Identifying from recent sediment records the effects of nutrients and climate on diatom dynamics in Loch Leven

1. Changes in nutrients and climate have occurred over approximately the same timescales in many European lake catchments. Here, we attempt to interpret the sedimentary diatom record of a large shallow lake, Loch Leven, in relation to these pressures using information gained from analysis of long‐term data sets of water quality, climate and planktonic diatoms. 2. The core data indicate the enrichment of Loch Leven starting in c. 1800–1850, most likely from agricultural practices in the catchment, with a more marked phase since c. 1940–1950 caused by increased phosphorus inputs from sewage treatment works, land drainage and a woollen mill. 3. While the recent diatom plankton remains are dominated by taxa associated with nutrient‐rich conditions, an increase in Aulacoseira subarctica relative to Stephanodiscus taxa since the mid‐1980s suggests that reductions in external catchment sources of nutrients (since 1985) may have resulted in partial recovery. This observation accords well with the long‐term monitoring series of water chemistry and phytoplankton. 4. On a decadal‐centennial scale, the eutrophication signal in the sediment record outweighs any evidence of climate as a control on the diatom community. However, at an inter‐annual scale, while the diatom data exhibit high variability, there are several changes in species composition in the recent fossil record that may be attributed to climatic controls. 5. The study highlights the value of a palaeolimnological approach, particularly when coupled with long‐term data sets, for developing our understanding of environmental change at a range of temporal scales. The diatom record in the sediment can be used effectively to track recovery from eutrophication, but requires greater understanding of contemporary ecology to fully interpret climate impacts. 6. The study illustrates the complexity of ecosystem response to synchronous changes in nutrients and climate, and the difficulty of disentangling the effects of these multiple, interacting pressures.     

Phosphorus dynamics in Danish lakes and the implications for diatom ecology and palaeoecology

SUMMARY 1. Seasonal phosphorus variability may be considerable in eutrophic lakes and patterns are influenced by internal, as well as external, nutrient loading. The strong seasonality of planktonic diatoms, with main growth periods in the spring and also in the autumn in meso‐ to eutrophic lakes, means that the measure of total phosphorus (TP) that is most relevant for diatom ecology may not be adequately assessed by single or few measurements during the year. 2. The diatom species assemblage of surface sediments can be used to infer in‐lake nutrient concentrations. Weighted averaging (WA) and weighted averaging partial least squares (WAPLS) regression and calibration models for diatoms and annual mean TP were developed for a 29‐site data set of Danish lakes based on (a) all diatom species and (b) planktonic species only. Jack‐knifed error statistics were: =0.37, root mean squared error of prediction (RMSEP)=0.28 log₁₀μg TP L^?1 and mean bias=0.04 log₁₀μg TP L^?1 for the WAPLS 2‐component model based on all species; =0.23, RMSEP= 0.32log₁₀μg TP L^?1 and mean bias=0.07 log₁₀μg TP L^?1 for the WA model with tolerance downweighting based on planktonic species only. These are comparable with similar, published data sets. 3. A subset of 23 sites was used to develop models based on seasonal TP measurements. Mean spring TP concentrations gave only slightly improved RMSEP values for models based on all species and plankton‐only (0.24 log₁₀μg TP L^?1 and 0.29 log₁₀μg TP L^?1, respectively). 4. Weighted averaging models derive environmental optima, for individual species, which are not necessarily of ecological relevance. However, good water chemistry data are required to model species' responses adequately and to develop calibration data sets.     

Climate drivers of diatom distribution in shallow subarctic lakes

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气候变化与人类活动双重驱动的冷水湖泊富营养化

富营养化对水生生态系统造成的负面影响已在世界范围内广泛发生,尤其对淡水水源地湖泊的水环境质量影响深远,进而引起当地居民的饮用水安全与健康隐患。在人类活动和气候变化的双重驱动下,富营养化辐射的范围不断扩大,从过去主要集中于温带大型浅水湖泊已经扩展到寒冷地区的冷水湖泊。分析了近年来世界范围内高寒地区冷水湖泊富营养化的趋势特征与研究进展,探讨了气候变化、人类干扰(农业活动、畜牧业生产、管理措施不当等)在不同地区冷水湖泊富营养化进程中的作用。在未来的研究中,应进一步加强对冷水湖泊富营养化机制的探讨,并对已有富营养化症状的湖泊进行生态修复,以确保冷水湖泊生态系统健康并改善饮用水源地的环境质量。     

Biological responses to permafrost thaw slumping in Canadian Arctic lakes

1. Rapid environmental change occurring in high‐latitude regions has the potential to cause extensive thawing of permafrost. Retrogressive thaw slumps are a particularly spectacular form of permafrost degradation that can significantly impact lake–water chemistry; however, to date, the effects on aquatic biota have received little attention. 2. We used a diatom‐based palaeolimnological approach featuring a paired lake study design to examine the impact of thaw slumping on freshwater ecosystems in the low Arctic of western Canada. We compared biological responses in six lakes affected by permafrost degradation with six undisturbed, reference lakes. 3. Slump‐affected lakes exhibited greater biological change than the paired reference systems, although all systems have undergone ecologically significant changes over the last 200 years. Four of the six reference systems showed an increase in the relative abundance of planktonic algal taxa (diatoms and scaled chrysophytes), the earliest beginning about 1900, consistent with increased temperature trends in this region. 4. The response of sedimentary diatoms to thaw slumping was understandably variable, but primarily related to the intensity of disturbance and associated changes in aquatic habitat. Five of the slump‐affected lakes recorded increases in the abundance and diversity of periphytic diatoms at the presumed time of slump initiation, consistent with increased water clarity and subsequent development of aquatic macrophyte communities. Slump‐affected lakes generally displayed lower nutrient levels; however, in one system, thaw slumping, induced by an intense fire at the site in 1968, ostensibly led to pronounced nutrient enrichment that persists today. 5. Our results demonstrate that retrogressive thaw slumping represents an important stressor to the biological communities of lakes in the western Canadian Arctic and can result in a number of limnological changes. We also show that palaeolimnological methods are effective for inferring the timing and response of aquatic ecosystems to permafrost degradation. These findings provide the first long‐term perspective on the biological response to permafrost thaw, a stressor that will become increasingly important as northern landscapes respond to climate change.     

Long‐term effects of climate change on carbon flows through benthic secondary production in small lakes

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A comparison of shallow Danish and Canadian lakes and implications of climate change

1. Winter temperatures differ markedly on the Canadian prairies compared with Denmark. Between 1 January 1998 and 31 December 2002, average weekly and monthly temperatures did not drop below 0 °C in the vicinity of Silkeborg, Denmark. Over this same time, weekly average temperatures near Calgary, Alberta, Canada, often dropped below −10 °C for 3–5 weeks and the average monthly temperature was below 0 °C for 2–4 months. Accordingly, winter ice conditions in shallow lakes in Canada and Denmark differed considerably. 2. To assess the implications of winter climate for lake biotic structure and function we compared a number of variables that describe the chemistry and biology of shallow Canadian and Danish lakes that had been chosen to have similar morphometries. 3. The Danish lakes had a fourfold higher ratio of chlorophyll‐a: total phosphorus (TP). Zooplankton : phytoplankton carbon was related to TP and fish abundance in Danish lakes but not in Canadian lakes. There was no significant difference in the ratio log total zooplankton biomass : log TP and the Canadian lakes had a significantly higher proportion of cladocerans that were Daphnia. These differences correspond well with the fact that the Danish lakes have more abundant and diverse fish communities than the Canadian lakes. 4. Our results suggest that severe Canadian winters lead to anoxia under ice and more depauperate fish communities, and stronger zooplankton control on phytoplankton in shallow prairie lakes compared with shallow Danish lakes. If climate change leads to warmer winters and a shorter duration of ice cover, we predict that shallow Canadian prairie lakes will experience increased survivorship of planktivores and stronger control of zooplankton. This, in turn, might decrease zooplankton control on phytoplankton, leading to ‘greener’ lakes on the Canadian prairies.     

Comparison of diatom communities in remote high-mountain lakes using index B and cluster analysis

Six remote high-mountain lakes in SE Switzerland were characterized on the basis of their water chemistry and the composition of their epilithic diatom communities. The index B (Renberg & Hellberg, 1982), calculated by summarizing the relative frequencies of occurrence in all five pH classes defined by Hustedt (1938/39), was found to be well correlated with pH values measured from August to October (5.8 to 7.0). Four lakes with pH values from 6.3 to 6.6 could not be distinguished from one another by means of the index B despite some differences in their physical, chemical and biological characteristics. However, it was possible to separate them by cluster analysis using the relative frequencies of the diatom species. We conclude that much ecological information is lost when applying the pH classification of Hustedt (1938/39).     

Diatom distribution and diatom inferred pH in the sediment of four alpine lakes

The diatom distribution in the sediment of four alpine lakes in the central alpine area (Ötztaler Alpen, Tyrolia, and Mölltal, Carinthia) has been analysed in order to assess possible influences of atmospheric pollutants on remote lakes. The lakes are located more than 2000 m above sea level on poorly buffered bedrock (granite, gneiss and mica schist) and are therefore susceptible to acidification by acid deposition. The pH history of the lakes has been calculated by using Index B from Renberg & Hellberg (1982), which has been adjusted to central European conditions and applied in this region for the first time. The results indicate that there have been no pH decreases in three of the lakes, while in one lake diatom composition and distribution change significantly towards the sediment surface. But taxonomical problems and difficulties in the pH classification of the rarely found diatoms complicate the interpretation of the results. Important taxa will be discussed in view of their ecological significance.     

Associations of diatoms in the surficial sediments of lakes and peat pools in Atlantic Canada

The distribution of diatom frustules in the surficial sediments of lakes and ombrotrophic peat pools in Atlantic Canada are examined especially in relation to pH and bog lake succession. Sedimentary diatom associations for peat pools, strongly acidic lakes, and weakly acidic lakes are characterized. These associations and the derived diatom-pH calibration functions will facilitate improved paleoecological interpretation of natural ontogenetic pathways and assessments of recent anthropogenic impacts.     

The effect of land use on the diatom communities in lakes

Data on recenr diatom community structure and relevant environmental characteristics from the lakes and their catchments have been collected from 151 oligotrophic lakes in eastern Finland. The pattern of frequency distribution of diatoms as a function of environmental variables, including land use in the catchment, differs between diatom taxa and indicates the optimum conditions and amplitude of occurrence for particular species. This kind of study should lead to increased understanding of the environmental requirements of diatom species and will be useful in the interpretation of historical changes in lakes as well as in forecasting possible future changes.     

Acidification and weathering processes in high mountain lakes in Southern Alps

About 200 lakes in Southern Alps (Italy and Switzerland) were studied in order to quantify their acidification. Although samplings were carried out in summer, long after the acid shock caused by snowmelt, some lakes were found to be acidic and 47% of them show alkalinity values of below 50 μeq 1^-1. Losses in alkalinity (acidification levels) were evaluated using a titration model with variable F-factor. Factor analysis shows that the main factors influencing water chemistry are related to rock weathering and nitrate uptake by vegetation. In watersheds containing carbonatic rocks the weathering of calcite is responsible for the most of the alkalinity production. We therefore focused on a set of 19 low-alkalinity lakes in the Maggia Valley lying in watersheds containing mainly acidic rocks. Results show that the weathering of silicate and calcite (present in small quantities, but highly soluble) and nitrate uptake account for most of the alkalinity production. Other watershed and in-lake processes may be important in some cases, but they never account for more than 27 % of the alkalinity production in these lakes.     

Hierarchical regulation of pelagic invertebrates in lakes of the northern Great Plains: a novel model for interdecadal effects of future climate change on lakes

Endorheic lakes of the northern Great Plains encompass a wide range of environmental parameters (e.g., salinity, pH, DOC, Ca, nutrients, depth) that vary 1000‐fold among sites and through the past 2000 years due to variation in basin hydrology and evaporative forcing. However, while many environmental parameters are known to individually influence zooplankton diversity and taxonomic composition, relatively little is known of the hierarchical relationships among potential controls or of how regulatory mechanisms may change in response to climate variation on diverse scales. To address these issues, we surveyed 70 lakes within a 100 000 km² prairie region to simulate the magnitude of environmental change expected to occur over 100–1000 years and to quantify the unique and interactive effects of diverse environmental parameters in regulating pelagic invertebrate community structure at that scale. Multivariate analyses showed that salinity was the principal correlate of changes in invertebrate composition among lakes, with a sequential loss of taxa between salinities of 4 and 50 g total dissolved solids L^?1 until one to two species predominated in highly saline systems. In contrast, changes in the concentrations of Ca²⁺ and other mineral nutrients exerted secondary controls of invertebrate assemblages independent of salinity, whereas lake depth provided a tertiary regulatory mechanism structuring species composition. In contrast to these large‐scale hierarchical patterns, seasonal surveys (May, July, September) of a subset of 21 lakes in each of 2003–2005 revealed that annual meteorological variation had no measurable effect on pelagic invertebrates, despite large differences in temperature, precipitation, and evaporation arising from regional droughts. Together these findings show that pelagic invertebrate communities in saline lakes are resilient to interannual variability in climate, but suggest that lakes of the northern Great Plains may provide a sensitive model to forecast centennial effects of future climate change.     

Acidification of small mountain lakes in the High Tatra Mountains,Poland

A hydrochemical investigation was carried out on eight small High Tatra mountain lakes (Poland). When comparing recent data with those from the period 1935–1965, a constant process of acidification of the lakes is found. The average pH of precipitation is 4.8 in the study area, but the lakes are in two stages of acidification: weak (pH 6.0–6.5) and intermediate (pH 5.5–5.8). The differences are due to differences in water sources.     

Recent environmental change in North African wetland lakes: diatom and other stratigraphic evidence from nine sites in the CASSARINA Project

All nine wetland lakes in the CASSARINA North African suite of sample sites have been disturbed strongly by human activity during the 20th century. Dated lake sediment core were used to provide evidence of the extent of recent environmental change at each site. Sedimentary diatoms at seven sites were useful for inferring salinity change trends during the last century. At two sites preservation problems severely degraded the sedimentary diatom record. Sediment core integrity was otherwise established.Lithostratigraphic measurements indicated some site specific changes in soil erosion and sediment composition but, for the Egyptian Delta lakes, no physical signal synchronous with Aswan High Dam construction was found. Sedimentary diatom assemblages were generally site characteristic and halophilous taxa were common. At two sites planktonic diatoms indicated some recent eutrophication but generally the assemblages were more indicative of salinity changes. Diatom-inferred salinity trends for the seven sites typically indicated that reductions in water salinity occurred sometime during the early or mid 20th century.Rather than climate, hydrological modification of water resources is implicated as the primary driver of salinity changes during most of the 20th century. In the western North African region these modifications were mainly local land drainage and water diversion programmes to alleviate winter flooding and/or promote summer water availability. In the Delta region, the Nile has been intensively exploited since antiquity and intensively so from the late 19th century to release more fresh water for agriculture. Here, diatom records indicate that freshening began well before the Aswan High Dam but salinity fluctuations have tended to diminish during the latter part of the 20th century. A small reversal in the water freshening trend in the 1980/90s was possibly a response to land subsidence/sea-level change or to reduced freshwater supply.Freshwater supply to the sites is generally diminishing as former freshwater surpluses switch to deficit. One site (Merja Bokka, Morocco) became completely dry in 1998 as agriculture encroached and Megene Chitane, the only acid lake in Tunisia, is currently affected by excessive inflow abstraction. At the beginning of the 21st century, eight of the nine CASSARINA sites persist as viable but modified aquatic ecosystems. They nevertheless continue to support valuable aquatic biodiversity, especially in the Delta sites and in Chitane. The modern diatom communities are clearly tolerant of considerable environmental change but the remaining sites are increasingly threatened by major hydrological disturbance. Base-line floristic data for the late 20th century are given but continuous biomonitoring combined with effective management is needed urgently to help conserve North Africa's diminishing natural wetland lake resources.     

Sensitivity and responses of diatoms to climate warming in lakes heavily influenced by humans

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A survey on water chemistry and plankton in high mountain lakes in northern Swedish Lapland

A helicopter survey was carried out on 56 water bodies in the Abisko mountains, Swedish Lapland, in August 1981. Water chemistry was found to be highly correlated with bedrock quality in the drainage area of the lakes. Low pH values (down to 5.1) appeared in the neighbourhood of sulphuric iron-ores. Natrium and chloride concentrations showed large scale patterns which can be explained by orographic rainfall. Biologically, northern high mountain conditions are reflected in species composition rather than in biomass or possibly in diversity. Small chrysomonades and dinoflagellates, as well as Keratella hiemalis and Cyclops scutifer characterize the most ‘arctic’ waters. A comparison with data from earlier investigations did not confirm expected signs of acidification.