlation analyses.Results There was a significant positive relationship between local persistence and abundance of diatoms in epilithon, epipelon and epiphyton. Furthermore, local abundance and regional occupancy showed a significant positive relationship at multiple spatial extents; that is, across ecoregions, within ecoregions and within a drainage system. The relationships between occupancy and abundance did not differ appreciably among impacted and near pristine‐reference sites. The occupancy–frequency distribution was characterized by a large number of satellite species which occurred at only a few sites, whereas core species that occurred at most sites were virtually absent.Main conclusions The positive relationship between local population persistence and abundance suggested that a high local abundance may prevent local extinction or that high persistence is facilitated by a high local cell density. High local persistence and local abundance may also positively affect the degree of regional occupancy in stream diatoms. The results further showed that anthropogenic effects were probably too weak to bias the relationship between occupancy and abundance, or that the effects have already modified the distribution patterns of stream diatoms. The small number of core species in the species–occupancy frequency distribution suggested that the regional distribution patterns of stream diatoms, or perhaps unicellular microbial organisms in general, may not be fundamentally different from those described previously for multicellular organisms, mainly in terrestrial environments, although average global range sizes may differ sharply between these two broad groups of organisms.

Late Pleistocene–Holocene paleoenvironmental changes inferred from the diatom record of the Ulleung Basin, East Sea (Sea of Japan)

The diatom floral record from two piston cores (00GHP-01 and 00GHP-07) taken from the southwestern margin of the Ulleung Basin, East Sea (Sea of Japan) reveals a series of well-defined changes in glacio-eustatic sea level and paleoceanographic conditions during the late Pleistocene–Holocene. Six assemblage zones and two barren zones in 00GHP-01 and four assemblage zones in 00GHP-07 are erected respectively on the basis of frequency of variations in, and occurrences of, biostratigraphically significant diatom species. All assemblage zones have been strongly influenced by the Tsushima Warm Current (TWC). The TWC has an important affect on controlling the distribution and composition of diatom flora, which in turn, reveal the history of the TWC in this area. The distribution pattern of diatoms in the diatom assemblages reveals sea surface temperature and salinity affected by the sediments type and terrigenous material input. Diatom temperature (Td) values show that the site of 00GHP-01 in the Ulleung Basin has been influenced by a relatively enhanced TWC twice during the deposition of lowermost and uppermost intervals. A relatively weak TWC was recorded in the middle interval since the latest Pleistocene. The diatom assemblages of 00GHP-07 represent an apparently continuous Late Quaternary record, spanning the Last Glacial Maximum, Bølling-Allerød, Younger Dyras and Holocene. High abundance of a low-salinity, coastal water diatom Paralia sulcata may reflect the influx of the East China Sea water through the Korean Strait to the East Sea (Sea of Japan). During the Last Glacial Maximum, the sea level was low enough that selected basin with shallow sills along the margin of the East Sea (Sea of Japan) became isolated from the Pacific Ocean. Salinity also decreased due to increased freshwater input from rivers draining the surrounding lands. The density-stratified water column may have prevented vertical mixing and resulted in anoxic bottom-water conditions.