Surface Sediment Diatom Research Articles

A diatom-based sea-ice reconstruction for the Vaigat Strait (Disko Bugt, West Greenland) over the last 5000 yr

A diatom-based sea-ice concentration (SIC) transfer function was developed by using 72 surface samples from west of Greenland and around Iceland, and validated against associated modern SIC. Canonical correspondence analysis on surface sediment diatoms and monthly average of SIC indicated that April SIC is the most important environmental factor controlling the distribution of diatoms in the area, justifying the development of a diatom-based SIC transfer function. The agreement between reconstructed SIC based on diatoms from West Greenland and the satellite and modelled sea-ice data during the last ~75yr suggests that the diatom-based SIC reconstruction is reliable for studying the palaeoceanography off West Greenland.Relatively warm conditions with a strong influence of the Irminger Current (IC) were indicated for the early part of the record (~5000–3860cal. yr BP), corresponding in time to the latest part of the Holocene Thermal Maximum. Between 3860 and 1510cal. yr BP, April SIC oscillated around the mean value (55%) and during the time interval 1510–1120cal. yr BP and after 650cal. yr BP was above the mean, indicating more extensive sea-ice cover in Disko Bugt.Agreement between reconstructed April SIC and changes in the diatom species suggests that the sea-ice condition in Disko Bugt was strongly influenced by variations in the relative strength of two components of the West Greenland Current, i.e. the cold East Greenland Current and the relatively warm IC. Further analysis of the reconstructed SIC record suggests that solar radiation may be an important forcing mechanism behind the historic sea-ice changes.

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.

Relationship between diatoms and water quality (TN, TP) in sub-tropical east Australian estuaries

Nutrient over-enrichment of estuarine environments is increasing globally. However, it is difficult to determine the eutrophication trend in estuaries over long periods of time because long-term monitoring records are scarce and do not permit the identification of baseline environmental conditions. In this study, preliminary diatom based transfer functions for the inference of total phosphorus (TP) and total nitrogen (TN) in east-Australian sub-tropical estuaries were developed to address the deficiency in knowledge relating to historical estuary water quality trends. The transfer functions were created from a calibration set consisting of water quality and associated surface sediment diatom assemblage data from fifty-two sub-tropical estuaries in New South Wales and Queensland, Australia. Following data screening processes, Canonical Correspondence Analysis confirmed that TP and TN both explained significant, independent variation in the diatom assemblages. Variance partitioning, however, indicated that the TP was confounded with and may receive some strength from TN. WA and WA-PLS 2 component models for TP that included all calibration set sites yielded statistically weak results based on the jack-knifed r 2 scores $$ \left( {r_{\text{jack}}^{{^{ 2} }} \, = 0.22\;{\text{and}}\;0. 2 2 {\text{ respectively}}} \right) $$ . Removal from the calibration set of 12 sites that had all PO4, NH4, NO2, and NOx concentrations below detection limit resulted in a substantial improvement in WA-PLS 2 component TP model scores $$ \left( {r_{\text{jack}}^{{^{ 2} }} \; = \;\,0.69} \right) $$ , indicating that this model is statistically robust, and thus suitable for down core nutrient reconstructions. Caution, however, is required when developing diatom based inference models in Australian estuaries as nutrient cycling processes may have the potential to influence diatom based transfer functions. The model reported on here provides a foundation for reconstructing nutrient histories in eastern Australian sub-tropical estuaries in the absence of monitoring data.

Diatom assemblages and limnological variables from 40 lakes and ponds on Bathurst Island and neighboring high Arctic islands

AbstractWe examined the influence of catchment geology, specifically differences in buffering capacity, on the limnological characteristics and surface sediment diatom assemblages from lakes and ponds from Bathurst Island, High Arctic Canada. Differences in buffering capacity exist on Bathurst Island due to a geological gradient that spans from carbonate‐bearing limestone in the east, to more stable quartz sandstone, siltstone, and shale in the west. We collected physical and chemical limnological data, as well as surface sediment diatom assemblages from nine ponds on the poorly buffered western portion of the island and combined these observations with a previously published dataset of 31 lakes and ponds, from the well‐buffered eastern region. The addition of these nine ponds expanded the pH gradient of the existing Bathurst Island dataset (pH 8.0–8.6) to pH 6.8–8.6. A regional, weighted average diatom‐inferred pH model was developed and showed strength similar to other Arctic calibration sets (, root‐mean‐squared‐error of prediction (RMSEP) = 0.298). Given the links between climate and pH shifts in the High Arctic, the ability to reconstruct pH should be a valuable tool for future paleolimnological studies.

The structure and species composition of the diatom communities in tropical volcanic lakes of eastern Mexico

Six nutrient-rich tropical volcanic lakes in eastern Mexico were investigated to document their characteristic diatom floras. Plankton and sedimentary diatom communities were compared to verify the representativity of the sediment assemblage by identifying the plankton species present in the sediment. Relationships between surface sediment diatom communities and 20 physico-chemical variables were evaluated using multivariate analysis. High phosphorus concentrations were measured in all the lakes sampled and chlorophyll a concentrations were found to be useful in describing lake trophic status which ranged from meso- to hypereutrophic. The results showed that the species composition and distribution of the diatom community in these lakes was most influenced by nutrient availability (P) and ionic composition. Canonical correspondence analysis showed that cation dominance explained a significant amount of the variation in the diatom community composition. It allowed the identification of three lake types: (1) Na+-dominated lakes where diatom communities had>50% Achnanthidium minutissimum (Grunow) Czarnecki; (2) Mg2+-dominated lakes, where the characteristic species were A. minutissimum (<30%) and Cymbella microcephala Grunow (<50%); and (3) one Ca2+-dominated lake, where Navicula arvensis Hustedt and Discostella pseudostelligera (Hustedt) Houk & Klee were the most abundant diatoms (>20%). The results suggest that some epiphytic diatoms (e.g., A. minutissimum, C. microcephala) can use other plankton algae as substrate; this is a strategy that allows them to remain within the euphotic zone in deep, eutrophic, turbid lakes. The lakes with the lowest nutrient levels had the best preserved terrestrial vegetation cover on the crater walls. This suggests that deforestation has been a factor that has accelerated the process of lake eutrophication during recent decades in this region.

Influence of environmental and spatial factors on the distribution of surface sediment diatoms in Chaohu Lake, southeast China

Abstract- The spatial distribution of surface sediment diatoms in Chaohu Lake (southeast China), and their relationships with environmental and spatial variables were analyzed in this study. The diatom assemblages were dominated by planktonic species. Three dominant speciesCyclostephanos dubius,Aulacoseira granulataandAulacoseira alpigenaare unevenly distributed across the lake. The distribution of surface sediment diatoms must be subject to trophic status, hydrodynamics and other spatial variables in the lake.

The Distribution of Surface Sediment Diatoms in Bering Sea and Chukchi Sea

Diatoms from surface sediment samples in the Bering Sea and Chukchi Sea were analyzed to reveal the distribution pattern of the surface sediment diatoms and their relationship with the ocean environment.Diatom was found to be absent in sediment samples from north of the Arctic Sea Ice Minimum,showing the growth and reproduction of diatoms are strongly inhibited by sea ice.In areas between the Arctic Sea Ice Minimum and Maximum,i.e.areas influenced by seasonally changed sea ice,the sea ice diatoms(mainly Fragilariopsis oceanica and Fragilariopsis cylindrus) was found to be the dominant diatom species.Arctic sea ice was therefore confirmed to be an important influencing factor for surface sediment diatoms in the Bering and Chukchi Seas.The Chaetoceros resting spores were more abundant in the Chukchi Sea than the Bering Sea,which was coincident with the composition of modern phytoplankton in these areas,and can possibly be attributed to stronger hydrodynamic condition in the Bering Sea.This species was least abundant on the northeast Bering shelf,possibly because of the lower water depth,stronger effect of the coastal current and river influx,relatively coarser sediment particles,and relatively strong wind and bottom current.The Arctic diatom group(dominated by Bacterosira bathyomphala,Thalassiosira antarctic v.borealis and Thalassiosira antarctic resting spore) was more abundant in the Bering basin and the central and north Chukchi Sea,while the coastal benthic diatom group(including Paralia sulcata and Delpheneis surirella) was mainly found on the northeast Bering Shelf and near Cape Lisburne in the Chukchi Sea.Thalassisira nordenskioledii was most plentiful nearby the Bering Strait,and Neodenticula saminae was only found in the Bering Sea,especially in the Bering basin,indicating close correlation with the Pacific waters.

THE INFLUENCE OF SUBMERGED MACROPHYTES ON SEDIMENTARY DIATOM ASSEMBLAGES(1).

Submerged macrophytes are a central component of lake ecosystems; however, little is known regarding their long-term response to environmental change. We have examined the potential of diatoms as indicators of past macrophyte biomass. We first sampled periphyton to determine whether habitat was a predictor of diatom assemblage. We then sampled 41 lakes in Quebec, Canada, to evaluate whether whole-lake submerged macrophyte biomass (BiomEpiV) influenced surface sediment diatom assemblages. A multivariate regression tree (MRT) was used to construct a semiquantitative model to reconstruct past macrophyte biomass. We determined that periphytic diatom assemblages on macrophytes were significantly different from those on wood and rocks (ANOSIM R = 0.63, P < 0.01). A redundancy analysis (RDA) of the 41-lake data set identified BiomEpiV as a significant (P < 0.05) variable in structuring sedimentary diatom assemblages. The MRT analysis classified the lakes into three groups. These groups were (A) high-macrophyte, nutrient-limited lakes (BiomEpiV ≥525 μg · L(-1) ; total phosphorus [TP] <35 μg · L(-1) ; 23 lakes); (B) low-macrophyte, nutrient-limited lakes (BiomEpiV <525 μg · L(-1) ; TP <35 μg · L(-1) ; 12 lakes); and (C) eutrophic lakes (TP ≥35 μg · L(-1) ; six lakes). A semiquantitative model correctly predicted the MRT group of the lake 71% of the time (P < 0.001). These results suggest that submerged macrophytes have a significant influence on diatom community structure and that sedimentary diatom assemblages can be used to infer past macrophyte abundance.

Relationships between freshwater sedimentary diatoms and environmental variables in Subarctic Icelandic lakes

The distribution patterns of surface sediment diatom assemblages from 49 lakes were used to explore the relationship between limnological variables and diatom assemblages as well as to assess an ecological classifi ca- tion system as a tool for the management and conservation of Icelandic freshwaters. Lakes were limnologically diverse ranging from deep, oligotrophic, ionically dilute lakes, to shallow lakes with a wide range of nutrient and ionic contents. Physical conditions (depth, surface area, surface water temperature) and nutrient and ion concentra- tions differed signifi cantly among ecological lake categories (i.e., plateau, spring-fed, direct-runoff, valley, glacial, and coastal lakes) (ANOVA, p 20 % in all but 4 of the lakes, most likely due to the generally cold lake water conditions in this subarctic region. Variation in diatom distributions was best explained by the combination of mean depth (infl uencing littoral versus planktonic habitats), surface water temperature, specifi c conductivity, alkalinity, total organic carbon, total nitrogen and SiO2 in a canonical correspondence analysis (CCA). However, these variables did not explain distribution patterns amongst small benthic Fragilaria taxa, although some weak relationships between some taxa and these variables were evident. Distinct diatom assemblages and limnological properties among ecological lake categories support the classifi cation of Icelandic freshwaters based on major topographic, geological and hydrological characteristics. More detailed inclusion of lake depth along with lake basin form, as well as more refi ned categories of lake water origin and topographical positioning to better approximate regional climatic conditions, may improve the ecological classifi cation of Icelandic freshwaters for conservation and management practices.

Surface sediment diatom assemblages and epilimnetic total phosphorus in large, shallow lakes of the Yangtze floodplain: their relationships and implications for assessing long‐term eutrophication

Summary1. The Yangtze floodplain (SE China) is characterized by a number of large shallow lakes, many of which have undergone eutrophication due to the intensification of agriculture and urban growth over recent decades. As monitoring data are limited and in order to determine lake baseline nutrient concentrations, 49 lakes were sampled, covering a total phosphorus (TP) gradient (c. 30–550 μg L−1) to develop a diatom‐based inference model.2. There are three dominant diatom assemblages in these shallow lakes with a marked change in assemblage structure near the boundary between eutrophic and hypereutrophic nutrient levels (as indicated by their TP value). Canonical correspondence analysis indicated that TP was the most important and significant variable in explaining the diatom distributions, independently accounting for 9.5% variance of diatoms.3. Forty‐three lakes were used to generate a transfer function using weighted averaging (WA) with inverse deshrinking. This model had low predictive error (root mean squared error of prediction; RMSEPjack = 0.12) and a high coefficient of prediction (R2jack = 0.82), comparable with regional TP models elsewhere. The good performance of this TP model may reflect the low abundance of benthic diatom species which are commonly regarded as the main error source in European shallow lake WA models.4. The WA model was used to reconstruct the past‐TP concentrations for Taibai Lake, a shallow hypereutrophic lake in Hubei province. The results showed that TP concentration varied slightly (43–62 μg L−1) prior to the 1920s, indicating an eutrophic state since the 1800s. A period of sustained eutrophication occurred after 1950, because of the development of agriculture, reflecting by maximum values of Aulacoseira alpigena and increased abundance of Cyclotella meneghiniana, C. atomus and Cyclostephanos dubius. The steep increase in nutrient concentration after 1970 was related to the overuse of chemical fertilizer and fish farming in the catchment.5. The shift in fossil diatoms from epiphytic to planktonic forms in the lake sediment core during 1950–70 provides useful information on the transformation from macrophyte‐dominated to alga‐dominated states. It is plausible that the TP concentration of 80–110 μg L−1 observed in this study is the critical range for switching between the two stable states in the lake.6. The regional diatom‐TP model developed in this study allows, therefore, the possibility of reconstructing historical background nutrient concentrations in lakes. It will provide an indication of the onset and development of eutrophication at any site. This is particularly important for the many lakes in the Yangtze floodplain where information about historical changes in water quality is lacking.

Diatom-based total phosphorus (TP) and pH transfer functions for the Irish Ecoregion

A 72-lake diatom training set was developed for the Irish Ecoregion to examine the response of surface sediment diatom assemblages to measured environmental variables. A variety of multivariate data analyses was used to investigate environmental and biological data structure and their inter-relationships. Of the variables used in determining a typology for lakes in the Irish Ecoregion, alkalinity was the only one found to have a significant effect on diatom assemblages. A total of 602 diatom taxa were identified, with 233 recorded at three or more sites with abundances ≥1%. Generally diatom data displayed a high degree of heterogeneity at the species level and non-linear ecological responses. Both pH and total phosphorus (TP) (in the ranges of 5.1–8.5 and 4.0–142.3 μg l−1 respectively) were shown to be the most significant variables in determining the surface sediment diatom assemblages. The calibration models for pH and TP were developed using the weighted averaging (WA) method; data manipulation showed strong influences on model performances. The optima WA models based on 70 lakes produced a jack-knifed coefficient of determination (r 2 jack) of 0.89 with a root mean squared error (RMSEP) of 0.32 for pH and r 2 jack of 0.74 and RMSEP of 0.21 (log10 μg l−1) for TP. Both models showed strong performances in comparison with existing models for Ireland and elsewhere. Application of the pH and TP transfer functions developed here will enable the generation of quantitative water quality data from the expanding number of palaeolimnological records available for the Irish Ecoregion, and thus facilitate the use of palaeolimnological approaches in the reconstruction of past lake water quality, ecological assessment and restoration.

Reconstructing changes in macrophyte cover in lakes across the northeastern United States based on sedimentary diatom assemblages

Macrophytes are a critical component of lake ecosystems affecting nutrient and contaminant cycling, food web structure, and lake biodiversity. The long-term (decades to centuries) dynamics of macrophyte cover are, however, poorly understood and no quantitative estimates exist for pre-industrial (pre-1850) macrophyte cover in northeastern North America. Using a 215 lake dataset, we tested if surface sediment diatom assemblages significantly differed among lakes that have sparse ( 40% cover; group 3) macrophyte cover. Analysis of similarity indicated that the diatom assemblages of these a priori groups of macrophyte cover were significantly different from one another (i.e., difference between: groups 1 and 3, R statistic = 0.31, P < 0.001; groups 1 and 2, R statistic = 0.049, P < 0.01; groups 3 and 2, R statistic = 0.112, P < 0.001). We then developed an inference model for macrophyte cover from lakes classified as sparse or extensive cover (145 lakes) based on the surface sediment diatom assemblages, and applied this model using the top-bottom paleolimnological approach (i.e., comparison of recent sediments to pre-disturbance sediments). We used the second axis of our correspondence analysis, which significantly divided sparse and extensive macrophyte cover sites, as the independent variable in a logistic regression to predict macrophyte cover as either sparse or extensive. Cross validation, using 48 randomly chosen sites that were excluded from model development, indicated that our model accurately predicts macrophyte cover 79% of the time (r2 = 0.32, P < 0.001). When applied to the top and bottom sediment samples, our model predicted that 12.5% of natural lakes and 22.4% of reservoirs in the dataset have undergone a ≥30% change in macrophyte cover. For the sites with an inferred change in macrophyte cover, the majority of natural lakes (64.3%) increased in cover, while the majority of reservoirs (87.5%) decreased in macrophyte cover. This study demonstrates that surface sediment diatom assemblages from profundal zones differ in lakes based on their macrophyte cover and that diatoms are useful indicators for quantitatively reconstructing changes in macrophyte cover.

Relationships between diatoms and environmental variables in wetlands in the Willamette Valley, Oregon, USA

The heterogeneous nature and widespread anthropogenic impacts to wetlands in the Willamette Valley, Oregon, pose challenges to bioassessment of these systems. The purpose of this study was to examine diatom species patterns in relation to environmental variables in wetlands, and to compare a diatom-based wetland classification to a hydrogeomorphic (HGM) wetland classification. Surface sediment diatoms, water quality, physical habitat, and surrounding land use characteristics were assessed for 92 wetlands. A total of 419 taxa were identified. Taxa richness was high (mean = 54, range 15–94) and dominance by a single taxon at a site was low (mean = 27%, range 7%–78%). Assemblages were dominated by tychoplanktonic taxa (e.g., Staurosira construens) and periphytic taxa (e.g., Fragilaria capucina and Achnanthidium minutissimum). Non-metric multidimensional scaling and correlational analysis showed that diatom assemblages were correlated with water depth, summer and winter total phosphorus, soluble reactive phosphorus, and turbidity. Diatom-based classification of wetlands produced four statistically significant groups that corresponded to within-wetland water depth, nutrient levels, and turbidity. Diatom-based wetlands classification did not agree with a priori HGM classification. Diatom assemblages and common species were similar between depressional and riverine-impounded wetlands. Our results suggest that, despite regional heterogeneity in both diatom assemblages and environmental conditions, diatoms can be useful indicators of water quality and habitat conditions in riverine and depressional wetlands. HGM classification may be too coarse to elucidate patterns of diatom community structure and conditions of the habitat in these anthropogenically-impacted wetlands.

Ecological characteristics of modern diatom assemblages from Axel Heiberg Island (High Arctic Canada) and their application to paleolimnological inference models

Relationships between modern surface sediment diatom assemblages and measured water chemistry variables were examined from 30 lakes and ponds on Axel Heiberg Island (Nunavut) in the Canadian High Arctic. Canonical correspondence analysis with forward selection and Monte Carlo permutation tests identified dissolved organic carbon, dissolved inorganic carbon, specific conductance, and pH as the measured environmental variables explaining significant proportions of the diatom variance. Canonical correspondence analysis axis 1 represented a gradient of specific conductance, and axis 2 was influenced primarily by pH. To increase the signal-to-noise ratio in our reconstructions, the species data sets were refined to include only taxa that had significant responses to either conductivity or pH, as determined by Huisman Olff Fresco models of species–environment relationships. Diatom-based inference models were subsequently developed for both lakewater specific conductance (r2boot= 0.75, root mean square error of prediction = 0.22) and lakewater pH (r2boot= 0.31, root mean square error of prediction = 0.57) using weighted averaging techniques. These data contribute to our understanding of diatom biogeography throughout the Canadian Arctic and have implications for regional paleoclimatic reconstructions in these climatically and environmentally sensitive regions.

Diatom assemblages in surface sediments of the northwestern Weddell Sea, Antarctic Peninsula

A study of 42 surface sediment samples from the northwestern Weddell Sea documents the relationship between the modern distribution of diatom assemblages and present oceanographic conditions in the region. Three diatom assemblages were identified using R-mode principal components analysis and cluster analysis, while Q-mode cluster analysis was used to distinguish two geographically distinct biofacies. Based on the relative abundance of diatom species, the biofacies of the former Larsen A Ice Shelf (LAIS) is more closely associated with a sea ice diatom assemblage, while the biofacies of the Prince Gustav Channel/Northern Peninsula (PGC/NP) is associated with two open coastal diatom assemblages. This division is consistent with variance observed in the current oceanographic setting, whereby the LAIS is generally characterized by a longer seasonal duration of sea ice coverage and lower primary productivity relative to the PGC/NP which is marked by a shorter seasonal duration of sea ice coverage and greater surface water productivity. This relationship is further reflected in the distribution of the paleoenvironmentally significant diatom species Fragilariopsis curta, Thalassiosira antarctica, and Chaetoceros subg. Hyalochaete spp. as well as in absolute abundance data. Specifically, F. curta and the morphological form T. antarctica T1 are representative of greater seasonal duration of sea ice coverage and lower productivity. Chaetoceros subg. Hyalochaete spp. and the morphological form T. antarctica T2 are associated with decreased seasonal duration of sea ice cover and greater surface water productivity, a setting that is further reflected in high absolute diatom abundances. These findings provide a basis for future paleoceanographic work in the region, especially as studies of surface sediment diatom assemblages are vital to establishing a link between downcore diatom assemblages and those presently found in the water column.

Diatom-based bioassessment in wetlands: How many samples do we need to characterize the diatom assemblage in a wetland adequately?

Diatom-based bioassessment in wetlands requires quantitative characterization of spatial and temporal variability of the diatom assemblages within each wetland. The purpose of this study was to examine surface-sediment diatom distributional patterns in a wetland to determine how best to sample these systems to capture spatial variability in the assemblage. Diatoms and environmental conditions were characterized from 29 sampling points within a wetland in the floodplain of the Columbia River, Oregon, USA. A total of 159 diatom taxa were identified in the surface-sediment samples. Species richness was high at each sampling point (median: 42, range: 23-57), and relative abundances of common taxa varied between 15 and 39% throughout the wetland. Assemblages contained taxa with both benthic (e.g., Staurosira construens, Nitzschia palea, Fragilaria capucina, Achnanthidium minutissimum )a nd planktonic (e.g., Aulacoseira granulata and Tabellaria spp.) preferences. Non-metric multidimensional scaling (NMDS) techniques detected differences in the low marsh and upper marsh sediment diatom assemblages. Geostatistical analysis showed spatial autocorrelation of the diatom assemblage in the wetland, measured as semivariance and Moran's I. A simulation procedure indicated that changes in diatom species richness stabilized after approximately five samples were composited. Our results suggest that the wetland surface sediment diatom assemblage is heterogeneous and that hydrologic gradients may be an important structuring force. Diatom-based bioassessment has the potential to be a useful tool in assessing wetland environmental conditions; however, the shallow nature and complex hydrology of these systems require careful sampling design to adequately characterize the diatom assemblage.

Diatom response to the Holocene climatic optimum on the North Icelandic shelf

A diatom record from core MD992275 on the North Icelandic shelf reveals palaeoclimatic and palaeoceanographic changes during the time interval 9000 to 5600 cal. year BP. The study period includes the Holocene Climate Optimum (HCO), as widely known in the North Atlantic region. A comparison with data for the last 2000 years and with the present-day surface sediment diatom assemblages shows that there was considerably stronger influence of Atlantic water masses in the area during the HCO than in the late Holocene and today. The results of cluster analysis and principal component analysis divide the studied period into three intervals, each characterised by a specific diatom assemblage. The assemblages indicate changes in the interaction of the relatively warm Irminger Current (IC) and the cold East Greenland Current (EGC) and East Icelandic Current (EIC). The North Icelandic shelf was strongly affected by the IC during the interval 8000–6800 cal. year BP, which may be the warmest period of the Holocene. During the intervals 9000–8000 and 6800–5600 cal. year BP, the influence of the IC was relatively weak and that of the EGC and EIC increased correspondingly. The well-known 8200 cal. year BP cooling event, shown in many records from the North Atlantic, is reflected by a relatively strong influence of the EGC and EIC between about 8300 and 8100 cal. year BP in our record. However, the diatom assemblages show that the inferred circulation change did not bring the strength of the IC down to the levels seen at present and during the last two millennia.

Calibration of Littoral Diatoms to Water Chemistry in Standing Fresh Waters (Flanders, Lower Belgium): Inference Models for Historical Sediment Assemblages

Relationships between littoral surface-sediment diatom assemblages and ambient limnological conditions were examined in 186 lentic fresh waters throughout lower Belgium (Flanders). Most of these waters were small, unstratified, alkaline and rich in nutrients. Using weighted-averaging techniques, robust and accurate transfer functions were developed for median pH-values ranging from 3.4 to 9.3 and dissolved inorganic carbon concentrations from <1.6 to 63 mg l−1 (jackknifed r 2≈ 0.87, RMSEP <10% of the observed range), while a less precise model was obtained for sodium (2–571 mg l−1; jackknifed r 2 0.69, RMSEP 9.9% of the range). Restricting the data set to circumneutral and alkaline sites (pH≥6.5) revealed the importance of additional variables, including calcium, silica, chemical oxygen demand and potential gross oxygen production (a proxy for metabolic activity and phytoplankton abundance). Calibration models for these variables were strong enough to be useful (jackknifed r 2 0.57–0.59, RMSEP 13.1–16.4% of the observed range), although estimations should not always be considered entirely independent. Except for the predominant pH gradient, removal of all taxa with a distribution unrelated to the variable of interest improved model performance. In general, such taxa were proportionally represented among taxa classified according to their principal habitat. Application of the present models to diatom assemblages of shallow-water sediments obtained from historical samples and, most importantly, herbarium-macrophyte specimens, will improve hindsight into regional freshwater conditions and add to base-line setting of ecological quality standards in a highly impacted region.

Using diatoms to assess the impacts of prehistoric, pre-industrial and modern land-use on Danish lakes

The impact of contemporary agriculture on Danish lakes is acknowledged to be extreme. In particular, high loading of nutrients from agricultural soils contributes to the eutrophic conditions found in many of Denmark’s lakes. Palaeolimnological studies have shown that human disturbance of the Danish landscape since the introduction of agriculture around 6,000 years ago has had a major impact on lake ecosystems. The European Union’s Water Framework Directive requires an evaluation of reference conditions for lakes, the conditions expected with only minimal human impact. Monitoring data and palaeolimnological studies of Danish lakes demonstrate that many of the most detrimental effects of eutrophication have been experienced in recent decades. A new study has suggested that the reference status for Danish lakes may be set to the status in ad 1850–1900, probably providing attainable, realistic restoration targets for many sites. The aims of this study were to explore the impacts of past and contemporary land-use on Danish lakes, and to consider how appropriate the use of 1850 as a date to define reference status is for these sites. Catchment land-cover data for ad 1800, taken from historical maps, and sedimentary diatom assemblages of the same age, from dated sediment cores, were used to assess the impact of pre-industrial land-use on 20 Danish lakes. Analysis of contemporary land-cover data and surface-sediment diatom assemblages for the 20 sites was also made. In-lake total phosphorus (TP) concentrations were estimated using the sedimentary diatom assemblages and an existing calibration dataset for Danish lakes. The percentage of the lake catchment that was agricultural land in ad 1800 explained 8.8% of the total variation in the diatom data. The land-cover variables ‘built-up areas’ and ‘plantations’, together explained 16.9% of the variation in the diatom data for the modern samples. Diatom-inferred TP concentrations were high for both ad 1800 (mean 112 μg TP L−1) and the present (mean 122 μg TP L−1), the latter estimates reflecting efforts in recent decades to reduce nutrient loading to Danish lakes following very high levels of nutrient enrichment post-1950. The data presented highlight the impact that human activities 200 years ago, particularly agriculture, had on Danish lake systems. The long cultural history and major anthropogenic disturbance of the Danish landscape mean that true reference conditions for lakes (or ‘baseline’ conditions, those found prior to human impacts) can be found only by considering century to millennial timescales.

COASTAL DIATOM–ENVIRONMENT RELATIONSHIPS FROM THE GULF OF FINLAND, BALTIC SEA1

Eutrophication of the Baltic Sea has become a serious concern in recent decades. To provide a potential means for quality assessments of coastal waters in this area, we collected a data set of 49 embayments in the Gulf of Finland, and explored the relationship between surface sediment diatom assemblages and 15 environmental variables, with special emphasis on nutrients. Total dissolved nitrogen, total phosphorus, depth, and salinity all accounted for significant and independent fractions of variation in the diatom data and explained 34% of the total variation. There were clear changes in diatom assemblage structures along the nutrient gradients. Although these changes were gradual, we could identify a number of taxa that were more abundant in a particular nutrient environment. These taxa could be used as potential indicators of the quality of coastal waters in the Baltic Sea. Diatom assemblages that were least affected by nutrient enrichment included a variety of benthic species and a relatively high species richness. Small planktonic taxa such as Cyclotella atomus Hustedt, Cyclotella meneghiniana Kützing and Thalassiosira pseudonana Hasle and Heimdal were good indicators of highly elevated nutrient concentrations (&gt;600 lg·L−1 total dissolved nitrogen and 60 lg·L−1 total phosphorus) together with low species richness. The first appearance of these small planktonic taxa in regular monitoring could be used as an early warning sign for deteriorating water quality. Diatoms could be applied to water quality classification and monitoring purposes in the coastal waters of the Baltic Sea area using techniques such as weighted‐averaging regression and calibration.