Forensic limnology

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Forensic limnology is the part of science that uses freshwater life to help solve crimes. It mainly focuses on diatoms, tiny algae with glassy shells, found in water and on people or objects involved in a case.

Diatoms have silica cell walls called frustules. There are about 12,000 species, and they live in different waters and change with the seasons. When diatoms die, their shells sink and leave a record in the water’s sediments. Over time, this creates a natural time series of which diatoms were present.

Scientists build diatom profiles for lakes, rivers, and other waters. They compare these profiles with diatoms found at a crime scene or on a victim. This helps determine where a body might have been or whether a person drowned in a particular body of water.

Diatoms are useful because they are diverse yet identifiable under a light microscope, and their shells preserve well. Some species prefer certain pH levels or salinities, and the presence and abundance of diatoms change with the environment and the time of year. Even if there isn’t a known water profile for a location, the types of diatoms found can still reveal something about the ecosystem involved.

There are limitations. Diatoms don’t infallibly indicate where a crime happened or exactly when, and they don’t always enter the body if there hasn’t been water inhalation or circulation. If a body is placed in freshwater after death, diatoms may not help determine the time of death. Frustules are durable and can be present for reasons other than the specific scene, which can complicate results. Collecting and analyzing diatoms is also time-consuming.

How evidence is collected and tested: to be reliable, samples usually need at least 20 diatoms in 100 microliters. For human tissue, five complete diatoms from at least two different organs (such as lung, bone marrow, liver, or spleen) are often used. Samples can come from body tissues or from the crime scene.

Extraction methods commonly use acid digestion to dissolve organic material and leave the diatom shells for analysis. Phase-contrast microscopy is used to observe and count diatoms, grouping them by species. The measured ratios of diatoms in the sample are compared to the water’s diatom profile to look for matches.

A diatom database began in 2006 as a kind of fingerprint system, where software tries to identify diatoms by shape and color. By 2012, the database was not complete and was not used in courtrooms.

Time-of-death estimates can be guided by how many diatom species are found. Fewer than about 20 species might suggest death within roughly the last week or two; more than about 50 species could indicate death occurred several weeks earlier. Some late-appearing diatoms or algae may help narrow the window further, but such indicators are not always precise.

Today, forensic limnology is used in only a small number of court cases. It can help estimate drowning location and give a rough time frame, aiding investigations. Even when not used in prosecution, the findings can deepen understanding of a crime.