Paleolimnology
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.
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.
As the hypolimnion of lakes gradually filled with sediments, oxygen depletion would promote the release of iron-bound phosphorus to the overlying water.
[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.
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.
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.
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.
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.
In paleolimnology, proxy data refer to preserved or fossilized physical markers which serve as substitutes for direct meteorological measurements.
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.
[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.
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.
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.
[29] These algal groups produce δ15N values that closely aligned with those of atmospheric N2, which allowed the researchers to draw conclusions about nutrient cycling and productivity in the lake by examining specific nitrogen isotopes of their sediment cores.
[30] 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.
[30] Lake deposits have a rich diversity of fossilized insects that trace back to middle Paleozoic era, further increasing in abundance during the Quaternary period.
Their head capsule and feeding structures are commonly fossilized in lake sediments,[32] allowing them to serve as a valuable paleoclimate proxy.
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.
[32] Researchers assessing chironomid distribution primarily examine the temperature, while considering supporting factors, such as pH, salinity, nutrient flow, and productivity, especially of the late Pleistocene/Holocene time period.
Chironomids are directly and indirectly affected by temperature during their entire life cycle, including larval emergence, growth, feeding and reproduction.
[9] According to research conducted in the high-altitude lake Lej da la Tscheppa, Switzerland, seasonal temperature reconstruction can be done with the help of independent chironomids and diatoms.
