Diatom assemblages in sediments of Lake Juusa, Southern Estonia with an assessment of their habitat

Diatom analysis of surface sediments and two sediment cores from different sedimentation areas of a small closed lake was undertaken with the aim of acquiring knowledge on the dependence of the distribution of diatom assemblages on lake bathymetry. Lake Juusa was selected for the study because we have for this lake a large data set about the lithological composition of sediments and macrofossil and cladoceran records for the Holocene. A high carbonate content (20–60%) in the sediment sequence indicates high carbonacity and relatively stable pH values during the Holocene. On the basis of comprehensive analysis, abrupt water-level fluctuations and changes in the trophic status were established. Results of this study showed that the fluctuations of the water-level were the leading factor determining the habitats of diatom assemblages in the lake. In the surface sediment samples planktonic species such as Cyclotella spp., Stephanodiscus spp. and Aulacoseira spp. had a depth optimum at 3–4 m and the most abundant periphytic taxa were distributed mostly at depths shallower than 3.5 m. The same regularity was established in sediment cores where a good correlation between planktonic species and lake water depth was found in sediments accumulated at water depths >4 m. Lake Juusa appears to be a proper site for detailed environmental reconstructions over the Holocene, and the results will give us a good opportunity to analyse the history of water-level fluctuations in other small Estonian lakes. Electronic Supplementary Material The online version of this article (doi:) contains supplementary material, which is available to authorized users. Handling editor: K. Martens     

Climate drivers of diatom distribution in shallow subarctic lakes

相似文献   

Sedimentary diatom concentrations and accumulation rates as predictors of lake trophic state

Diatom concentrations in surface sediments are positively correlated with limnetic chlorophyll a concentrations in Florida (USA) lakes. Using this relationship, I examine models that provide quantitative inferences for trophic state in historical applications.The best model predicts chlorophyll a trophic state index (TSI) values from log-transformed diatom concentrations and explains approximately half the variance in the dependent variable. Diatom accumulation rates are not better than sedimentary diatom concentrations as predictors of TSI. The entire diatom assemblage is as sensitive an indicator of TSI as are the planktonic diatoms alone. A model that considers the ecological preferences of specific taxa was found to be a better predictor than the model based on total diatom concentration.The sedimentary diatom concentration model provides a useful method for assessing historical changes in primary productivity, except in lakes where factors (e.g., silica limitation, blue-green bacterial inhibition) limited diatom production, or post-depositional changes removed sedimentary diatoms. TSI inferences are presented for sediment cores from two Florida lakes, one of which demonstrates a problematic application, and the other of which does not.     

Monitoring lake recovery from point-source eutrophication: the use of diatom-inferred epilimnetic total phosphorus and sediment chemistry

1. Diatom and geochemical responses to reduced nutrient loading were followed in a small, monomictic eutrophic lake in Northern Ireland by use of short sediment cores taken c. 15 years after redirection of creamery waste away from the lake. 2. Epilimnetic total phosphorus (TP) concentrations (fig TP 1-^?1) were estimated for the period 1850–1990 using weighted averaging regression and calibration. Background TP levels, inferred using the diatom model, were c. 35μg TP 1-^?1 and increased to >140μg TP 1-^?1 in the late 1960s to early 1970s. Total P concentrations dropped to 80 μg TP 1-^?1 within 5 years of waste diversion (c. 1978–79), but varied between 1980 and 1990 (range 70–140 μg TP 1-^?1), perhaps due to internal loading, occasional continued disposal from the creamery and natural variations in stream P load. 3. Diatom-inferred TP concentrations were compared with monitored data where available, and the diatom model tended to overestimate TP concentrations by about 25 fig TP 1-^?1. Possible reasons for this are discussed (errors in the diatom model, stratigraphic variability, variability in the monitoring data). 4. Post-1980 geochemistry profiles (concentrations and accumulation rates) indicated some changes when compared with sediments deposited before 1980, perhaps reflecting the redirection of the creamery waste and reduced productivity of the lake (e.g. reduced calcium deposition). Phosphorus concentrations in the sediments changed very little over the last 150 years and, while sedimentary TP fluxes (g cm^?2 yr^?1) increase steadily up-core, they do not record the effluent redirection in the mid-1970s. There is, however, some indication of a slight lowering of P retention in the most recent sediments (1985–90). 5. The general implications of such an approach to monitoring (i.e. the use of short cores) are discussed and the value of diatom-inferred TP assessed. Diatom models offer the possibility of determining background TP concentrations and indicate that, despite the redirection of the creamery waste over 15 years ago, the pre-creamery epilimnetic TP concentrations have not yet been reached.     

Diatoms and their relationship to environmental variables from lakes and ponds on Bathurst Island, Nunavut, Canadian High Arctic

Baseline and historical environmental data are sparse in the High Arctic, however diatom assemblages preserved in high arctic lake and pond sediment profiles can provide proxy data of past environmental changes. Ecological data, however, are still sparse. Diatom taxa preserved in the surficial sediment of lakes and ponds on Bathurst Island (75° 42 N, 97° 21 W), Nunavut, Canadian High Arctic, and their relationship to 34 environmental variables were examined using multivariate statistics. A total of 148 diatom taxa were identified from the surface sediments of 29 study sites. Five environmental variables, Fe³⁺, Total Phosphorus (Unfiltered) (TPU), Total Nitrogen (TN), Temperature (TEMP) and pH significantly (P0.05) accounted for most of the variation in the diatom assemblages. TN was also significantly correlated to other variables (e.g. TPF, DOC, POC). A CCA constrained to TN indicated that this variable best explained the species distributions, and a weighted-averaging (WA) model was developed to infer nutrient levels from the relative abundances of the 58 dominant taxa. Interestingly, previous limnological work indicated that nitrogen most likely limited algal production in some of these high arctic sites. This model may be used to quantitatively estimate past TN levels from diatom assemblages preserved in sediment cores from Bathurst Island, and may provide a means to track past environmental changes in the High Arctic.     

Planktonic diatoms and silica in Loch Leven, Kinross, Scotland: a one month silica budget

Diatom populations and silica concentrations were monitored at frequent intervals in the shallow, eutrophic Loch Leven over a 27-day period (October 1972) and the influences of the inflows, outflow and the sediment were assessed. Changes in dissolved and particulate silica are accounted for by incorporating the results into a silica budget. During this period processes affecting silica within the loch were more important than those outside. The incorporation of diatom frustules into the sediments and the release of dissolved silica from the sediments appeared to be of particular importance. Evidence suggests that dissolution of the frustules of some planktonic diatom species was also important.     

Distribution of diatom surface sediment assemblages within Effingham Inlet, a temperate fjord on the west coast of Vancouver Island (Canada)

Twenty-nine surface sediment samples from Effingham Inlet, a small fjord on the west coast of Vancouver Island, British Columbia, were analyzed for diatoms. This fjord has been selected for paleoceanographic investigation due to the presence of laminated sediments resulting from the dysoxic to anoxic bottom water conditions in the inner and outer basins of the inlet. Distributional patterns of the diatom microflora reflected proximity to littoral regions, phytoplankton production, and marine influence from outside the fjord. Principal components and cluster analyses of the microflora established four diatom assemblages with a clear separation between the inner and outer basin diatom floras. Inner basin stations were characterized by elevated absolute abundance with assemblages dominated by spring–early summer bloom taxa including Skeletonema costatum, Thalassiosira nordenskioeldii, and Thalassiosira pacifica. Chaetoceros spp. resting spores were abundant throughout Effingham Inlet, with the exception of the stations closest to the fjord head. Stations located in the outer basin and towards the fjord mouth had relatively lower absolute abundance yet showed a higher relative and absolute abundance of Thalassionema nitzschioides, Rhizosolenia setigera, Coscinodiscus radiatus, Ditylum brightwellii, Odontella longicruris, and Paralia sulcata in relation to the inner basin. Many of these latter taxa are often associated with late summer and autumn conditions in fjords along coastal British Columbia. Oceanographic data for Effingham Inlet suggest that increased offshore penetration is more likely to occur from summer to early fall, with a more restricted offshore influence in the inner basin. Diatom surface sediment assemblages in Effingham Inlet appear to reflect incursions of offshore waters into the fjord. Absolute abundance estimates and the preservation of lightly silicified taxa suggest excellent preservation of fossil material in the sediments of the predominantly anoxic inner basin. Preservation in the outer basin is reduced, reflecting more frequent recharge by oxygenated waters spilling over the outer sill into the basin. Our findings suggest the inner basin should be an optimal site for reconstruction of diatom production, with records from the outer basin providing more consistent information about offshore influence and coastal upwelling conditions over the Holocene. Estimates of diatom abundance within the inner basin sediment may serve as a good proxy of production, although proxy tracers of bottom water conditions and sedimentological analyses must be coupled to the diatom record to ensure depositional conditions were not influencing valve preservation or abundance. Our results suggest that fjords can serve as good environments for paleoceanographic reconstructions of both inshore and offshore conditions although careful site selection and understanding of processes affecting the microfossil record are essential.     

Comparison of radiolarian and sedimentologic paleoproductivity proxies in the latest Miocene–Recent Benguela Upwelling System

Estimating past ocean productivity from ocean sediments often gives different results depending on the measurement used. We have examined a suite of paleoproductivity proxies in latest Miocene–Recent sediments from DSDP Site 532 and ODP Site 1084, two deep-sea sections underlying the Benguela Upwelling System off the Atlantic coast of southern Africa. The productivity history of this system has been previously established via organic carbon concentration, diatom floras and alkenone based estimates of surface water temperature, and shows a change from low productivity in the early Pliocene to sustain high productivity in the late Pliocene–Recent. Each of our samples was split and simultaneously analysed for several proxies of ocean productivity, including organic carbon (TOC%), carbonate, abundance of opaline radiolarians, accumulation rate of benthic foraminifera (BFAR); the radiolarian faunal composition indices Upwelling Radiolarian Index (URI) and the Water Depth Ecology index (WADE); other proxies for opal and carbonate dissolution, plus stable isotopes of benthic foraminifera. Comparisons between proxies in the same measured samples, between sites in downcore plots and to the published productivity record for this region suggest that TOC and radiolarian faunal composition, particularly the WADE index, are good indicators of past productivity, albeit with different sensitivities (log–linear correlation WADE–TOC% r = 0.78, n = 65, p < 0.01). In contrast, carbonate, and carbonate-based proxies such as BFAR primarily reflect changes in dissolution. Radiolarian faunal composition indices do not appear to be affected by bulk opal accumulation or changes in opal preservation. WADE analysis of radiolarian faunas and TOC% measurements appear to be useful proxies for productivity in late Neogene sediments, particularly for sections where opal or carbonate dissolution is significant.     

The Diatoms,Productivity and Morphometry of 43 Lakes in New York State,U.S.A

Diatoms were examined in surface sediments collected from 43 lakes throughout New York State to determine the interrelationships among the abundances of various diatom taxa,' lake productivity and lake morphometry. Diatom ratios, including the A/C, C/P and five new quotients, were not well correlated with lake productivity, probably because of the diversity of the taxa which they employ. The abundance of individual genera or combinations of genera were better indicators of productivity. Cyclotella was the best single indicator of productivity being most abundant in low productivity lakes. Several fairly distinct diatom associations were identified by correlations between all possible pairs of diatom taxa. Finally, lake productivity was found to be significantly correlated with morphometry, being highest in small/shallow lakes.     

Effects of increased salinity on the diatom assemblage in Fonda Lake,Michigan

A salt storage facility has been located adjacent to Fonda Lake since 1953. In February 1981 a core was taken from the profundal sediments of the lake and analyzed to determine the effects of salt perturbation on the diatom community over a 32-year period. Diatom assemblages from different levels were compared using multivariate techniques including cluster analysis and principal component analysis. Shifts in diatom composition related to salinification were revealed most clearly by subdominant taxa. Five distinct groups of diatom taxa were found to correspond with 5 depth intervals. The diatom component of the lake up to 1960 included two groups of taxa which were alkaliphilous and chloride indifferent. A reduction in species diversity beginning in 1960 may indicate a salt effect. By 1968, when diversity reached a minimum, a variety of halophilic taxa (including Diatoma tenue, Navicula gregaria and Synedra fasciculata) attained their highest relative abundances. At the top of the core, diversity increased slightly and some halophilic taxa decreased in relative abundance, which suggests a possible decrease in salt loading to the lake.     

Diatoms and their preservation in the sediments of Lake Neuchâtel (Switzerland) as evidence of past hydrological changes

Core LNO3 is 1136 cm long and includes uninterrupted sediments from the Oldest Dryas to historic time which have been submitted for geological, geochemical, palynological and diatom analysis. Thirty samples have been investigated for diatoms, focussing especially on their preservation, which changed during major hydrological events. In the lower part of the core (Oldest Dryas to beginning of Atlantic), with low carbonate, but high clastic silicate content, the diatoms are rare but are well preserved. In the upper well carbonated part of the core (Atlantic to historical periods), the diatoms are abundant but show traces of dissolution.Between 1136 and 909 cm (Oldest Dryas) the flora is made up of benthic species principally of the genus Fragilaria. This flora is often the first to develop after the last glaciation. Between 879 and 569 cm (Bølling to beginning of Atlantic), littoral and terrestrial species adapted to eutrophic conditions are dominating. They suggest a strong fluvial impact on sedimentation, which corroborates the hypothesis of past inflows of the River Aar into the lake. Between 529 cm and 1 cm (Atlantic to historical periods) planktonic species of the genera Cyclotella, Aulacoseira or Stephanodiscus are dominating. This suggests that the upper well carbonated sediments were mainly formed by pelagic precipitation, without influence of the River Aar. The trophic level of the lake seems to have decreased, then increased again during prehistoric deforestation and development of agriculture in the watershed.     

Climate-lake interactions recorded in varved sediments from a Swedish boreal forest lake

A 1100-year long record of lake ecosystem response to climate and catchment change with precise chronological control is reported. Diatom and pollen assemblages of an annually laminated (varved) sediment from a northern Swedish lake (Kassjön, Våsterbotten) were used as records of lake diatom communities and catchment vegetation. These data were compared with summer temperature estimates based on tree-ring records of the same geographical area to identify the effects of climate change and catchment disturbance on diatom assemblages in the lake. In a canonical ordination, 23% of the variability in the total diatom assemblages for the period AD1040–1804 was accounted for by changes in pollen data which reflect agricultural development in the catchment. Diatom species richness, however, exhibited a stronger relationship with summer temperature and, significantly, declined with the lower temperatures associated with the Little Ice Age minimum (early 17th century). Summer temperature accounted for 23% of the variability in diatom species richness 20 years later. The mechanism behind this time-lag is unclear, but may be related to catchment-mediated effects, given recent evidence for lags in the response of boreal-forest vegetation regeneration cycles to climatic variability. These results suggest that climate-related effects on lakes occurring over medium timescales can be resolved in lake sediments. Moreover, it is possible to identify these effects despite cultural-related signals, but as the latter become more extreme in the late 20th century the climate signal is obscured.     

Post-sampling dissolution and the consistency of nannofossil diversity measures: A case study from freshly cored sediments of coastal Tanzania

This study investigates the potentially deleterious effect of prolonged sample storage on the reliability of calcareous nannofossil species abundance and community diversity measures and their subsequent interpretation as paleoceanographic proxies. Nannofossil assemblages from two freshly cored successions of hemipelagic clay were documented over a one-year period using a series of paired smear-slides, the first made from sediment samples within a day of coring and the second either 6 months or 1 year later. Diversity measured by Shannon's H is consistent between the two sets for the majority of samples, however significant changes, both positive and negative, were recorded in a number of cases and appear to be largely due to variations in smear-slide thickness and not to post-sampling dissolution. There is no significant change in nannofossil fragmentation or abundance over time, although the relative abundances of small, dissolution susceptible placoliths of Reticulofenestra minuta do appear to decrease with time in some samples. Counts of mineral grains show that there is a significant loss of fine, 1–2 μm, cubic pyrite within months of drilling, and this, combined with the decline in abundance of R. minuta, suggests that limited pyrite oxidation coupled to carbonate dissolution is occurring. Geochemical analyses confirm that these sediments contain significant concentrations of sulphate one year after sample recovery, which may be the result of gypsum formation associated with the oxidation of pyrite. Estimates of pyrite oxidation suggest that up to 3% of the original calcium carbonate has dissolved in the year after coring, which could account for the observed loss of small placoliths. Effects of this kind are almost certainly confined to hemipelagic sequences rich in organic matter and/or reduced iron but are frequently offset by the exceptional preservation of calcareous microfossils that can occur within such sediments.     

Problems with the application of diatom-total phosphorus transfer functions: examples from a shallow English lake

1. A diatom‐total phosphorus transfer function has been applied to a sedimentary diatom sequence from Groby Pool, a small shallow lake in Leicestershire, U.K. 2. Extensive aquatic plant records exist for Groby Pool dating back over two centuries. These records, in conjunction with selected aquatic pollen and herbarium diatom data, provide independent, qualitative evidence for the progression of eutrophication and its effects on aquatic plant communities and habitat structure. 3. Before 1800, Groby Pool was probably mesotrophic with clear water and a diverse submerged macrophyte community, but subsequently it experienced considerable nutrient enrichment. Key evidence for this includes: (i) historical plant records indicating the loss of species associated with mesotrophic waters and their replacement by others typical of eutrophic conditions, (ii) a significant increase in the percentage of planktonic diatoms in the lake sediment record (particularly Cyclostephanos dubius) after 1890, and (iii) increases in percentages of Stephanodiscus parvus and Cocconeis placentula in the second half of the twentieth century. 4. Diatom‐inferred total phosphorus (DI‐TP) estimates were inconsistent with the qualitative evidence for eutrophication at Groby Pool. In particular the DI‐TP profile was thought to overestimate phosphorus during the period of dominance by small Fragilaria spp. before 1890, and to misjudge the timing and direction of subsequent changes in nutrient loading. 5. This study highlights some of the problems associated with the application of diatom‐TP transfer functions to sedimentary diatom sequences from shallow lakes. The major problem relates to the frequent dominance of non‐planktonic diatoms in the sediments of these systems, many species of which (particularly small Fragilaria spp.) appear to be more sensitive to changes in habitat availability than to phosphorus. Potential ways of improving diatom‐TP models via altered approaches to sampling are suggested.     

Geochemical environments of continental shelf-upper slope sediments in the northern Gulf of Mexico

Geochemical environments were characterized for 14 sites along the northern Gulf of Mexico continental shelf and upper slope, in an effort to examine the relationship between sediment geochemistry and carbonate shell taphonomy in a long-term study—Shelf and Slope Experimental Taphonomy Initiative (SSETI). Three groups of environments of preservation (seep, near-seep, and shelf-and-slope) were identified based on their geochemical characteristics (i.e., oxygen uptake rate and penetration depth, pore-water saturation states, and carbonate dissolution fluxes). Diffusive oxygen uptake rate increased in the order of shelf-and-slope, near-seep, and seep, although carbonate dissolution flux did not show significant correlation with O₂ flux, presumably due to non-diffusive behavior at some sites. Using pore-water saturation indices with respect to aragonite and calcite and sedimentation rates, we defined a semi-quantitative parameter, carbonate dissolution index (CDI), to predict carbonate preservation potential during the taphonomic processes. Our limited database suggests that both the seep and the shelf-and-slope sediments may have higher carbonate preservation potential than the near-seep sediments.     

Feeding effects of the keystone deposit feeder Ilyanassa obsoleta (Neogastropoda,Gastropoda) on sedimentary diatoms

Microphytobenthos (MPB), typically comprised mainly of diatoms, is a key contributor to nearshore energy flow and nutrient cycles. Deposit-feeding invertebrates are known to alter the structure and activity of MPB. The eastern mud snail Ilyanassa obsoleta can reach extremely high densities in estuaries of the northwestern Atlantic, and their deposit-feeding and locomotion strongly influence other invertebrates and microbes. Our objective was to explore quantitative and qualitative effects of this keystone deposit-feeder on diatoms of intertidal sediments. We gathered snails from mudflat and sandflat habitats and collected their fresh fecal pellets in the laboratory. DNA metabarcoding allowed us to characterize diatom assemblages of ingested sediments and feces. We noted selective feeding such that reduction in MPB biomass with gut passage was difficult to quantify. Diatom α-diversity was reduced with gut passage in snails from both sedimentary regimes. Mudflat and sandflat diatom assemblages were distinct and differed markedly between feces and sediment in mud-feeding snails, whereas the difference in sand-feeding snails was minor. The sandy habitat was dominated by a mix of epipelic and epipsammic diatoms. In contrast, mudflat samples were dominated by epipelic and planktonic diatoms. Compositional differences between sediment and feces reflected preferential removal of planktonic taxa. Our results suggest the importance of phytodetritus to the mud snail diet, particularly in hydrodynamically quiescent environments. Due to the natural spatial patchiness of the snails and the capacity for rapid microbial recolonization, field experiments are recommended to determine whether MPB community changes attributed to gut passage are manifested at the landscape scale.     

Diatom succession trends in recent sediments from Lake Baikal and their relation to atmospheric pollution and to climate change

Recent environmental change in Lake Baikal has been attributed to anthropogenic influences on the ecosystem, especially through pollution and cultural eutrophication. These hypotheses are tested in this paper principally by diatom analyses in 20 short sediment cores. Most of the cores were collected with a new type of box corer specifically designed for use on Lake Baikal. Most cores contain a good sediment record but turbidites occur in some sediment profiles which may be best recognized using a combination of techniques, such as radiometric dating and percentage dry weight analyses. The most recent sediments, especially those in the southern basin and in the very north of Baikal, contain a record of anthropogenic contamination in the form of lead and spheroidal carbonaceous particles, which confirms that the southern basin of Baikal is most affected by atmospheric sources of pollution. However, there is no sedimentary diatom evidence indicating offshore water quality deterioration in Baikal owing to air pollution or eutrophication. Small increases in diatoms which indicate nutrient enrichment (e.g. Stephanodiscus minutulus, Synedra acus v. radians and Synedra acus v. acus) may reflect local eutrophication of the shallow waters close to the Selenga Delta and certain coastal sites in the southern basin near to the Baikalsk paper and pulp mill. By using numerical techniques, Lake Baikal can be split into at least four regions on the basis of its surface sediment flora: the south, middle and north basins, and the shallow waters surrounding the Selenga Delta region. Diatom analyses reveal that the endemic flora of Lake Baikal has been constantly changing over at least the last 2000 years and that these fluctuations are probably responses to natural climatic variability. Recent sediments of Baikal may be affected by taphanomic processes (e.g. dissolution) and turbidite deposition, and these must be taken into account when interpreting the sedimentary diatom record. The diatom flora of the lake is currently dominated by several species, such as Aulacoseira baicalensis, A. islandica, Cyclotella minuta and Stephanodiscus binderanus v. baicalensis. All these species, except for C. minuta, have become more common in the lake in approximately the last 130 years, and we hypothesize that these changes may be attributed to a number of different processes linked to an ameliorating climate after the end of the Little Ice Age. The results presented here have important implications for this recently designated World Heritage Site, with regard to future pollution controls and catchment management policies.     

Increase in silicon concentrations and release from suspended solids and bottom sediments in Lake Kasumigaura, Japan

Increasing trends of dissolved Si measured by a colorimetric method and ICP (DSi_col and DSi_ICP, respectively) and total Si concentrations were detected at the center of Lake Kasumigaura during 1980–2006 (mean DSi_col concentration in the 1980s and 2000s was 1.3 and 4.0 mg l⁻¹, respectively). The observation of such trends is rare; therefore, the elucidation of the causes could be useful to understanding silicon dynamics in inland waters. Based on statistical analysis, we found that the increases in DSi_col and lithogenic Si accounted for most of the total Si increase (44 and 45%, respectively) and that biogenic Si, consisting of diatom frustules, also increased with them. Increases in DSi_ICP concentration were not detected near the mouth of the inflowing rivers, suggesting that the increase was caused by in-lake processes. Because the increase in suspended solids (SS) caused by sediment resuspension had been observed in the lake for the same period, we assumed that the Si release from SS containing diatom frustules contributed to the increase. The results of the laboratory experiments in which surface sediments were stirred in lake waters showed that the change in DSi_col concentration depended mainly on SS concentration, water temperature and the elapsed time of diatom frustules dissolution. An estimation of the released amount of Si from SS using the sediment resuspension model was (1.0–2.7) × 10⁹ g year⁻¹ in the 2000s, which was about 30–90% of the increase in the DSi_col outflow of 3.0 × 10⁹ g year⁻¹ from the 1980s to the 2000s. We also determined the Si release rates from bottom sediments through laboratory experiments. The Si amount released from bottom sediments in the lake in the 2000s was estimated to be 4.3 × 10⁹ g year⁻¹, which was about 2–4 times higher than the estimated Si amount released from SS. These findings suggest that the sediment resuspension might be the cause of the latest DSi increase.     

Seasonal and microhabitat influences on diatom assemblages and their representation in sediment traps and surface sediments from adjacent High Arctic lakes: Cape Bounty,Melville Island,Nunavut

The spatial (i.e. microhabitat) and temporal (i.e. seasonal) characteristics of diatom assemblages in adjacent High Arctic lakes were studied intensively June–August 2004. These baseline data are used to improve understanding of modern diatom community dynamics, as well to inform paleoenvironmental reconstructions. Diatoms were collected approximately weekly through the melt season from each principal benthic substrate (moss/macrophyte, rock scrapes, littoral sediment), plankton, and sediment traps, and were compared to the uppermost 0.5 cm of a surface core obtained from the deepest part of the lake where sediment cores are routinely collected. Water samples were collected concurrently with diatom samples to investigate species–environment relationships. The lakes share approximately half of their common taxa, the most abundant overall in both lakes being small Cyclotella species. Results of detrended correspondence analysis (DCA) indicate that the largest gradient in species turnover existed between benthic and planktonic communities in both lakes, and that sediment trap and the surface core top samples most closely resemble the planktonic assemblage, with an additional contribution from the lotic environment. Our results indicate clear micro-spatial controls on species assemblages and a degree of disconnection between the benthos and deep lake sediments that manifests as an under-representation of benthic taxa in deep lake surface sediments. These findings are particularly relevant in the context of interpreting the paleoenvironmental record and assessing ecosystem sensitivity to continued climate change.     

A palaeolimnological record of human disturbance from Harvey's Lake, Vermont: geochemistry, pigments and diatoms

• 1 Stratigraphic analyses of inorganic geochemistry, pigments and fossil diatoms in a 0.7 m core of profundal sediments are used to reconstruct the limnological history of Harvey's Lake, Vermont, over the last 1000 years. The lake is moderately productive, deep (44 m) and clear, and the phytoplankton today is dominated by the blue-green alga, Oscillatoria rubescens. Sedimentary pigments unique to blue-green algae, oscillaxanthin and myxoxanthophyll, provide a detailed history of changes in the O. rubescens population. Accurate sediment chronology is derived from ²¹⁰Pb, ¹³⁷Cs and ¹⁴C dating and from the stratigraphy of pollen and sawmill wastes.
• 2 Primary production increased in Harvey's Lake in 1780 following European settlement and again after 1945, as shown by greater accumulation of sedimentary pigments and diatom frustules, and changes in fossil algal assemblages. Blue-green algae first appeared in abundance about 1945, indicating nutrient enrichment from dairy wastes and shoreline development. Increased deposition of elements associated with classic minerals also suggests greater soil erosion during both of these intervals.
• 3 Two episodes of increased sedimentary anoxia (1820–1920 and 1945–present) are marked in the sedimentary record by enhanced pigment preservation, changes in authigenic Fe and Mn stratigraphy,’and the development of laminated sediments. The earlier episode of oxygens depletion is correlated with the discharge of sawmill wastes into the lake, and the later episode is associated with increased primary production.
• 4 Based on these data a new model for Fe and Mn sediment stratigraphy is proposed for lakes that do not undergo complete hypolimnetic anoxia.
• 5 Fine-scale resolution of recent diatom and oscillaxanthin stratigraphy provides historical evidence for a long-term negative interaction between diatom and blue-green algal populations in Harvey's Lake.