Abstract
<jats:p>Dissolved iron and low pH are widespread stressors in streams draining iron-rich soils, wetlands and former lignite-mining areas. The biological effects of ferrous iron are difficult to assess from iron concentration alone, because toxicity depends on pH, exposure duration, chemical form and species sensitivity. This study examined the combined effects of dissolved ferrous iron (Fe²⁺) and pH on the survival of brown trout (Salmo trutta), common dace (Leuciscus leuciscus) and European eel (Anguilla anguilla) exposed in flow-through cages in Danish lowland streams.Groups of five fish were exposed under field conditions, and survival was recorded after approximately one week and one month. Mean Fe²⁺ concentration and mean pH during each exposure period were related to survival using observed Fe²⁺–pH distributions and binomial generalised linear models. The final dataset comprised 128 one-week and 102 one-month observations for brown trout, and 21 observations for each exposure period for common dace and European eel.Brown trout showed the clearest response. Survival decreased consistently with increasing Fe²⁺ concentration and decreasing pH after both one week and one month. Common dace showed a more variable response, with weaker statistical support, especially after one month. European eel was the most tolerant species, with high survival across much of the observed Fe²⁺–pH range, although mortality occurred under some low-pH and high-iron conditions.The results show that no single Fe²⁺ concentration can be used as a universal survival threshold. Assessment of iron-rich streams should consider Fe²⁺ concentration together with pH, exposure duration, species and life stage.</jats:p>