Exposure to grassland fire ash alters the life-history traits and causes morphological deformities in the aquatic insect Chironomus xanthus.

Wildfires generate large amounts of ash that can contaminate watercourses, with potentially harmful effects on aquatic biodiversity that are not fully understood. This study experimentally evaluated the influence of environmentally relevant concentrations of grassland ash on aquatic biota, using the freshwater non-biting midge Chironomus xanthus as a model organism. Specifically, we assessed the toxicity of ash on oviposition and emergence patterns, changes in body and head capsule sizes, and deformities in the mentum and mandible of 4th-instar larvae and the wings of adult C. xanthus. The tested ash concentrations were 0, 0.01, 0.1, 1, and 10 g/L, allowing, for the first time, the use of a geometric morphometric approach to evaluate deformities in immature and adult chironomids as sublethal implications of ash exposure. Our results showed a concentration-dependent decrease in the proportion of individuals undergoing pupation and emerging as adults. In addition, water contamination by ash led to reduced body size and more frequent morphological deformities in larval mouthparts, with a consistent narrowing pattern in the mentum at ash concentrations. Geometric morphometry also revealed that males' and females' wings were significantly smaller and narrower, with females exhibiting asymmetries between the right and left wings under higher ash concentrations. These findings demonstrate that environmentally relevant concentrations of grassland ash impair the development of C. xanthus larvae and adults, reducing body size and reproductive capacity. Given the key ecological role of chironomids as prey for higher trophic levels and as benthic ecosystem engineers, such impairments raise concerns about population declines and potential disruptions to freshwater food web dynamics after grassland fires.