Paleolimnology
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Paleolimnology (from
processes.Paleolimnological studies are mostly conducted using analyses of the
History
Lake ontogeny
Most early paleolimnological studies focused on the biological productivity of lakes, and the role of internal lake processes in lake development. Although Einar Naumann had speculated that the productivity of lakes should gradually decrease due to leaching of catchment soils, August Thienemann suggested that the reverse process likely occurred. Early midge records seemed to support Thienemann's view.[1]
Hutchinson and Wollack suggested that, following an initial oligotrophic stage, lakes would achieve and maintain a trophic equilibrium. They also stressed parallels between the early development of lake communities and the sigmoid growth phase of animal communities – implying that the apparent early developmental processes in lakes were dominated by colonization effects, and lags due to the limited reproductive potential of the colonizing organisms.[1]
In a classic paper,
Deevey and Lindemann's ideas were widely accepted. Although these ideas are still widely held by some limnologists, they were refuted in 1957 by Deevey's student Daniel A. Livingstone.[5] Mel Whiteside[6] also criticized Deevey and Lindemann's hypothesis; and paleolimnologists now think that a host of external factors are equally or more important as regulators of lake development and productivity. Indeed, late-glacial climatic oscillations (e.g., the Younger Dryas) appear to have been accompanied by parallel changes in productivity, which shows that lake development is not a unidirectional process, and climatic change can have a profound effect on lake communities.
Anthropogenic eutrophication, acidification, and climate change
Interest in paleolimnology eventually shifted from esoteric questions of lake ontogeny to applied investigations of human impact. Torgny Wiederholm and Bill Warwick, for example, used
From 1980 to 1990 the primary focus of paleolimnologists' efforts shifted to understanding the impact human activity had (e.g.,
In recent years paleolimnologists have recognized that climate is a dominant force in aquatic ecosystem processes, and have begun to use lacustrine records to reconstruct
Recent studies in the
Paleoclimate proxies
![](http://upload.wikimedia.org/wikipedia/commons/thumb/9/94/2016._Lake_sediment_core._Forlorn_Lakes._Gifford_Pinchot_National_Forest%2C_Washington._%2838853497655%29.jpg/314px-2016._Lake_sediment_core._Forlorn_Lakes._Gifford_Pinchot_National_Forest%2C_Washington._%2838853497655%29.jpg)
Paleoclimatology (the study of past climates) uses proxy data in order to relate elements collected in modern-day samples to climatic conditions of the past. In paleolimnology, proxy data refer to preserved or fossilized physical markers which serve as substitutes for direct meteorological measurements.[14]
Sediment cores
Sediment cores are one of the primary tools for studying paleolimnology because of the role lake and river sediments play in preserving biological information.
Pollen records
![](http://upload.wikimedia.org/wikipedia/commons/thumb/0/0c/Trout_Lake_pollen_temperature_chart_1.svg/497px-Trout_Lake_pollen_temperature_chart_1.svg.png)
Pollen and spores of terrestrial vegetation around a lake are often found within sediment cores and can be analyzed in a lab setting to determine the taxonomy of the pollen grains.[17] The distribution of these pollen grains can offer insight into the historical distribution of vegetation around the lake.[18][17] Pollen records derived from paleolimnological assessments also allow researchers to track the distribution and density of different vegetation classes across large periods of time, and allow modeling of the successive ecologies of the surrounding landscape.[19] Several studies have been able to assess transitions in vegetation profiles by examining the relationship between different types of land cover. For instance, an increase in the presence of fern pollen and herbaceous plant pollen coinciding with a decrease in grassland pollen often indicates a major disturbance or significant land clearance.[19] Another trend that can be observed from historical pollen records is rates of soil erosion around the lake, as arboreal pollen rates often increase with soil erosion due to increased pollen levels in surface soils.[18][19]
Vegetation profiles derived from historical pollen analysis are also seen as a key tool in assessing historical climate trends and disturbances. Pollen analysis offers a historical record of vegetation profiles that are sensitive to abrupt changes in climate conditions. Therefore, historical climate events, including human-induced climate change, can shift vegetation profiles relatively rapidly compared to natural transitions. For example, the quantity of poplar pollen increased dramatically at the beginning and end of the
Diatoms
The taxonomic assemblages of
Organic matter analysis
Examinations of the deposition and makeup of organic matter in the sediments of lakes has often been utilized in paleolimnological assessments.[25] A variety of factors are taken into consideration when examining deposited organic matter, including the quantity, origin, and variety of isotopes and biomarkers.[25] Diagenesis can have a significant impact on these factors, and thus careful consideration of such impact is required when drawing conclusions about records of organic matter.[25]
Quantity
The quantity of organic matter from a sediment core can offer a variety of insights into paleolimnological conditions of a body of water. It often serves as an indicator of primary productivity levels as well as terrestrial nutrient input,
Origin
By determining the origins of fossilized organic matter, researchers can make assessments about the vegetation profile in and around the lake, as well as gain a better understanding of microbial density within lake sediments.[25] A key technique in determining the origin of deposited organic matter is to examine the carbon-to-nitrogen ratio (C:N). Aquatic plants are largely non-vascular, which results in their organic matter having a relatively low C:N ratio relative to that of vascular terrestrial plants.[25] This disparity is usually quite large; and although it is lessened by alterations to the C:N ratio during diagenesis, the original disparity is still evident enough to allow researchers to accurately assess from C:N ratios the origin of the organic matter in the lake.[25] This helps researchers determine algal density and terrestrial organic matter inputs during specific historical periods. Several biomarkers also aid in the determination of organic matter origin. Lipid extraction, in particular, is a common practice, as it can reveal acids and alcohols characteristic of algal plants, as well as diagnostic lipids generated in the waxy cuticle of terrestrial plants.[25] Lignin phenols also serve as key biomarkers, helping researchers distinguish the source, plant type, tissue type, and age of organic matter.[27] Lignin is particularly useful in distinguishing between angiosperms and gymnosperms, as well as between woody and non-woody tissue types, which help researchers further develop their knowledge of the surrounding vegetation.[27] It is also important to note that both biomarkers and the C:N ratio can be altered by microbial interactions, some of which can serve as metrics for measuring microbial density, further adding to the breadth of paleolimnological information that can be derived from examinations of organic matter.[25]
Carbon isotope analysis
Three main
Nitrogen isotope analysis
Nitrogen, like carbon, shows characteristic isotope shifts, depending on the fixation pathway, that can be used to assess certain paleolimnological indices. However, also like carbon, a variety of factors go into the nitrogen isotope composition of lake sediments, which makes assessments derived from this method somewhat speculative.
Chironomids
![](http://upload.wikimedia.org/wikipedia/commons/thumb/d/d7/Mikrofoto.de-Zuckmueckenlarve3.jpg/318px-Mikrofoto.de-Zuckmueckenlarve3.jpg)
Chironomids as a paleoclimate proxy
Lake deposits have a rich diversity of fossilized insects that trace back to middle Paleozoic era, further increasing in abundance during the Quaternary period. Among the diverse array of aquatic invertebrates, different families of aquatic fly larvae can be extracted from sediments of the Quaternary era. Among them, Chironomids, two-winged flies that belong to the family Chironomidae, are of greatest ecological importance due to their diverse feeding habitats and their role as an important component of the food web. Chironomids complete their larval stage in the water, with their adult life stage outside of the water lasting only a very short time. During their larval stages, Chironomids play an important part in the degradation of material in the aquatic ecosystem.[31] Ecologically they are considered bottom dwellers and are very responsive to any fluctuation in the surrounding environment. Their head capsule and feeding structures are commonly fossilized in lake sediments,[32] allowing them to serve as a valuable paleoclimate proxy.
Factors influencing chironomid distribution and abundance
One of the major factors that affect chironomid distribution is the climate conditions at local, regional, and global scales. Changes in these conditions are preserved as a fossil record over large periods of time. Through paleolimnological methods, including chironomid assessment, these changes can be extrapolated to predict future climate change. Being very responsive to any fluctuation in the surrounding environment, Chironomids are good indicators of a variety of factors, including
A variety of disparate factors have influenced the abundance and distribution patterns of chironomids in recent history. Therefore, it is important to be careful when making broader interpretations from their fossil records. The impact of temperature on chironomid abundance and diversity, along with other associated factors, has recently been debated. Accurate interpretations of chironomid fossil records must consider a wide array of associated factors within the ecosystem. In order to understand the different forces that have been affecting the fossil data of a lake, it is important to reconstruct the
Chironomids and reconstruction of quantitative change in Holocene climate
Researchers assessing chironomid distribution primarily examine the temperature, while considering supporting factors, such as pH,
Use of chironomids in assessments of anthropogenic climate change
According to the fifth
References
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- ^ Deevey, E. S., Jr. 1955. The obliteration of the hypolimnion. Mem. Ist. Ital. Idrobiol., Suppl 8, 9-38.
- ^ Walker, I. R. 2006. Chironomid overview. pp.360–366 in S.A. EIias (ed.) Encyclopedia of Quaternary Science, Vo1. 1, Elsevier, Amsterdam
- ^ Livingstone, D.A. 1957. On the sigmoid growth phase in the history of Linsley Pond. American Journal of Science 255: 364–373.
- ^ Whiteside. M. C. 1983. The mythical concept of eutrophication. Hydrobiologia 103, 107–111.
- ^ Battarbee, R. W. 1984. Diatom analysis and the acidification of lakes. Philosophical Transactions of the Royal Society of London 305: 451–477.
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