The potential of algae as indicators of post mortem submersion interval in a standing body of freshwater

Sloane, Natalie
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While the succession of arthropods is used to determine the post mortem interval (PMI) in a terrestrial environment, there is currently no method to determine the post mortem submersion interval (PMSI) of a submerged body. Recently, however, the succession of algae colonising submerged remains has received increased attention as a possible PMSI determinant. The aim of this study was to determine whether algae could be used as PMSI determinants. The decomposition of piglet carcasses were considered. The stages of decomposition and the ADD for each determined. The algae colonizing the substrates (piglet and glass microscope slides) were recorded and any changes in their communities or growth tested to see whether this can be aligned with the submersion period. The effect of seasonality was analyzed using glass microscope slides as changes in the algal community growth and composition would be in response to reasons other than decomposition. This research was conducted in a freshwater pond in South Africa over three periods (winter 2015, summer 2015 and winter 2018). In 2015 the substrates were submerged for 42 days. The winter study was repeated in 2018 to accommodate complete decomposition. The pH and dissolved oxygen was measured every second day in the winter 2018 study. Samples were obtained every 6 days from a pre-marked surface area of 2,5cm x 2cm along the carcasses’ vertebrae and the glass microscope slides. The sampled algae were identified up to the genus level, classified as planktonic (free floating) or benthic (attached to substrate) and a count of each taxon was carried out. Decompositional changes were documented every two days and five stages of decomposition were determined. The defining characteristics of each stage were present in each study but the duration of the stages differed. The ADD values and stage of decomposition alone were insufficient PMSI indicators. The ADD derived from the TAD scores decreased in accuracy with increasing submersion period. The variation of equations produced in different seasons (current study) and compared with other studies indicates that a new equation will need to be iii determined for each scenario. The successful colonisation of the carcasses by algae, the patterns in algal succession and the general bell-shaped curve in algal diversity over time supports the theory that algae, in conjunction with the ADD and degree of decomposition (stages of decomposition), are useful PMSI estimates. The absence of trends between algal diversity and the TAD score prevents the TAD score being used to gauge the degree of decomposition. The bimodal fluctuation of the Shannon-Weiner Index (considers abundance and diversity) is a promising approach in estimating the PMSI but will need to be considered along with the degree of decomposition. The importance of considering season when using algae as PMSI determinants was confirmed by the difference in algal growth and community composition on the glass microscope slides. The variation between seasons and year indicate the need to consider each scenario by itself until such time that the effect of environmental factors can be accounted for. Further research into the effects of seasonal variation and the impact of decomposition on the colonising community could allow ADD, decomposition (TADS and stages) and the algae colonising a submerged body to be used collectively to provide a more informed and complete estimate of the PMSI.
A dissertation submitted in fulfilment of the requirements for the degree of MSc(Med) in Forensic Science to the Faculty of Health Sciences; University of the Witwatersrand, Johannesburg, 2020