Freshwater and estuarine diatom composition and seasonal variation: the influence of environmental factors in supporting forensic applications

Diatoms are an integral part of aquatic ecosystems and have proved valuable in forensic investigation in providing evidence in drowning cases or in assisting geolocation. This preliminary study investigated the seasonal composition of diatom community in three aquatic systems in northern Portugal with regard to different hydrodynamic and anthropogenic pressures. The aim was to understand how environmental factors influence diatom distribution and to establish a first step towards developing a database for forensic applications. Water samples were collected seasonally from 2022 to 2023 and physicochemical parameters were measured. Diatom identification and analysis were performed using optical microscopy. Distinct seasonal patterns emerged. The genus Pinnularia preferred alkaline conditions. The estuary showed increased nutrient levels and turbidity in autumn and spring, favouring genus such as Melosira. Diatom density was low in wells, where benthic genera (Navicula and Nitzschia) correlated with conductivity and nitrites, Gomphonema preferred oligotrophic conditions. These preliminary findings reinforce the value of diatoms as bioindicators in forensic investigations. Correlation with parameters (nitrites and conductivity) supports their use in environmental monitoring and forensic analysis. Developing a comprehensive seasonal database enhances forensic accuracy, especially in drowning or geolocation cases. Future research should consider the influence of climate change on diatom distribution, given its ecological and forensic relevance.