Threshold-mediated diatom community reorganization during multi-stressor recovery in a mining-impacted stream

Ecological recovery in mining-impacted streams is commonly interpreted as gradual biological improvement following chemical remediation, yet few studies have tested whether recovery in multi-stressor systems proceeds through identifiable ecological thresholds. We evaluated 13 years of data from Silver Bow Creek (Montana, USA), a historically metal-contaminated system undergoing remediation and wastewater treatment upgrades after 2016. Although dissolved metals declined relative to historical conditions, the environmental drivers of biological recovery remained uncertain. Gradient Forest (GF) analysis was used to identify drivers of diatom community turnover, after which Threshold Indicator Taxa Analysis (TITAN) was applied to detect species-level thresholds along the key drivers. TITAN sensitivity classifications were used to construct a Sensitive-Tolerant Index (STI) to evaluate structural recovery, while functional traits were examined using community-weighted metrics and redundancy analysis. GF identified total nitrogen (TN), total phosphorus (TP), and conductivity (COND) as the primary drivers structuring assemblage turnover. TN exhibited the most coherent species thresholds, converging near ∼1 mg L⁻¹. Crossing this threshold following wastewater treatment upgrades in 2016 coincided with an increase in the TITAN-derived STI (median -50.6 pre-2016 to -21.0 post-2016), indicating structural improvement. However, communities did not converge toward sensitive-dominated states. TP responses were more diffuse, and COND remained elevated due to persistent conservative ions (e.g., Na⁺, Ca²⁺, SO₄²⁻), with community thresholds near ∼400-500 µS cm⁻¹ indicating a secondary constraint. Trait composition showed redistribution rather than convergence toward low-disturbance strategies, indicating phased improvement in which nutrient-driven threshold crossing initiated structural improvement but persistent ionic enrichment limited full functional stabilization.