How are diatoms identified?
i did research on these a while back In general diatoms can be used to trace a variety of environmental phenomena, from changes in sea level, (whether brought about by climate change or tectonic activity), breaches of coastal barriers, (as a result of storms and/or sea-level rise), to the evaporation of lakes, (increasing salinity determining diatom assemblages). Below is an outline of their most prevalent uses. # MarineSome species are restricted to a very narrow range of salinities and are know as stenohaline species, others have no such restrictions and are known as cosmopolitan species. As a result, this causes zonation, which is particularly evident in estuaries, where a spectrum (and a gradient for such a spectrum) can be calculated from coastal to offshore species. This has applications in determining palaeo-fluvial environments, and sediment focusing. # Freshwater.Some freshwater species will tolerate a little salt, and are known as halophilic, occurring in coastal lakes, or where the groundwater is rich in salts. However most freshwater species are stenohaline and will not tolerate salt. There are several ways of deducing palaeotrophic status using diatoms: # Total Diatom Count - This is relatively simple, the more diatoms there are in your sample, the more productive a given body of water is. # Centric:Pennate Ratio - The more centrics there are in your sample, the more productive the environment is. (With the exception of a species called Cyclotella.) # Indicator Species - Certain species are typical of certain conditions, for example Stephanodiscus is typical of eutrophic (abundant nutrient) conditions, and Tabellaria of oligotrophic (very low nutrient) conditions. # Planktonic:Non-planktonic Ratio - Planktonic forms are more common in eutrophic lakes. # Diversity Indicators - A low overall diversity amongst diatoms indicates stressful conditions, for example extreme trophic status (hyper-oligotrophic or hyper-eutrophic). However this could also indicate a source of pollution etc. This perhaps the most important and most widely used application of diatom studies. Diatoms are highly sensitive to pH and can illustrate differences of as little as 0.1 pH units. To accomplish this species are classified as either: * Acidobiontic (Acid Living) pH < 7 * Acidophilous (Acid Preferring) pH ≤ 7 * Circumneutral pH = 7 * Alkaliphilous (Alkali Preferring) pH ≥ 7 * Alkalibiontic(Alkali Living) pH > 7 This method is highly dependant upon knowing the pH preference for all of the diatoms present, as the percentage of each of the above groups is measured and the ratios used to calculate a log index of the given population. With the use of some complicated mathematics this, in turn, can then be used to determine the palaeo-pH. Obviously, it is not always possible to know the preference of all of the species in your sample, and therefore this method can not always be applied. Diatoms are not very useful in determining changes in palaeo-temperature, due to the fact that the large majority of species will tolerate very wide ranges of temperature, typically from 0oC to 20oC. That said, different assemblages arepresent when comparing warm and cold waters. However, this is almost certainly due to other overriding factors such as: incident solar radiation, water chemistry, pH, and nutrient availability.
