Evaluating the effectiveness of biotic indices for long-term ecological quality assessment in a heavily polluted estuary.

Marine ecosystems, particularly estuaries, are increasingly threatened by anthropogenic pressures. The Odiel Estuary has suffered severe contamination from acid mine drainage and industrial activities. Since 1986, mitigation efforts have been implemented, yet their long-term ecological effectiveness remains under-evaluated. This study assesses environmental recovery by analyzing six biotic indices over 18 years (1998-2016). Our objectives were to determine spatial gradients and temporal improvements in ecological status within the estuary using these indices, and to identify the most reliable monitoring tools for transitional ecosystems. Results revealed a clear spatial gradient, with inner estuary sites in poor condition and outer marine zones showing better status. A detectable improvement in benthic community structure and water quality was observed over time, particularly in 2016. M-AMBI and BENFES correlated strongly with species diversity and effectively captured environmental gradients, whereas AMBI, BENTIX, and BOPA/BO2A showed lower sensitivity. Despite observed improvements, persistent heavy metal contamination requires continued monitoring and management. The study underscores the challenge of distinguishing natural estuarine variability from anthropogenic impacts, advocating for multi-index approaches to enhance ecological assessments. These findings provide insights for future monitoring and standardized protocols in transitional waters globally.