Ecotoxicological effects of post-fire runoff on aquatic microbial decomposers and invertebrate shredders
In Mediterranean forests, wildfires are a major disturbance that pose a threat to life, human goods, and natural resources in fire-prone forest areas and their frequency is not expected to decrease due to climate change. Wildfires are a diffuse source of contamination of freshwaters affecting water quality through the production of pyrolytic substances, such as polycyclic aromatic hydrocarbons (PAHs), and the input of metals associated with ash/soil loads. Although PAHs and metals are recognized by their toxicity, environmental persistence, and bioaccumulation, there is a lack of knowledge on the impacts of post-fire runoff on stream biota and ecological processes they drive.
We used post-fire runoff from high-severity wildfires in Pinus and Eucalyptus forests, and an adjacent stream in Sever-do-Vouga (Portugal) to evaluate their effects on microbial decomposer diversity (based on DNA profiles) and activity (leaf mass loss), and on the feeding behavior and biochemical responses of invertebrate shredders.
Leaves were immersed in a stream, to allow microbial colonization, and then were exposed for 20d in microcosms to increasing concentrations of post-fire runoff in the absence/presence of the shredder Allogamus sp. Biochemical responses were assessed by measuring oxidative stress and neuronal biomarkers in invertebrates exposed for 96h to post-fire runoff.
Post-fire runoff from plant forests and stream water affected leaf decomposition by microbes: effects of post-fire runoff from Pinus and Eucalyptus forests varied with concentration and time, while effects of post-fire stream water varied with concentration. The structure of bacterial and fungal communities, assessed by PCR-DGGE (Denaturing Gradient Gel Electrophoresis), was mainly affected by exposure time. Post-fire runoff decreased leaf-associated fungal biomass and effects varied with concentration and time. Leaf consumption by invertebrates was inhibited in all treatments, particularly after exposure to post-fire runoff from Eucalyptus forest.
After exposing invertebrate shredders to post-fire runoff from all origins, the activity of oxidative stress enzymes (catalase, glutathione reductase, glutathione S-transferase and glutathione peroxidase) increased in a concentration-dependent manner, while cholinesterase activity decreased. This suggests that post-fire runoff induce oxidative and neuronal stress in invertebrate shredders.
Overall, results suggested that post-fire runoff affects microbial decomposers and the behavior and physiology of invertebrate shredders with potential impacts to plant-litter decomposition in streams.