Patterns of ecological quality in Portuguese transitional waters over the last five decades: a framework based on benthic foraminifera for the Sado estuary

This study aimed to examine the validity of benthic foraminifera-based indices – Foraminiferal Stress Index (FSI), diversity index Exp(H'_bc), and Ecological Quality Ratio (EQR) – for evaluating estuarine environmental conditions and supporting palaeoecological reconstructions, focusing on the Sado estuary (southwest Portugal). Located near heavily industrialized urban areas and designated as a Nature Reserve, this estuary faces human pressures and high natural values, leading to management conflicts. Foraminiferal assemblages were analysed in surface sediment samples, with known concentrations of several metal(loid)s, and in dated core samples. Correlations were found between metal(loid)s, organic matter, grain size, and salinity (as a proxy for tidal influence) controls on foraminiferal distribution. Despite such hydrodynamic controls, statistical analyses also confirmed that metal(loid) concentrations and enrichment factors (as proxies for ecological quality) influenced foraminiferal distribution. A significant direct correlation was observed between lower ecological quality status (EcoQS), as reflected by FSI and Exp(H'_bc), and higher metal(loid) concentrations, particularly for Exp(H'_bc) and Ni, Cr, Cu, Zn, Pb, and As, suggesting they can be used in preliminary surveys with varying accuracy. Reconstructions applying Exp(H'_bc) and EQR aligned with Portuguese economic cycles, showing that Palaeo-EcoQS tended to be “Good”/“High” during downturns and “Poor”/“Moderate” in recovery and peak periods. While this analysis has limitations, the cost-effective use of foraminiferal data can offer reliable early-stage environmental quality assessments, with the added benefit of supporting historical reconstructions from sedimentary archives. Overall, this work contributes to the ongoing efforts to validate benthic foraminifera as a biological quality element in assessing EcoQS in marine and transitional habitats.