Changes in diatom assemblages since pre-industrial times in 40 reference lakes from the Experimental Lakes Area (northwestern Ontario,Canada)

Diatom assemblages in recent versus pre-industrial sediments were examined in 40 relatively undisturbed lakes from the Experimental Lakes Area (ELA). The ELA region of northwestern Ontario receives low amounts of acidic deposition and the lakes have been minimally disturbed by watershed development or other human activities. Consequently, this region represents an important location to detect possible changes in lakes due to climate change. In over half of the lakes, planktonic taxa (especially Discostella stelligera) increased between 10 and 40% since pre-industrial times. Changes in diatom assemblages are consistent with taxa that would benefit from enhanced stratification and a longer ice-free season. We hypothesized that there should be a relationship between stratification and measured chemical and physical characteristics of the study lakes. Multiple correlation analysis was undertaken to see the relationship between planktonic taxa and D. stelligera since pre-industrial times and the physical and chemical characteristics of the study lakes. Lake depth was consistently identified as an important variable. The timing of the increase in planktonic taxa within cores from these lakes will be needed to rule out other possible regional changes that may also be occurring in the ELA region.     

Factors affecting diatom distribution in floodplain lakes of the southeast Murray Basin,Australia and implications for palaeolimnological studies

Diatom assemblages of surface sediments in 46 billabongs from four river floodplains in the southeast Murray-Darling Basin, Australia were sampled to investigate drivers of species distribution. The principal purpose of the study was to derive information to aid interpretation of diatom-based palaeoecological studies of these systems and of floodplain lakes more generally. Patterns in billabong diatom assemblages in relation to river reach, hydrology and farming intensity on surrounding land were examined, as were correlations with water quality variables. Seasonal variation in billabong water quality was high relative to spatial variation, and spatial patterns in billabong water quality were weak. In contrast, strong patterns were evident in diatom assemblages. Three main patterns were observed: (1) a distinction between billabongs dominated by planktonic diatoms from those dominated by benthic and attached forms; (2) differences in diatom assemblages in billabongs on different river reaches; and (3) differences in assemblages in billabongs with different hydrology. Of all water quality variables tested, total phosphorus (TP), total nitrogen (TN) and pH exerted the strongest independent influence on diatom distribution; however, only TP remained an important variable when species variation due to river reach, hydrology, and aquatic plant cover was taken into account. The weak influence of water quality on diatom distribution is interpreted as reflecting the dichotomy between plankton and non-plankton-dominated billabongs, the influence of hydrology and biogeography, the lack of strong spatial water quality gradients and the high degree of temporal variability in water quality. The findings show that diatom records from billabong sediments can provide evidence of long-term changes in the abundance of aquatic macrophytes and hydrology. They also suggest that merging calibration data sets across regions for the purpose of improving diatom transfer functions for water quality reconstruction is of limited value for floodplain lakes, and that performance is more likely to be gained by boosting site numbers within regions.     

Planktonic diatom communities in temperate South-Central Chilean lakes with a focus on Asterionella formosa and the genus Aulacoseira

Journal of Paleolimnology - Paleolimnological records from temperate South-Central Chilean lakes revealed a recent and marked increase in the relative abundances of the key-planktonic diatom...     

Mid-Holocene hemlock decline and diatom communities in van Nostrand Lake, Ontario, Canada

Terrestrial ecosystem disturbances inferred from the fossil hemlock pollen decline (ca. 4,800 BP) and recovery (ca. 3,800 BP) affected van Nostrand Lake, including the diatom communities. Ecological models suggest the lake responded by eutrophying, reflecting higher nutrient influx resulting from increased erosion. A decline in lake productivity followed as the forest vegetation recovered and erosion slowed. Lastly, as the forest switched from early and middle successional species to mature species, lake productivity increased as erosion increased nutrient inflow, especially phosphorus. However, this eutrophication response was delayed or buffered, perhaps due to wetland development surrounding the lake. The lake never fully returned to its initial, pre-hemlock decline state, but oscillated between more eutrophic and more mesotrophic or oligotrophic stages, possibly reflecting other disturbances in the catchment and climatic change.     

Representation of fish communities by scale sub-fossils in shallow lakes: implications for inferring percid–cyprinid shifts

The palaeoecological potential of fish scales was assessed by comparing contemporary population data with scale remains obtained from littoral (n = 10) and open water (n = 10) surface sediment samples in two English shallow lakes, Selbrigg Pond and Cockshoot Broad. Scales and/or scale fragments were present, in low numbers (<20 per 100 cm³ wet sediment) in 34 of 40 sediment samples. In accordance with fish population data, higher densities of scale remains were found in Selbrigg compared to Cockshoot, and in littoral compared to open water samples. Taxonomic difficulties, exacerbated by scale fragmentation, made it impossible to assign the majority of remains to individual species. Most remains could, however, be placed into one of two groups: (i) percids – represented by both scales and scale fragments; and (ii) cyprinids – largely represented by scale fragments. To allow comparison of fish population and sedimentary scale data, both were converted to percentages of the aggregate percid–cyprinid total. Whole scales recovered were almost exclusively percid (45 of 48), thus bore little resemblance to the contemporary fish data. Nevertheless, percentages of scale fragments (Selbrigg: 34 and 66%; Cockshoot: 13 and 87% percid and cyprinid, respectively) and of whole scales and fragments combined (Selbrigg: 54 and 46%; Cockshoot: 46 and 54% percid and cyprinid, respectively) reflected the presence of the numerically dominant fish groups and the broad inter-site differences in their relative abundance (Selbrigg: 36 and 64%; Cockshoot: 10 and 90% percid and cyprinid, respectively). A running mean of scales per sediment volume indicated that some 400 cm³ of sediment was required to accurately characterise the remains present. This study suggests that, with the appropriate methodological considerations (e.g., collection of large sediment samples), fish scale remains may be used to determine the past presence–absence and relative abundance of percid and cyprinid species. As such, this technique may be a valuable supplementary tool for establishing longer-term changes in the fish communities of shallow lakes.     

The relationship between sedimentary diatom assemblages and lakewater pH in 35 Quebec lakes,Canada

Surface sediment diatoms from 35 soft-water lakes in southern Quebec were studied to examine the relationship with lakewater pH. The lakes ranged in pH from 5.25 to 7.66. The species composition and the relative abundance of diatoms in the study lakes was found to be closely related to pH and/or factors closely associated with pH. Predictive models were developed to infer lakewater pH using simple linear regression equations of index alpha, index B, and multiple regressions using pH preference categories. Among the predictive models examined here, the multiple regression technique provided the highest correlation coefficient (r²=0.88) and the lowest standard error (±0.26 of a pH unit) in computing diatom-inferred pH. This model appears to be the most appropriate to reconstruct lake pH histories in Quebec region.     

A diatom dataset and diatom-salinity inference model for southeast Australian estuaries and coastal lakes

To quantify the relationship between diatom species assemblages and the water chemistry of southeast Australian estuaries and coastal lakes, a new dataset of 81 modern diatom samples and water chemistry data was created. Three hundred and ninety-nine species from 53 genera were identified in 36 samples from 32 coastal water bodies in eastern Tasmania and 45 samples from 13 coastal water bodies in southern Victoria. Multivariate statistical analyses revealed that the sampling sites were primarily distributed along salinity and nutrient gradients, and that salinity, nitrate + nitrite, phosphate and turbidity explained independent portions of variance in the diatom data. Species salinity optima and tolerances were determined and a diatom-salinity inference model (WAinv r ² = 0.72, r ²jack = 0.58, RMSEP = 0.09 log ppt) was developed. This new information on diatom species’ salinity preferences provides a useful tool for quantitatively reconstructing salinity changes over time from diatom microfossils preserved in the sediments of a range of estuaries and coastal lakes in southeast Australia. This is valuable for studies investigating long-term human impacts and climate change in the region.     

Biogeochemistry of silica in Devils Lake: implications for diatom preservation

Diatom-salinity records from sediment cores have been used to construct climate records of saline-lake basins. In many cases, this has been done without thorough understanding of the preservation potential of the diatoms in the sediments through time. The purpose of this study was to determine the biogeochemistry of silica in Devils Lake and evaluate the potential effects of silica cycling on diatom preservation. During the period of record, 1867–1999, lake levels have fluctuated from 427 m above sea level in 1940 to 441.1 m above sea level in 1999. The biogeochemistry of silica in Devils Lake is dominated by internal cycling. During the early 1990s when lake levels were relatively high, about 94% of the biogenic silica (BSi) produced in Devils Lake was recycled in the water column before burial. About 42% of the BSi that was incorporated in bottom sediments was dissolved and diffused back into the lake, and the remaining 58% was buried. Therefore, the BSi accumulation rate was about 3% of the BSi assimilation rate. Generally, the results obtained from this study are similar to those obtained from studies of the biogeochemistry of silica in large oligotrophic lakes and the open ocean where most of the BSi produced is recycled in surface water. During the mid 1960s when lake levels were relatively low, BSi assimilation and water-column dissolution rates were much higher than when lake levels were high. The BSi assimilation rate was as much as three times higher during low lake levels. Even with the much higher BSi assimilation rate, the BSi accumulation rate was about three times lower because the BSi water-column dissolution rate was more than 99% of the BSi assimilation rate compared to 94% during high lake levels. Variations in the biogeochemistry of silica with lake level have important implications for paleolimnologic studies. Increased BSi water-column dissolution during decreasing lake levels may alter the diatom-salinity record by selectively removing the less resistant diatoms. Also, BSi accumulation may be proportional to the amount of silica input from tributary sources. Therefore, BSi accumulation chronologies from sediment cores may be effective records of tributary inflow.     

Environmental change in Northern Egyptian Delta lakes during the late Holocene,based on diatom analysis

Quantitative and qualitative diatom analyses from the north Nile Delta lakes sediments of Egypt were used to evaluate the paleoenvironmental development of the lakes and climatic changes during the late Holocene. We analyzed 565 samples taken from 19 cores from Manzala, Burullus and Edku lakes. A total of 263 diatom species and varieties were identified. Multivariate statistical analyses distinguished 17 ecological groups that reflect changes in water salinity, lake-level and trophic state of the lakes, which in turn are mainly related to climatic changes and anthropogenic impacts. Manzala and Burullus lakes experienced a series of alternation between fresh, brackish and marine episodes, which were associated with wet and dry climates. Edku Lake cores, however, contained only three ecological groups that are characteristic of brackish water conditions. The general depositional regime in the lakes indicated five environmental phases: (a) a deep freshwater phase when the Nile flood water reach the lakes during humid warm climate; (b) a shallow freshwater phase with some macrophytes during a dry climate; (c) a shallow brackish water phase when Nile floodwater ceased during a dry climate and the lakes shifted to brackish conditions; (d) a mixed environmental phase when the seawater mixed with freshwater from drains and canals (water salinity fluctuated widely from freshwater to estuarine and full marine conditions); (e) a fully marine phase when seawater entered the lakes at all stages of the tide.     

Nutrients as a link between ionic concentration/composition and diatom distributions in saline lakes

Diatom profiles in closed-basin lake sediments are commonly used to reconstruct climate change based on the observed correlations between salinity (ionic concentration) and modern diatom assemblages. Diatom assemblages are strongly correlated not only with salinity but also anion composition, with certain taxa characteristic of carbonate systems and others sulfate-dominated waters. Although strong correlations exist, the actual mechanisms behind these correlations are unknown. Here we briefly review the influence of salinity and ionic composition on nutrient dynamics in saline lakes and suggest that these interactions may drive shifts in diatom species composition along gradients of ionic concentration/composition. We discuss the influence of salinity and anion composition on nutrient availability, as well as on nutrient requirements and uptake by diatoms.     

Spatial variability of diatom assemblages in surface lake sediments and its implications for transfer functions

The variability of diatom species composition in lake surface sediments was studied along transects in four lakes in northeastern Germany. Three dimictic lakes (Dudinghausener See, Tiefer See, and Cambser See) and one shallow lake (Groß Peetscher See) were sampled. Large differences in diatom composition were found between adjoined samples from different depths within one lake. These differences were mainly displayed by planktonic species. For example, the relative frequency of Stephanodiscus alpinus varied between 4% and 43% within the surface sediment samples of the open-water region of Dudinghausener See. Using transfer functions for total phosphorus (TP) based on the European Diatom data-base (EDDI) combined TP data-set and a local data-set, the inferred TP values differed strongly within one lake when using Weighted Averaging-Partial Least Squares (WA-PLS) regression. In Tiefer See (average of measured TP: 30 μg l^?1), the inferred TP values range from 45 to 110 μg l^?1 using the transfer function based on WA-PLS regression and the EDDI data-set; and from 16 to 100 μg l^?1 using WA-PLS and a local data-set. Performing Maximum Likelihood (ML) regression reduced the difference between measured and inferred values. For Tiefer See, the inferred TP values range between 16 and 45 μg l^?1 using ML regression and the local data-set. Therefore, it seems that ML regression can deal better with the natural variability in species composition than WA-PLS regression. In general, it was shown that by using ML regression and the local data-set, the error of the inferred values was significant lower for all lakes than using WA-PLS regression and the EDDI data-set. The Root Mean Square Error of Prediction (RMSEP) was not useful in selecting the most stable transfer function.     

Sediment chemistry and diatom stratigraphy of two high arctic isolation lakes,Truelove Lowland,Devon Island,N.W.T., Canada

Three sediment cores from two lakes, Fish Lake and Phalarope Lake, in Truelove Lowland, Devon Island, N.W.T. were analyzed for diatoms and chemical composition. Multivariate statistical techniques using a range of chemical variables successfully isolated three sediment groupings in the cores. Allochthonous and autochthonous chemical components in the sediments have been used to reconstruct paleoenvironmental conditions. The two lakes began approximately 10600 years ago as shallow marine lagoons that were isolated from the sea as a result of glacio-isostatic rebound. Based on the presence of distinctive diatom assemblages, the three stratigraphic zones are identified as a basal marine zone, an intermediate and transitional brackish/marine zone and an upper freshwater zone. Following isolation from the sea, the lakes were flushed with freshwater produced by snow and ice melt. In Fish Lake, the period of transition from marine to freshwater, which began approximately 7000 years ago, lasted approximately 800 years. In Phalarope Lake, which was isolated from the sea approximately 5000 years ago, flushing by fresh water was completed only within the last 300 years. Fe, Cr, and Mo in the sediments are associated with the isolation phase when lake sedimentation is sensitive to the presence of brackish water and erosion within the lake catchments. In particular, the precipitation of Mo as MoS₂ reflects the presence of hypolimnetic anoxia associated with lake isolation. During the early post-isolation phase the response of lake biota to an influx of nutrients is reflected in an increase in biological silica and organic carbon in the lake sediments. On the other hand, the generally low organic content of the sediments indicates that sedimentation in these lakes has been largely determined by variations in non-biogenic factors through time. During the mid Holocene the progressive stabilization of surface materials within the lake catchments is marked by decreasing Cr, As and Na in the sediments. At the same time, an increase in allochthonous Mn and Fe is attributed to progressive soil development. During the last 2500 years the catchments have experienced decreased erosion resulting in a decrease in both allochthonous clastic input and lake productivity.     

Impact of climate change on dynamic of plant communities, lakes and swamps in Beijing area during Holocene period

IntroductionInPAGESofIGBPcoreprojectstudy',pollenanalysisinsbeenoneoftheimportantmethodstoreco~ctQuaternarypaleoclilnate.Recently,tviththedeepeningofglobalchangeresearches,manyresearchersactounderstandindetaildiechangeofclimatebystUdyingpollenrecordsoflandsoilsandlakesandswamps.Foralongbine,theChinesescienhstshavepaidgreatattenhontotheStUdyofthehistoricalclinistcandenvironmentchangesinNorthChina.particularlyinBedingarea.Daingthepastthreedecades,thescienhstShavemadesurveysandstUdiesonth…     

Comparing palaeolimnological and instrumental evidence of climate change for remote mountain lakes over the last 200 years

This paper compares the palaeolimnological evidence for climate change over the last 200 years with instrumental climate data for the same period at seven European remote mountain lakes. The sites are Øvre Neådalsvatn (Norway), Saanajärvi (Finland), Gossenköllesee (Austria), Hagelseewli (Switzerland), Jezero v Ledvici (Slovenia), Estany Redó (Spain, Pyrenees), and Niné Terianske Pleso (Slovakia). We used multiple regression analysis to transfer homogenised lowland air temperature records to each of the sites, and these reconstructions were validated using data from on-site automatic weather stations. These data showed that mean annual temperature has varied over the last 200 years at each site by between 1 and 2 °C, typical of the high frequency variability found throughout the Holocene, and appropriate, therefore, to test the sensitivity of the various proxy methods used. Sediment cores from each site were radiometrically dated using ²¹⁰Pb, ¹³⁷Cs and ²⁴¹Am and analysed for loss-on-ignition, C, N, S, pigments, diatoms, chrysophytes, Cladocera and chironomids. Comparisons between the proxy data and the instrumental data were based on linear regression analysis with the proxy data treated as response variables and the instrumental data (after smoothing using LOESS regressions) as predictor variables. The results showed few clear or consistent patterns with generally low or very low r² values. Highest values were found when the data were compared after smoothing using a broad span, indicating that some of the proxy data were capturing climate variability but only at a relatively coarse time resolution. Probable reasons for the weak performance of the methods used include inaccurate dating, especially for earlier time periods, the influence of confounding forcing factors at some sites e.g., air pollution, earthquakes, and the insensitivity of some methods to low amplitude climate forcing. Nevertheless, there were trends in some proxy records at a number of sites that had a relatively unambiguous correspondence with the instrumental climate records. These included organic matter and associated variables (C and N) and planktonic diatom assemblages at the majority of sites and chrysophytes and chironomids at a few sites. Overall for longer term studies of the Holocene, these results indicate the need to be cautious in the interpretation of proxy records, the importance of proxy method validation, the continuing need to use reinforcing multi-proxy approaches, and the need for careful site and method selection.     

The detection of Rhizosolenia (Bacillariophyceae) in sediment of Ontario lakes and implications for paleoecology

The detection of the freshwater genus Rhizosolenia (Bacillariophyceae) remains in sediments is affected by the oxidants used in the digestion procedures. The Rhizosolenia counts decrease from 30% H₂O₂>50% HNO₃>50% chromic acid>100% HNO₃>100% H₂SO₄. Rhizosolenia is mostly found in Ontario lakes with summer average pH ranging from 5.6 to 8.3, Gran alkalinity from 0.2 to 120 mg L^–1 as CaCO₃ and water colour from 4 to 105 Hazen units. Large Rhizosolenia populations are generally found in clear lakes, neutral pH and low alkalinity.     

An expanded weighted-averaging model for inferring past total phosphorus concentrations from diatom assemblages in eutrophic British Columbia (Canada) lakes

Eighteen lakes were added to a published training set of 46 British Columbia (BC) lakes in order to expand the original range of total phosphorus (TP) concentrations. Canonical correspondence analysis (CCA) was used to analyze the relationship between diatom assemblages and environmental variables. Specific conductivity and [TP] each explained significant (P0.05) directions of variance in the distribution of the diatoms. The relationship between diatom assemblages and [TP] was sufficiently strong to warrant the development of a weighted-averaging (WA) regression and calibration model that can be used to infer past trophic status from fossil diatom assemblages.The relationship between observed and inferred [TP] was not improved by the addition of more eutrophic lakes, however the [TP] range and the number of taxa used in the transfer function are now superior to the original model. Diatom species assemblages changed very little in lakes with TP concentrations greater than 85 µg 1^–1, so we document the development of a model containing lakes with TP85 µg 1^–1. The updated model uses 59 training lakes and covers a range of species optima from 6 to 41.9 µg 1^–1 TP, and a total of 150 diatom taxa.The updated inference model provided a more realistic reconstruction of the anthropogenic history of a highly eutrophic BC lake. The model can now be used to infer past nutrient conditions in other BC lakes in order to assess changes in trophic status.     

Framing realistic river rehabilitation targets in light of altered sediment supply and transport relationships: lessons from East Gippsland, Australia

Since European settlement, the Cann River in East Gippsland, Victoria has experienced a 700% increase in channel capacity, a 150-fold increase in the rate of lateral channel migration, a 45-fold increase in bankfull discharge and a 860-fold increase in annual sediment load. Over the last century, and primarily the last 40 years, channel incision has removed the equivalent of around 1500 years of floodplain deposition. A numerical floodplain evolution model is presented which suggests that under a best case scenario, infilling the incised channel trench will take 31,000 years and this is predicated on the full recovery of the immediate riparian vegetation and the in-channel loading of woody debris. The asymmetry in the recovery time following rapid channel change, compared with the original deposition of the material, is explained by a combination of the sediment-starved character of the catchment and the altered hydraulic conditions within the channel, principally associated with the role of woody debris. These factors have major implications for geomorphic recovery potential, constraining what can be realistically achieved in river rehabilitation.     

Diatom taphonomy and silica cycling in two freshwater lakes and their implications for inferring past lake productivity

Diatom preservation can be a major taphonomic issue in many lakes but is often unrecognised and its impacts on qualitative and quantitative inferences (such as productivity and biodiversity estimates) from sedimentary archives are seldom explored. Here two palaeolimnological case studies of 20th-century anthropogenic eutrophication of freshwater lakes in Northern Ireland (Lough Neagh and Lough Augher) are re-visited and new data presented on diatom preservation. Assessing problems of taphonomy challenges previous interpretations of silica dynamics and diatom productivity at these sites. Diatom preservation was assessed in both sediment trap material and sediment cores from Lough Neagh, and in sediment cores from Lough Augher. Preservation data, combined with geochemical analysis (Si, Fe), provide an insight into silica cycling and diatom accumulation over a range of temporal scales from these lakes. Diatom preservation was generally good for the Lough Neagh material, although differential (better) preservation of the smaller Aulacoseira subarctica compared to the larger Stephanodiscus neoastraea sensu lato valves was clear, especially in sediments. Porewater silica showed a complex seasonal pattern in the upper sediment, against expectations of steady-state. The Lough Augher material was generally poorly preserved, although preservation (dissolution) was significantly (and positively) correlated to bulk sedimentation rate, and was found to be a major control on (net) diatom accumulation rate across the basin. Past seasonal and severe anoxia at Lough Augher did not improve diatom preservation, contrary to some previous studies, which may be due to extreme changes in sedimentary redox conditions. Finally, using published experimental relationships between dissolution and diatom valve loss, correction factors were applied to previously published profiles of diatom accumulation over the last ~150 years (biovolume from Lough Neagh and frustule accumulation rate from Lough Augher), which suggest that diatom productivity estimates from sedimentary records are underestimated by a factor of 2–4 due to dissolution effects alone. The results clearly have implications for the reliability and accuracy of diatom-based inferences made from sediment records, both qualitative and quantitative, especially for those that employ diatoms as direct measures of productivity or biodiversity.     

Distribution of diatom assemblages and their relationship to environmental variables in the surface sediments of three northern Egyptian lakes

Surficial sediments of three northern Egyptian lakes (Manzala, Burullus and Edku) show differences in diatom assemblages deposited in different sites of these lakes. A total of 172 species and varieties belonging to 58 genera were identified and counted from 62 samples. Of these, 163 diatom taxa were recorded from Manzala Lake sediments, 147 taxa were found in Burullus Lake sediments, and 117 taxa were identified in Edku Lake sediments. The considerable variation in the composition and distribution of the diatom assemblages among these lakes was mainly related to differences in the water quality, salinity, the concentration of nutrients and climatic changes. The planktonic diatom Cyclotella meneghiniana was dominant in the majority of the samples from Manzala Lake, but dominant in only a few samples from the middle parts of Burullus and Edku lakes. The non-planktonic epiphytic taxa Cocconeis placentula and Epithemia sorex were the subdominant species in the surface sediments, especially in shallow and marginal parts of the lakes. Multivariate statistical techniques (hierarchical ascending clustering and canonical correspondence analysis) were used to identify ecological groups of diatoms and to investigate which environmental variables were important in explaining the variation between these groups. Eight ecological groups containing distinctive diatom assemblages reflect current environmental conditions; especially saltwater intrusion in the north and nutrient-rich freshwater in the south.