Paleolimnology (from Greek: παλαιός, palaios, "ancient", λίμνη, limne, "lake", and λόγος, logos, "study") is a scientific sub-discipline closely related to both limnology and paleoecology. Paleolimnological studies focus on reconstructing the past environments of inland waters (e.g., lakes and streams) using the geologic record, especially with regard to events such as climatic change, eutrophication, acidification, and internal ontogenic processes.
Paleolimnological studies are mostly conducted using analyses of the physical, chemical, and mineralogical properties of sediments, or of biological records such as fossil pollen, diatoms, or chironomids.
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.
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.
In a classic paper, Raymond Lindeman outlined a hypothetical developmental sequence, with lakes progressively developing through oligotrophic, mesotrophic, and eutrophic stages, before senescing to a dystrophic stage and then filling completely with sediment. A climax forest community would eventually be established on the peaty fill of the former lake basin. These ideas were further elaborated by Ed Deevey,Deevey, E. S., Jr. 1955. The obliteration of the hypolimnion. Mem. Ist. Ital. Idrobiol., Suppl 8, 9-38. who suggested that lake development was dominated by a process of morphometric eutrophication. As the hypolimnion of lakes gradually filled with sediments, oxygen depletion would promote the release of iron-bound phosphorus to the overlying water. This process of internal fertilization would stimulate biological productivity, further accelerating the in-filling process.Walker, I. R. 2006. Chironomid overview. pp.360–366 in S.A. EIias (ed.) Encyclopedia of Quaternary Science, Vo1. 1, Elsevier, Amsterdam
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.Livingstone, D.A. 1957. On the sigmoid growth phase in the history of Linsley Pond. American Journal of Science 255: 364–373. Mel WhitesideWhiteside. M. C. 1983. The mythical concept of eutrophication. Hydrobiologia 103, 107–111. 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 chironomid fossils to assess the impact of increased, human-caused nutrient loading (anthropogenic eutrophication) on lake communities. Their studies revealed pronounced changes in the bottom fauna of North American and European lakes as a consequence of severe oxygen depletion.
From 1980 to 1990 the primary focus of paleolimnologists' efforts shifted to understanding the impact human activity had (e.g., acid rain) versus natural processes (e.g., soil leaching) as drivers of pH change in northern lakes.Battarbee, R. W. 1984. Diatom analysis and the acidification of lakes. Philosophical Transactions of the Royal Society of London 305: 451–477. The pH-sensitivity of diatom communities had been recognized as early as the 1930s, when Friedrich Hustedt developed a classification for diatoms, based on their apparent pH preferences. Gunnar Nygaard subsequently developed a series of diatom pH indices. By calibrating these indices to pH, Jouko Meriläinen introduced the first diatom-pH transfer function. Using diatom and chrysophyte fossil records, research groups were able to clearly demonstrate that many northern lakes had rapidly acidified in consequence of increased industrialization. Although lakes also showed a tendency to acidify slightly during their early (late-glacial) history, the pH of most lakes had remained stable for several thousand years prior to their recent human-driven acidification.
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 paleoclimates. Detailed records of historical climate change have been developed from a variety of indicators, including, for example, paleotemperature reconstructions derived from chironomid fossils, and paleosalinity records inferred from diatoms.
Recent studies in the Arctic show that changes in biodiversity are largely due to warming, rather than other associated factors, such as human alteration and acidification. In the Himalayas, bodies of water are not only affected by the anthropogenic disturbances but also impacted by the different types of pollutants that are transferred to the area from afar. Therefore, it is vital to understand all the associated factors acting on aquatic biodiversity, while analyzing the impact of climate change over the years, with the help of lake sediments.Sharma, C.M; Sharma, S; Gurung, S; Bajracharya, R.M; Jüttner, I; Pradhan, N.S (2009). "Global Climatic Change and High Altitude Lakes: Impacts on Aquatic Biodiversity and Pollution Status". Natural Resources Management: Reviews and Research in the Himalayan Watersheds. 44 (977): 103–122. It is also important to consider that the impact of climate-change varies depending on an ecosystem's sensitivity to change, when assessing climate change from a paleolimnological perspective.
Paleoclimate proxies
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.
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. Paleolimnologists collect sediment cores and observe various proxy indicators in order to reconstruct the past limnology of an area. Such proxy data include geochemical markers and isotope data as well as fossilized pollen, charcoal, diatoms, chironomids, and other organic matter. These proxies show distributions and characteristics that can indicate past limnological conditions. In order to calibrate the proxy data extracted from sediment cores, the new core is compared to a group of those from around 40 or more calibration lakes. This allows researchers to assess key differences in the limnological conditions of the lake from which the core is taken. Lake-sediment cores, in particular, facilitate a more comprehensive analysis of an area because of the continual accumulation of sediment as well as other organic matter such as pollen and charcoal. Sediment cores can also be dated quite accurately, often allowing for lake histories to be reconstructed in chronological sequence.
Pollen records
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. The distribution of these pollen grains can offer insight into the historical distribution of vegetation around the lake. 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. 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. 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.
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 Younger Dryas period, serving as a biological marker for that time period. Comparing historical vegetation profiles also allows researchers to compare successive changes in vegetation between two specific regions and correlate these differences with the corresponding climates of each region. A recent study conducted at Shudu Lake in the Hengduan Mountains of Yunnan, , southwestern China, was able to correlate changes in temperature and humidity with the development of needleleaf forests, as well as model recent anthropogenic effects on vegetation distribution in the area.
Diatoms The taxonomic assemblages of diatoms reflect many aspects of the temperature, chemical, and nutrient environment of a lake. Diatoms are particularly suited to paleolimnology, due to their silica-based frustules, which are preserved in sufficient condition, and in large enough quantities, to be extracted from sediment cores and identified at the species level. Diatoms have also been examined in conjunction with chrysophycean statospores to estimate nutrient conditions of prehistoric temperate lakes. Such estimations can be derived due to the fact that the predominance of either algal group varies depending on the nutrient conditions of their limnic environment. Diatoms show a high degree of success in water with a high nutrient content as opposed to chyrsophytes, which are better in water with a lower nutrient content. Certain species of diatoms also demonstrate a preference for specific aquatic pH, which allows researchers to estimate the historical pH conditions of a body of water by analyzing the species of diatoms within a sediment core. This makes diatom samples well suited for determining the impact of acid rain on a specific body of water, as diatom inference techniques are advanced enough to estimate relatively small numeric ranges of nutrient levels and pH values, as well as fluctuations in these measurements over a certain paleolimnological period.
Organic matter analysis