Paleolimnology

J P Smol, Queen's University, Kingston, ON, Canada © 2009 Elsevier Inc. All rights reserved.

Introduction

The ecological and environmental conditions in lakes and rivers change on a variety of time scales. However, one of the greatest challenges faced by limnolo-gists (as well as other environmental and ecological scientists) is the general lack of long-term data. Without such data, it is difficult to determine if changes are occurring in an aquatic system, and if so, to identify the likely drivers of these ecosystem changes. Such long-term data are critical in, for example, contrasting the effects of human activity versus natural environmental change on ecosystems.

Although direct instrumental data on limnological changes are often lacking, an important archive of past environmental and ecological conditions is preserved in the sediments of lakes and some river systems. Although incomplete, relatively comprehensive records of past limnological changes, as well as changes occurring in the surrounding terrestrial catchment and airshed, can often be reconstructed from information preserved in sediment profiles using paleolim-nological techniques.

Paleolimnology can be broadly defined as the mul-tidisciplinary science that uses the physical, chemical, and biological information preserved in aquatic sediments to track past changes in ecosystems. Paleolim-nology has enjoyed considerable progress over the last three decades, with important advances in the ways paleolimnologists can retrieve and section sediment cores, new approaches to provide geochrono-logical control for these sediment profiles, major advances in the quantity and quality of proxy information that paleolimnologists have learned to retrieve from sediments, as well as major improvements in the ways that paleolimnologists can interpret these stratigraphic changes in a statistically robust and defendable manner.

Examples using lakes will be emphasized throughout this article; however, similar approaches are being used in river and marine ecosystems. The overall focus is on paleolimnological approaches rather than applications.

Sediments

Lakes slowly fill up with sediments. Typically, 24 hours a day, every day of the year, sediments are slowly accumulating at the bottom of lakes. The overriding principle in paleolimnology is that lake sediments accumulate in an organized fashion, with older material occurring deeper in the sediment core, and the most recent material at the surface of the core (i.e., the Law of Superposition). Of course, not all sediment profiles are ideal for historical analyses, as some mixing processes, such as bioturbation (i.e., the mixing of the sediments by benthic organisms), may occur in some profiles. Nonetheless, many of these potential problems can be assessed (e.g., using dating techniques) and it is now clear that the vast majority of lakes archive valuable records of past environmental change in a stratigraphically intact manner.

Incorporated in sediments is a tremendously rich library of information about the processes and the biological communities from within or external to the lake. Paleolimnologists typically divide the sources of lake sediments into two broad categories: (1) allochtho-nous sources, which refer to material originating from outside the lake basin (e.g., soil particles, pollen grains from trees, pollution from industries); and (2) autochthonous sources, which refer to material originating from within the water body itself (e.g., dead algae or invertebrates, chemical precipitates). The amount of sediment that has accumulated can vary tremendously between lakes, and even within a single lake basin. A typical glaciated temperate lake may contain 4 or 5 m (or even 10 m or more) of sediment, representing the lake's history since the time of the lake's formation following the retreat of the ice sheets at the end of the last ice age (c. 12 000 years ago). In contrast, a lake of similar age from a polar region may have far less sediment accumulated, as sedimentation rates are typically very low. In some regions, such as the rift valley lakes of Africa, records spanning several hundred thousand years are contained in 100s of meters of sediment.

A major advantage of paleolimnological approaches is that researchers can, to a large extent, set the time scale. For example, by taking longer sediment cores, a longer time frame can be studied. Similarly, the researcher can set the temporal resolution of the study. For example, by slicing the sediment core at finer intervals, more detailed records of past environmental changes can be inferred. Of course, there are also potentially serious limitations. For example, the sediment accumulation rate may be so low that even fine-resolution sampling will still not provide sufficiently fine temporal resolution to answer certain questions.

From the information preserved in sediments, paleolimnologists can reconstruct the history and development of aquatic ecosystems. There are two key steps involved in any paleolimnological investigation: (1) retrieval of the sedimentary material required for study, and (2) establishment of the depth-time scale of the profile using geochronological techniques. These first two critical steps are described in the following section.

Retrieving Sediment Cores

A large number of ingenious sediment samplers have been developed to collect sediment cores in an undisturbed manner. Each sampler has various advantages and drawbacks, and so it is first important to determine the scientific questions that researchers hope to address. For example, what time frame will be required for the paleolimnological study? Will the program include a detailed analysis of the more recent sediments, which may be used to study, for example, the effects of recent human interventions, such as the effects of agriculture or acid rain, on the lake system? Or is the main focus on long-term changes, such as broad climatic shifts over millennial time scales? What temporal detail (e.g., annual, decadal) will be needed to answer these questions? The answers to these questions will determine which lake(s) will be chosen for study, as well as what type of equipment will be required to retrieve and section the most suitable sediment samples.

The earliest corers were developed to answer questions related to long-term (millennial) scale changes in lake ontogeny. Many of the original designs were inspired by the work of M. Jull Hvorslev (1895-1989), whose seminal work on sampling soil profiles for the U.S. Army Corps of Engineers contains many of the basic principles that paleolimnologists still use today to collect sedimentary profiles. Using some of these concepts, Daniel Livingstone published a piston coring design in 1955 that is still widely used today (often with some minor modifications). The typical piston corer consists of three components: the piston and cable assembly, the core tube, and the drive head and drive rods. Many variations of the original design are available, including those that can be used on a rope. Piston corers can remove, sequentially, sediment profiles of about 1 m in length, until the required depth of sample penetration is reached (Figure 1).

Since the mid-1980s, there has been an increased emphasis on using lake sediments to study more recent changes in lake histories, such as the effects

Kajak Sediment Core Sampler
Figure 1 Lake sediment coring on Lake Chala (Kenya), using a Livingstone-type piston corer (modified by UWITEC). Photograph courtesy of B. Cumming (Queen's University).

of anthropogenic impacts on lake ecosystems. This requires the use of a corer that will carefully sample the most recent, watery sediment records, which may be disturbed or lost if collected using standard piston corers. Two major types of samplers are used: gravity corers and freeze-crust corers.

A variety of gravity corers have been developed to specifically sample the most recent sediments. Many of these follow the general design of a benthic invertebrate sampler that Z. Kajak (1929-2002) developed; however, significant modifications have been made to the original design, such as those developed and modified by John R. Glew (Figure 2). In its simplest form, a surface sediment gravity corer is a hollow coring tube that is carefully lowered into the recent sediments on a rope from the surface, using its own weight for penetration. However, when sampling stiffer sediments, additional weight may be needed to achieve adequate penetration. A brass messenger is then delivered down the coring line to trigger a closing mechanism that seals off the top of the coring tube. The gravity corer, with the tube of collected sediment, is then retrieved to the surface for subsequent sectioning and subsampling.

Freeze-crust coring is another sampling method used for retrieving relatively undisturbed recent sediments. This process is also quite simple. A freeze-crust corer can take many designs, but most are either a metal box or tube that is filled with a coolant, such as frozen carbon dioxide (i.e., dry ice), as well as a fluid, such as ethanol. The filled freeze-crust corer, which would now be very cold, is then slowly lowered on a rope into the sediments where in situ freezing of the sediments onto the corer will occur. During this time, the sampling rope must be anchored on a solid

Sediment Sampler Paleolimnology
Figure 2 Retrieving surface sediment cores with a Glew gravity corer from Lake Opinicon (Canada). Photograph courtesy of B. Cumming (Queen's University).

platform at the lake surface, such as the ice cover, to prevent it from sinking further into the sediments. After a certain period of time (the time that the sampler is left in the sediments depends on the design used, but typically from 10 to 15min), a crust of sediment has been frozen onto the surface of the sampler. The sampler can then be retrieved, and the crust of frozen sediment can be removed and sectioned.

A large number of other coring designs are available, including those specifically developed to take long sediment cores in deep waters, such as the Kullenberg sampler. Other designs include corers designed to penetrate compact sediment profiles, such as hammer corers and vibracorers. Suffice to say that almost any lake system can now be sampled for its sediment history.

Just as there are many types of sediment samplers, there are also many ways to section the sediments. However, most researchers use an extrusion system for recent sediments, whereby discrete intervals of sediment can be sliced off the core for further analyses (Figure 3). For some paleolimnological analyses, researchers may want to keep the core intact until some preliminary analyses are completed, such as X-raying the sediment profile.

Dating Sediment Cores

A critical factor in deciphering lake sediment histories is to establish an accurate depth-time profile (i.e., geochronology) of the sediment core. Without this information, it would not be possible to place past environmental changes in a proper temporal perspective. A variety of approaches are available depending on the type of sediments, the location of the study lake, and the approximate age of the sediments themselves.

In some lakes, such as certain meromictic lakes, sediments may be annually laminated (i.e., varved), and superficially may resemble annual tree rings. A typical biogenic varve is composed of a couplet of laminae representing one year of sediment accumulation. The reason why couplets form in some sediment profiles vary from lake to lake, but varves represent seasonal differences in the materials (biological, chemical, or mineral) reaching the sediment surface. If varved sediments can be identified, the paleolim-nologist can often attain a very high degree of temporal resolution (e.g., even subannual changes in some cases by splitting varves into seasons). However, the annual nature of varves must first be confirmed using other dating techniques (see later), as false laminae and missing laminae may obscure the record.

In most cases, lake sediments are dated using radio-metric techniques. For older sediments (e.g., on millennial scales), radiocarbon-14 (14C) dating is often used. As 14C has a radioactive half-life of 5730 ± 40 years, it is useful for dating sediments on millennial time scales up to approximately 45 000 years of age. In the older paleolimnological literature, many 14C dates were described as 'bulk dates,' as the 14C content of bulk sediment was often used for dating. However, over the last 20 years, advances in accelerator mass spectrometry (AMS) technology have allowed paleolimnologists to date much smaller organic samples, such as a single seed or pine needle. As with all techniques, care must be taken to use the most appropriate material for dating, because a variety of potential error sources exist. For example, one of the most common problems with 14C dating is the so-called 'hard water effect,' whereby 'old carbon' from the local bedrock can dilute the 14C content of a bulk sediment sample. Ideally, paleolimnologists try to radiocarbon date material such as terrestrial

Subsample Sediment Core Dna
Figure 3 An example of a vertical extrusion system used for sectioning recent sediment at close intervals. (a) A1 cm slice of sediment has been extruded from the core tube. (b) The sediment section is sliced off and removed. Photographs courtesy of N. Michelutti (Queen's University).

macrofossils (e.g., seeds, twigs), which are not affected by some of these problems.

As noted previously, there has been a shift in some areas of paleolimnology to focus on the more recent histories of lakes. Because of the relatively long radioactive half-life of 14C, it is not appropriate for dating recent sediments. Instead lead-210 (210Pb), a naturally occurring isotope, is often the dating method of choice. 210Pb has a half-life of about 22.3 years, and so it is ideal for dating sediments over the last century or so. In addition, other isotopes, such as cesium-137 (137Cs), which is a by-product of nuclear explosions, can be used in some sedimentary sequences to further refine depth-time profiles. As stratospheric nuclear bomb testing became more common in the mid-1950s, a rise in 137Cs can often be seen in sediment profiles at this time. Bomb testing by the former Soviet Union and the United States peaked in 1963, and this is often seen as a peak in the 137Cs profile, followed by a marked decline in concentrations, with the signing of the nuclear Test Ban Treaty in that year. However, in some parts of Europe and elsewhere, a second peak in 137Cs can be found, marking the 1986 Chernobyl nuclear accident in the Ukraine.

Other dating methods include identifying ash layers (tephras) from known volcanic eruptions, as well as other episodic events, such as sedimentary charcoal from known forest fires. Accurate dating of sediment profiles remains a major challenge for many studies, and so, when possible, several dating methods should be used simultaneously in order to increase the confidence of the geochronology.

The 'Top-Bottom' or 'Before and After' Approach

As noted earlier, paleolimnologists often have the important advantage that they can set the time scales for analysis. Typically, many sediment sections in a core are analyzed for a suite of indicators. However, this is also quite time consuming, as many paleolim-nological analyses are time intensive (e.g., taxonomy, microscopy). For some research questions, though, it may be more important to study a larger number of lakes to obtain a regional assessment of environmental change. In such cases, detailed paleolimnological analyses are not practical because of time and resource restraints. Instead, paleolimnologists have developed a simple, regional assessment tool, which is often referred to as the 'top-bottom approach.'

The top-bottom approach is straightforward, and was originally developed to study the effects of recent human influences (e.g., acidification, eutrophication) on lake ecosystems. In this approach, surface sediment cores spanning the last several hundred years, such as those collected with gravity corers, are taken from the lakes included in the study. However, in contrast to a standard paleolimnological assessment, where many sediment samples would be analyzed from each core, typically only two sediment samples are analyzed for indicators: the surface (or top) sediment sample, representing recent environmental conditions; and a sediment sample that predates the period of marked human impacts (or bottom sample). For example, if one was attempting to assess the differences in lakewater pH as a result of acidic precipitation for a region, the bottom sample should predate c. 1850 AD. By comparing ecosystem changes, as inferred from the paleolimnological indicators present in the surface sediment (i.e., recent) and deeper sediments (i.e., preimpact), a regional assessment of environmental change can be attained. A major advantage of the top-bottom approach is that only two samples (not including samples to establish reproducibility) are analyzed for each core, as opposed to perhaps 20 or more sediment sections of a typical paleolimnological assessment of recent environmental change. The major disadvantage is that the top-bottom approach does not provide information on the trajectory or timing of environmental changes—it is simply a 'snapshot' approach.

Paleolimnological Indicators

Lake sediments contain a wide array of physical, chemical, and biological indicators of past environmental change. Only the most commonly used indicators are discussed below.

Important information is contained in the physical structure and composition of the sediments themselves. For example, a simple visual inspection of the sedimentary sequence can provide information on color changes, texture, presence of laminae, etc. A variety of sediment logging and recording techniques are now available, especially with the advent of digital technology. In addition, radiography and X-ray techniques can often reveal important bedding features that may not be visible with the naked eye. Other procedures that are commonly used include particle size analyses of the sediments, which provide important insights to help determine the process, and, to some extent, the source of detrital sedimentation. Lake sediments also archive a magnetic signal, which can be used in a number of applications, including deciphering past erosion events.

Typically, one of the first analyses a paleolimnolo-gist will undertake is to determine the relative proportions of water, organic matter, carbonate, and siliciclastic material (i.e., clastic sediment composed of silicon-bearing minerals such as quartz, feldspars, clay minerals) in the sediment sections. This is often done using weight loss techniques by successively exposing the sediment to higher temperatures, and then using the differences in sample weights to estimate the percentage of the various fractions. For example, wet sediment can be dried in a drying oven set at approximately 80-100 °C, and the percentage of water can be estimated by weight loss. Combusting the same sediment in a muffle furnace at about 450 °C for a few hours will provide an estimate of the organic matter content. A further ashing to about 950 °C can be used to determine the carbonate content. The remaining material represents mainly siliciclastic sediments. These types of data have many applications, although they also have to be interpreted cautiously. For example, intuitively, one might expect that the percentage of organic matter in a sediment profile would be directly related to the amount of organic production occurring in the lake basin. Although to some extent this relationship holds, it is important to consider that these variables are typically expressed as percentages, and if, for example, human activities in a catchment have resulted in erosion as well as nutrient export, the siliciclastic material deposited in the lake as a result of the erosion may well obfuscate any signal of increased autochthonous production.

Lake sediments also archive a wide spectrum of geochemical information that can be used to interpret ecosystem changes. Typical approaches include analyses of various elements that can be used to track past biogeochemical changes in the lake and its catchment. Other geochemical data can be used to reconstruct past anthropogenic influences, such as mercury and lead, as well as other metals. Persistent organic pollutants, such as DDT or PCBs, also provide important records of contaminant trajectories to lake sediments. These pollution profiles can be strengthened by also studying the past deposition of fly-ash particles, such as spheroidal carbonaceous particles and inorganic ash spheres, resulting from various industrial activities.

The isotopic composition of sediments provides additional paleoenvironmental information. These include isotopes of carbon that have been used in, for example, lake eutrophication work, sulfur isotopes to track the effects of anthropogenic acidification, oxygen isotopes in climatic and hydrologic change research, and nitrogen isotopes to reconstruct past changes in sources and cycling of nitrogen (and even sockeye salmon populations). More recent approaches include determining the isotopic composition of specific indicators found in lake sediments, such as the carapaces of some invertebrates.

One of the most active areas of paleolimnological research, however, deals with biological indicators. A central theme in ecology has always been the attempt to link the distributions of organisms to environmental conditions. A diverse array of biological indicators is preserved in lake sediments, either as morphological remains (e.g., diatom valves) or as biogeochemical fossils (e.g., pigments). It is therefore not surprising that biological paleolimnology has provided important insights into past ecosystem changes. Below, I summarize some of the dominant indicators.

Pollen grains and spores are among the most common morphological indicators used in paleoenviron-mental studies. The study of pollen grains and spores is a large scientific discipline, called palynology, which has many applications besides paleolimnology, such as in archeology and forensic science. Plants can produce large numbers of pollen grains, which are often well preserved in sediments. Similarly, the so-called lower plants, such as mosses, produce spores that are also well represented in sediment profiles. Although most pollen grains and spores are from terrestrial vegetation, aquatic macrophytes also produce pollen. As different species of plants produce morphologically distinct pollen grains (often identifiable to the genus level, but sometimes even to the species level), it is possible to reconstruct, at least in a general way, the composition of past forests and other vegetation. This provides important information on past climate and soil development, as well as the succession of different plant species. Some pollen grains can also assist in dating sediment cores. For example, in northeastern North America, the arrival of European settlers, and their related activities of clearing forests and initiating European style agriculture, resulted in the increase in pollen grains from ragweed (Ambrosia).

Plants are also represented in sedimentary deposits by macroscopic remains, typically referred to as macrofossils, which include seeds, twigs, and pieces of bark. Although pollen grains can be transported long distances via wind and other vectors, plant macrofossils are less easily transported, and so they more accurately reflect local vegetation changes. In addition, as noted earlier, they are also important sources for material used in 14C dating.

Algae and cyanobacteria (i.e., blue-green algae) represent major primary producers in most aquatic systems, and fortunately almost all of them leave reliable morphological or biogeochemical fossils. Foremost among these are the siliceous cell walls of diatoms (Bacillariophyceae, Figure 4). A diatom cell wall is composed of two similar halves (or valves) joined together; two valves forming a full diatom cell are called a frustule. Diatoms, which include many thousands of species, often dominate the plankton and periphyton of most lake systems. They have many characteristics that make them ideal paleoindicators. For example, they are abundant and diverse, and their taxonomy is based on the size and sculpturing of the intricate glass cell walls that characterize individual species. As their cell walls are siliceous, they are very well preserved in most lake environments. The distributions of many diatom species are closely linked to specific limnological conditions, such as lakewater pH, salinity, nutrient levels, and so forth, and so are excellent paleoindicators.

Chrysophytes (classes Chrysophyceae and Synuro-phyceae) similarly can be tracked using siliceous microfossils. About 15% of chrysophyte taxa (including important genera such as Mallomonas and Synura) are characterized by an armor of overlapping siliceous scales. Similar to the diatom valves described earlier, these scales are species specific and can be used by paleolimnologists to track past populations of some groups. In addition to the scales, which are only produced by some taxa, all chrysophytes are characterized by the endogenous formation of siliceous resting stages, called stomatocysts (or stato-spores in the older literature). These cysts also appear to be species specific and are well preserved in sediments, However, unlike chrysophyte scales, the taxonomic identity of many cyst morphotypes have not yet been linked to the species that produced them, and instead cyst morphotypes are often simply referred to by temporary number designations.

Other algae also leave morphological microfossils, but are less widely used. This includes vegetative or reproductive cells of certain green (Chlorophyta) algae (e.g., Pediastrum colonies), or the akinetes and heterocysts of some blue-green algae. However, all algal and cyanobacterial groups are also represented in lake sediments by a variety of biogeochemical indicators. Fossil pigments (e.g., chlorophylls, carotenes, xanthophylls), which characterize certain groups and can be identified using high performance

Figure 4 Freshwater diatom valves. (a) Eunotia, (b) Cymbella, (c) Cyclotella, (d) Semiorbis, (e) Brachysira, (f) Aulacoseira, and (g) Diploneis. Photographs courtesy of B. Ginn (Queen's University).

Figure 4 Freshwater diatom valves. (a) Eunotia, (b) Cymbella, (c) Cyclotella, (d) Semiorbis, (e) Brachysira, (f) Aulacoseira, and (g) Diploneis. Photographs courtesy of B. Ginn (Queen's University).

liquid chromatography (HPLC) or other techniques from sediments, are most commonly used.

In addition to some of the primary producers described earlier, a large suite of zoological indicators is identifiable in sediments. Cladocera (Figure 5) are represented in sediments primarily by their chitinized body parts: head-shield, shell or carapace, and postabdominal claws. Trained taxonomists can determine the species affinities of many of these body parts. In addition, the sexual resting stages (i.e., ephippia) of Cladocera are commonly encountered.

Lakes are also the habitat for many insect larvae, with midges or chironomids (Chironomidae, Diptera) dominating many assemblages. Fossil chironomids can be identified via their chitinized head capsules (Figure 6), which can often be identified to the generic level, or even species level. Chironomid head capsules are most often used in climate reconstructions and in tracking past deepwater oxygen levels. Other insect

Eubosmina Longispina
Figure 5 Part of the chitinous exoskeleton of the cladoceran Eubosmina longispina. Photograph courtesy of J. Sweetman (Queen's University).
Stomatocyst
Figure 6 A head capsule of the chironomid Chironomus. Photograph courtesy of J. Sweetman (Queen's University).

remains include the chitinized mandibles of Chao-borus (phantom midge) larvae. Because certain taxa cannot coexist with fish predators, these mandibles have been used to document collapses of past fish populations (such as from lake acidification).

The Ostracoda (ostracods, also spelled ostracodes) is another widely used group of invertebrates in paleolimnological studies. Ostracods are characterized by two calcitic valves or shells that can be used to track a wide spectrum of environmental variables. Recent work has also focused not only on changes in species but also on the trace-element chemistry and stable isotope signatures of individual valves, as a memory of past limnological conditions is preserved in the chemistry and isotopic composition of ostracod valves.

In contrast to algal and invertebrate remains, vertebrates tend to be poorly represented in lake sediments. However, in certain environments, fish scales are common, and the inner ear bones of fish (i.e., otoliths) are increasingly being used in paleolimnolo-gical reconstructions.

Determining the Environmental Optima of Indicators

The previous section summarized some of the key indicators used in paleolimnology. However, in order to use, for example, diatom or chironomid species assemblages to infer environmental conditions, the environmental optima of the indicator taxa must first be determined. Some information on the ecological optima and tolerances of indicators is available in the scientific literature from previous biological surveys or similar studies. However, given the large number of taxa present, as well as the myriad of environmental variables that can influence species distributions, it can be a daunting task to link species distributions to environmental variables. The most commonly used paleolimnological approach to provide estimates of the environmental optima of indicator taxa are to use surface sediment calibration sets (or sometimes referred to as training sets).

Surface sediment calibration sets have become common components of many paleolimnological studies. The approach is fairly straightforward. A set of calibration lakes is chosen (perhaps 80 or so in number) that reflect the gradients of limnological conditions that might be expected in the paleolimno-logical study being undertaken. For example, suppose the goal of the study is to reconstruct lakewater pH in a series of lakes affected by acidification. Lakes with present-day pH values ranging from 4.5 to 8.0 would represent a broad gradient in pH values. The first component of the calibration set would be a matrix of limnological and other environmental data available for the calibration lakes. The second step is to determine the distribution of indicator taxa in the 80 calibration lakes. This is done by taking surface sediment cores from each lake and identifying and enumerating the indicators in questions (e.g., diatoms) from the surface 0.5 or 1.0 cm of sediments (i.e., representing recently deposited biota). This represents the second matrix of data required for developing the calibration set, namely the species matrix. The next step is to use a variety of statistical techniques to link the species distributions in the surface sediments to the measured environmental variables for the calibration lakes. Multivariate techniques, such as canonical correspondence analysis (CCA), are often used to determine which environmental variables exert the greatest influence on species distributions. Transfer functions can then be constructed relating species distributions (e.g., diatom percentages) to the variables of interest (e.g., lakewater pH). The latter is typically done using statistical approaches such as weighted averaging regression and calibration. The resulting transfer functions linking species distributions to environmental variables are often quite robust.

Although quantitative approaches, such as those described earlier, have become commonplace in paleolimnological studies, many environmental inferences can still be accomplished using qualitative approaches.

Paleolimnological Applications

Paleolimnological approaches have now been used to assess a wide spectrum of limnological changes. Early work focused on long-term studies of lake ontogeny; however, a considerable volume of more recent research has focused on determining the effects of human impacts on aquatic ecosystems. Much of this applied paleolimnology began in the 1980s with work on acidic precipitation, where paleolimnology was used to determine if lakes had acidified, and if so, when and by how much. For example, Figure 7 shows a typical diatom stratigraphic profile for an acidified lake in Nova Scotia (Canada). The change in the

210Pb date

Core depth (cm)

2002

0.00-

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1998

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1991

4.00-

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1983

6.00-

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1974

8.00-

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1966

10.00

-10.25-

1957

12.00

-12.25-

1942

14.00

-14.25-

1915

16.00

-16.25-

1868

18.00

-18.25-

~1820

20.00

J"

0cP 0cP

20 0

Figure 7 A summary diagram of the dominant diatom changes (as percentages) in the recent sediments of Kejimkujik Lake (Nova Scotia, Canada). The dates of the sediment slices are estimated using 210Pb dating (left of figure). To the right is the diatom-inferred pH profile, which was generated using a transfer function developed from diatoms preserved in the surface sediments of a calibration set of east coast lakes. Reproduced from Figure 3 from Ginn BK, Stewart LJ, Cumming BF, and Smol JP (2007) Surface-water acidification and reproducibility of sediment cores from Kejimkujik Lake, Nova Scotia, Canada. Water, Air and Soil Pollution 183: 15-24, with permission from Springer Science and Business Media.

dominant diatom assemblages (shown as percentages) clearly shows a shift in species assemblages (beginning c. 1940, as estimated from 210Pb dating) to taxa characteristic of more acidic waters. By using transfer functions developed from surface sediment calibration sets, the pH optima of the individual taxa can be used to provide quantitative estimates of past lakewater pH (as shown to the right of Figure 7). These types of analyses played a critical role in demonstrating the effects of acid precipitation on lake ecosystems. Other water quality issues can similarly be studied using these approaches.

Another major research focus is to use paleolimno-logical approaches to study long-term changes in climate, which has been receiving heightened attention with recent concerns about greenhouse gas induced warming. As climatic variables influence, to some extent, all biological distributions, it is not surprising that indicators such as chironomids, diatoms, and other proxies have been demonstrated to track climate variables, either directly (e.g., temperature reconstructions) or indirectly (e.g., via tracking past lakewater salinity changes which can be linked to past precipitation and evaporation ratios, past lake ice covers, etc.).

Many new paleolimnological studies are being initiated and are now often multidisciplinary, including a wide spectrum of techniques to provide more robust environmental inferences. In addition, important synergies between different groups of scientists (e.g., the study of fossil DNA in cladoceran ephippia) have heightened the interest in many paleolimnologi-cal approaches. As environmental problems continue to be identified, and as little monitoring data exist, it is clear that paleolimnological studies will continue to provide critical information on environmental change.

Glossary

Allochthonous material - Material originating from outside the lake basin (e.g., soil particles, pollen grains from trees).

Autochthonous material - Material originating from within the water body (e.g., dead algal remains, chemical precipitates).

Bioturbation - The mixing of the sediments by ben-thic organisms.

Paleolimnology - The multidisciplinary science that uses the physical, chemical, and biological information preserved in aquatic sediments to track past changes in ecosystems.

Palynology - The study of pollen grains and spores.

Siliciclastic sediment - Clastic sediment composed of silicon-bearing minerals such as quartz, feldspars, and clay minerals.

See also: Benthic Invertebrate Fauna, Lakes and Reservoirs; Effects of Climate Change on Lakes; Eutrophication of Lakes and Reservoirs; Lake and Reservoir Management; Lakes as Ecosystems; Meromictic Lakes; Mixing Dynamics in Lakes Across Climatic Zones; Origins of Types of Lake Basins.

Further Reading

Battarbee RW, Gasse F, and Stickley CE (eds.) (2004) Past Climate Variability through Europe and Africa. Dordrecht: Springer.

Birks HJB (1998) Numerical tools in palaeolimnology - Progress, potentialities, and problems. Journal of Paleolimnology 20: 307-332.

Cohen AS (2003) Paleolimnology: The History and Evolution of Lake Systems. Oxford: Oxford University Press.

Francus P (ed.) (2004) Image Analysis, Sediments and Paleoenvir-onments. Dordrecht: Springer.

Hvorslev MJ (1949) Subsurface Exploration and Sampling of Soils for Civil Engineering Purposes. Vicksburg, MS: American Society of Civil Engineers, Waterways Experiment Station, Corps of Engineers, U.S. Army.

Last WM and Smol JP (eds.) (2001) Tracking Environmental Change Using Lake Sediments. Volume 1: Basin Analysis, Coring, and Chronological Techniques. Dordrecht: Kluwer Academic.

Last WM and Smol JP (eds.) (2001) Tracking Environmental Change Using Lake Sediments. Volume 2: Physical and Geo-chemical Methods. Dordrecht: Kluwer Academic.

Leng MJ (ed.) (2006) Isotopes in Palaeoenvironmental Research. Dordrecht: Springer.

Maher BA and Thompson R (eds.) (1999) Quaternary Climates, Environments and Magnetism. Cambridge: Cambridge University Press.

Pienitz R, Douglas MSV, and Smol JP (eds.) (2004) Long-Term Environmental Change in Arctic and Antarctic Lakes. Dordrecht: Springer.

Smol JP (2008) Pollution of Lakes and Rivers: A Paleoenviron-mental Perspective, 2nd edn. Oxford: Blackwell.

Smol JP, Birks HJB, and Last WM (eds.) (2001) Tracking Environmental Change Using Lake Sediments. Volume 3: Terrestrial, Algal, and Siliceous Indicators. Dordrecht: Kluwer Academic.

Smol JP, Birks HJB, and Last WM (eds.) (2001) Tracking Environmental Change Using Lake Sediments. Volume 4: Zoological Indicators. Dordrecht: Kluwer Academic.

Stoermer EF and Smol JP (eds.) (1999) The Diatoms: Applications for the Environmental and Earth Sciences. Cambridge: Cambridge University Press.

Relevant Websites

http://www.paleolim.org/ - International Paleolimnology Association.

http://www.pages.unibe.ch/ - PAGES - Past Global Changes.

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  • phillipp
    What can you use to clear acid spelt in a lake or river?
    1 year ago

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