Multi-proxy data from Kaksoislammi Lake in Finland: dramatic changes in the late Holocene cladoceran assemblages

A small lake, Kaksoislammi (60° 3830N, 24° 4550E), in southern Finland was studied for Cladocera, diatoms and pollen from a core which covers the entire Holocene. The diatom remains indicate a steady development from alkaliphilous taxa towards the dominance of acidophilous forms and lowering pH in the late Holocene. About 1800–1700 BP, dramatic changes took place in the microfauna, mainly the planktonic Cladocera. Bosmina longirostris, the dominant species, suddenly disappeared, and Daphnia, Chydorus sphaericus and Chaoborus increased. The change is simultaneous with a decline of the diatom-inferred pH to 4.8. It is probable that there was a sudden, profound change in predator-prey relationships. The acidity of the lake water probably increased to such a low level that it led to the disappearance of even the most acid-tolerant fish. Consequently invertebrate predators increased and quickly altered the species composition in the lake. There is also pollen evidence of the onset of Iron Age cultivation and grazing almost simultaneously with the faunal change. Therefore, it cannot be ruled out that the sudden lowering of pH was indirectly caused by prehistoric human activity; possibly the acidic peatland surrounding the lake was disturbed.     

A palaeoenvironmental study of Fairfax Lake,a small lake situated in the rocky mountain foothills of west-central Alberta

Fairfax Lake is a small, oligotrophic to mesotrophic headwater lake situated in the Foothills of the Rocky Mountains of west-central Alberta (Latitude 52° 58 N; Longitude 116° 34 W). Through acquisition of a sediment core, and analyses of the diatoms, chrysophyte stomatocysts, pollen and sedimentary pigments, including myxoxanthophyll and oscillaxanthin, a palaeoenvironmental history of the lake has been determined. The sedimentary record spans ca. 13 200 years. An open tree-less vegetation existed in the region ca. 13 200–ca. 11 600 years BP. Maximum oscillaxanthin and myxoxanthophyll concentrations, hence the largest blue-green algal populations, occurred during the same interval. With increasing temperature pioneering parkland vegetation appeared ca. 11 600 years BP but was replaced ca. 10 100 years BP by spruce forest. Pine appeared ca. 7800 years BP and this marked the development of the present day montane boreal forest. Diatoms were not found until ca. 11 255 years BP. Benthic taxa dominated but by ca. 10 100 years BP planktonic taxa had become more prominent. Lake levels are interpreted as having risen, and the lake water was probably more transparent. Maximum chlorophyll and total carotenoid concentrations occur ca. 11 255 to ca. 7000 years BP corresponding to the warm early to mid-Holocene period. Lake nutrient levels appear to have been higher prior to ca. 7000 years BP, and the lake has changed from being eutrophic during the early Holocene to its present status as an oligotrophic to mesotrophic lake. Subtle hydrological changes have also occurred in the catchment as water levels do not appear to have remained constant.     

Holocene diatom flora and stratigraphy from sediment cores of two Antarctic lakes (King George Island)

The diatom stratigraphy of Holocene sediment cores from two Antarctic lakes (Mondsee, Tiefersee; 62° 10 S/58° 50 W) of King George Island was investigated. The diatom assemblages were dominated by cosmopolitan species. The flora was composed of three main components: 1) taxa of submersed bryophytic habitats, which also occur in the bryophilic diatom flora of southern South America; 2) species of various terrestrial habitats, including some specific subantarctic taxa; 3) species distributed in coastal inland waters influenced by sea-spray.Changes in sediment pattern and diatom species composition seemed to reflect climatic changes. At least three different core sections were distinguished in both lakes: Two sections rich in mosses occur between approx. 7000–4700 B.P. and from 3200 B.P. to present, in general resembling present day conditions. In between is a section of high allochthonous content with generally low diatom concentrations and rising percentages of aerophilic, halophilic and alkaliphilic diatoms. An increase of melt water discharge, possible sea-spray influence, and lake- level fluctuations are discussed.Three new taxa are described: Achnanthes metakryophila nov. spec., Achnanthes renei nov. spec., Navicula australomediocris nov. spec. The taxonomy of selected taxa is discussed.     

Siliceous microfossil succession in the recent history of two basins in Lake Baikal, Siberia

As part of the international cooperative Baikal Drilling Project, siliceous microfossil assemblage succession was analyzed in two short ( 30-cm) sediment cores from Lake Baikal. One core was recovered from the north basin (Core 324, 55°15N, 109°30E), a second from between the central and southern basins (Core 316, 52°28N, 106°5E). The northern core had higher amounts of biogenic silica (40 g SiO₂ per 100 g dry weight sediment) compared to the southern core, and increased deposition in the more recent sediments. Weight percent biogenic silica was lower in the southern core, ranging from approximately 20–30 g SiO₂ per 100 g dry weight sediment throughout the entire core. Trends in absolute microfossil abundance mirror those of biogenic silica, with generally greater abundance in the northern core (86–275×10⁶ microfossils g^–1 dry sediment) compared to the southern core (94–163×10⁶ microfossils g^–1 dry sediment).Cluster analyses using relative abundance of the dominant diatom and chrysophyte taxa revealed four zones of microfossil succession in each core. Microfossil assemblage succession in the north basin may be reflecting shifts in nutrient supply and cycling driven by climatic changes. The most recent sediments in the northern basin (Zone 1,c. 1890's–1991 A.D.) were characterized by an increased abundance ofAulacoseira baicalensis andAulacoseira spore. Zone 3 (c. 1630's–1830's A.D.) was dominated by the endemicCyclotella spp. and reduced abundance of theAulacoseira spp. Zone 3 corresponds approximately to the Little Ice Age, a cooler climatic period. The microfossil assemblages between Zones 1 and 3 (Zone 2,c. 1830's–1890's A.D.) and below Zone 3 (Zone 4,c. 830's–1430's A.D.) are similar to one another suggesting they represent transitional intervals between warm and cold periods. Southern basin sediments record similar changes in the endemic taxa. However, the increased abundance of non-endemic planktonic taxa (e.g.Stephanodiscus binderanus, Synedra acus, Cyclostephanos dubius) during two periods in recent history (post World War II and late 1700's) suggests evidence for anthropogenic induced changes in southern Lake Baikal.     

Postglacial development of a marl and peat complex on the Precambrian Shield of northwestern Ontario

This paper presents the first paleolimnological study of the postglacial development of a marl and peat complex on the Canadian Precambrian Shield. Ring Lake (48° 46 N, 85° 51 W), situated within the carbonate glacial drift area of northwestern Ontario, originated about 9000 BP in a basin exposed by the retreating waters of proglacial Lake Superior. The development of Ring Lake was interpreted from pollen and diatom analysis of one sediment core from the littoral zone and another core from near the lake centre.The sequence of postglacial vegetation development parallels published accounts of forest history in northern Ontario. The predominant diatom throughout the littoral core was the alkaliphilous Cymbella diluviana. The central core was dominated by circumneutral and alkaliphilous species of Achnanthes Navicula, Fragilaria, and Cymbella, except in recent samples where acidophilous species of Anomoeoneis were common.Diatom-inferred (DI) pH shows that the early lake was alkaline because of drainage from base-rich tills. The presence of marl in the littoral core indicates deposition of calcareous materials until the site dried out during the Hypsithermal period. There is evidence that beaver activity around 5000 BP caused a temporary change in lake hydrology. A decline in DI pH over much of the postglacial reflects gradual exhaustion of carbonates in the drainage area. An increase in acidophilous diatoms in samples representing the past 3500 y is consistent with gradual acidification of the system and development of a littoral peatland in a cooler neoglacial climate.     

Recent diatom assemblage changes in subarctic Lake Saanajärvi, NW Finnish Lapland, and their paleoenvironmental implications

Diatoms were analysed from a 30-cm long sediment core obtained from remote subarctic Lake Saanaärvi (69°03N, 20°52E) in order to trace possible changes in the lake. Diatom assemblages were relatively constant throughout the core, except in the top 4–5 cm (approx 1850 A.D.) where relative frequencies of Aulacoseira italica subsp. subarctica, A. lirata var. biseriata, Cyclotella comensis and C. glomerata increased markedly. No significant trends were observed in the weighted averaging (WA) reconstructed pH values. Several hypotheses, including (i) airborne pollution, (ii) climatic change, and (iii) catchment disturbances have been put forth to explain the recent changes in diatom assemblages. The diatom change coincides with a marked increase in mean annual temperature that has been documented in the area since the termination of the Little Ice Age. Our evidence favours climate change as the main causative mechanism for the observed diatom compositional changes, although other explanations cannot be ruled out.     

Late glacial — early holocene palaeosalinity in Alberta,Canada — climate implications

Palaeolimnological and palynological records from climatically variable central Alberta, Canada, document periods of hypersaline lake conditions indicative of late glacial and early Holocene drought. The sensitivity of palaeolimnological indicators for inferring palaeoclimates is examined by comparing records from two sites at opposite ends of the regional precipitation gradient. Palaeosalinity is identified by the presence of Ruppia pollen, a hypersaline aquatic plant not presently growing in either lake, and diatom assemblages comprising both saline epipelic and planktonic species. Goldeye Lake (52° 27 N; 116° 12 W), in the relatively moist Rocky Mountain Foothills remained saline from its inception before ca 14500 years BP until ca 10400 years BP by which time pioneering forests had replaced tundra vegetation; however, freshwater planktonic diatoms dominated ca 12500 to 11500 years BP. However, dating problems endemic to the Foothills region make this chronology only tentative. Moore Lake (54° 30 N; 110° 30 N), in dry, east-central Alberta contained Ruppia only between ca 9000 and 6000 years BP. Freshwater diatoms dominated until ca 10000 years BP when they were succeeded by taxa characteristic of saline water. The lake remained saline until ca 6000 years BP. The late glacial period of palaeosalinity at Goldeye Lake occurred because the lake was surrounded by Cordilleran and Laurentide glacial ice, and therefore, cut off from moisture sources until the early Holocene by which time significant ice recession had occurred. Factors causing the second period of salinity remain unknown at this time. In contrast, by the early Holocene, Moore Lake was influenced by drought caused by high summer insolation induced by orbital fluctuations. Freshwater conditions were maintained through the Holocene in the Foothills region of west-central Alberta, but occurred consistently only over the last 4000 years in central and east-central Alberta. The warmer, drier climate during the early Holocene did affect lake levels in at least one headwater Foothills lake (Fairfax Lake — 52° 58 N; 116° 34 W). The severity of the drought increased in an easterly direction across the province.This publication is the fourth of a series of papers presented at the Conference on Sedimentary and Palaeolimnological Records of Saline Lakes. This Conference was held August 13–16, 1991 at the University of Saskatchewan, Saskatoon, Canada. Dr. Evans is serving as Guest Editor.     

Limnological studies of Amarga Lagoon,Chile: A saline lake in Patagonian South America

Amarga Lagoon, lat. 50°29 S and long. 73°45 S, it is located at the Torres del Paine National Park, Chile. The physical, chemical, and biological features of the lake were studied. According to salinity, the lake is mesosaline. Sodium and chloride were the dominant ions. Nitrogen was potentially limiting for phytoplankton growth. One Cyanophyceae species andArtemia were the predominant species in the plankton.Artemia provided abundant food for flamingoes.     

Late Quaternary Palaeoenvironment of Spring Lake,Alberta, Canada

Palaeoenvironmental investigations based upon sediment cores taken from Spring Lake in the Peace River District of Alberta, Canada (latitude 55° 31 N; longitude 119° 35 W) show that the sedimentary record spans the Holocene period. Chemical and diatom changes coincide with regional climatic change since deglaciation (about 11 000 yr. B.P.). Calcite laminations in the basal 3 metres of the cores are evident, and were probably formed through elevated water temperature although photosynthetic removal of CO₂ undoubtedly contributed. The disappearance of the laminations, and concurrent decrease in calcite X-ray diffraction peaks ca. 5000 yr. B.P. may have been caused by a change from a partially meromictic to a dimictic lake as the climate changed. Benthic and alkaline diatoms dominate before 5000 yr. B.P., while the planktonic Stephanodiscus hantzschii dominated between 5100 and 4200 yr. B.P. probably owing to increased nutrient levels. From ca. 4200 yr. B.P. until the present, benthic Fragilaria spp. and more circumneutral diatoms were dominant. Palaeoproduction, as measured by chlorophyll derivates, was highest in the early developmental stages of the lake, decreased coinciding with a major disturbance of the sediments and lowered water levels, and then gradually increased again until present. Only during the earlier period of peak production does biogenic calcite formation appear more important than calcite deposition caused by high water temperatures.     

Paleohydrology of Andean saline lakes from sedimentological and isotopic records, Northwestern Argentina

The paleohydrological evolution of several high altitude, saline lakes located in the southernmost Altiplano (El Peinado and San Francisco basins, Catamarca province, NW Argentina) was reconstructed applying sedimentological, geochemical and isotopic techniques. Several playa lakes from the San Francisco basin (26° 56 S; 68° 08 W, 3800-3900 m a.s.l.) show evidence of a recent raise in the watertable that led to modern deposition of carbonate and diatomaceous muds. A 2 m - long core from El Peinado Lake (26° 29 59 S, 68°05 32 W, 3820 m a.s.l.) consists of calcitic crusts (unit 3), overlaid by an alternation of macrophyte-rich and travertine clast- rich, laminated muds (unit 2), and topped by travertine facies (unit 1). This sedimentary sequence illustrates a paleohydrological evolution from a subaerial exposure (unit 3) to a high lake stand (unit 2), and a subsequent smaller decrease in lake level (unit 1). The ¹³C_{organic matter}record also reflects the lake transgression between units 3 and 2. Although there is a general positive correlation between ¹⁸O_carbonate and salinity proxies (Na, Li and B content), the large data dispersion indicates that other factors besides evaporation effects control chemical and isotopic composition of lakewater. Consequently, the oxygen isotopic composition cannot be interpreted exclusively as an indicator of salinity or evaporation ratio. The degassing of CO₂ during groundwater discharge can explain the enriched ¹³C values for primary carbonates precipitated. The carbon budget in these high altitude, saline lakes seems to be controlled by physical rather than biological processes.The Altiplano saline lakes contain records of environmental and climatic change, although accurate ¹⁴C dating of these lacustrine sediments is hindered by the scarcity of terrestrial organic material, and the large reservoir effects. Sedimentologic evidence, a ²¹⁰Pb-based chronology, and a preliminary U/Th chronology indicate a very large reservoir effect in El Peinado, likely as a result of old groundwaters and large contributions of volcanic and geothermal ¹⁴C-free CO₂ to the lake system. Alternative chronologies are needed to place these paleorecords in a reliable chronological framework. A period of increased water balance in the San Francisco basin ended at about 1660 ± 82 yr B.P. (calendar yr U/Th age), and would correlates with the humid phase between 3000 and 1800 yr B.P detected in other sites of the southern Altiplano. Both, ²¹⁰Pb and preliminary U/Th dating favor a younger age for the paleohydrological changes in El Peinado. The arid period reflected by subaerial exposure and low lake levels in unit 3 would have ended with a large increase in effective moisture during the late 17th century. The increased lake level during deposition of unit 2 would represent the period between AD1650 - 1900, synchronous to the Little Ice Age. This chronological framework is coherent with other regional records that show an abrupt transition from more arid to more humid conditions in the early 17th century, and a change to modern conditions in the late 19th century. Although there are local differences, the Little Ice Age stands as a significant climatic event in the Andean Altiplano.     

A high resolution proxy-climate record from an arctic lake with annually-laminated sediments on Devon Island, Nunavut, Canada

Sediments from a 3 ha lake (75 °34.34N, 89 °18.55W) from the coastal region of northern Devon Island, Nunavut, Canada, contain discrete laminations in the deepest part of the basin. The laminations are varves as indicated by the correspondence between counts and thickness measurements of the couplets and ²¹⁰Pb dating. A 14 cm core representing 150 years of sedimentation contained laminated couplets consisting of a lighter inorganic layer with a higher percentage of calcium and magnesium, alternating with fine darker bands, typically more cohesive, and comprising higher proportions of silica and carbon. A reddish oxidation zone with higher iron and aluminum frequently separates the laminations. The dark layer represents a biogenic component deposited in summer and is made cohesive by bacterial filaments among the other particles. The light inorganic layer represents clastic deposition from allochthonous sources. Deposition rates were relatively consistent through the core with an increase in varve thickness in the 1950s. Diatom concentrations in the sediments increased by two orders of magnitude in this century, with major increases in the 1920s and 1950s. The increase in varve thickness and diatom abundance coincides with an increase in summer melt percentage in an ice core from the Devon Island Ice Cap (Koerner, 1977). The relatively high sedimentation rate (0.15 cm yr^-1) coupled with the consistency of deposition makes this lake a significant indicator for recent climate changes of the Canadian Arctic Archipelago.     

Pulp and paper mill pollution and subsequent ecosystem recovery of a large boreal lake in Finland: a paleolimnological analysis

Sedimentary diatom frustules and chironomid remains, in addition to the chemical stratigraphy of 32 elements and resin acids, were studied from short-core samples taken from two basins and a sheltered bay of Lake Päijänne in southern Finland (about 61 °10–62 °15 N, 25 °30 E). The lake was formerly oligotrophic but has been subject to effluent loading from pulp and paper mills during the past century since the opening of the first pulp mill in the 1880s. Four developmental phases were distinguished and named mainly according to the effects found in the basin close to the pollution source: (1) the pre-industrial phase (from the 1800s to ca. 1920), including the early years of industrialization, with low impacts; (2) the phase of increasing pollution (ca. 1920–1969); (3) the phase of severe pollution (1970–1981); and (4) the water protection phase (1982 onwards).Distinct differences were observed between pelagic (diatoms) and benthic (chironomid) ecosystems in their response to pollution. Effluent loading from the pulp and paper mills had a pronounced impact on chironomid assemblages, but only a slight influence on diatoms in the basin close to the pollution source. In the southern basin, approximately 60 km downstream, an increase in acidophilous species was observed in the diatom assemblages during the early years of pollution, but changes in chironomids in this basin were negligible. The water protection work of the past 30 years, which has led to an improvement in water quality and a marked recovery of the profundal biota, has also resulted in a slight increase in primary production of the lake. The decrease in brown-coloured effluent water has led to an increase in thickness of the lake's photic productive layer, while the toxicity of the water has simultaneously diminished. Rapid changes in chironomid assemblages and sedimentation observed in the sheltered bay highlight the importance of local land-use activities as causes of environmental change in this area, and especially their effects on sedimentation and benthic life.     

Palynology of Lagoa dos Patos sediments,Rio Grande do Sul,Brazil

With the purpose of studying the vegetation and climatic changes in the last millenia of the Coastal Plain of Rio Grande do Sul, Brazil, a palynological study was made of the sediments of the northern part of Lagoa dos Patos lagoon. Twenty-four samples from a 2.26 m core taken at a depth of 7.70 m (30° 50 50 S and 50° 59 05 W) were collected.The analyses revealed marine transgression at 5170±120 years B.P., giving rise to local vegetation consisting chiefly of xerophytes and halophytes. Vegetation characteristic of a humid environment was present along adjacent portions of the Coastal Plain at this time. Transgression increased at about 4080±110 years B.P., when the greatest level of tidewater was reached. This coincided with the beginning of forest vegetation development along the inner portions of the Coastal Plain. These data suggest that marine transgression may have been a consequence of higher temperatures and more humidity. After 4000 years B.P., regression occurred, resulting in fresh waters characteristics in the northern portion of the lagoon. The development of forest vegetation began at this time.This is the fourth in a series of papers published in this issue on Paleolimnology in Southern South America. Dr. C. A. Fernández served as guest editor for these papers.     

Environmental changes in an alpine lake (Gossenköllesee, austria) over the last two centuries – the influence of air temperature on biological parameters

Changes in microfossils (diatoms, chrysophytes, chironomids and cladocera remains), geochemistry and deposition of atmospheric pollutants have been investigated in the sediment records of the alpine lake Gossenköllesee (Tyrol, Austria) spanning the last two centuries. The sediment records were compared with seasonal and annual air temperature trends calculated for the elevation (2417 m a.s.l.) and the geographical position (47° 1346N, 11° 0051E) of the lake, and with precipitation records available since 1866 from Innsbruck. Temperature trends followed a 20–30 year oscillation between cold and warm periods. Regarding long-term changes, temperature trends showed a U-shaped trend between 1780 and 1950, followed by a steep increase since 1975.Physical, geochemical, and organic parameters were not controlled by air temperature. Among the biological records only diatoms and chrysophytes reacted to air temperature changes: the relative abundance of planktonic diatoms increased during warm periods and changes in mean annual alpine air temperature explained 36.5% of their variation. The relation between abundance of seasonal stomatocyst types and air temperature varied on two different time scales: while summer stomatocysts were influenced by short term temperature fluctuations, the autumn stomatocysts were affected only by the long term changes. Other biological parameters exhibited a constant species composition (chironomids, pigments) or changes were small and independent of temperature (cladocera). Spheroidal carbonaceous fly-ash particles, and trends in Pb and Cr indicated increasing deposition of atmospheric pollutants but had no detectable effects on the biological parameters either. In respect to temperature variations over the last 200 years, this alpine lake is much less sensitive than expected and has thus to be regarded as a well buffered site. However, temperature alone is not sufficient to understand changes in species composition and other biogeochemical processes with unknown historical patterns might have affected species composition more strongly.     

Diatom responses to late Quaternary vegetation and climate change,and to deposition of two tephras in an alpine and a sub-alpine lake in Yoho National Park,British Columbia

The late Quaternary diatom records from alpine Opabin Lake (altitude 2285 m a.s.l.) and sub-alpine Mary Lake (altitude 2054 m a.s.l.), located in Yoho National Park, British Columbia (lat. 51 ° 21N; long. 116 ° 20), have been analyzed, and changes in these records have been used to reconstruct lake histories. The results have also been related to independently inferred vegetation and climate changes. Following deglaciation, when both lakes were receiving high inputs of clastic materials, benthic diatom taxa dominate the records of these two shallow lakes with small species ofFragilaria being particularly prominent. During the early to mid-Holocene period, when treeline was at a higher elevation than today, the diatom flora of both lakes became more diverse with previously minor species becoming more prominent.Cyclotella radiosa occurs in cores from both Mary Lake, and much deeper, neighbouring Lake O'Hara during the warm early Holocene, and may reflect this warmer climate, a longer ice-free season than presently, and perhaps less turbid water, or its presence may reflect a subtly higher nutrient status of the lake water during this period. The Neoglacial is marked by increased amounts of sediments originating from glacial sources in Opabin Lake, which undoubtedly led to very turbid water, and by the presence ofEllerbeckia arenaria f.teres andCampylodiscus noricus v.hibernica in Opabin Lake; however, these species are absent from Mary Lake which has not been influenced by either glacial activity since the recession of the glaciers prior toc. 10 000 years BP or water originating from Opabin Lake. The impact of the two tephras during the Holocene was dramatic in terms of increased diatom production, as exemplified by the increases in diatom numbers, but there was little effect upon species composition. The diatom records and changes in the diatom:cyst ratio suggest that the chemical status of these two small, shallow lakes has changed little during the Holocene, other than after deposition of the two tephras. These results provide evidence that shallow alpine and high sub-alpine lakes are sensitive recorders of past environmental changes.     

Recent environmental history of a large,originally oligotrophic lake in Finland: a palaeolimnological study of chironomid remains

The sedimentary chironomid stratigraphy in short-core samples covering approx. the past 150 years was studied in the northernmost basin of Lake Päijänne, southern Finland (62° 11 N, 25° 48 E). The basin has received effluent loading from the wood-processing industry and municipal waste water. Four developmental stages were distinguished based on the changes in chironomid assemblages: 1. Pre-industrial stage (dated by the²¹⁰Pb method as covering approximately the period 1838–1936), 2. Stage of increasing pollution (approx. 1944–1973), 3. The black decade, or the period of worst pollution (approx. 1973–1983), and 4. Water protection stage (approx. 1983 onwards).During the first stage the basin suffered very little human interference and was oligotrophic, with values of 4.00–4.28 for Wiederholm's Benthic Quality Index (BQI). During the second stage it altered between weak mesotrophy and moderate mesotrophy, and the former profundal assemblages characterized byHeterotrissocladius subpilosus andMicropsectra were replaced bySergentia coracina andChironomus anthracinus gr. The BQI ranged from 2.75 to 3.50. This process led to the extirpation ofH. subpilosus. Eutrophication of the basin reached its climax during the black decade, representing moderate eutrophy with a BQI of 2.15. The profundal assemblages were composed mainly ofChironomus anthracinus gr. andC. plumosus gr. The adoption of more effective waste water purification processes in both the wood-processing industry and the municipal treatment plant markedly reduced effluent loading, especially BOD loading during the fourth stage, and this led to a recovery in the basin. At present it is mesotrophic, with a BQI of 2.90–3.00, and its biological conditions resemble those of the second stage in the 1950's and 1960's.     

Paleolimnological environment indicated by the diatom and pollen assemblages in an alpine lake in Taiwan

Yuanyang Lake (24°35N, 121°24E), located at an altitude of 1,670 m within a nature preserve in northern Taiwan, is an acidic lake. Remains of diatoms and pollen from a 3.72-m sediment core were used to elucidate the relationships between the vegetation of the watershed and the paleolimnological environment. Past pH, saprobity level, and total P of the lake were inferred from the diatom assemblages and were analyzed with respect to changes in the terrestrial vegetation. The inferred pH values fluctuated only slightly, whereas the inferred saprobity level increased markedly towards the sediment surface. In the topmost sediment, a slight drop in the inferred pH was associated with a lowering in the saprobity index. This was interpreted as a possible result of recent anthropogenic acidification and changes in productivity related to changes in acidity. Based on pollen analyses, we conclude that Chamaecyparis persisted over at least the last four thousand years in the watershed. The vegetation in the watershed changed little during this period of time, which is consistent with the constancy of inferred pH values. A positive correlation between the inferred pH and ¹³C values of the sediments was found.     

A high resolution Holocene pollen record from Lago do Pires,SE Brazil: vegetation,climate and fire history

Lago do Pires (17° 57 S, 42° 13 W) is situated at 390 m a.s.l. in the foothills of the Serra do Espinhaço, 250 km from the Brazilian Atlantic coast. The original vegetation of the study area has been almost destroyed by pastoral activity. Relicts of a dense 20–30 m tall tropical semidciduous forest are present only on a few hill tops. The dry season of the Lago do Pires region lasts for 4 months and the annual precipitation is 1250 mm. A high resolution pollen record from a 16 m long sediment core, composed of 77 samples, subdivided in 7 zones and 4 subzones, allows a reconstruction of Holocene paleoenvironments. For the early Holocene (9720-8810 B.P.), the results indicate that the region surrounding the lake was dominated by a herb savanna (campo cerrado) withCuratella americana (cerrado tree) and high fire frequency. Species ofCecropia, Urticales and a few others, form small gallery forests along the water courses. This vegetation pattern is consistent with a long dry season (perhaps 6 months) and a low annual precipitation. Between 8810 and 7500 years B.P. gallery forests expanded in the valleys and suggest a period of higher rainfall with shortened dry season (perhaps 5 months). Fire was less frequent. Reduction of gallery forests followed (7500-5530 B.P.), probably related to a return of drier climatic conditions (5–6 months dry season, lower precipitation). Fires were more frequent. Between 5530 and 2780 years B.P. in the vallyes were forests and on the hills still an open cerrado. The dry season probably was about 5 months and the rainfall was higher than in the previous period. Later (2780-970 B.P.) the more open cerrado on the hills changed to more closed cerrado. A dense and closed semideciduous forest existed in the region only in the latest Holocene period (since 970 B.P.) under the current climatic conditions. The vegetation was no longer influenced by fire. A very strong human impact by deforestation and use of fire occurred in the last decades. Today cerrado vegetation is generally restricted to central Brazil and exists in several small isolated Islands (Hueck, 1956) in the area of semideciduous forest in SE Brazil which were more widespread during the drier periods of the Holocene. The wettest period of Holocene occurs in the present millenium.This is the 4th in a series of papers published in this special AMQUA issue. These papers were presented at the 1994 meeting of the American Quaternary Association held 19–22 June, 1994, at the University of Minnesota, Minneapolis, Minnesota, USA. Dr Linda C. K. Shane served as guest editor for these papers.     

Holocene environmental change at Lake Njulla (999 m a.s.l.), northern Sweden: a comparison with four small nearby lakes alongan altitudinal gradient

We assess Holocene environmental change at alpine Lake Njulla(68°22N, 18°42E, 999 m a.s.l.) innorthernmost Sweden using sedimentary remains of chironomid head capsules anddiatoms. We apply regional calibration sets to quantitatively reconstruct meanJuly air temperature (using chironomids and diatoms) and lake-water pH(using diatoms). Both chironomids and diatoms infer highest temperatures(1.7–2.3°C above present-day estimates, includinga correction for glacio-isostatic land up-lift by0.6°C) during the early Holocene (c.9,500–8,500 cal. yrs BP). Diatoms suggest a decreasing lake-waterpH trend (c. 0.6 pH units) since the early Holocene. Usingdetrended canonical correspondence analysis (DCCA), we compare the Holocenedevelopment of diatom communities in Lake Njulla with four other nearby lakes(Lake 850, Lake Tibetanus, Vuoskkujávri, Vuolep Njakajaure) locatedalong an altitudinal gradient. All five lakes show similar initial DCCA scoresafter deglaciation, suggesting that similar environmental processes such ashigh erosion rates and low light availability associated with high summertemperature appear to have regulated the diatom community, favouring highabundances of Fragilaria species. Subsequently, the diatomassemblages develop in a directional manner, but timing and scale ofdevelopment differ substantially between lakes. This is attributed primarily todifferences in the local geology, which is controlling the lake-waterpH. Imposed on the basic geological setting, site-specific processessuch as vegetation development, climate, hydrological setting andin-lake processes appear to control lake development in northernSweden.     

Importance of diffuse nutrient loading and lake level changes to the eutrophication of an originally oligotrophic boreal lake: a palaeolimnological diatom and chironomid analysis

The recent environmental history of Lake Lappajärvi in western Finland (63°00 N, 23°30 E, area 149 km²), a humic, brown water lake with an average phosphorus content of ca. 20 g l^–1, was studied from short core sediment samples taken from the two main basins of the lake. Based on the stratigraphy of diatoms and chironomids and the sediment quality it was possible to distinguish four developmental stages during the past century: (1) a pre-industrial stage covering the time up to about 1935; (2) a stage of increasing nutrient loading (ca. 1936–1960); (3) a stage of pronounced erosion from lake level regulation and extensive ditching of the catchment area (ca. 1960–1970); and (4) a meso-eutrophic stage from ca. 1970 onwards.Acidophilous Aulacoseira distans coll. and other species typical of dystrophic, nutrient-poor lakes characterized the diatom assemblages during the first stage, and the profundal zoobenthic assemblages, characterized by Heterotrissocladius subpilosus and Micropsectra, indicated good hypolimnetic oxygen conditions and a low sedimentation of organic matter (approx. less than 50 g m^–2 a^–1). The increased loading rapidly led to changes both in diatoms and chironomids (e.g., to an early extinction of H. subpilosus in the 1950s). The process finally led to eutrophication with a successive proliferation of diatom species such as Asterionella formosa followed by Aulacoseira ambigua, Fragilaria crotonensis, and finally Melosira varians. The relative proportion of alkaliphilous species reached a maximum in the final stage and the original profundal chironomid fauna was replaced by Chironomus anthracinus gr. and C. plumosus which are typical of profundal areas suffering from temporal oxygen deficit. It is notable that the considerable decrease in waste water loading from the point sources (80–86% ) during the past two decades has not led to a recovery in the lake. This highlights the importance of diffuse loading from agriculture, forestry and other human activities even to this comparatively large lake.