Where conditions are conducive, diatom remains will usually accumulate on lake/sea beds,
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and will often exhibit mixed assemblages, (i.e. consist of both benthic and planktonic forms, in addition to those brought in from tributary river/stream systems, and from soil in-wash). The best preservation conditions in terms of diatoms are those with any mixture of fine grained, anaerobic, and slightly acidic sediments.
Sampling is most frequently carried out by random core samples of a given area, as this preserves changes in the diatom assemblages over time. Where cores are sampled from beneath existing lakes, care should be taken to disturb the sediment-water interface as little as possible. Often a rich organic mud called gyttja, (typical of interglacial periods) will have accumulated, consisting of mainly faecal debris, animal and plant remains, along with some clastic component, (sand/silt/clay). Gyttja will retain a record of the most recent diatom activity.
Uses Of Diatoms
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.
Indicators of Salinity
1. Marine
Some 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.
2. 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.