Impact on lake development of changed agricultural watershed exploitation during the last three centuries

The history of the development of Lake Hejrede Sø (Denmark) and the related history of the watershed management were studied based on analyses of macrofossil content, chemical composition and Clostridium perfringens content in sediment cores. Depth-age relations of ecological changes were established through Pb²¹⁰ analyses, and a systematic search for written sources describing the watershed history was carried out.Lake Hejrede Sø used to be a humic, acid but relatively clearwater lake with an extensive submersed vegetation. Owing to deforestation and reclamation of bogs and meadows, the accumulation of nutrients in these ecotones vanished. Subsequently, in early 1800 the lake developed into a clearwater alkaline lake, and later became more eutrophic which resulted in the decline of the submersed vegetation. At the time the annual sediment phosphorus accumulation increased more than twofold, and influx of mineral matter increased drastically. In mid-1900, submersed vegetation had disappeared. The Clostridium perfringens analysis and the historical evidence show that no sewage was discharged to the lake until mid-1900, which implies that changed agricultural watershed exploitation and ecotone reclamation during the 19th century was the cause of the increased nutrient influx, and the deterioration of the lake.Written historical sources confirm the palaeolimnological interpretations and describe the agricultural practices responsible for the development.     

High variability in iron-bound organic carbon among five boreal lake sediments

Being both stable carbon sinks and greenhouse gas sources, boreal lake sediments represent significant players in carbon (C) cycling. The release of dissolved organic carbon (DOC) into anoxic water is a widespread phenomenon in boreal lakes with impact on sediment C budgets. The association of OC with iron (Fe) is assumed to play an important role for this anoxic OC release via the dissimilatory reduction of Fe, but also to influence the stabilization of OC in sediments. To investigate the role of Fe–OC association for OC dynamics in different boreal lake sediments, we compared the content of Fe-bound OC [Fe–OC, defined as citrate bicarbonate dithionite (CBD) extractable OC] and the extent of reductive dissolution of solid-phase Fe and OC at anoxia. We found high among-lake variability in Fe–OC content, and while the amount of Fe–OC was high in three of the lakes (980–1920 µmol g^?1), the overall contribution of Fe–OC to the sediment OC pool in all study lakes was not higher than 11%. No linkages between the amount of the Fe–OC pool and lake or sediment characteristics (e.g., pH, DOC concentration, sediment OC content, C:N ratio) could be identified. The observed release of OC from anoxic sediment may be derived from dissolution of Fe–OC in the lake sediments with high Fe–OC, but in other lake sediments, OC release during anoxia exceeded the sediment Fe–OC pool, indicating low contribution of reductive Fe dissolution to OC release from these lake sediments. The range of the investigated boreal lakes reflects the high variability in the size of the sediment Fe–OC pool (0–1920 µmol g^?1) and CBD-extractable Fe (123–4050 µmol g^?1), which was not mirrored in the extent of reductive dissolution of Fe (18.9–84.6 µmol g^?1) and OC (1080–1700 µmol g^?1) during anoxia, suggesting that Fe-bound OC may play a minor role for sediment OC release in boreal lakes. However, studies of redox-related OC cycling in boreal lake sediments should consider that the amount of Fe–OC can be high in some lakes.     

70 Use of molecular probe to detect taste/odor-causing cyanobacteria in the phoenix drinking water distribution system

Upper Klamath Lake (UKL) is the largest lake in Oregon (area 287 km², avg. depth 4.2 m). It is naturally eutrophic and regularly suffers nuisance summer blooms of cyanobacteria, principally Aphanizomenon flos-aquae (AFA). Sediment coring studies show that AFA was absent or minimal until about 1880 when a steady increase began, culminating in the blooms of recent decades. These studies show concomitant increases in sediment N (∼20%) and P (∼50%) along with shifts in the algal flora indicating increased eutrophication. These changes correlate with increased human impacts, such as deforestation, construction, roadbuilding etc., and especially the ditching, diking and draining of adjacent wetlands for conversion to agriculture. Agricultural nutrient runoff, especially P, has been often cited as the cause of the AFA blooms, and most attention has been focused on the dynamics of UKL during the summer bloom. We propose that a more significant factor may be the loss of early-season suppression of AFA because of the loss of the lake-associated wetlands, which originally constituted 42% of the lake area, and which have declined in area by 66.3% since the late 1800's. The melting of snow and ice in the spring would flush into the lake a surge of wetland plant decomposition products, most significantly organic acids and humic substances. We propose that formerly these wetland effluents caused a complex of effects on lake pH, solar UV transparency, photochemical interactions, nutrient availability, and Daphnia grazing dynamics, which would have combined to prevent the development of any AFA bloom.     

Temporal and geographical variation in lake trophic status in the English Lake District: evidence from (sub)fossil diatoms and aquatic macrophytes

1. A sediment core (representing 250–300 years) was taken from each of three lakes of conservation interest and contrasting trophic status in the English Lake District: Wastwater, Bassenthwaite Lake and Esthwaite Water. Lithostratigraphic analyses, radiometric dating and analysis of fossil diatoms were carried out.
2. Transfer functions, based on the diatoms, were used to reconstruct total phosphorus (TP) and, thus, eutrophication at the study lakes. In Wastwater, changes in lake pH were also reconstructed.
3. The lakes were also classified according to their present macrophyte flora, the latter being compared with previous records.
4. The fossil diatoms of Wastwater were continuously dominated by taxa typical of oligotrophic, circumneutral waters, indicating that the lake has not been enriched or acidified in the last 250 years. The aquatic macrophyte flora has probably remained unchanged since before the Industrial Revolution.
5. The diatom assemblages of both Bassenthwaite Lake and Esthwaite Water began to change in the mid-1800s. Further change occurred from the 1960s, at the onset of a recent period of eutrophication. These two lakes have experienced continued nutrient enrichment throughout the 1970s, 80s and 90s, largely associated with increasing phosphorus inputs from sewage effluent. There is no evidence of any recovery in response to recent reductions in external nutrient loads.
6. Only in Esthwaite Water has the change in aquatic macrophytes been pronounced.
7. Palaeolimnological reconstruction is useful in determining background conditions and natural variation in lake ecosystems.     

Long-term trends in limnological characteristics in the Aurora trout lakes, Sudbury, Canada

Paleolimnological techniques were employed to document the limnological histories of the aurora trout lakes, located in the Sudbury region of Ontario. Two of these lakes are of special interest to fisheries managers, as they represent the only known native habitats of a rare strain of brook trout: the aurora trout. These lakes were limed as part of restoration efforts. Stratigraphic changes in diatom and chrysophyte assemblages from dated lake sediment cores indicate that all the lakes have been impacted by anthropogenic acidification, although the timing and the magnitude of acidification were different amongst the lakes. For example, Whirligig Lake was likely the most naturally acidic lake in the past, but it had further acidified since about 1960. This lake was limed in 1989 and then again in 1993. In Whitepine Lake, acidification started 1940; however, in the most recent sediments (1992), some recovery in lakewater acidity has occurred. In Little Whitepine Lake (a reference lake), acidification started earlier (1920) and the lakewater pH continued to decline until about 1990. This lake was limed in 1989. The chrysophyte paleoindicators suggest a recent recovery in this lake. The successful re-introduction of aurora trout in Whirligig and Whitepine lakes is undoubtedly related to the improved water quality through liming but, based on our paleolimnological indicators, the lakes' limnological characteristics (e.g. pH and metal concentrations) are still different from those present before atmospheric deposition of strong acids from the Sudbury smelters.     

From deciduous forest to open landscape: application of new approaches to help understand cultural landscape development in western Norway

Development of the cultural landscape in a village situated by the inner fjords of western Norway is investigated by pollen analysis and quantitative reconstruction methods. Pollen samples from lake sediments and a soil profile were analysed and represent different spatial scales. The Landscape Reconstruction Algorithm (LRA) is applied to a large and a small lake to convert pollen percentages from the small lake into estimated local vegetation cover in selected time periods starting from 2800 cal bc (Middle Neolithic A). This reconstruction shows that estimated forest cover has fluctuated through time, and changes in openness related to human impact are distinct from the Early Bronze Age (1800–1200 cal bc). Pollen analyses from the soil profile indicate forest clearances from the Late Neolithic (2300–1800 cal bc). Gradual intensification of farming is recognized in both pollen diagrams throughout the Bronze and Iron Ages with increasing openness and spatial differentiation in land-use practices. Presence of pollen of cereals and flax record the cultivation of these plants from the Iron Age, and intensification of land-use may have caused erosion and re-sedimentation in the lake in medieval times. To identify a possible landscape in the past, HUMPOL software has been used with the Late Neolithic as a case study. The LRA-based estimates of forest cover are supported by the HUMPOL simulations, but several solutions to the Late Neolithic landscape pattern exist. The results clearly demonstrate how implementation of LRA and HUMPOL improve the understanding of cultural landscape development.     

68 A new paradigm for the aphanizomenon bloom problem in upper klamath lake

Upper Klamath Lake (UKL) is the largest lake in Oregon (area 287 km², avg. depth 4.2 m). It is naturally eutrophic and regularly suffers nuisance summer blooms of cyanobacteria, principally Aphanizomenon flos‐aquae (AFA). Sediment coring studies show that AFA was absent or minimal until about 1880 when a steady increase began, culminating in the blooms of recent decades. These studies show concomitant increases in sediment N (～20%) and P (～50%) along with shifts in the algal flora indicating increased eutrophication. These changes correlate with increased human impacts, such as deforestation, construction, roadbuilding etc., and especially the ditching, diking and draining of adjacent wetlands for conversion to agriculture. Agricultural nutrient runoff, especially P, has been often cited as the cause of the AFA blooms, and most attention has been focused on the dynamics of UKL during the summer bloom. We propose that a more significant factor may be the loss of early‐season suppression of AFA because of the loss of the lake‐associated wetlands, which originally constituted 42% of the lake area, and which have declined in area by 66.3% since the late 1800's. The melting of snow and ice in the spring would flush into the lake a surge of wetland plant decomposition products, most significantly organic acids and humic substances. We propose that formerly these wetland effluents caused a complex of effects on lake pH, solar UV transparency, photochemical interactions, nutrient availability, and Daphnia grazing dynamics, which would have combined to prevent the development of any AFA bloom.     

Subfossil chironomids (Diptera,Chironomidae) in three Tatra Mountain lakes (Slovakia) on an acidification gradient

Three lakes were studied in the High Tatra Mountains at altitudes from 2000 to 2157 m a.s.l., which represent three categories of acidity status recognised in the Tatra lakes in the 1980s: non-acidified, acidified, and strongly acidified. Subfossil chironomid remains from dated sediment cores covering ca. throughout 200 years were analysed. The chironomid thanatocoenoses of all cores layers reflected ultra-oligotrophic non-acidified conditions in L’adové pleso. Nevertheless, the finding of the acid-tolerant species Zalutschia tatrica in littoral samples in 1980s indicates that the littoral zone was more influenced by acidification than deeper areas of the lake. Three stages of lake developmental history can be distinguished in the acidified lake Vy?né Wahlenbergovo pleso based on the chironomid subfossil record: the pre-acidification stage before the 1920s, the anthropogenic acidification stage between the 1920–1980s, and the recovery of the lake from the end of the 1980s. The period of acidification was accompanied by a decline in chironomid numbers, while chironomid fauna composition was unchanged during the whole investigated history of the lake. None of the chironomid taxa present prior to the acidification period disappeared during the peak of acidification. The most apparent change in the sediment record taken from the strongly acidified lake Starolesnianske pleso was recognised in the layers corresponding to the period 1960–1980. It is characterised by the disappearance of the dominant acid-sensitive Tanytarsus lugens group and the dominance of acidtolerant Tanytarsus gregarius group. The most recent chironomid assemblage probably benefits from the amount of food resources as a result of increased lake productivity induced by acidification.     

Seasonal changes in sediment and water chemistry of a subtropical shallow eutrophic lake

A field study was conducted (May 1981 to June 1982) to develop a data-base on seasonal changes of water and sediment chemistry of Lake Monroe (4 000 ha surface and ca. 2 m deep) located in central Florida, USA. This shallow eutrophic lake is a part of the St. Johns River. Quantitative samples of lake water and sediments were collected on a monthly basis from 16 stations and analyzed for various physico-chemical parameters. Relatively high levels of dissolved solids (mean electrical conductivity (EC) = 1832 µS cm¹) prevailed in the lake water, and seasonal changes in EC were probably associated with hydrologic flushing from external sources, such as incoming water from upstream as well as precipitation. Average monthly levels of total N and P during the study period were 1.82 and 0.21 mg l^–1, respectively. Nutrient concentrations in the water did not show any strong seasonal trends. Organic matter content of lake sediments ranged from 1 to 182 g C kg^–1 of dry sediment, reflecting considerable spatial variability. All nutrient elements in the sediments showed highly significant (P < 0.01) correlations with sediment organic C, though little or no significant relationship appeared at any sampling period between water and sediment chemistry of the lake. Temporal trends in water and sediment chemical parameters may have been concealed by periodic hydrologic flushing of the St. Johns River into Lake Monroe.Florida Agricultural Experiment Stations Journal Series No. 7836.     

Natural and anthropogenic forcing of aquatic macrophyte development in a shallow Danish lake during the last 7000 years

Aim To investigate the long‐term changes in aquatic vegetation in a lowland, shallow lake, and to assess the relationship between aquatic vegetation and natural and anthropogenic catchment changes. Location Gundsømagle Sø, Zealand, Denmark: a shallow (mean depth 1.2 m), hypereutrophic lake (mean annual total phosphorus (TP) c. 700 μg TP L^?1) located in a predominantly agricultural catchment (88% cultivated land). The lake is presently devoid of macrophytes. Methods One hundred and forty‐seven contiguous samples from a sediment core (taken in 2000) were analysed for macrofossil remains together with loss‐on‐ignition and dry weight. From an earlier sediment core (taken in 1992), 67 samples were analysed for pollen and the two cores were correlated using the ignition residue profiles. Core chronology was determined by ²¹⁰Pb and ¹³⁷Cs dating of the recent lake sediments, while older sediments were dated by pollen‐stratigraphical correlation, as ¹⁴C dating proved problematical. Aquatic macrofossil abundance was used to reconstruct past changes in the lake's plant community and water‐level. The contemporary catchment land‐use change was inferred from sedimentary pollen data, and soil erosion to the lake was deduced from the minerogenic content of the lake sediments. Results The macrofossil record covers the last 7000 years, but aquatic plant remains were scarce prior to c. 1300 bc . After this date the abundance of submerged and emergent macrophyte remains increased dramatically, paralleled by an increase in sediment minerogenic matter and non‐arboreal pollen (NAP). Aquatic plant remains were abundant for more than 3000 years until the mid 1900s. Macrofossils of Linum usitatissimum (L.) (flax) and high pollen percentages of ‘Cannabis type’ (hemp) were recorded in periods between c. 1150 bc and 1800 ad . Main conclusions Our study suggests that, between c. 5000 bc and 1300 bc , the submerged plant community was confined to the littoral zone. From 1300 bc onwards, the submerged macrophyte vegetation expanded rapidly across the lake bed, presumably as a response to lake shallowing caused by a combination of climatic‐induced water‐level lowering and enhanced erosional infilling of the lake basin due to intensified anthropogenic activities in the catchment. The lake was meso‐eutrophic and had an extensive and diverse aquatic flora for more than 3000 years, until the middle of the twentieth century. In periods between c. 1150 bc and 1800 ad , the lake experienced direct anthropogenic impact from retting of fibre plants (Linum and Cannabis). Over the last 200 years, erosional infilling of the lake basin increased drastically, probably as a result of agricultural intensification. In the twentieth century, the lake was strongly affected by nutrient enrichment from both point sources (sewage from built‐up areas) and diffuse agricultural run‐off which led to hypertrophic conditions and the collapse of the submerged vegetation c. 1950–60. The concept of ‘naturalness’ and the implications for lake conservation are discussed.     

Environmental changes in Lake Taihu during the past century as recorded in sediment cores

The Lake Taihu drainage basin is an economically developed area with some of the highest population densities in China. The lake has deteriorated due to ecological destruction and eutrophication. Three short sediment cores from eastern, northeastern and southwestern Lake Taihu were collected. Total organic carbon (TOC), total nitrogen (TN), pigments, elements and particle size were analyzed for the purpose of understanding past trophic status and pollution levels. Sedimentation rates were based on ¹³⁷Cs or ²¹⁰Pb methods. Results indicated that sediment particle size became coarser since the 1920s, and the lake was contaminated by heavy metals, such as Cu and Zn, since the 1970s. A remarkable increase in eutrophication since the 1980s due to increased loading of untreated effluents from industry, agriculture and urbanization is reflected by total organic carbon, total nitrogen and pigments in the studied cores. However the onset times of eutrophication in different parts of Lake Taihu were not synchronous.     

Palaeoenvironmental Changes Inferred from Biological Remains in Short Lake Sediment Cores from the Central Alps and Dolomites

In this paper we review research on eutrophication, acidification and climate change based on studies of lake sediments in the Alps. Studies on fossil diatoms, chrysophytes, plant pigments, Cladocera and carbonaceous particles in a number of high altitude lakes in Italy and Switzerland have been used to track environmental changes. We present an original study on fossil Cladocera in sediment cores from 29 lakes that have shown changes in biodiversity from the pre-industrial period (ca. pre AD 1850) to the present. These data show that altitude, ca. 60 years of acidification impacts and fish stocking are the most important factors that affect their distribution and abundance. We review further case studies from two morphometrically and chemically different lakes (Tovel and Paione Superiore). Their lake sediment records span 400 and 150 years, respectively; multi-core, multi-proxy analyses show their pH, trophic and climatic evolution over time.     

Factors influencing the variability of pigments in the surface sediments of mountain lakes

1. Surface sediment biofilm samples from 82 Pyrenean lakes were analysed for marker pigment composition using high performance liquid chromatography (HPLC). 2. Variability in the pigment composition among lakes was investigated by multivariate statistical analyses using a large data set of factors describing lake chemical, physical, morphological and catchment characteristics. 3. Due to the widely varying light penetration in the lakes, the most significant gradient of pigment composition extended from a benthic to a planktonic signal. The most important pigments in the gradient were alloxanthin (cryptophytes marker pigment, planktonic signal) and diatoxanthin (diatoms marker pigment, benthic signal). The molar ratio between these two marker pigments was positively correlated with lake depth. 4. Chlorophyll‐a preservation was found to be positively related to light penetration and the development of an autothrophic biofilm on the surface sediment and negatively related to decreasing pH and the percentage of alpine meadows in the lake catchments. 5. Zooplankton marker pigments in the surface sediment, including grazing by‐products (e.g. phaeophorbides) and carotenoids (astaxanthin, canthaxanthin, echinenone) incorporated into their tissues, were correlated with the areal abundance of zooplankton. 6. Marker pigments for photosynthetic bacteria, BChl‐e and okenone, were found mainly in relatively shallow lakes with large catchments that are forested, probably because of their higher loading of allochthonous organic matter. 7. The evaluation of a preservation index (Chl‐a expressed as a percentage of a‐phorbins) and the alloxanthin/diatoxanthin ratios throughout the sediment record of mountain lakes can provide evidence of historical changes in the relative importance of planktonic versus benthic primary production and might ultimately be interpreted in terms of climatic or environmental changes.     

Paleolimnological reconstruction of the effects of atmospheric deposition of acids and heavy metals on the chemistry and biology of lakes in New England and Norway

Sediment cores from nine lakes in southern Norway (N) and six in northern New England (NE) were dated by ¹³⁷Cs, ²¹⁰Pb and in NE also by pollen, and were analyzed geochemically and for diatoms. Cores from two N and three NE lakes were analyzed for cladocerans. ¹³⁷Cs dating is unreliable in these lakes, probably due to mobility of Cs in the sediment. In Holmvatn sediment, an up-core increase in Fe, starting ca. 1900, correlates with geochemical indications of decreasing mechanical erosion of soils. Diatoms indicate a lake acidification starting in the 1920's. We propose that soil Fe was mobilized and runoff acidified by acidic precipitation and/or by soil acidification resulting from vegetational succession following reduced grazing. Even minor land use changes or disturbances in lake watersheds introduce ambiguity to the sedimentary evidence relating to atmospheric influences. Diatom counts from surface sediments in 36 N and 31 NE lakes were regressed against contemporary water pH to obtain coefficients for computing past pH from subsurface counts. Computed decreases of 0.3–0.8 pH units start between I890 and I930 in N lakes already acidic (pH 5.0–5.5) before the decrease. These and lesser decreases in other lakes start decades to over a century after the first sedimentary indications of atmospheric heavy metal pollution. It is proposed that the acidification of precipitation accompanied the metal pollution. The delays in lake acidification may be due to buffering by the lakes and watersheds. The magnitude of acidification and heavy metal loading of the lakes parallels air pollution gradients. Shift in cladoceran remains are contemporary with acidification, preceding elimination of fishes.     

Paleoecological reconstruction of changes in the productivity of a small,meromictic lake in Southern Ontario,Canada

Some aspects of the paleoproductivity of meromictic Crawford Lake, near Toronto, are inferred from a study of its sedimentary pigments, and diatoms. Several stages of lake development are observed over the 35 cm-deep sediment core removed from the center of Crawford Lake. Evidence of changes in lake productivity during the last 300 years was reflected by significant stratigraphic sediment pigment changes which were associated with European settlement in the Crawford Lake watershed and recent alterations associated with the area's operation by the Conservation Authority (1969 — present). One of the most important factors correlated with paleoproductivity was land clearance (mainly logging of white oak and pine). Deforestation during the last century is correlated with an increase in the amount of algal pigments deposited in the lake's sediments during the 1800's. During the last 10 years a striking increase in the accumulation of chlorophyll derivatives was observed. This is correlated with a dramatic increase in the number of visitors to the lake.Stratigraphic changes in the ratio of cyanobacterial to phototrophic bacterial pigment accumulation are used to infer changes which occurred during the shift from mesotrophy to eutrophy in Crawford Lake.     

Quantifying denudation and sediment–accumulation systems (open and closed lakes): basic concepts and first results

The quantitative aspects of denudation in the drainage areas and sediment accumulation in the corresponding closed or open lakes are approached using two methods: (1) from known mean rates of mechanical and chemical denudation and the ratio of the drainage/lake area, average terrigenous and potential chemical (biogenic and evaporites) sedimentation rates in the lake are determined; or (2) from known sedimentation rates the average denudation rate is found. In closed systems the total mass denuded during a certain unit of time is deposited in the lake as terrigenous, biogenic and evaporitic sediments, whereas open lake systems lose a major part of the dissolved river input derived from chemical denudation. Applying elementary equations describing these relationships and using ratios between mechanical and chemical denudation (weathering ratio) established in other regions, e.g. chemical weathering in the lake drainage can be estimated. Likewise, the knowledge of the ‘bio-index' (ratio of biogenic sediment/total dissolved mass) allows the calculation of the loss of dissolved matter through the outflow from the lake, etc. In a first step, existing data from modern lake systems are evaluated. Later, idealized lake systems in various climatic and morphotectonic zones can be simulated and the results utilized to better understand ancient lake systems. Modern Alpine lakes reflect increased mechanical denudation rates (100 to ≥300 mm/ka) in highly elevated drainage areas of the Central Alps (mainly crystalline rocks) and high chemical denudation (up to ca. 100 mm/ka) in carbonate rocks of the Northern Calcareous Alps. Mechanical denudation is significantly enhanced by glaciation, but transport of the detritus into lakes may be delayed until the time immediately after glacial retreat. Average rates of terrigenous lake sedimentation amount to 5 to ≥15 mm/a (crystalline rocks, high relief), or are ≤5 mm/a (carbonate rocks). Delta outbuilding was a main factor in the filling of many Alpine lakes. Lakes draining low-altitude, semi-arid parts along the eastern margin of the Alps (Lake Balaton in Hungary) have low sedimentation rates and reflect the influence of increased temperature and vegetation. In East Africa, the transition from the Late Pleistocene (arid) to the early Holocene (humid) and again back to somewhat drier conditions caused substantial rises in lake levels and changes from closed to open lake systems. In such cases, the calculation of denudation from accumulation rates is biassed as a result of intermittent sediment storage at the lake margins (low sedimentation rates in the lake centre during lake highstands) or redeposition of sediment during lowstands (lowstand shedding). The modern denudation rates of East African rift lake systems vary by a factor of ≥15 (Lake Tanganyika, crystalline rocks and more dense vegetation cover, total denudation rate ca. 4 mm/ka; Lake Turkana, young volcanic rocks and tephra, sparse vegetation, mechanical denudation rate ca. 60 mm/ka).     

Characteristic seasonal variation in dissolved uranium concentration induced by the change of lake water pH in Lake Biwa,Japan

Epilimnion-dominated profiles of dissolved uranium (U) have been observed during summer in an oxygenated Japanese lake, Lake Biwa, contrary to the commonly accepted view that U shows conservative behavior in oxygenated seas and lakes. Monthly observations were conducted to reveal the mechanism for such characteristic distribution and geochemical behavior of dissolved U in the lake. In the surface water, dissolved U concentration started to increase in spring, peaked in summer, and decreased from autumn to winter. In contrast, the concentration remained almost constant in the middle layer (40 m depth) and decreased slightly in the bottom layer (70 m depth) throughout the stagnation period. Mass balance calculations of U suggest that the major mechanism for seasonal variations in the surface layer is the supply of U, not via water inflow from the watershed, but by internal chemical reactions within the lake. A laboratory experiment using the lake water and sediment demonstrated that U was desorbed from and adsorbed onto sediment in response to variations in lake water pH. From these results, it is inferred that the seasonal variation in the concentration of dissolved U in the epilimnion results mainly from the desorptive/adsorptive processes of U between sediment/water interface in response to variation in water pH, which is affected by biological activity in the lake.     

Geochemical and physical characteristics of river and lake [2pt] sediments at Naivasha,Kenya

Prior studies on Lake Naivasha relevant to understanding sediment dynamics include a bathymetric map, a paleolimnological study of fossil invertebrate assemblages in lake sediment, an overview of lake level fluctuations throughout the 20th century, and identification of a dynamic assemblage of macrophyte zones that has responded both to these changes in lake level and to more recent, alien species. Sediment samples collected from the rivers systems and the lake were examined physically and chemically. River sediment characteristics reflect geology and geomorphological processes in the catchment, whereas lake sediment stratigraphy has responded to past lake level changes. Such changes have caused significant changes in aquatic vegetation assemblages. Present day sediment dynamics in the lake are governed by the presence of river point sources in the north and wave-induced re-suspension, such that sediments introduced by rivers are transported in easterly and southerly directions, and are eventually deposited in the eastern, central and southern parts of the lake. Sedimentary deposition is also occurring in northern areas that once were protected by papyrus swamp vegetation but now only have a narrow fringe, highlighting the important role of swamp vegetation in filtering out suspended particulates and thereby controlling water quality in the lake. Geochemical analyses of river and lake sediments indicate that they represent fairly undisturbed background conditions. Higher-than-expected concentrations of cadmium, iron, nickel and zinc found in both river and lake sediment are likely to derive from volcanic rocks and/or lateritic soils found in the lake catchment.     

Interpretation of caesium-137 profiles in lacustrine and other sediments: the role of catchment-derived inputs

The caesium-137 profiles obtained in many investigations which have attempted to use caesium-137 measurements as a means of assessing the rate of accumulation of recent lake sediments, have not conformed to the classic shape expected from the record of fallout input. Such deviations have been accounted for in terms of post-depositional mobility of the caesium-137 input (e.g. bioturbation) and delayed inputs from the drainage basin. There have, however, been few attempts to determine the likely character of the drainage basin input and to analyse the role of such inputs in influencing the precise form of the caesium-137 profile. This paper presents the results of an attempt to employ existing knowledge concerning the behaviour of caesium-137 in soils and the processes of sediment mobilization to predict the likely form of the record of caesium-137 input to a lake or river floodplain from its drainage basin. The influence of this input on the profile shape will depend on the relative importance of the atmospheric fallout to the lake or floodplain surface and the drainage basin input to the total caesium-137 inventory in the sediment core, and on the land use and sediment sources in the drainage basin. By incorporating the drainage basin input into a simple model of caesium-137 accumulation in lake and floodplain sediments, it was possible to account for the profile shapes measured in four cores investigated by the authors and therefore to verify their utility for assessing rates of sediment accumulation.     

Spheroidal carbonaceous particles as a marker for recent sediment distribution

We have tested the hypothesis that spheroidal carbonaceous particles (SCP), emitted from oil and coal combustion, can be used as a marker for recent sediment distribution in lake basins. Sediment traps have been used to study when and how the particles are introduced into a lake, a laboratory experiment has been made to study the depositional behaviour of the particles in relation to other sedimentary constituents, and spatial SCP distributions have been surveyed in three lakes. The results support the hypothesis that SCP can be used as a marker, and also indicate that acidification can change sediment distribution in lakes.