The field of forensic limnology relates to the study of freshwater ecology in a legal scenario, particularly the study of diatoms. Diatoms are microscopic algal organisms plentiful in many aquatic environments, including both flowing and standing water systems. The abundance and uniqueness of diatoms allows for their study to be of great use in certain legal investigations.
These diverse, asexually-reproducing organisms occur in a great variety of colours, sizes and shapes, most being unicellular but some forming simple colonies. Not only are they found in aquatic environments, but also in soils and a range of other moist environments. Typically less than 200 microns in size, diatoms are encased in a silica-based cell wall which causes significant variety in the morphological features of different species of diatom. This occurrence allows for diatoms to be easily identified and distinguished from one another, as well as being particularly resistant to decay. The species and numbers of diatoms present in a body of water can vary depending on the environment and even the time of year, with diatom populations increasing or decreasing seasonally.
Numerous features of diatoms make these organisms an ideal focus for analysis in forensic investigations. Their minute size means that they can be readily transferred from the crime scene by objects or people, with perpetrators unlikely to be aware of the presence of these organisms. The distinctive morphology of diatoms allows for species to be distinguished from one another, and their abundance and variation results in different bodies of water developing unique assemblages of diatoms.
Currently, the primary application of the study of diatoms in forensic investigations is in the diagnosis of drowning as the cause of death.
Drowning is not an uncommon cause of death, however it must be considered that a body found in water may not have actually drowned in that water, particularly if certain pathological indicators of death by drowning are not present. Furthermore, confirming cause of death may be particularly problematic if the remains are badly decomposed. It is vital to establish whether the victim was alive when they entered the water or if they had died elsewhere before being moved to the water.
When an individual drowns, water is inhaled and subsequently the diatoms in that water reach the lungs and can be circulated around the body to other internal organs and bodily tissues, including the brain, kidneys and bone marrow. To an extent, diatoms could reach the individual’s lungs even if they were no longer breathing, but without blood circulation they would not be readily transported around the body. It must however be taken into account that diatoms can be present in the bodily tissues and internal organs by other means, as diatoms are present elsewhere in the environment, including in the air. Due to the silica wall possessed by diatoms, they are resilient to decay and digestion by the body, therefore will often persist in the body even in later stages of putrefaction.
Samples taken from the body of the deceased can be examined in order to compare the diatoms in the body with the diatoms found in the body of water in which the remains were found. By comparing species, abundance and distribution of diatoms, it may be possible to establish whether or not the victim is likely to have drowned in the suspected drowning medium. If the same diatoms are found in both the remains and the water, it may be concluded that the victim is likely to have drowned in that water. In the event that the diatoms prevalent in the water do not match those found in the body, it may be possible to use the diatoms to establish the actual site of drowning if samples are available for comparison.
As previously stated, diatoms vary seasonally, a fact which can be of use to a forensic investigation involving submerged human remains that have been in the water for a significant period of time. To an extent, the study of diatoms may be able to provide insight into post-mortem interval. As previously discussed, diatom populations vary throughout the year, thus it may be possible to establish the time period in which an individual drowned in the body of water based upon the diatoms found in the body and how they compare to diatom populations in the water at different times of the year. However this could only be used as an estimation and would not be useful if a more specific post-mortem interval is required.
The analysis and comparison of diatoms is typically conducted using light microscopy techniques. The silica wall of the diatom results in the various species exhibiting distinctive morphological features that can be used to distinguish between diatoms of different species or determine that two diatoms are the same. Identifying the species can generally be achieved using a light microscope, though scanning electron and transmission electron microscopy (SEM and TEM respectively) can also be used. Both of these forms of microscopy utilise electron beams to visualise the sample, however SEM is typically preferred when viewing the organism as a whole, whereas TEM can produce images with greater magnification and resolution, thus may be more ideal for focussing on specific features of the diatom’s morphology. The analysis will often involve the microscopic examination and comparison of diatoms recovered from a suspect body of water (for instance, a lake in which some human remains were found) and diatoms retrieved from the human remains. The extraction of diatoms from human remains is often achieved through acid extraction of the tissue sample or bone marrow, for instance using nitric or sulphuric acid.
Unfortunately the diatom test to refute or corroborate cause of death by drowning has been placed under much scrutiny, and is not viewed as a reliable indicator of cause of death worldwide. Further research in this area is needed.
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