Recalibrating the BMWP-CR index: a methodological proposal for updating tolerance scores and water quality classifications

The BMWP-CR index, integrated into Costa Rican legislation in Minae, 2007 , requires recalibration to more accurately reflect local ecological conditions. This study updated the index using data from the National Water Quality Monitoring Plan (2015–2020) across 107 rivers, employing an integrated methodological framework for assigning tolerance values (TVs) through expert surveys, Principal Component Analysis (PCA), and Threshold Indicator Taxa Analysis (TITAN2). To ensure robustness, a consensus-based criterion was applied: TVs were modified only when at least two of the three methods aligned in the direction of change. TITAN2 enabled the quantification of specific ecological thresholds against stressors, including biochemical oxygen demand (BOD), oxygen saturation percentage, and total suspended solids (TSS). Consequently, TVs for 41 families were refined, 13 new taxa were incorporated, and genus-level differentiations were established for four families, thereby enhancing the index's taxonomic representativeness. Comparative analysis with the Minae, 2007 version revealed significant shifts in quality classifications: 14 families showed increased tolerance scores, 27 decreased, and water quality category thresholds were redefined, notably raising the “Excellent” category limit from >120 to >131 points. This recalibration enhances the sensitivity of the BMWP-CR as a management tool, providing a more rigorous technical framework for the protection of freshwater rivers in Costa Rica. • The adjustment of the BMWP-CR index enhances its accuracy and utility in water quality assessment. • Adapting biological indices to local ecological characteristics is emphasized. • The refinement of the index strengthens its applicability in the management and conservation of freshwater resources. • The proposed methodological approach could be replicated in any country using the BMWP index for freshwater biomonitoring.