Estuarine health assessed by sediment biotic and geochemical indices: insights into sustainable decision-making in the Sado transitional system (SW Portugal).

Balancing socio-economic development with environmental quality in estuaries requires reliable tools for ecological assessment and informed management. Although various biological and (geo)chemical indices have been formulated to evaluate ecological quality status (EcoQS), transitional systems such as estuaries remain challenging to assess due to steep natural gradients and intense anthropogenic pressures, which can compromise the effectiveness of conventional indices. This study applied a practical, multi-criteria sediment assessment to evaluate benthic EcoQS in the Sado estuary, SW Portugal - a socio-ecological system strongly influenced by human activity. The methodology combined five indices - two based on benthic foraminiferal taxa and three on metal(loid) concentrations - each reclassified into a binary EcoQS of either "Acceptable" or "Unacceptable" to aid timely environmental management decisions. Most sampling stations were assessed as "Unacceptable", underscoring the need for restoration, with metal(loid) contamination/pollution identified as a major contributor to the final EcoQS. Reliability analysis revealed varying levels of agreement among indices, with the Modified Pollution Index (MPI) and the foraminiferal diversity index Exp(H') evidencing the highest suitability for this estuarine context. Importantly, the use of the composite index - integrating all five metrics - provided a more robust and comprehensive assessment than any individual measure, confirming previous research that advocates for integrated approaches in dynamic and heterogeneous environments. From a management perspective, the applied framework offers many advantages: it translates complex, multidimensional environmental data into actionable guidance, accounts for both biological responses and chemical stressors, identifies priority areas for meeting Water Framework Directive (WFD) goals, and supports consistent monitoring of restoration progress.