Contemporary Lake Research Articles

Spatial and Temporal Genetic Analysis of Walleyes in the Ohio River

AbstractPrevious genetic analyses have shown that Walleyes Sander vitreus in the upper Ohio River comprise two distinct genetic strains: (1) fish of Great Lakes origin that were stocked into the Ohio River basin and (2) a remnant native strain (Highlands strain). Resource agencies are developing management strategies to conserve and restore the native strain within the upper reaches of the Ohio River. Hybridization between strains has impacted the genetic integrity of the native strain. To better understand the extent and effects of hybridization on the native strain, we used mitochondrial DNA and microsatellite markers to evaluate the spatial (river sections) and temporal (pre‐ and poststocking) genetic diversity of Ohio River Walleyes. Contemporary Lake Erie Walleyes and archival museum specimens collected from the Ohio River basin were used for comparison to contemporary Ohio River samples. Although there was evidence of hybridization between strains, most of the genetic diversity within the Ohio River was partitioned by basin of origin (Great Lakes versus the Ohio River), with greater similarity among river sections than between strains within the same section. Results also suggested that the native strain has diverged from historical populations. Furthermore, notable decreases in measures of genetic diversity and increased relatedness among native‐strain Walleyes within two sections of the Ohio River may be related to stocking aimed at restoration of the Highlands strain. Our results suggest that although the Highlands strain persists within the Ohio River, it has diverged over time, and managers should consider the potential impacts of future management practices on the genetic diversity of this native strain.

Collapse of Lake WhitefishCoregonus clupeaformis(Mitchill, 1818) species pair in Como Lake, Ontario

Lake Whitefish (Coregonus clupeaformis) species pairs exist in a small number of northern temperate lakes in North America. Sympatric populations of dwarf and normal-sized whitefish represent a continuum of morphological and genetic divergence among these lakes. In this study, we used morphological and age data to assess the status of the Lake Whitefish species pair in Como Lake, Ontario (Canada). Whitefish were collected with gill-nets from spawning shoals during the fall of 2012 and 2014 and compared with archived (circa 1989) dwarf and normal-sized specimens. Our study indicates that the two previously collected forms have been replaced by a single form with a different morphology and age structure. In comparison to archived specimens, contemporary Lake Whitefish were larger, deeper-bodied and older; with more gill rakers, lateral line scales, smaller eyes and a wider interorbital width. We hypothesize that the recent invasion of Como Lake by Spiny Waterflea (Bythotrephes longimanus) has affected whitefish growth rates and changed the ecosystem conditions that previously maintained the species pair.

Air–lake boundary layer and performance of a simple lake parameterization scheme over the Tibetan highlands

The Tibetan Plateau (TP) is covered by thousands of lakes which affect the regional and global heat and mass budget with important implications for the current and future climate change. However, the heat and mass budget of TP lakes and the performance of contemporary lake models over TP have not been quantified to date. We utilise 3-yr observations from Ngoring Lake, the largest lake in the Yellow River source region of TP, to investigate the typical properties of the lake–air boundary layer and to evaluate the performance of a simplified lake scheme from the Community Land Model version 4.5 (SLCLM) as one of the most popular lake parameterization schemes in atmospheric models. The strong boundary layer instability during the entire open-water period is a distinguishing feature of the air–lake exchange, similar to the situation over tropical and subtropical lakes, while contrasting to the generally stable atmospheric conditions commonly observed over ice-free temperate and boreal lakes from spring to summer. The rather simple algorithm of SLCLM demonstrated good performance in these conditions. A series of sensitivity simulations with SLCLM revealed strong shortwave solar radiation and cold air temperatures because of high altitude as the primary factors causing the boundary layer instability. The outcomes of the study (1) demonstrate the role of TP lakes as accumulators of shortwave solar radiation releasing the heat into the atmosphere during the entire open-water period; (2) justify the use of simple lake models for the Tibetan highlands, while revealing remarkable uncertainties in the estimations of the latent heat flux; (3) qualify the strong cool-skin effect on the lake surface which results from permanent negative air–lake temperature difference, and should be taken into account when interpreting remote sensing data from highland areas.

Aeolian Activity on the Northern Coast of Lake Peipsi, North-Eastern Estonia

ABSTRACT Ratas, U.; Raukas, A.; Rivis, R., and Tavast, E., 2015. Aeolian activity on the northern coast of Lake Peipsi, north-eastern Estonia. Lake Peipsi is one of the largest inland water bodies in Europe. In the geological past, it was much bigger, flooding vast areas to the north of the contemporary lake. In the proglacial lakes formed behind the retreating glacier, large amounts of sand and silt accumulated, providing source material for various aeolian deposits and landforms, such as coversand hillocks, blowouts, deflation hollows, foredunes, inland and coastal dunes, and wind-eroded plains. Because of land uplift they are of different age (from the Late Glacial up to the present) and structure. The oldest dunes in the area were formed in severe climatic conditions of the Younger Dryas and the Early pre-Boreal. Near the contemporary shoreline of Lake Peipsi, the rhythmically changing water level has strongly influenced the aeolian redistribution of sediments. Here specific “basket-trap” dunes were f...

A Bayesian network incorporating observation error to predict phosphorus and chlorophyll a in Saginaw Bay

Empirical relationships between lake chlorophyll a and total phosphorus concentrations are widely used to develop predictive models. These models are often estimated using sample averages as implicit surrogates for unknown lake-wide means, a practice than can result in biased parameter estimation and inaccurate predictive uncertainty. We develop a Bayesian network model based on empirical chlorophyll-phosphorus relationships for Saginaw Bay, an embayment on Lake Huron. The model treats the means as unknown parameters, and includes structure to accommodate the observation error associated with estimating those means. Compared with results from an analogous simple model using sample averages, the observation error model has a lower predictive uncertainty and predicts lower chlorophyll and phosphorus concentrations under contemporary lake conditions. These models will be useful to guide pending decision-making pursuant to the 2012 Great Lakes Water Quality Agreement.

Of Fur and Fins: Quantifying Fur Trade Era Fish Harvest to Assess Changes in Contemporary Lake Whitefish (Coregonus clupeaformis) Production at Lac La Biche, Alberta

The history of fisheries exploitation in Canada has significant ties to the development and westward expansion of the fur trade. Understanding the scale and nature of this relationship is important when assessing the developmental or evolutionary history of a system. This study uses estimates of human population size and subsistence lake whitefish (Coregonus clupeaformis) consumption to estimate annual fish harvest at Lac la Biche, Alberta(54o52'N, 112o05'W) during the fur trade era and to assess the magnitude and potential influence of historic harvest on contemporary harvest potential. Historic (1800-1911) lake whitefish harvest increased approximately 10-fold, from 74,000 kg in 1800 to 811,000 kg in 1875, immediately preceding a lake whitefish population collapse in 1878. Following the initiation of a formal commercial fishing industry, contemporary (1912-2009) harvest peaked at 424,000 kg, about one half the previous estimated maximum. The persistence of low contemporary harvest biomasses suggests a shift from a system of high- to low-lake whitefish productivity, likely resulting from decreasing ecosystem resilience with increasing harvest pressure. Knowledge of historic fish harvest can minimize the impacts of the shifting baseline syndrome by elucidating the magnitude and impacts of historic harvests on future harvest potential and potential production.

Temporal changes in the benthic invertebrate community of Lake Simcoe, 1983–2008

The Lake Simcoe ecosystem has been subjected to multiple stressors during the past century, including changes in nutrient loads and the introduction of non-native species. To evaluate the effects of these changes on the Lake Simcoe offshore benthic community, we collected benthic samples in 2008 at sites allowing statistical comparisons with detailed surveys in 1983 and qualitative comparisons with others focussing more on nearshore collections in 2005 and 1926. Total abundance of benthic invertebrates declined significantly from 1983 to 2008, and correspondence analyses revealed major changes in community composition. Typical of the overall pattern, three of the major benthic taxa (Nematoda, Oligochaeta and Ostracoda) declined from 1983 to 2008, particularly at depths greater than 10 m. Chironomids showed little change in abundance from 1983 to 2008 but their biomass increased because body length nearly doubled during this period. Oligochaete abundance declined but biomass remained the same due to increases in body length from 1983 to 2008. Ostracod abundance and biomass both declined because body size distributions did not change from 1983 to 2008. In contrast to total invertebrate abundance, that of dreissenid mussels, amphipods, isopods, gastropods and sphaeriids increased and these taxa also occupied greater depths (up to 30 m) in 2008 compared with historical surveys. Our results suggest a strong effect of both water quality changes and dreissenid establishment in shaping the contemporary Lake Simcoe benthic invertebrate community.

Microbial communities of the stratified soda Lake Doroninskoe (Transbaikal region)

The physicochemical properties, species composition, and vertical distribution of microorganisms in the water column, shoreline microbial mat, and small shoreline mud volcanoes of the stratified soda Lake Doroninskoe were investigated in September 2007. The lake is located in the Transbaikal region, in the permafrost zone (51°25′N; 112°28′E). The maximal depth of the contemporary lake is about 6 m, the pH value of the water is 9.72, and the water mineralization in the near-bottom horizon is 32.3 g l−1. In summer, the surface oxygen-containing horizon of the water column becomes demineralized to 26.5 g l−1; at a depth of 3.5–4.0 m, an abrupt transition occurs to the aerobic zone containing hydrosulfide (up to 12.56 g l−1). Hydrosulfide was also detected in trace quantities in the upper water horizons. The density stratification of the water column usually ensures stable anaerobic conditions until the freezing period (November and December). The primary production of oxygenic phototrophs reached 176–230 μg l−1. High rates of dark CO2 assimilation (61–240 μg l−1) were detected in the chemocline. Within this zone, an alkaliphilic species of sulfur-oxidizing bacteria of the genus Thioalkalivibrio was detected (104 cells ml−1). Lithoheterotrophic bacteria Halomonas spp., as well as bacteriochlorophyll a-containing aerobic anoxygenic phototrophic bacteria (AAP) Roseinatronobacter sp. capable of thiosulfate oxidation, were isolated from samples collected from the aerobic zone (0–3 m). The water transparency in September was extremely low; therefore, no visible clusters of anoxygenic phototrophic bacteria (APBs) were detected at the boundary of the hydrosulfide layer. However, purple sulfur bacteria which, according to the results of the 16S rRNA gene analysis, belong to the species Thioalkalicoccus limnaeus, Ectothiorhodospira variabilis, “Ect. magna,” and Ect. shaposhnikovii, were isolated from samples of deep silt sediments. Ect. variabilis and Ect. shaposhnikovii were the major APB species in the shoreline algo-bacterial mat. The halotolerant bacterium Ect. shaposhnikovii, purple nonsulfur bacteria of the genus Rhodobacter, and AAP of Roseococcus sp. were isolated from the samples collected from mud volcanoes. All these species are alkaliphiles, moderate halophiles, or halotolerant microorganisms.

Long‐term dynamics of submerged macrophytes and algae in a small and shallow, eutrophic lake: implications for the stability of macrophyte‐dominance

Summary1. Submerged macrophyte and phytoplankton components of eutrophic, shallow lakes have frequently undergone dynamic changes in composition and abundance with important consequences for lake functioning and stability. However, because of a paucity of long‐term survey data, we know little regarding the nature, direction and sequencing of such changes over decadal–centennial or longer timescales.2. To circumvent this problem, we analysed multiple (n = 5) chronologically correlated sediment cores for plant macro‐remains and a single core for pollen and diatoms from one small, shallow, English lake (Felbrigg Hall Lake, Norfolk, U.K.), documenting 250 years of change to macrophyte and algal communities.3. All five cores showed broadly similar stratigraphic changes in macrophyte remains with three distinct phases of macrophyte development: Myriophyllum–Chara–Potamogeton (c. pre‐1900), to Ceratophyllum–Chara–Potamogeton (c. 1900–1960) and finally to Zannichellia–Potamogeton (c. post‐1960). Macrophyte species richness declined from at least 10 species pre‐1900 to just four species at the present day. Additionally, in the final Zannichellia–Potamogeton phase, a directional shift between epi‐benthic and phytoplankton‐based primary production was indicated by the diatom data.4. Based on macrophyte–seasonality relationships established for the region, concomitant with the final shift to Zannichellia–Potamogeton, we infer a reduction in the seasonal duration of plant dominance (plant‐covered period). Furthermore, we hypothesise that this change in species composition resulted in a situation whereby macrophyte populations were seasonally ‘sandwiched’ between two phytoplankton peaks in spring and late summer as observed in the contemporary lake.5. We suggest that eutrophication‐induced reductions in macrophyte species richness, especially if the number of plant‐seasonal strategies is reduced, may constrict the plant growing season. In turn, this may render a shallow lake increasingly vulnerable to seasonal invasions of phytoplankton resulting in further species losses in the plant community. Thus, as part of a slow (over perhaps 10–100s of years) and self‐perpetuating process, macrophytes may be gradually pushed out by phytoplankton without the need for a perturbation as required in the alternative stable states model of plant loss.

Weathering, Sr fluxes, and controls on water chemistry in the Lake Qinghai catchment, NE Tibetan Plateau

AbstractStrontium (Sr) concentrations and isotopic ratios have been measured in a series of water and rock samples from most of the major tributaries of the Lake Qinghai basin on the north‐eastern Tibetan Plateau. Dissolved Sr and 87Sr/86Sr show ranges of 488–12 240 nmol/l and 0·710497–0·716977, respectively. These data, together with measurements of major cations and anions in rivers and their tributaries and various lithologies of the catchment, were used to determine the contributions of Sr and its isotopic expense to rivers and lakes. Our results demonstrate that the chemical components and 87Sr/86Sr ratios of the alkaline waters are derived from mixing of carbonate and silicate sources, with the former contributing 72 ± 18% dissolved Sr to rivers. The difference in tributary compositions stems from the lithology of different river systems and low weathering intensity under a semi‐arid condition. Variation in 87Sr/86Sr ratios places constraint on the Sr‐isotopic compositions of the main tributaries surrounding Lake Qinghai. The water chemistry of the Buha River, the largest river within the catchment underlain by the late Paleozoic marine limestone and sandstones, dominates Sr isotopic composition of the lake water, being buffered by the waters from the other rivers and probably by groundwater. However, the characteristic chemical composition of the lake itself differs remarkably from the rivers, which can be attributed to precipitation of authigenic carbonates (low‐magnesium calcite, aragonite, and dolomite), though this does not impact the Sr isotope signature, which may remain a faithful indicator in paleo‐records. Regarding the potential role of groundwater input within the Lake Qinghai systems in the water budget and water chemistry, we have also determined the Sr concentration and 87Sr/S6Sr ratio of groundwater from diverse environments. This has allowed us to further constrain the Sr isotope systematic of this source. A steady‐state calculation gives an estimate for the groundwater flux of 0·19 ± 0·03 × 108 m3/yr, accounting for about 8% of contemporary lake Sr budget. Copyright © 2010 John Wiley & Sons, Ltd.

Fish fossils as paleo-indicators of ichthyofauna composition and climatic change in Lake Malawi, Africa

Numerous biological and chemical paleorecords have been used to infer paleoclimate, lake level fluctuation and faunal composition from the drill cores obtained from Lake Malawi, Africa. However, fish fossils have never been used to examine changes in African Great Lake vertebrate aquatic communities nor as indicators of changing paleolimnological conditions. Here we present results of analyses of a Lake Malawi core dating back ∼ 144 ka that describe and quantify the composition and abundance of fish fossils and report on stable carbon isotopic data ( δ 13C) from fish scale, bone and tooth fossils. We compared the fossil δ 13C values to δ 13C values from extant fish communities to determine whether carbon isotope ratios can be used as indicators of inshore versus offshore pelagic fish assemblages. Fossil buccal teeth, pharyngeal teeth and mills, vertebra and scales from the fish families Cichlidae and Cyprinidae occur in variable abundance throughout the core. Carbon isotopic ratios from numerous fish fossils throughout the core range between − 7.2 and − 27.5‰, similar to those found in contemporary Lake Malawi benthic and pelagic fish faunas. These results are the first paleo-record of fish fossils from a Lake Malawi sediment core and the first reported δ 13C values from Lake Malawi fish fossils. This approach provides a new methodology and framework for interpreting pelagic versus inshore fish faunas, lake level fluctuations and the evolution of the Lake Malawi fish assemblages.

Surface water acidification and critical loads: exploring the F-factor

Abstract. As acid deposition decreases, uncertainties in methods for calculating critical loads become more important when judgements have to be made about whether or not further emission reductions are needed. An important aspect of one type of model that has been used to calculate surface water critical loads is the empirical F-factor which estimates the degree to which acid deposition is neutralised before it reaches a lake at any particular point in time relative to the pre-industrial, steady-state water chemistry conditions. In this paper we will examine how well the empirical F-functions are able to estimate pre-industrial lake chemistry as lake chemistry changes during different phases of acidification and recovery. To accomplish this, we use the dynamic, process-oriented biogeochemical model SAFE to generate a plausible time series of annual runoff chemistry for ca. 140 Swedish catchments between 1800 and 2100. These annual hydrochemistry data are then used to generate empirical F-factors that are compared to the "actual" F-factor seen in the SAFE data for each lake and year in the time series. The dynamics of the F-factor as catchments acidify, and then recover are not widely recognised. Our results suggest that the F-factor approach worked best during the acidification phase when soil processes buffer incoming acidity. However, the empirical functions for estimating F from contemporary lake chemistry are not well suited to the recovery phase when the F-factor turns negative due to recovery processes in the soil. This happens when acid deposition has depleted the soil store of BC, and then acid deposition declines, reducing the leaching of base cations to levels below those in the pre-industrial era. An estimate of critical load from water chemistry during recovery and empirical F functions would therefore result in critical loads that are too low. Therefore, the empirical estimates of the F-factor are a significant source of uncertainty in the estimate of surface water critical loads and related calculations for quantifying lake acidification status, especially now that acid deposition has declined across large areas of Europe and North America.

Addition of round gobies to the prey community of Lake Ontario and potential implications to thiamine status and reproductive success of Lake Trout

The addition of round goby to the Lake Ontario prey fish community can, because it appears to lack thiaminase, ameliorate the effects of a diet high in thiaminase-rich alewives associated with a thiamine deficiency in Lake Trout. However, the effects of round goby predation may negate such effects. We conducted studies to assess the proportion, by weight, of round gobies in the diet of contemporary Lake Ontario Lake Trout and predicted its effect on egg thiamine concentration, and early mortality syndrome (EMS) and growth retardation, two indicators of thiamine deficiency. We compared these parameters to that for the historic prey community, as well as possible future diet scenarios with progressively greater proportions of round goby (e.g. 50, 75 and 100%). To assess the combined and separate effects of the thiamine deficiency we used a modeling approach to determine the effect of EMS (70%) typical of Lake Ontario Lake Trout, growth retardation, density of Lake Trout egg (e.g. 100, 500, 1000, and 5000 eggs m−2) and round goby (e.g. 1, 9, 18, 35, 75, 150 gobies m−2), three separate durations of round goby residence on Lake Trout spawning habitat (e.g. 30, 60 and 213 days), and at either a shallow water or deepwater temperature regime. Eight years after first invading Lake Ontario, the proportion of round gobies in Lake Trout diets remains low, although the highest observed (25%) would result in significant amelioration of the thiamine deficiency from past conditions. Modeling indicated that the negative effects associated with round goby predation on eggs and fry could easily exceed any positive effects resulting from amelioration of the thiamine deficiency by a diet containing round goby. The effects of goby predation were found to be dependent on density of both Lake Trout eggs and round gobies, and while predation effects were minimally affected by temperature regime, they were highly dependent on the period of round goby residence on Lake Trout spawning habitat.

Empirical evidence of monsoon influences on Asian Lakes

Limited information is available on how the Asian monsoon influences contemporary lake processes, seasonal patterns and interannual variation. Data sets from Nepal (Indian monsoon region) and South Korea (East Asian monsoon region) along with published literature show a wide range in the response of Asian lakes to the seasonal monsoon. Cross-system patterns show nutrients and algal biomass peak in most lakes in response to monsoon inflow, which suggests nonpoint sources dominate external loading. This seasonal increase may be the general phenological model for nutrients and phytoplankton in lakes in monsoon regions. Major ions show dilution by surface runoff during the monsoon and post-monsoon increases. Dilution of nutrients in fertile systems during the monsoon suggests point source inputs dominate peak loads during the non-monsoon period. Peak monsoon runoff can enter the water column of large stratified lakes as a density-dependent interflow at depth in the water column with little direct influence on the mixed layer. In these water bodies nutrients in surface water can be lower during strong monsoon seasons because of sub-surface loading. Short-interval data from individual lakes show the linkage between nutrients and phytoplankton biomass can be uncoupled by physical processes and light limitation, resulting in considerable temporal variation. Collectively these mixed responses limit our ability to generalize patterns or predict conditions in individual lakes during the monsoon.

Physical and Chemical Limnology of a Subsaline Athalassic Lake in West Greenland

Physical and chemical profiles of a shallow (c. 12-m-deep) subsaline (total dissolved solids 2.3-2.8 g l−1) closed-basin lake in the continental area of southwestern Greenland are described for the first time. Watercolumn data for every 5th consecutive day between April 20 and October 6, 2001, and continuous recordings of lake water level and meteorological conditions are used to infer controls on contemporary lake functioning, sediment formation and climate-lake interactions. Limnological observations demonstrate the importance of lake-ice formation and its role in haline convection and the development of meromixis. Observed lake cycling suggest that the lake at present is in a state of near-meromixis where stagnant bottom waters de-stratify through deep penetration of weak haline convective cells by the end of June. From this study, the primary reasons the shallow Greenlandic low salinity lakes develop meromixis are:(i) lack of an outflow (ii) meltwater dilution and chemical stratification of surface waters, (iii) insubstantial wind mixing, (iv) a weak winter thermohaline convective cell forced by cryoconcentration, and (v) biogeochemically enhanced solute concentrations near the sediment bed. Throughout the open water period the hydrological balance is dominated by evaporative losses. Lake surface water conductivities change from 2110 to 2890 μS cm−1 due to the combined effects of open water evaporation, meltwater dilution, diffusive exchanges over the seasonal pycnocline, and boundary mixing. Freeze-out of salts and resulting deep haline convection increase overall water column salinity during winter. Owing to deep convective mixing, plant nutrients are relatively high in the upper watercolumn with a dominant internal source of phosphorous. Extreme productivity pulses of phytoplankton are observed as soon as sub-ice radiation levels increase and directly after ice-out when sufficient wind mixing can support an intense monospecific diatom bloom of Diatoma spp. leading to the rapid depletion of dissolved silica.

Climatic and non-climatic effects on the δ 18O and δ 13C compositions of Lake Awassa, Ethiopia, during the last 6.5 ka

A comparison of a 6450 14C yr δ 18O and δ 13C record of authigenic calcite from Lake Awassa, Ethiopia, with other proxy climate records in the area suggests that the lake records long-term regional climate changes. Co-varying and increasing δ 18O and δ 13C values from ∼4800 BP suggest an aridification of climate after the early Holocene insolation maximum. After 4000 BP, humid conditions return until after ∼2800 BP when δ 18O increases again, reflecting more arid conditions recorded elsewhere in Ethiopia. In addition to these long-term changes, there are abrupt decreases in both δ 18O calcite and δ 13C calcite immediately after tephra layers. The likeliest explanation for these abrupt decreases in isotopes is the effect of tephra on the lake's catchment vegetation. δ 18O, δ 13C and lake-level measurements from Lake Awassa since the 1970s suggest that the lake is currently isotopically sensitive to short-term (annual–decadal) climate change. However, during this period, the catchment has undergone progressive deforestation that may have caused an increase in runoff. Caution is therefore required when reconstructing palaeoclimates as a contemporary lake may not always be a good analogue for lake hydrology in the past.

New geological aspects for freshwater seepage and formation in Eckernförde Bay, western Baltic

The subsurface geology relevant to the submarine freshwater seepage in Eckernförde Bay has been investigated using shallow seismic instruments and vibrocoring. Detailed surveying revealed that the pockmarks are aligned like pearls on a string or densely clustered in furrow-like structures depending on the glacial and postglacial setting of the underlying strata. Two possible aquifers have been verified: The older Miocene sand aquifer is partly sealed by a till unit forming the central part of the Mittelgrund. The younger aquifer consists of a mixture of glacial till and meltwater sediments partly sealed by till and partly by lateglacial galciolacustrine silt and clay sediments. The investigations imply that connections exist between the aquifers and that groundwater leakage takes place in the marginal zones of the bay due to thinning and coarsening of the sediment composition of the lateglacial seal. Within the seepage areas, the pockmarks are restricted to areas covered by unconsolidated Holocene mud of low thickness’ that are easy to penetrate by artesian groundwater. Macrofossil studies and AMS 14C dating of the lateglacial and Holocene units reveal that the Mittelgrund shoal of glacial origin has been modified by coastal processes and formation of cuspate foreland deposits during the subsequent palaeo-lake phases of 15–20 m below the present sea level (b.s.l.). The lake phases correlate in time with the regional Baltic Ice Lake highstand about 10,000 14C years BP and the Ancylus Lake highstand about 9200 14C years BP. This means that local contemporary lakes existed or the western margin of the regional lakes can be moved considerably further west than expected hitherto. In the earliest phase of the Littorina Sea transgression, the Mittelgrund shoal was exposed to coastal erosion once more before the final drowning and the initiation of mud sedimentation in the surrounding basins took place.

Application of AVHRR to monitoring a climatically sensitive playa. case study: Chott El Djerid, Southern Tunisia

The importance of monitoring changes in the levels of lakes within endorheic basins using remotely sensed data as a means of assessing changes in regional aridity is noted. Large salt playas are highlighted as ephemeral lakes that can display extreme sensitivity to changes in regional rainfall patterns, and which commonly do not have extensively managed catchments. To explore the application of high temporal frequency monitoring of salt playas using remote sensing, the Chott el Djerid, a large salt playa situated in southern Tunisia was targeted. A short time series of 39 Advanced Very High Resolution Radiometer (AVHRR; resolution 1·1km at nadir) images of the Chott el Djerid (spanning 36 months between 1987 and 1990) were compiled along with climate information from a weather station at Tozeur. Using image histogram manipulation, lake areas were extracted from the time series. A good level of agreement was observed between recorded rainfall events and the presence of surface water on the playa, and for a limited sample of large flood events it was found that there were significant relationships between rainfall, evaporation and estimated lake areas (r 2 =98·5, p50·001 ). Overall, these data suggest that contemporary lake formation is largely controlled by temporal changes in effective precipitation within the basin. In addition, it was found that the coefficient of variation of the time series, and a combination of temporal reflectance profiles extracted from it, could be used to give a direct indication of which sedimentary surfaces on the playa are affected by large flood events, and the extent to which these events may be preserved within the recent sedimentary record at these sites. Copyright # 1999 John Wiley & Sons, Ltd.

Examination of Contemporary Lake Sediments by Means of Thermal Analysis: The Dabie lake near Szczecin

An attempt was made to use thermal analysis (DTA, TG, DTG) for identification of the mineralogical composition of contemporary lake sediments. Sediments were investigated both in the natural state and after ignition at 300°C. Selected fractions of the sediments in the natural state were also studied. The thermoanalytical curves of mollusca shells, reed and plankton were also recorded for use in the interpretation of the thermoanalytical curves of sediments.

Sedimentary purpurins: oxidative transformation products of chlorophylls

Examination of sediments from three contemporary lakes (Priest Pot, Loch Ness and Lake Baikal) reveals the presence of oxidative transformation products of chlorophyll a in two of the settings. Purpurin-18 phytyl ester, not previously reported in sediments, occurs in the two lakes that have deep water columns. The free acid counterpart, purpurin-18, occurs in only one of the samples, that from Loch Ness. Although, where present, the abundance of purpurin-18 phytyl ester is similar to that of individual chlorin steryl esters there appears not to be a correlation between their abundances in the sediments, suggesting that formation of the purpurins is not mediated by herbivory. The presence of oxidative transformation products of chlorophyll in contemporary lake bottom sediments indicates their formation during the earliest stages in chlorophyll degradation. Their preservation in sediments demonstrates that oxidative transformation reactions in the aquatic environment lead not only to destruction of the chlorophyll macrocycle, but also to structures modified in ring E. This lends support to the view that sedimentary aetioporphyrins originate mainly, though not exclusively, from transformations of chlorophylls that involve oxidative cleavage of the five-membered exocyclic ring.