Diatoms

Can be centric (circular) or pennate (oblong)
Scanning electron microscope alllows much more precise analysis, lots of revisions, splitting of genera based on this approach
Light microscopes don't allow as much detail
Diatoms can be arranged into beautiful shapes (Klaus Kemp diatom arrangements)

What are diatoms?

Microscopic single celled silivsuos algae
Can live alone or in colonies
They can live anywhere light and moisture are available - even the catacombs of Rome after artificial light was brought in for tourists
They bind 20-25% of organic carbon, despite being <1% of biomass
Oldest diatom found dates to Jurassic, but according to genetic research they may have emerged in the Permian or Triassic

Many species have very specific physical, chemical and biological factors
Since variables are correlated with each other it's sometimes hard to determine which has the strongest effect

One of the most important variables controlling their occurence
Most of the known species live in saline waters, but they struggle in hypersaline waters (although some salt lake species are more tolerant)
The Kolbe and Husted classification for salinity is used to assign them into groups

Very important for freshwater diatom community structure
They can live anywhere from 2.5 to 9 pH (very extreme), almost nothing else lives at these conditions
Diatom biodiversity (species richness) is highest in pH7 neutral water, this is common in northern Finnish lakes
Below 5.5 pH the planktonic component (free floating photosynthesising diatoms) slowly dissapear. Periphytic (adhering to surfaces like rocks or submerged logs) diatoms outcompete planktonic diatoms at this pH
In lakes with lower nutrients and higher water clarity, periphytic diatoms also outcompete diatoms
Higher pH means more plankton and less periphytic taxa

Nitrogen and phosphorous are most important
- Limiting nutrient in oceans - usually nitrogen
- In lakes - phosphorous
Increasing nutrient loading increases primary production, eutrophication and unfavorable conditions for benthic algae and other species
Primary production in eutrophied lakes is mostly controlled by planktonic algae. In oligotrophic lakes its benthic algae

Other chemical (nutrient) factors like dissolved carbon, dissolved oxygen, silica, calcium, K, Mg, Fe...
Physical factors like water depth, turbulence, light, water temp
Bio factors like zooplankton abundance and parasites (parasites can infect the diatoms!)

Can use a qualitative (descriptive) or quantitative (statistical analysis, training set) approach.

Comparing time periods or different lakes
- e.g. relative abundance of acid vs alkaline species
- fresh water species vs saline species to study ocean transgressions or tsunamis, or changes in salinity in endorheic basins
- plankton vs littoral species
Vegetation history can be indirectly studied by looking at total carbon and dissolved organic carbon reconstruction - certain species are diven by carbon content of the lakes
Hydrological changes - ocean sediments showing higher levels of freshwater species can be used to measure river flow

Archaeology - reconstructing past landscapes at archaeological sites
- Clay-pot origin (?)
Oil and gas -can be used for dating sediment layers for oil and gas search
Forensic science
- Can find out if someone found dead in a lake drowned by seeing if there are diatoms in their lungs (meaning they inhaled water)
- Diatoms in clothes indicate the source location - Jan Weckström is the person who examines diatoms in clothes after drowning incidents
Diatomite
- Porous structure with a good ability to absorb water, used as filter for beer and wine brewing, explosives, filtering drinking water, toothpaste, polishing

Diatoms are the most commonly used biotic indicator in palaeolimnological research