Diatom surface sediment assemblages around Iceland and their relationships to oceanic environmental variables

Canonical correspondence analysis of diatoms from surface sediment samples and oceanographic environmental variables shows that summer and winter sea-surface temperatures, water depth and winter sea-surface salinity are the main environmental factors affecting diatom distribution around Iceland. Of these, summer sea-surface temperature is the most important. Five diatom assemblages are distinguished and the distribution of these assemblages is clearly correlated with oceanic current patterns in the region. The sea-ice diatom assemblage is limited to the area where the East Greenland Current (Polar Water) has its strongest influence, and the cold diatom assemblage is basically controlled by the less cold East Icelandic Current (Modified Polar Water). The mixing diatom assemblage results from the interaction between the cold East Greenland and East Icelandic Currents and the warm Irminger Current. The warm diatom assemblage is located in the area dominated by the Irminger Current and may be used as an indicator of warm-water masses (Atlantic Water). The coastal diatom assemblage is the only one strongly influenced by both water depth and summer water temperatures.     

Associations between wintering avian predators and schooling fish in the Skagerrak-Kattegat suggest reliance on predictable aggregations of herring Clupeaharengus

We investigated the distributional dynamics of piscivorous seabirds in relation to the distribution of young fish outside the breeding season in the Skagerrak and Kattegat, using synoptic bottom trawl and seabird surveys. Kittiwake Rissa tridactyla , Common Guillemot Uria aalge and Razorbill Alca torda were significantly correlated with herring Clupea harengus abundance at coarse spatial scales (>20 km). The association between the three seabird species and other abundant fish species like sprat Sprattus sprattus , whiting Merlangius merlangus and Norway pout Trisopterus esmarki was considerably weaker. In the Skagerrak and Kattegat, the location of aggregations of immature herring during winter is relatively stable and predictable. The correlation with herring and the lack of correlation with the abundant, but less stable occurrence of sprat of similar caloric value suggest that the tendency among piscivorous seabirds to depend on predictable concentrations of prey during the breeding season is a common feature during the non-breeding season as well. The results are discussed in relation to ecological and winter habitat characteristics of wintering piscivorous seabirds in the eastern North Atlantic, and in relation to the management and conservation of nursering herring in the Skagerrak and Kattegat.     

DISTRIBUTION OF DIATOMS IN THE PLANKTON OF YAQUINA ESTUARY,OREGON1

The distributional patterns of diatoms in the plankton of the Yaqnina Estuary, Oregon, were, investigated and related to selected climatic and hydrographic factors. Distribution was strongly influenced by seasonal patterns of rainfall resulting in the introduction of a large volume of fresh water into the estuary during fall and winter. Plankton assemblages in spring, summer and fall had fewer diatom species and exhibited a more rapid rate of change in species composition than in winter. Winter assemblages were further characterized by many pennate diatoms, apparently dislodged from the benthos during periods of high freshwater discharge and silt loads. A statistical measure of community difference indicated an increase in taxonomic homogeneity among assemblages throughout the estuary with the onset of the rainy season in late fall and a gradual transition to a more heterogeneous system again during late spring. Canonical correlation ordered 20 prominent diatom taxa along the salinity gradient and identified possible relationships among certain taxa and selected environmental variables, namely visible light energy and temperature. Redundancy in the species data given the environmental data was only 40%, emphasizing the difficulty in demonstrating a quantitive relationship between plankton dynamics in the field and concurrent measurements of chemical and physical variables.     

Comparison of contemporary and fossil diatom assemblages from the western Antarctic Peninsula shelf

During the past ten years, the Antarctic Peninsula has been identified as the most rapidly warming region of the Southern Hemisphere and it is important to place this warming in the context of the natural climate and oceanographic variability of the recent geological past. Many biological proxies, such as marine diatom assemblages, have been used to determine Southern Ocean palaeoceanographic conditions during the Late Quaternary, however, few investigations have attempted to link observations of modern floras with the fossil record. In this study we examine a suite of modern austral spring (December 2003) and summer (February 2002) surface water samples from along the western Antarctic Peninsula (WAP) continental shelf and compare these to core-top, surface sediment samples. Using detrended correspondence analysis (DCA) and principal component analysis (PCA) of diatom abundance data we investigate the relationship of contemporary diatom floras with the fossil record. This multivariate analysis reveals that our modern assemblages can be divided into three groups: summer southern WAP sites, summer northern WAP sites, and spring WAP sites. Sea surface temperature (SST) is an important environmental variable for explaining seasonal differences in diatom assemblages between spring and summer, but sea surface salinity (SSS) is more important for understanding temporally-equivalent regional variations in assemblage. Our summer diatom samples are more reminiscent of early season assemblages, reflecting the unusually late sea ice retreat from the region that year. When the modern assemblages are compared to the fossil record, it is clear that most of the important diatoms from the summer assemblage are not preserved into the sediments, and that the fossil record more closely reflects spring assemblages. This observation is important for any future attempts to quantitatively reconstruct palaeoceanographic conditions along the WAP during the Late Quaternary and highlights the need for many more such studies in order to address longer timescales, such as interannual variability, in the context of the fossil record.     

Development of diatom-based salinity models for paleoclimatic research from lakes in British Columbia (Canada)

Diatoms were identified and enumerated from the surface sediments of 65 lakes located on the Cariboo and Chilcotin Plateaux (British Columbia, Canada). These lakes span a large gradient in lakewater ionic concentration (fresh through hypersaline) and composition, as well as other physical/chemical variables. Almost all of the study lakes had higher salinities in the late-summer than in the spring. The lakes with spring salinities >8 g l^–1 showed the largest seasonal increases in salinity. Ionic composition was similar in the spring and late-summer for most lakes. Both ionic concentration (i.e. salinity) and composition were important environmental variables that could account for the different diatom floras in the lakes. Diatom assemblages characteristic of carbonate-dominated and sulfate-dominated waters were identified. Other variables such as water depth and phosphorus concentration were also important.The majority (87%) of diatom taxa had estimated salinity optima < 3 g l^–1 Halophilic diatom taxa had broader tolerances to salinity when compared to the fresh water taxa, however taxa with narrow and broad tolerances could be identified across the salinity gradient. Species diversity was weakly but significantly correlated to lakewater salinity (r ² = 0.18 to 0.3, P < 0.05).Salinity inference models were developed based on the relationship between the diatom assemblages and the spring, late-summer and average salinity. The correlations between the measured and diatominferred salinity, based on the spring (r = 0.95), late-summer (r = 0.94) and average (r = 0.95) salinity data, are high because there was an extremely strong correlation (r = 0.98) between the log transformed spring and late-summer measured salinities. These salinity reconstruction models provide a tool that can be used to infer past climatic changes as part of paleolimnological studies from appropriate closed-basin lakes in British Columbia.     

Local environment and connectivity are the main drivers of diatom species composition and trait variation in a set of tropical reservoirs

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Comparison study of the modern ostracod associations in the Kara and Laptev seas: Ecological aspects

Recent ostracod assemblages were investigated from coretop sediment samples collected in the eastern Kara Sea from water depths down to 300 m. A total of 45 species were identified, 27 of them were reported for the Kara Sea for the first time. The Kara Sea data were compared with our results on the distribution of ostracods in the eastern Laptev Sea. The spatial distribution of recent taxa and the ecological groupings demonstrate a clear relation to dominant environmental factors which range from estuarine to full-marine conditions. Four assemblages related to average summer bottom water salinities were established: (1) a freshwater assemblage from the inner estuaries of the Ob' and Yenisei rivers with salinities less than 2 and from thermokarst lagoons of the southern Laptev Sea coast with strong salinization in winter; (2) a brackishwater assemblage of the outer estuaries of the Ob' and Yenisei rivers with salinities up to 26; (3) a mixed euryhaline–marine assemblage dominated by euryhaline species Paracyprideis pseudopunctillata and Heterocyprideis sorbyana from the inner shelf river-affected zone of the Kara and Laptev seas, where salinities range between 26 and 32; (4) a taxonomically diverse marine assemblage dominated by shallow-water marine taxa from the northern parts of the Kara and Laptev shelves and upper continental slope with stable bottom environments and a salinity higher than 32. Abundant euryhaline species found at greater water depths are identified as part of an ice-rafted assemblage. They are possibly entrained into the newly formed fast ice during autumn storms and freeze-up period and then transported to the distal open-sea areas during summer.     

Patterns of macroalgal diversity, community composition and long-term changes along the Swedish west coast

This study analyses the complicated patterns of vertical distribution of the macroalgal vegetation in an area where brackish and marine waters meet and mix. Variables used to record vegetation characteristics are algal cover, species composition and diversity. The data set includes 64 diving profiles, all from sites exposed to wave action, along a ca. 260 km long coastline. The profiles belong to four categories: coastal sites in the Skagerrak (more marine), coastal sites in the Kattegat (more brackish), coastal sites in the Kattegat after a toxic phytoplankton bloom, and submerged offshore stone reefs in the Kattegat. The highest species diversity was found at the reefs, which are not affected directly by land runoff. At the reefs the 18 most common perennial species penetrate 2–11 m (on average 5.5 m) deeper than at the coastal sites. The virtual absence of sedimentation, and thus the availability of substratum, at the reefs may explain the differences so that the lower limit for the algae is determined by light penetration or by recruitment problems caused by strong currents at the reefs, whereas sedimentation limits the settlement of algae in coastal sites. Ordination analysis based on species composition reveals that the major environmental gradients structuring the algal vegetation in the Kattegat and the Skagerrak are salinity and water depth. The large data set of this study made it possible to quantify the downward dislocation of Atlantic intertidal species to the sublittoral along the Swedish west coast. For example, the mean upper limit of Corallina officinalis is 2 m in the Skagerrak but 12.5 m in the Kattegat and the mean occurrence interval of Fucus serratus is 0.9–2.7 m in the Skagerrak, but 1.1–6.3 m in the Kattegat. This downward dislocation is suggested to be the result of decreased competition when species successively disappear with lower salinity. Comparisons of the present study's results with those of previous investigations show that eight common red algal species have moved upwards compared to the situation before the large-scale eutrophication started in the 1960s, e.g. Cystoclonium purpureum and Polysiphonia elongata by ca. 8 m, Phycodrys rubens and Delesseria sanguinea by ca. 5 m. A toxic phytoplankton bloom affected macroalgal community composition on the whole only slightly, but it had a negative effect on algal cover and species richness below a water depth of ca. 5 m, the algae were visibly damaged and the lower vegetation limit temporarily moved upwards.     

On the influence of water conductivity,pH and climate on bryophyte assemblages in Catalan semi-natural springs

Bryophytes are some of the most sensitive biological indicators of environmental change. Springs have a significant presence of bryophytes and so are ideal habitats for studying their relationship with the environment. We tested whether bryophyte assemblages can be explained with macro-, meso- and micro-ecological variables (i.e. seasonal climate, altitude, water pH and conductivity) sampling bryophytes from 198 semi-natural springs distributed along montane regions in the north-eastern Iberian Peninsula. We tested the influence of environmental variables on bryophyte assemblages in springs using sparse Partial Least Squares. Our results show that variability in bryophyte assemblages is explained by seasonal climate (temperature and precipitation from winter, spring, summer and autumn and temperature and precipitation seasonality), altitude and water conductivity. The results obtained by the present study will be useful for predicting bryophyte diversity in springs using simple and easy to obtain variables such as climate, water pH and conductivity.     

Regional distribution of diatom assemblages in the headwater streams of Luxembourg

The aim of this study was to determine the influence of environmental variables on the structure of benthic diatom assemblages, and to propose type assemblages of diatoms characterizing unpolluted headwater streams of Luxembourg. A total of 289 diatom samples were collected in the headwater streams of Luxembourg. At each sampling site, physical and chemical variables were also measured. The relationships between environmental variables and the distribution of the taxa were estimated using univariate and multivariate statistical analyses. On the basis of geology and stream water chemistry, Luxembourg could be separated in two regions. The streams in the schistose northern part of Luxembourg presented a low carbonate hardness (median 3.5° F); the streams in the southern part of Luxembourg presented a high carbonate hardness (median 21.8° F) notably connected to the presence of sandstone and limestone substrata. A Twinspan classification carried out on diatom assemblages defined two groups of samples corresponding to these two regions. The carbonate hardness that is related to the nature of the geological substratum appeared to be the major structuring variable for the assemblage composition. Anthropogenic pollution was a secondary structuring variable for diatom assemblages since each group could be subdivided in subgroups presenting statistically different nutrient and organic matter concentrations. The diatom assemblages of these subgroups were characterised by differences of saprobic and trophic preferences. On the basis of these results, two type assemblages of diatoms are proposed for the unpolluted headwater streams in the two regions of Luxembourg. The presence of these different type assemblages in Luxembourg shows the necessity to adapt diatom bioindication to the different regions of the country.     

Ecology of the Major Periphytic Diatom Communities from the Mesta River,Bulgaria

Epilithon, epiphyton, epipelon, epipsammon and plocon diatom samples and water chemistry samples were collected bimonthly from 11 stations along the Mesta River, Bulgaria between December 1989 and April 1991. Principal component analysis (PCA), correlation, and dominance analysis were employed for describing the seasonal dynamics of diatom assemblages and estimating the correlation between diatom distribution and the physico-chemical parameters. All periphytic communities were compared by PCA, MANOVA and Fisher's LSD multiple comparison test. Ionic strength and eutrophication were the major abiotic factors affecting diatom distribution in the epilithon, epiphyton, epipelon and epipsammon. Current velocity had an important influence on the diatom assemblages from the epilithon and epiphyton. Epilithon showed the lowest correlation with most of the environmental factors and had significantly lower species diversity than epiphyton, epipelon and epipsammon. Multivariate analysis of the diatom data suggested that in all benthic habitats of the Mesta River seasonal replacement of diatom assemblages was controlled by the water chemistry rather than substratum, current velocity or discharge. The oligotrophic, mesotrophic and eutrophic diatom communities in the Mesta River comprised early-, mid- and late-successional colonizers respectively. Multiple comparisons of all periphytic communities showed that habitat specificity was positively correlated with current velocity.     

Effect of spatial differences in growth on distribution of seasonally co‐occurring herring Clupea harengus stocks

The mechanisms most likely to determine the distribution of the two major herring Clupea harengus stocks in their common early summer feeding ground in the eastern North Sea, Skagerrak and Kattegat were investigated through analysis of acoustic survey data from six consecutive years. No change was detected in biomass of North Sea autumn spawning C. harengus (NSAS) over time, whereas the biomass of western Baltic spring spawning C. harengus (WBSS) declined severely. Analyses of centre of abundance by stock showed no change in NSAS distribution, whereas the WBSS changed to a more western distribution over time. Contrary to previous perception of the juvenile migration, NSAS were found to leave the study area at the age between 1 and 2 years and WBSS 1 year olds were encountered in the Skagerrak. The estimated parameters of von Bertalanffy growth equations showed marked differences between areas with fish in the eastern part of the area having the lowest size at age at all ages. Further, their growth conditions appeared to deteriorate progressively over the period studied. Both NSAS and WBSS showed the highest condition in the North Sea and Skagerrak while condition was substantially lower in age Kattegat. The westward movement of spring spawners over time suggests that growth rate and possibly density of conspecifics influence the migration pattern and distribution of C. harengus in the area. In contrast, there was no evidence to suggest that distribution was constant over time within stocks or that distribution reflected size‐dependent limitations on migration distance.     

Relationship Between Surface Sediment Diatoms and Summer Water Quality in Shallow Lakes of the Middle and Lower Reaches of the Yangtze River

The relationship between surface sediment diatoms and summer water quality was investigated at 49 lakes in the middle and lower reaches of the Yangtze River. Lakes ranging from oligomesotrophic to hypereutrophic were examined, providing an obvious nutrient gradient. With the shift from mesotrophic to eutrophic levels, diatom multi-ecotypes dominated by epiphytic and facultative planktonic taxa were replaced by nutrient-tolerant planktonic taxa, such as Cyclotella meneghiniana Skvortzow, C. atomus Hustedt,Cyclostephanos Round, and Stephanodiscus Ehrenberg etc., reflecting the nutrient changes in the lake.The relationship between diatoms and summer water quality indices was explored further using numeric analysis. Canonical correspondence analysis (CCA) with forward selection and a Monte Carlo permutation test revealed that of all 25 summer water environmental variables, total phosphorus (TP), chlorophyll a (Chzl a), Secchi depth (SD), dissolved inorganic phosphorus, C1-, SO42-, Mg2 , CO32-, and water depth were significant variables (P＜0.05) in explaining diatom distributions. Of these, TP, Chl a, SD, and C1-, were the most important variables. The result of the correlation analysis also showed that a significant correlation exists among these variables, implying that these indices are either interconnected or independent in explaining the diatom data. For phosphorus-limited sites, TP was the most significant variable affecting the diatoms, also affecting changes in Chl a, SD, and iron concentrations. The independence of Chl a may be related to algal competition induced by lake eutrophication, resulting in the feedback to diatom community.In addition to TP, SD can be related to sediment disturbance by wave action and the growth of macrophytes in large shallow lakes. These relationships between diatom ecotypes and water quality provide the basis for a future quantitative reconstruction of historic lake nutrient evolution in the study area and will also provide a wealth of modern ecological knowledge that can be used to interpret fossil diatom records.     

DIATOM ASSEMBLAGES AS INDICATORS OF LAKE TROPHIC STATUS IN SOUTHEASTERN ONTARIO LAKES1

Canonical correspondence analysis (CCA) was used to explore the relationship between measured environmental variables and surficial diatom (Bacillariophyceae) assemblages in alkaline lakes from southeastern Ontario. Total nitrogen (TN), watershed area, alkalinity, and maximum depth each explain significant (P ≤ 0.05) directions of variance in the distribution of diatom taxa. TN was highly correlated to total phosphorus (TP) (r = 0.92), chlorophyll a (r = 0.86), and Secchi depth (r =0.77). When a series of CCAs were run with the first axis constrained to each of these variables in turn, the ratio of the eigenvalue of the first axis to that of the second axis (λ1/λ2) was highest for TN, indicating that TN best explained the distribution of the diatom assemblages in this set of lakes. Furthermore, results of Monte Carlo permutation tests indicated that these four variables did not act independently on the diatom assemblages. Therefore, TN was selected to represent these four closely related variables to infer lake trophic status. Weighted-averaging regression and calibration (with classical deshrinking) were used to develop transfer functions to infer TN from the relative abundances of 83 diatom taxa recovered from the surficial sediments of 51 lakes. There was a good correlation between diatom-inferred TN concentrations and measured TN concentrations (r²= 0.75, n = 51). The weighted-averaging regression and calibration model was used to infer lake trophic status (represented by TN) from diatom assemblages presented in the sediments from Little Round Lake, Ontario. These data were used in conjunction with historical land-use data in order to quantify the sequence and extent of nutrient enrichment related to human activity in the watershed area.     

Relationship Between Surface Sediment Diatoms and Summer Water Quality in Shallow Lakes of the Middle and Lower Reaches of the Yangtze River

The relationship between surface sediment diatoms and summer water quality was investigated at 49 lakes in the middle and lower reaches of the Yangtze River. Lakes ranging from oligomesotrophic to hypereutrophic were examined, providing an obvious nutrient gradient. With the shift from mesotrophic to eutrophic levels, diatom multi-ecotypes dominated by epiphytic and facultative planktonic taxa were replaced by nutrient-tolerant planktonic taxa, such as Cyclotella meneghiniana Skvortzow, C. atomus Hustedt, Cyclostephanos Round, and Stephanodiscus Ehrenberg etc., reflecting the nutrient changes in the lake. The relationship between diatoms and summer water quality indices was explored further using numeric analysis. Canonical correspondence analysis (CCA) with forward selection and a Monte Carlo permutation test revealed that of all 25 summer water environmental variables, total phosphorus (TP), chlorophyll a (Chl a), Secchi depth (SD), dissolved inorganic phosphorus, Cl–, SO42–, Mg2+, CO32–, and water depth were significant variables (P<0.05) in explaining diatom distributions. Of these, TP, Chl a, SD, and Cl–, were the most important variables. The result of the correlation analysis also showed that a significant correlation exists among these variables, implying that these indices are either interconnected or independent in explaining the diatom data. For phosphorus-limited sites, TP was the most significant variable affecting the diatoms, also affecting changes in Chl a, SD, and iron concentrations. The independence of Chl a may be related to algal competition induced by lake eutrophication, resulting in the feedback to diatom community. In addition to TP, SD can be related to sediment disturbance by wave action and the growth of macrophytes in large shallow lakes. These relationships between diatom ecotypes and water quality provide the basis for a future quantitative reconstruction of historic lake nutrient evolution in the study area and will also provide a wealth of modern ecological knowledge that can be used to interpret fossil diatom records.     

东海表层沉积硅藻组合与环境关系研究

对东海表层沉积硅藻分析研究,共发现有103种和变种,根据其中分布特征和规律,结合因子负荷变化曲线,把东海表层沉积分为六个硅藻组合和分布区,并探讨影响硅藻分布的环境因素,为恢复古环境提供科学依据。     

PERIPHYTIC DIATOM ASSEMBLAGES FROM HIGH ARCTIC PONDS1

Epiphytic, epilithic, and surface sediment diatom assemblages were identified and enumerated from 35 study ponds on CapeHerschel (78°37″N, 74°42″W), east-central Ellesmere Island, Canada. All the sites are shallow (maximum depth <2 m), clear, oligotrophic, and freshwater. The ponds freeze completely for 10 months of the year. Major ion concentrations are relatively similar among the 35 sites, although environmental gradients exist. Over 130 diatom taxa from 28 genera were identified in the periphyton samples. Marked differences in species composition were evident among the ponds. Moreover, many of the diatoms exhibited varying degrees of microhabitat specificity. Variance partitioning by canonical correspondence analysis showed that 26% of the total variance exhibited by diatom species composition could be accounted for by the measured environmental variables (i.e. 10.2% by habitat and 15.8% by water chemistry). Pondwater alkalinity best explained the distribution of taxa, and weighted averaging regression and calibration were used to develop a transfer function to infer pondwater alkalinity from the diatom assemblages. Other important environmental variables included [Na⁺] for the epilithic and [SiO₂] for the epiphytic assemblages.     

FRESHWATER DIATOMS FROM THE CANADIAN ARCTIC TREELINE AND DEVELOPMENT OF PALEOLIMNOLOGICAL INFERENCE MODELS1

Relationships between surface sediment diatom assemblages and measured environmental variables from 77 lakes in the central Canadian arctic treeline region were examined using multivariate statistical methods. Lakes were distributed across the arctic treeline from boreal forest to arctic tundra ecozones, along steep climatic and environmental gradients. Forward selection in canonical correspondence analysis determined that dissolved inorganic carbon (DIC), dissolved organic carbon (DOC), total nitrogen (TN), lake surface area, silica, lake‐water depth, and iron explained significant portions of diatom species variation. Weighted‐averaging (WA) regression and calibration techniques were used to develop inference models for DIC, DOC, and TN from the estimated optima of the diatom taxa to these environmental variables. Simple WA models with classical deshrinking produced models with the strongest predictive abilities for all three variables based on the bootstrapped root mean squared errors of prediction (RMSEP). WA partial least squares showed little improvement over the simpler WA models as judged by the jackknifed RMSEP. These models suggest that it is possible to infer trends in DIC, DOC, and TN from fossil diatom assemblages from suitably chosen lakes in the central Canadian arctic treeline region.     

Assessing the vulnerability of endemic diatom species in Lake Baikal to predicted future climate change: a multivariate approach

Diatoms in Lake Baikal exhibit significant spatial variation, related to prevailing climate, lake morphology and fluvial input into the lake. Here we have assessed the threats to endemic planktonic diatom species (through the development of empirical models), which form a major component of primary production within the lake. Multivariate techniques employed include redundancy analysis (RDA) and Huisman–Olff–Fresco (HOF) models. Our analyses suggest that eight environmental variables were significant in explaining diatom distribution across the lake, and in order of importance these are snow thickness on the ice, water depth, duration of days with white ice, suspended matter in the lake, days of total ice duration, temperature of the water surface in July, concentration of zooplankton and suspended organic matter. Impacts on dominant phytoplankton diatom species are highlighted using t‐value biplots. Predictions of future climate change on Lake Baikal are likely to result in shorter periods of ice cover, decreased snow cover across the lake in spring, increased fluvial input into the lake, and an increase in the intensification of surface water stratification during summer months. All these factors are likely to impact negatively on the slow‐growing, cold‐water endemics such as Aulacoseira baicalensis and Cyclotella minuta, which currently dominate diatom assemblages. Instead, taxa that are only intermittently abundant, at present, in offshore areas (e.g. Stephanodiscus meyerii) are likely to become more frequent. However, given the climatic gradient across the lake, the timing and extent of changes in community structure are likely to vary. Moreover, palaeolimnological records show that Lake Baikal diatom assemblages have been dynamic throughout the Holocene, with both endemic and cosmopolitan species exhibiting periods of dominance. Effects of climate change on the entire lake ecosystem may yet be profound as the structure of the pelagic food web may change from one based on endemic diatom taxa to one dominated by nondiatom picoplankton, and as limnological functioning (e.g. stratification and mixing) affects deepwater oxygen availability, nutrient cycling and trophic linkages.     

SALINITY TOLERANCE OF DIATOMS FROM THALASSIC HYPERSALINE ENVIRONMENTS

Thirty-four benthic diatom strains were isolated from thalassic hypersaline marine environments and their salinity tolerance characterized in growth experiments conducted at salinities ranging from 0.5% to 17.5% (weight of total salts per volume, g·100 mL ^{− 1}). The results were compared with the patterns of diatom species distribution and abundance in hypersaline evaporation ponds and tidal channels of Guerrero Negro, Baja California Sur, Mexico. The isolated strains were representative of the diatom assemblages present in the saltern ponds but were less so of natural assemblages in tidal channels. In general, we found a clear decreasing trend of diatom diversity in the field and in the isolated strains with increasing salinity. With some exceptions, the upper limit of salinity tolerance in cultivated strains corresponded to their distribution in field samples. However, the relative abundance of species in the field was not correlated with growth rates achieved in culture for the same salinities. Most cultured strains exhibited extreme euryhalinity growing well from brackish to hypersaline conditions, but the particulars of salt tolerance were quite diverse among strains. The most halotolerant taxa, two Amphora species, Amphora cf. subacutiuscula Schoeman, Nitzschia fusiformis Grunow, and Entomoneis sp., grew well in salinities ranging from 0.5% to 15%. Three strains of Pleurosigma strigosum W. Smith that were unable to grow in salinities less than 5% total salts represent the only true halophilic diatoms ever reported. The fact that many strains displayed a remarkable halotolerance, with optimal or near-optimal growth rates at salinities as high as three times that of seawater, implies that diatoms from hypersaline environments are evolutionarily highly adapted to such environments.