Using machine learning models to assess the population dynamic of the freshwater invasive snail Physa acuta Draparnaud, 1805 (Gastropoda: Physidae) in a tropical urban polluted streams-system

Abstract

The poor understanding of the spread conditions and the geographical patterns of the exotic freshwater snail Physa acuta in sub-Saharan Africa urban streams makes it difficult to design and implement sustainable management tools, especially in deteriorating global and local environments. This study aimed to determine the ecological preferences and the major environmental variables associated with P. acuta abundance, occurrence and distribution in Douala urban streams-system using modelling approaches based on discriminant factor analysis, canonical Spearman correlation and random forest tree models. We inductively define four clusters for snail’s distribution along a geographical and upstream–downstream pollution gradient. Snail’s specimens were collected monthly over a 13-month period in two forest and ten urban sites using a long-handled net (30x30cm side, 400-µm mesh). Meanwhile, measurements of the physicochemical properties of water and streambed sediment were taken. In addition, ten metal contents were determined in P. acuta tissues to assess its ability of bioaccumulation of heavy metals. Results revealed that urban streams of Douala are adversely impaired, especially downstream of industrial effluents. Discriminant factor analysis and random forest regression trees model showed that dissolved oxygen, biochemical oxygen demand, alkalinity, salinity, pH, water depth, wetted width, zinc, iron, cadmium, encumbrance rate of the sediment and sediment types (high sandy mud, sandy mud and muddy sand) were the major environmental variables responsible for structuring P. acuta distribution. Abundances decline drastically following pollution gradient, with great decrease observed in juvenile size classes. High contents of metals were detected in P. acuta tissues, especially downstream of industrial effluents. This study highlights that P. acuta spread in Douala urban-streams system is structured by key environmental variables related to an upstream–downstream pollution gradient. Therefore management plans should emphasize on proper disposal ways of industrial wastes, and stakeholders should take responsible actions in water pollution control and implement the polluter-payer principle.