Determinism of anatoxin-a production in Phormidium-dominated biofilms of a regulated river (Ain, France)

Abstract

Benthic cyanobacteria have been increasingly held responsible for bird and mammal deaths along riverbanks in the last decades. However, little is known about the environmental conditions that favor their development, spatiotemporal dynamics and associated toxins. In the summer of 2020, we monitored physical, chemical and biological variables, quantified benthic cyanobacteria and measured anatoxin-a concentration in the biofilms of four reaches of the dam-regulated Ain River, France. The main potential anatoxin producer was Phormidium, often associated with Oscillatoria mats in the same field campaigns. Anatoxin-a detectable concentrations ranged from 0.5 to 197µg/g of dry weight showing a high spatial heterogeneity. According to structural equation modelling, anatoxin-a concentrations increased with water depth in a range of 20 to 80 cm. In contrast, the chlorophyta biovolume and the flow velocity were the main factors negatively correlated with anatoxin-a concentrations. Although flow velocity did not limit the biovolume of potential anatoxin producers, it could induce stress to the cyanobacterial mats within the river, causing a limitation of anatoxin-a concentrations. Finally, the effects of these drivers on anatoxin-a production demonstrated a high variability over the summer. This high temporal variability of both benthic cyanobacteria biomass and anatoxin-a concentration makes risk assessment relatively challenging.