Changes In Diatom Assemblages Research Articles

Recovery of UK lakes from acidification: An assessment using combined palaeoecological and contemporary diatom assemblage data

We assess the recovery of UK lakes from acidification using the combined data from sediment cores and sediment traps to track changes in diatom assemblages in 11 UK upland lakes from pre-acidification times (prior to ca. 1850 AD) to the present (2008 AD). We projected the data into a Principal Component Analysis (PCA) of diatom assemblage data from 121 low-alkalinity lakes in the UK to show how the floristic composition of the core and trap diatom assemblages for each site has changed through time. The results show that the degree of recovery from acidification varies amongst sites but in all cases its extent is limited when compared with the pre-acidification reference. In most cases the recovery, although usually slight, is characterised by a decline in acid tolerant taxa and a return towards taxa that occurred previously at each respective site. In a few cases, however, the floristic composition of recent samples is different from those that occurred during and before the acidification phase. The reasons for this are not yet clear but it is possible that nutrient enrichment from atmospheric N deposition and/or climate change is beginning to play a role in driving water quality as acidity decreases. More generally the results show that annually recovered samples from sediment traps can be successfully combined with sediment core data to provide a continuous record of environmental change in lake systems, and that diatoms collected in sediment traps can be used to provide a very powerful lake monitoring tool.

Gradual, catastrophic and human induced environmental changes from a coastal lake, southern New Zealand

Cores obtained from a coastal freshwater lake (Lake Waihola, South Island, New Zealand) show a >4000year history of environmental changes driven by significant variations in marine influence and anthropogenic modification of the catchment and minor contributions such as possible palaeotsunami inundation, all taking place against a background of sea level and landscape change. A marked shift from estuarine conditions (salinity >14ppt) to a predominantly freshwater state occurred around the mid-Holocene. This shift is recorded as a distinct change in diatom assemblages and by the disappearance of the estuarine bivalve Austrovenus stutchburyi from the sedimentary record. More recently, a marked change in sediment characteristics and changes in the pollen record correlate with rapid anthropogenic land clearance and conversion of the catchment to intensive agriculture in the mid 1800's. The mean rate of sediment accumulation between c. 4000yr BP and the start of major European settlement (c. AD1860) were around 0.18±0.01mmyr−1. From c. AD1860 onwards, this has increased markedly by about 30 fold following major land use changes in the catchment. Based on current predictions of sea level rise and sediment accumulation, various future sea level rise scenarios will more than compensate for infilling, resulting in the persistence of this aquatic system, which is expected to revert to an estuarine state.

A short-term climate oscillation during the Holsteinian interglacial (MIS 11c): An analogy to the 8.2 ka climatic event?

To gain insights into the mechanisms of abrupt climate change within interglacials, we have examined the characteristics and spatial extent of a prominent, climatically induced vegetation setback during the Holsteinian interglacial (Marine Isotope Stage 11c). Based on analyses of pollen and varves of lake sediments from Dethlingen (northern Germany), this climatic oscillation, here termed the “Older Holsteinian Oscillation” (OHO), lasted 220years. It can be subdivided into a 90-year-long decline of temperate tree taxa associated with an expansion of Pinus and herbs, and a 130-year-long recovery phase marked by the expansion of Betula and Alnus, and the subsequent recovery of temperate trees. The climate-induced nature of the OHO is corroborated by changes in diatom assemblages and δ18O measured on biogenic silica indicating an impact on the aquatic ecosystem of the Dethlingen paleolake. The OHO is widely documented in pollen records from Europe north of 50° latitude and is characterized by boreal climate conditions with cold winters from the British Isles to Poland, with a gradient of decreasing temperature and moisture availability, and increased continentality towards eastern Europe. This pattern points to a weakened influence of the westerlies and/or a stronger influence of the Siberian High. A comparison of the OHO with the 8.2ka event of the Holocene reveals close similarities regarding the imprint on terrestrial ecosystems and the interglacial boundary conditions. Hence, in analogy to the 8.2ka event, a transient, meltwater-induced slowdown of the North Atlantic Deep Water formation appears as a plausible trigger mechanism for the OHO. If correct, meltwater release into the North Atlantic may be a more common agent of abrupt climate change during interglacials than previously thought. We conclude that meltwater-induced climate setbacks during interglacials preferentially occurred when low rates of summer insolation increase during the preceding terminations facilitated the persistence of large-scale continental ice-sheets well into the interglacials.

A marine diatom record from the Amundsen Sea — Insights into oceanographic and climatic response to the Mid-Pleistocene Transition in the West Antarctic sector of the Southern Ocean

The goal of this study is to assess the changes that have occurred during the Mid-Pleistocene Transition, an important transitional period in cryosphere evolution, by examining the siliceous microfossil record of sediments collected proximal to a major ice drainage outlet for the West Antarctic Ice Sheet. Core PS58/254 was collected from a sediment drift on the upper continental rise in the Amundsen Sea, directly offshore from Pine Island Bay, one of the three main discharge areas for the West Antarctic Ice Sheet (WAIS). Published data on physical properties, geochemical composition, grain size and clay mineral assemblages are complemented here by a high-resolution record (sample spacing 10 cm) of the siliceous microfossil assemblages (diatoms and silicoflagellates). Between 1200 ka and 621 ka, the assemblage is relatively diverse, with Actinocyclus ingens, Thalassiothrix antarctica and Fragilariopsis kerguelensis dominating the assemblages, but diatom abundance is variable from low to barren. Additionally, the occurrence of A. ingens, Thalassiosira elliptipora and Thalassiosira fasciculata is used to confirm and further refine the existing age model and extend it back to 1200 ka. Species composition during the last ca. 621 ka is dominated by F. kerguelensis, which consistently comprises 80-90% of the assemblage. A clear relationship between diatom abundance and glacial/interglacial variability is apparent after 621 ka, which resembles the glacial-interglacial variability previously observed in other proxy data. A significant change in both sediment composition and diatom assemblages is observed at 621 ka. This change concurs with the last abundant occurrence of A. ingens and the end of the Mid-Pleistocene Transition (MPT, i.e. the onset of modern eccentricity/precession-paced glacial cycles around 650 ka). We suggest that during interglacial periods after 621 ka the Amundsen Sea Low pressure system shifted seasonally southwestwards towards the shelf and thereby increased the advection of relatively warm Circumpolar Deep Water (CDW) onto the Amundsen Sea shelf, which is a major factor for present ice-sheet melting in this part of West Antarctica.

兵庫県播磨灘北部沿岸域における過去数百年間の珪藻群集と海洋環境の変化

兵庫県赤穂市坂越湾と相生市沖で採取した柱状試料を用いて，過去約数百年間の珪藻群集および堆積物の全有機炭素（TOC）・全窒素（TN）濃度の時空間的変化を明らかにした．さらに，珪藻群集と最近の人為汚染・改変との関連性，および1980年代以降の環境保全対策が珪藻群集に与えた影響に関して検討した．各種分析を行った結果によると，播磨灘北部沿岸域では1930年代以降に水域の富栄養化が始まり，その初期段階には珪藻群集が増加しはじめ，1960年代の高度経済成長期において水域の富栄養化がさらに進行した．1980年代以降，坂越湾と相生市沖では，異なる環境変遷をたどったことが示唆され，1990年代以降に坂越湾ではマガキ養殖の影響，相生市沖では播磨灘北部の水質改善の影響を受けたと考えられる．

珪藻化石群集に基づく最終氷期極大期以降の南大洋における海氷と基礎生産力の変動史

珪藻化石群集に基づく最終氷期極大期以降の南大洋における海氷と基礎生産力の変動史

Response of diatom assemblages to 130 years of environmental change in Florida Bay (USA)

Coastal ecosystems around the world are constantly changing in response to interacting shifts in climate and land and water use by expanding human populations. The development of agricultural and urban areas in South Florida significantly modified its hydrologic regime and influenced rates of environmental change in wetlands and adjacent estuaries. This study describes changes in diatom species composition through time from four sediment cores collected across Florida Bay, for the purposes of detecting periods of major shifts in assemblage structure and identifying major drivers of those changes. We examined the magnitude of diatom assemblage change in consecutive 2-cm samples of the 210Pb-dated cores, producing a record of the past ~130 years. Average assemblage dissimilarity among successive core samples was ~30%, while larger inter-sample and persistent differences suggest perturbations or directional shifts. The earliest significant compositional changes occurred in the late 1800s at Russell Bank, Bob Allen Bank and Ninemile Bank in the central and southwestern Bay, and in the early 1900s at Trout Cove in the northeast. These changes coincided with the initial westward redirection of water from Lake Okeechobee between 1881 and 1894, construction of several canals between 1910 and 1915, and building the Florida Overseas Railroad between 1906 and 1916. Later significant assemblage restructurings occurred in the northeastern and central Bay in the late 1950s, early 1960s and early 1970s, and in the southwestern Bay in the 1980s. These changes coincide with climate cycles driving increased hurricane frequency in the 1960s, followed by a prolonged dry period in the 1970s to late 1980s that exacerbated the effects of drainage operations in the Everglades interior. Changes in the diatom assemblage structure at Trout Cove and Ninemile Bank in the 1980s correspond to documented eutrophication and a large seagrass die-off. A gradual decrease in the abundance of freshwater to brackish water taxa in the cores over ~130 years implies that freshwater deliveries to Florida Bay were much greater prior to major developments on the mainland. Salinity, which was quantitatively reconstructed at these sites, had the greatest effect on diatom communities in Florida Bay, but other factors—often short-lived, natural and anthropogenic in nature—also played important roles in that process. Studying the changes in subfossil diatom communities over time revealed important environmental information that would have been undetected if reconstructing only one water quality variable.

Two hundred years of environmental change in Picos de Europa National Park inferred from sediments of Lago Enol, northern Iberia

We present a study of two short sediment cores recovered from Lago Enol, in the Picos de Europa National Park, Cantabrian Mountains, northern Iberia. We inferred past climate conditions and anthropogenic impacts using geochemical and biological (pollen and diatoms) variables in the dated sequences, in conjunction with temperature and precipitation data collected since 1871 at meteorological stations in the region. The record provides evidence of environmental changes during the last 200 years. At the end of the Little Ice Age (~1800–1875 AD) the region was characterized by an open landscape. Long-term use of the area for mixed livestock grazing in the mountains, and cultivation of rye during the nineteenth century, contributed to the expansion of grassland at the expense of forest. Warmer temperatures since the end of the nineteenth century are inferred from a change in diatom assemblages and development of the local forest. Socioeconomic transformation during the twentieth century, such as livestock changes related to dairy specialization, planting of non-native trees, mining activities, and management of the national park since its creation in 1918, caused profound changes in the catchment and in the lake ecology. The last several decades (~1970–2007 AD) of the Lago Enol sediment record are strikingly different from previous periods, indicating lower runoff and increasing lake productivity, particularly since AD 2000. Today, the large number of tourists who visit the area cause substantial impacts on this ecosystem.

Holocene migration of oceanic front systems over the Conrad Rise in the Indian Sector of the Southern Ocean

AbstractThere has been limited previous research about Holocene climate variability in the Indian Sector of the Southern Ocean. Here we examine centennial‐scale changes in diatom assemblages and stable isotopic ratios since 10 000 cal a BP in a high‐accumulation‐rate sediment core from the Conrad Rise. Although abundances of dominant diatom taxa (Fragilariopsis kerguelensis and Thalassiothrix antarctica) are comparatively constant, relative abundances of secondary taxa fluctuate. Before c. 9900 cal a BP, winter sea‐ice and cold water covered the Conrad Rise. Following deglaciation the sea‐ice retreated from the Conrad Rise, lagging that of the Atlantic and eastern Indian Sectors by about 1500 a. The Polar Front moved southward during the early Holocene optimum and north Antarctic Zone waters covered the Conrad Rise for about 650 a. After 9300 cal a BP, solar insolation strongly influenced sea surface temperature and primary productivity in the Southern Ocean. In the high‐latitude Indian Sector, productivity increased 1500 a after the onset of late Holocene neoglaciation. Periodic δ18O and cold‐water diatom taxa spikes (at intervals of 200 and 300–500 a, respectively) occurred after 9300 cal a BP, probably associated with solar activity. Fluctuations in short‐term sea surface temperature and cold‐water taxa are synchronous with changes in δD observed in an east Antarctic ice core. Copyright © 2011 John Wiley & Sons, Ltd.

A diatom record of environmental change in Fryxell Basin, Taylor Valley, Antarctica, late Pleistocene to present

We present a diatom record from a sediment core taken in Lake Fryxell, Taylor Valley, Antarctica. Six zones were defined using diatom assemblage changes that indicate varying limnological conditions. The early lake stage, ca. 35,000 cal years BP, is characterized by Mayamea atomus f. permitis, a species rarely reported in modern Antarctic Dry Valley environments. An extended period from ca. 35,000 to 19,000 cal years BP is characterized by low diatom abundance, with dominant taxa Luticola spp., Muelleria spp., and Diadesmis contenta. The modern assemblage was established ca. 13,000 cal years BP, after two relatively brief transitional stages. One key species for this recent period, Navicula lineola var. perlepida, is absent in surface sediments and the modern environment, indicating an environmental change within the last several centuries. The diatom assemblage is compared to modern diatom communities in Dry Valley streams, which provide the most complete information on diatom distributions in this region. Although precise environmental interpretation of the core is hampered by limited knowledge of environmental constraints on many of the diatom taxa present in the lake core, the data provide important new insights into the history of Glacial Lake Washburn.

Correlation of Early Pliocene diatomite to low amplitude Milankovitch cycles in the ANDRILL AND-1B drill core

Abstract In the austral summer of 2006/7 the ANDRILL MIS (ANtarctic geological DRILLing-McMurdo Ice Shelf) project recovered a 1285 m sediment core from beneath the Ross Ice Shelf near Ross Island, Antarctica in a flexural moat associated with volcanic loading. The upper ~ 600 m of this core contain sediments recording 38 glacial/interglacial cycles of Early Pliocene to Pleistocene time, including 13 discrete diatomite units (DUs). The longest of these, DU XI, is ~ 76 m-thick, and has been assigned an Early to Mid-Pliocene age (5–3 Ma). A detailed record of the siliceous microfossil assemblages in DU XI is used in conjunction with geochemical and sedimentological data to subdivide DU XI into four discrete subunits of continuous sedimentation. Within each subunit, changes in diatom assemblages have been correlated with the δ18O record, providing a temporal resolution up to 600 yr, and allowing for the construction of a detailed age model and calculation of associated sediment accumulation rates within DU XI. Results indicate a productivity-dominated sedimentary record with greater proportions of hemipelagic mud accumulating during relatively cool periods. This implies that even during periods of substantial warmth, Milankovitch-paced changes in Antarctic ice volume can be linked to ecological changes recorded in diatom assemblages.

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.

Paleolimnological evidence of the effects of recent cultural eutrophication during the last 200 years in Lake Malawi, East Africa

To assess the long-term water quality changes in Lake Malawi, paleolimnological reconstructions of four radiometrically dated sediment cores collected in 1997/98 along a longitudinal transect of the lake were based on preserved diatom assemblages, as well as stratigraphies of organic carbon, total nitrogen, total phosphorus and biogenic silica concentrations. Population growth, deforestation and intensive agriculture, especially in the southern catchments, have accelerated soil erosion causing rivers to transport greater sediment and nutrient loads into Lake Malawi. Southern cores contain evidence of nutrient enrichment starting around 1940, as indicated by increased silica, carbon and nitrogen influxes. By 1980, increased rates of sedimentation, phosphorus influx, diatom influx, and relative abundances of eutrophic diatom taxa are attributable to accelerated nutrient enrichment accompanying soil erosion. The succession of diatoms in southern Lake Malawi begins with dominance by Aulacoseira nyassensis and Fragilaria africana, followed by a shift in 1980 towards Stephanodiscus, Cyclostephanos and Nitzschia, diatom taxa that have reduced silica requirements. Paleoecological results indicate that patterns of diatom assemblage change are not uniform lake-wide. Evidence of eutrophication extends to central Lake Malawi, but is not observable in the paleo-record from the deeper northern basin. The recent cultural eutrophication of the southern lake has impacted the biogeochemical cycling of silica, the available silica to phosphorus ratios, and the diatom communities of a large portion of Lake Malawi. These results provide a warning that eutrophication of this great lake is underway and will continue unless changes are made to current land use practices within the lake's catchment.

Use of sedimentary diatoms from multiple lakes to distinguish between past changes in trophic state and climate: evidence for climate change in northern Germany during the past 5,000 years

Fossil diatoms from lake sediments have been used to infer both past trophic state and climate conditions. In Europe, climate reconstructions focused on northern and alpine regions because these areas are climatically sensitive and anthropogenic impact was low. In contrast, anthropogenic impact was often high in the central European lowlands, such as northern Germany, beginning in the Neolithic Age, ~3700 BC. Since that time, trophic state change was the main factor that affected diatom assemblages in central European lowland lakes. Therefore, it was considered difficult or impossible to identify climate changes in the region using sedimented diatoms. We used diatom assemblage changes, diatom-inferred total phosphorus concentrations and the relative abundance of planktonic diatoms from sediments of three lakes that differ in their location, size, morphology, catchment area and current trophic state to test whether we could distinguish between trophic state and climate signals over the past 5,000 years in northern Germany. In this study, changes in trophic state and climate were well differentiated. In the study lakes, relative abundance of planktonic diatoms seems to be linked to the length of lake mixing phases. Planktonic diatom abundance decreased during years with shorter mixing duration, and these shorter mixing times probably reflect colder winters. The diatom-inferred periods of short mixing phases from 1000 BC to AD 500 and from AD 1300 to 1800 coincide well with two known cooling phases in Europe and the North Atlantic region.

Palaeolimnological assessment of lake acidification and environmental change in the Athabasca Oil Sands Region, Alberta

Exploitation of the Athabasca Oil Sands has expanded hugely over the last 40 years. Regional emissions of oxidised sulphur and nitrogen compounds increased rapidly over this period and similar emissions have been linked to lake acidification in other parts of North America and Europe. To determine whether lakes in the region have undergone acidification, 12 lakes within the Regional Municipality of Wood Buffalo and the Caribou Mountains were selected to cover chemical and spatial gradients and sediment cores were obtained for palaeolimnological analyses including radiometric dating, diatom analysis, isotopic analysis of bulk sediment 13C and 15N, and spheroidal carbonaceous particles (SCPs). All lake sediment cores show evidence of industrial contamination based on SCPs, but there is no clear industrial signal in stable isotopes. Most lakes showed changes in diatom assemblages and sediment C:N ratios consistent with nutrient enrichment over various timescales, with potential drivers including climatic change, forest fires and anthropogenic nitrogen deposition. Only one of the 12 lakes investigated showed strong evidence of acidification with a decline in diatom-inferred pH from 6.3 to 5.6 since 1970 linked to increasing relative abundances of the acidophilous diatom species Actinella punctata, Asterionella ralfsii and Fragilariforma polygonata. Analysis of mercury (Hg) in the acidified lake showed increasing sediment fluxes over the last 20 years, a possible indication of industrial contamination. The acidified lake is the smallest of those studied with the shortest residence time, suggesting a limited capacity for neutralisation of acid inputs in catchment soils or by inlake processes.

Records of environmental changes during the Holocene from Isla de los Estados (54.4°S), southeastern Tierra del Fuego

Southernmost Patagonia, located at the relatively narrow passage between Antarctica and South America, is a highly sensitive region for recording meridional and zonal changes in the pattern of oceanic and atmospheric circulation. The island of Isla de los Estados, situated at 54.5°S, 64°W, east of Argentinean Tierra del Fuego, provides an exceptional possibility, to investigate terrestrial records of atmospheric conditions in an oceanic setting during the last deglaciation and the Holocene. Here we present geochemical and diatom analyses from 10 600 to c. 1500 cal BP of one sequence (LGB) with peat, lake sediments and lagoon deposits at the northern coast of the island, and a lake sediment sequence (CAS) 3 km further inland. The data comprise TC, TN, loss on ignition analyses and continuous XRF scanning as well as age–depth modeling based on AMS- 14C dating on both cores. Diatom analysis of the CAS record complements the geochemical proxies. During the Holocene, our two sites have been impacted by two different forcings: changes in the regional climate regime largely influenced by the varying strength and position of the Southern Hemisphere Westerlies (SHW), while relative sea-level changes affected the deposits of the coastal site. In concert with the onset of the Antarctic thermal optimum, our data suggest fairly warm conditions and the establishment of denser peat and forest vegetation on the island c. 10 600 cal BP. Between 8500 and 4500 cal BP geochemistry and diatoms from the CAS record indicate stronger Westerlies at this latitude, which means higher wind speed or higher storm frequency and more precipitation, resulting in more pronounced surface run-off. After 4500 cal BP, the geochemical proxies and large changes in diatom assemblages indicate a decrease in precipitation, weaker winds and possibly cooler conditions, probably as an effect of weaker SHW and/or a latitudinal shift. The depositional environment of CAS changed from gyttja to peat around 1000 cal BP. At LGB, the onset of gyttja sedimentation around 7900 cal BP shows that the former wet land with peat accumulation had become a lake with a fairly rapid sedimentation. The chemical data from LGB imply a gradually increasing marine influence, probably as an effect of both rising sea level and increased impact of storms and maximum high tides. After a marine high-stand during the mid-Holocene, the LGB site returned into a peat bog again around 3400 cal BP. Our data suggest that since then relative sea level first dropped back below present sea level followed by a rise to present day sea level.

Watershed-scale reconstruction of middle and late Holocene paleoenvironmental changes on Melville Peninsula, Nunavut, Canada

Arctic regions have heightened sensitivity to climatic changes, however regional scale differences exist in the magnitude and timing of biotic responses. Melville Peninsula in the east-central Canadian Arctic is a transitional area with respect to modern climate, with a continental climate to the west, and more maritime influences to the east. This, and the presence of Thule archaeological sites, make the Peninsula an area of high importance for paleoclimate reconstructions. Lake sediment cores from two adjacent ultra-oligotrophic and very dilute lakes (SP02 and SP04) from interior Melville Peninsula (68°33′N and 83°17′W, 220 m a.s.l.) were analyzed for diatom assemblages (microscopic algae) to track paleoclimatic changes. High magnetic susceptibility, coarse-grained sediments, low organic matter, and lack of diatoms in sediments older than 6000 years BP confirm earlier suggestions of a persistent ice cap on the Peninsula. Peak diatom concentration and biogenic silica, and maximum values for sediment organic matter, between 4400 and 2900 years BP may indicate the Holocene Thermal Maximum (HTM). The timing of the HTM is similar to that of the Baffin region to the east. Decreases in these proxies, as well as small shifts in diatom assemblages following 2900 years BP indicate the advancement of Neoglacial cooling. The changes of the pre-industrial portion of the last millennium were of smaller magnitude than those of the HTM, but subtle changes in diatom assemblages after 750 years BP may be indicative of the Little Ice Age, the timing of which agrees well with that of Boothia Peninsula to the west. The Anthropocene is well marked in these records, with increases in diatom species richness from a pre-industrial maximum of 28–51 diatom species in the modern assemblage at Lake SP02. Planktonic diatoms appear in the 20th century, signalling profound ecological changes associated with longer ice-free seasons.

Changes in benthic diatom assemblages in shrimp culture ponds.

Although diatoms constitute part of the shrimp diet, the species composition and structure ofdiatom assemblages that proliferate in the ponds of shrimp farms had not been studied. Because the ponds are manipulated to maintain constant the phytoplankton characteristics (dominated by diatom), here we contrasted the hypotheses that: 1) a single diatom assemblage would be present during a production cycle in a shrimp farm and 2) that the assemblage would be different from those from the natural habitat. Thus, the objective of this investigation was to determine if time variation of diatom assemblages occurs under typical production conditions characteristic of culture ponds in a shrimp farm. Monthly samples where taken (JuneOctober) from the bottom (microphytobenthos) and surface (net plankton) of the ponds. In the bottom samples 100 diatom species and varieties within 46 genera were identified. The best represented genera were Nitzschia (9 species), Amphora (7) and Navicula (6). Variations in diversity values were observed ranging from H´= 1.5 (June) to H´= 4.3 (September and October). In the surface samples 120 taxa (50genera) were identified. Tychoplanktonic forms were dominant, sharing 70% of the taxa with the bottom assemblage. Two temporal assemblages where distinguished in bottom and surface samples: the first one included the June-July samples which showed high dominance and lowest richness, mainly characterized by Amphora coffeaeformis var. salina and Catacombas gaillioni. The second one, September-October, showed lower numbers of the previous taxa plus the appearance of Navicula cryptocephala var. subsalina and Neodelphineis pelagica. The observed changes in the structure of the summer diatom assemblage to an autumn assemblage, similar to that of natural shallow environments, reject the proposed hypotheses.

Diatom changes in two Uinta mountain lakes, Utah, USA: responses to anthropogenic and natural atmospheric inputs

Diatom assemblages in sediments from two subalpine lakes in the Uinta Mountains, Utah, show asynchronous changes that are related to both anthropogenic and natural inputs of dust. These lakes are downwind of sources of atmospheric inputs originating from mining, industrial, urban, agricultural and natural sources that are distributed within tens to hundreds of kilometers west and south of the Uinta Mountains. Sediment cores were retrieved from Marshall and Hidden lakes to determine the impacts of atmospheric pollution, especially metals. Paleolimnological techniques, including elemental analyses and 210Pb and 239+240Pu dating, indicate that both lakes began receiving eolian inputs from anthropogenic sources in the late 1800s with the greatest increases occurring after the early 1900s. Over the last century, sediments in Marshall Lake, which is closer to the Wasatch Front and receives more precipitation than Hidden Lake, received twice the concentrations of metals and phosphorus as Hidden Lake. Comparison of diatom and elemental data reveals coeval changes in geochemistry and diatom assemblages at Marshall Lake, but not at Hidden Lake; however, a major shift in diatom assemblages occurs at Hidden Lake in the seventeenth century. The change in diatoms at Marshall Lake is marked by the near disappearance of Cyclotella stelligera and C. pseudostelligera and an increase in benthic, metal-tolerant diatoms. This change is similar to changes in other lakes that have been attributed to metal pollution. The marked change in diatom assemblages at Hidden Lake indicates a shift in lake-water pH from somewhat acidic to circumneutral. We hypothesize that this change in pH is related to drought-induced changes in input of carbonate-rich desert dust.

Diatom-inferred climatic and environmental changes over the last ∼ 9000 years from a low Arctic (Nunavut, Canada) tundra lake

Diatom assemblage changes over the Holocene were examined from a 14C-dated sediment core retrieved from Lake TK-2, a small low Arctic lake located ca. 200 km north of the forest–tundra ecotone in mainland Nunavut, Canada. Notable changes in the diatom assemblages were recorded throughout the core, suggesting that the Holocene epoch in this region has been environmentally and climatically dynamic. The earliest diatom assemblages (ca. 9000 cal yr BP) were dominated by taxa that are atypical of post-glacial assemblages commonly recorded throughout the Arctic, and may suggest that early Holocene conditions at Lake TK-2 were relatively warmer. A shift to dominance by small, benthic, opportunistic Fragilaria taxa followed (after ca. 8550 cal. yr BP), more typical of Arctic assemblages during initial lake ontogeny, suggesting the onset of cooler, more alkaline conditions. An abrupt and short-lived marked decrease in diatoms between ca. 8550 and 8500 cal. yr BP, with corresponding changes in physical and chemical indicators (e.g., sedimentation rate, siliciclastic content, % organic matter content), provides potential evidence for the 8.2 k cooling anomaly, an event rarely recognized from other paleolimnological studies in the Canadian north. Following ca. 7000 cal. yr BP, a substantial shift occurred to a more complex and diverse diatom assemblage that now included more acidophilic taxa. This compositional change is likely indicative of a natural, long-term loss of alkalinity in the lake, and marks the onset of the Holocene Thermal Maximum, consistent with the timing of this warm period for this region. The relatively stable diatom assemblage composition during the Neoglacial period was punctuated by fluctuations in key species potentially correlative with the so-called Medieval Warm Period and Little Ice Age. In the most recent sediments, the post-industrial expansion of the small, planktonic Cyclotella stelligera complex, and a concurrent decline in the heavily-silicified Aulacoseira lirata complex, are similar to shifts that are increasingly being recognized as geographically widespread diatom responses to recent climate warming. The Lake TK-2 diatom record provides important insights into the Holocene environmental history of this understudied region of the Canadian Arctic. Furthermore, it is one of the few Arctic lakes in which the 8.2 k cold event is possibly expressed.