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Paleolimnology (Greek: paleon=old, limne=lake, logos=study) is a scientific subdiscipline closely related to both limnology and paleoecology. Palaeolimnological studies are concerned with reconstructing the paleoenvironments of inland waters (lakes and streams; freshwater, brackish, or saline) – and especially changes associated with such events as climatic change, human impacts (e.g., eutrophication, or acidification), and internal ontogenic processes.

Paleolimnological studies are commonly based on meticulous analyses of sediment cores, including the physical, chemical and mineralogical properties of sediments, and diverse biological records (e.g., fossil diatoms, cladocera, ostracodes, molluscs, pollen, pigments, or chironomids). One of the primary sources for recent paleolimnological research is the Journal of Paleolimnology.

History

Lake ontogeny

Most early paleolimnological studies focused especially on the biological productivity of lakes, and the role of internal lake processes in directing 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 & 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 colonising organisms.^[1]

In a classic paper, Raymond Lindeman^[2] outlined a hypothetical developmental sequence, with lakes progressively developing through oligotrophic, mesotrophic, and eutrophic stages, before senescing to a dystrophic stage and 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,^[3] 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.^[4]

Deevey and Lindemann's ideas were widely, if uncritically, accepted. Although these ideas are still widely held by some limnologists, they were effectively refuted in 1957 by Deevey's student Daniel A. Livingstone.^[5] Mel Whiteside^[6] also criticized Deevey and Lindemann's proposal, and palaeolimnologists now consider 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, illustrating that 1) lake development is not a unidirectional process, and 2) climatic change can have a profound effect on lake communities.

Limnic Environment

Limnology is study of inland water bodies including: lakes, streams, rivers, estuaries, wetlands and more.^[7]

Biological properties

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 impacts of increased nutrient loading (anthropogenic eutrophication) on lake communities. Their studies reveal pronounced changes in the bottom fauna of North American and European lakes, a consequence of severe oxygen depletion in the eutrophied lakes.

From 1980 to 1990 the primary focus of paleolimnologists efforts shifted to understanding the role of human impacts (acid rain) versus natural processes (e.g., soil leaching) as drivers of pH change in northern lakes.^[8] The pH-sensitivity of diatom communities had been recognised since at least 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 parallel with increased industry and emissions.^{[citation needed]} 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, anthropogenic acidification.

In recent years palaeolimnologists have recognised that climate is a dominant force in aquatic ecosystem processes, and have begun to use lacustrine records to reconstruct paleoclimates. Sensitive records of climate change have been developed from a variety of indicators including, for example, paleotemperature reconstructions derived from chironomid fossils, and palaeosalinity records inferred from diatoms.^{[citation needed]}

Freshwater

Freshwater macroinvertebrates (invertebrates that can be see by naked eye; > 0.25 mm) are a very diverse group of small animals present in rivers, streams, lakes and wetlands.^[9] These animals include crustaceans, insects, molluscs, annelids and arachnids. They comprise a functionally and taxonomically diverse group that plays an important role in linking the food web in the aquatic ecosystem. They serve as a main source of food for the fishes and they depend on different types of algae and bacteria for the food.^[10]

Most aquatic macro-invertebrates are small while some are quite large such as freshwater crayfish. They are very sensitive to different physical and chemical conditions and respond to it accordingly due to their limited mobility. For example, their community will change when a pollutant enters into the water or due to the change of water quality. Therefore, the water body which is rich with a macro-invertebrate community can be used to provide an estimate of water body health.^[11]

Macro invertebrates termed "benthos" inhabit surfaces of vegetation, rocks and other aquatic sediments. They are widely distributed in the aquatic environment and present at a range of trophic levels. Most macroinvertebrates taxa are dependent on a range of abiotic factors and biotic factors and therefore community structure can be used to represent the local ecosystem. These factors may be the availability and quality of food, pH, temperature, salinity, land use pattern and water flow etc. These factors are interconnected and influence each other. For example, any change in the land use affects the riparian vegetation that may be increased or decreased which changes the inputs into the aquatic ecosystem, affecting the abundance and distribution of macroinvertebrate taxa.^[12] For all of these reasons, macroinvertabrates are considered as one of the best indicators of water quality.

Taxonomic groups of macroinvertebrates

Taxonomic groups of macroinvertebrates vary in their tolerance to changes in the aquatic environment.^[13][14]

Macroinvertebrates as an indicator of water quality

Macroinvertebrateas are commonly and widely distributed in many types of water bodies and respond with a range of sensitivities to many kinds of stressors. They are commonly used for toxicity testing and ecological assessments in water bodies and many of them complete their life cycles in a single water body so they are exposed directly to physical, chemical, and biological stressors and they also integrate the long and short term changes. When a freshwater source is degraded, the most sensitive invertebrate species are lost initially, followed by species more tolerant to pollution.These species either sustain their existence or their number decreases in response to the change.^[15]

Macro-invertebrates are an essential part of the food chain. Their strategies for feeding can be clustered into several "functional feeding" groups. For example, shedders feed on leaves and woody materials, some depends on the vegetation growing in the water body. Examples of shredders are amphipods, isopods, freshwater crayfish, and caddisfly larvae.

Paleoclimate proxies

Paleoclimatology (or the study of past climates) uses proxy data in order to relate elements collected in modern-day samples to suggest the climatic conditions of the past.

Sediment cores

Sediment cores are one of the primary tools for studying paleolimnology because of the role in which lake and river sediments play in preserving biological information.^[16] Paleolimnologists collect sediment cores and observe various proxy indicators in order to reconstruct past limnology of an area.^[16] Some of these indicators include geochemical markers and isotope data which are found within the various sections of the sediment core.^[16] In order to calibrate data, a group of around 40 or more calibration lakes are used to reconstruct what the conditions of a new core are likely to reflect.^[16] The use of these calibration lakes is necessary to identify dramatic differences in a newly collected core compared to normal variations which would be depicted in the calibration lakes. Through the use of sediment cores as a technique, paleolimnologists are able to use other proxies (such as pollen, charcoal, diatoms, chironomids, and other organic matter) in order to reconstruct the past climatic conditions of an area.^[16] Lake sediment cores in particular offer a more comprehensive analysis of an area because of the continual accumulation of sediment as well as other organic matter like pollen and charcoal.

Pollen records

Pollen and spores can be analyzed within a lab setting to determine the family, genus, or species which the pollen grains belong. The overall taxonomic assemblage of pollen types reflects the surrounding vegetation of the lake. Pollen analysis is a major field in paleoecology. A range of statistical and qualitative treatment of pollen records can be used to reconstruct past vegetation and climate.^[16]

Diatoms

The taxonomic assemblages of diatoms reflect many aspects of the lake temperature, chemical, and nutrient environment.

Organic matter characteristics

Organic matter may be characterized in terms of 1) amount (accumulation rate, reflecting productivity), 2) origin (aquatic or terrestrial determined from the C:N ratio), 3) carbon isotopic composition (reflecting plant photosynthetic pathways of C3 vs C4 plants, and reflecting drought and/or aquatic productivity), 4) nitrogen isotopic composition (reflecting sources of nitrogen and nitrogen cycling). There are a range of biomarkers widely used, as well as compound-specific isotopic analyses. Diagenesis is an important factor to consider in interpreting organic matter records.

Chironomids

Chironomids as a proxy of climate change: paleolimnology and chironomids

One of the major factor that affects the flora and fauna of the earth is the change of climate at the local, regional and global scale. These changes are preserved over the period of time as a fossil record and through paleolimnological methods, these changes can be traced out and also helpful for the prediction of the future climate change. Chironomids are one of the best indicators of climate change. These are two winged flies and belongs to family Chironomidae with 500 to 15,000 species. Ecologically they are considered as bottom dwellers and are very responsive to any fluctuation in the surrounding environment. They are considered as a good indicator of salinity, water depth, stream flow, aquatic productivity, oxygen level, lake acidification, pollution, temperature, and overall ecosystem health. As the assemblage of chironomids is dependent on several environmental factors, they can be easily related to those changing factors using a transfer function to connect a particular group of organisms to a specific environmental variable.^[17]

Lake deposits have a rich diversity of fossilized insects that trace back to middle Paleozoic era and more abundance can be found in the Quaternary period.Among aquatic invertebrates, different families of aquatic fly larvae can be traced out in the lake sediments of Quaternary era. Among them, Chironomids are of greatest ecological importance due to their diverse habitat in the aquatic ecosystem, feeding habitat, important component in the food web and above all their usage as a plaeolimnological studies because their head capsule and feeding structures are fossilized in the lake sediments.^[18] Chironomids complete their larval stage in the water and its life cycle continues up to several years while adult life span is very short. They have an important role in the degradation of material in the aquatic ecosystem and serve as an important part of the food web during their whole life cycle.^[19]

Climate change

According to IPCC (2007) reports one of the vital aspect of environment is the continuous climate change due several factors over the period of time which is influential in shaping the biodiversity.
Macroinvertebrates, especially chironomids have been considered as an important indicator of the climate change especially temperature due to their presence in the abundance in the lake sediment as a fossil. There is a strong correlation between the chironomids assemblage and the water temperature, lake depth, salinity and, nutrients of the lake.Lake water levels over the past time period can be inferred from the chironomids assemblage due to their correlation. It is due to the fact that climate change resulted in the water level change of the lake that is related to change in the pattern of chironomids distribution and abundance. Their strong correlation helps to build the information about the evaporation and precipitation in the past. Past climate change is reconstructed based on the paleolimnology with the help of different fossilized records especially the lake sediments that helps to find out the regional and local climate change. It is important to differentiate between the impacts that a lake store due to the regional and local level climate change.^[19]

Recent studies in Arctic shows that biodiversity affect more due to the arctic warming than other associated factors like human alteration and acidification.^[20] In the Himalayan region water bodies are not only affected by the anthropogenic disturbances but also impacted by the different types of pollutants that are transferred to this area over a longer distance. It is vital to understand all the associated factors acting on the aquatic biodiversity while analyzing the impact of climate change over the past years with the help of lake sediments.^[21] It is also important to consider that the impact and intensity of different changes due to climate change may have totally different affect on different ecosystems depending upon its sensitivity to that change.^[22]

Factors influencing chironomids distribution and abundance

There are several factors that have been affecting the abundance and distribution pattern of chironomids over the past years. Therefore, it is important to be careful while making interpretations from their fossil records. There are several debates for the last many years on the impact of temperature on their abundance and diversity along with other associated factors. For the correct interpretation of the fossil records, it is important to understand all the associated factors of an ecosystem. In order to understand the different forces that have been affecting the fossil data of lake, it important to reconstruct physical, chemical and the nutrient content of the lakes that actually shapes the communities. Their distribution and abundance are highly influenced by the combination of human disturbance and change in the climate. Both of them influence the catchment area that resulted into the changing of vegetation, hydrology, and nutrients cycle. Any change at the regional level especially the temperature affects the water quality at the local scale and then at the end it affect the habitat that is species-specific.^[18]

Chironomids and reconstruction of quantitative change in Holocene climate

There is ongoing debate about the main factor that controls the distribution and abundance of chironomids. Several factors are considered as the controlling factor especially temperature that in turn control other supporting factors including pH, salinity, nutrient flow and productivity especially in the late Pleistocene/Holocene timer period. For many years research has been carried out on the relationship between temperature and chironomids distribution pattern due to its influence on its emergence. Chironomids are directly and indirectly affected by temperature during the whole life cycle including their larval emergence, growth, feeding and reproduction.^[18] According to Eggermont and Heiri there is indirect impact of temperature on different physic chemical aspects of water resulted into determination of their distribution and abundance. There is also a strong relationship between the chironomids abundance, emergence and distribution with the mean temperature of the air and water.^[23]

According to a research conducted in high altitude lake Lej da la Tscheppain Switzerland, seasonal temperature reconstruction can be done with the help of independent chironomids and diatoms. Any change in the assemblage of chironomids reflects change in the temperature and duration of ice cover of that specific water body due to climate change. According to their findings, chironomids respond mostly to the change in the summer temperature, so seasonal variation in temperature can be inferred from different cores of the sediments.^[24]

Various resources for application of chironomid analysis to lake sediments

1. US Geological Survey collected sediment cores from 56 lakes across USA, as a part of "National Water Quality Assessment" form 1992 to 200. This report has details about the methods of coring sediments.^[25]
   


2. The Glew sediment corer (a common means of collecting a short sediment core) ^[26]
   


3. Following are several sources that can be used for the identification of chironomids.
   
   
   
   1. Royal Entomology Society ^[27]
      
   
   
   2. 
      BugGuide ^[28]
      
   
   
   3. Identification of Chironomidae.^[29]
      
   
   
   4. 
      Museum Victoria. This website has several reports about the Diptera and their identification keys ^[30]
      
   
   
   
   

See also

• Paleobotany


• Paleophycology

References

External links

• International Paleolimnology Association (IPA)


• Journal of Paleolimnology (JOPL)


• NOAA online Paleolimnology data collection


• http://dx.doi.org/10.18268/BSGM2018v70n1a10