A structural equation model to predict macroinvertebrate-based ecological status in catchments influenced by anthropogenic pressures

Abstract

A Partial Least Squares-Path Model (PLS-PM) was developed for the Ave River Basin (North of Portugal), and the results used in a scenario analysis. The data for PLS-PM comprised a set of anthropogenic pressures, water quality parameters, and a macroinvertebrate-based biodiversity index (IPtI_N) used to assess the ecological status of streams. These groups of measured parameters (called latent variables) were given the names "Pressures", "Contamination" and "Ecological Integrity". Besides, latent variables were connected through path coefficients representing potential causal effects among them. In a large portion of Ave the ecological status of streams is currently bad or poor. Nitrate and coliforms were the most weighted measured variables of latent variable "Contamination", with w ≈ 0.7 and w ≈ 0.2, respectively. The highest "Pressures" weights were ascribed to livestock farming (0.7) and population density (0.4). The connections "Pressures"-"Contamination" and "Contamination" - "Ecological Integrity" exposed a sequence of direct negative effects between the three variables, expressed in the corresponding path coefficients (pc = 0.87 and pc = -1.11). Paradoxically, a direct negative effect of "Pressures" over "Ecological Integrity" was absent (pc = 0.29). Therefore, the poor ecological status of local stream waters might not be directly related to the presence of potentially threatening contaminant sources (the "Pressures"), but to ineffective monitoring of livestock farming and wastewater treatment activities that potentiate (accidental) releases of contaminants into the streams. The lack of a direct link "Pressures" - "Ecological Integrity" supported the results of pressure change versus IPtI_N change scenarios. Regardless of some significant reductions of anthropogenic activity and population density until 2027, announced by the Portuguese Environmental Agency, the scenarios could not predict improvement of ecological status beyond the "moderate" category. The study recommendations were therefore to prevent contamination through proper implementation and monitoring of existing watershed management plans. The adequate treatment of domestic effluents and the control of livestock farming residues are urgent.

Keywords: Anthropogenic pressures; Ecological status; Livestock farming; PLS-PM; Stream water contamination; Urban sewage.