Paleolimnology

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. 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, 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. 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. Mel Whiteside 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. 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. 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 ==

, Washington 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. Pollen records derived from a variety of pollen species proxy data. 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. 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. 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 have also been examined in conjunction with chrysophycean statospores to estimate nutrient conditions of prehistoric temperate lakes. 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 Examinations of the deposition and makeup of organic matter in the sediments of lakes has often been utilized in paleolimnological assessments. A variety of factors are taken into consideration when examining deposited organic matter, including the quantity, origin, and variety of isotopes and biomarkers. as well as serving as a bridge between paleolimnology and geochemistry in demonstrating the relationship between lake geochemistry and organic matter deposition. For instance, a study in eastern China found that larger and deeper highstand lakes in warmer, more humid climates tended to show higher levels of organic matter deposition than lowland lakes in cooler, arid climates. 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. 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. In particular, δ15N values can vary based on productivity levels in aquatic ecosystems. A study that reconstructed lake conditions of Lago Taypi in Cordillera Real, Bolivia, found that when Nitrogen served as the limiting nutrient, levels of nitrogen-fixating algae significantly rose. Human and animal waste, as well as synthetic fertilizers, have diagnostic isotopic shifts that allow researchers to characterize specific nitrogen inputs and track potential human-induced changes in nutrient flux, using δ15N measurements. 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, 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 salinity, water depth, stream flow, aquatic productivity, oxygen level, lake acidification, pollution, temperature, and overall ecosystem health. Chironomid distribution can be related to those factors using a transfer function to connect a particular group of organisms to a specific environmental variable. 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 physical, chemical, and nutrient content that actually shape the lake communities. Their distribution and abundance are highly influenced by the combination of human disturbance and changes in climate, both of which influence the catchment area that resulted in changing vegetation, hydrology, and nutrient cycles. Any change at the regional level, especially temperature, affects local water quality and then ultimately has a species-specific effect on habitat. Any change in the assemblage of chironomids reflects change in the temperature and duration of ice cover of that body of water due to climate change. According to their findings, chironomids respond mostly to change in summer temperature, so seasonal variation in temperature can be inferred from sediment cores. Macroinvertebrates, especially chironomids, have been considered an important indicator of past climate change, in particular with regard to temperature. There is a strong correlation between the chironomid assemblage and water temperature, lake depth, salinity, and nutrient concentrations. Therefore, the impact of climate change on lake water levels can be related to changes in the pattern of chironomid distribution and abundance. This strong correlation indicates the evaporation and precipitation profiles of the lake in the past. Past climatic conditions are reconstructed based on paleolimnology with the help of different fossilized records, especially lake sediments that help differentiate regional and local climate change. ==References==