Spatial and seasonal changes in a naturally acidified river (Río Agrio, Argentina) and their effect on epilithic algae

Abstract

This study investigated seasonal changes in the longitudinal pH gradient of a naturally acidified river and how the attributes of epilithic algae (biomass, abundance, diversity) react to these variations. Five sites were selected on the Río Agrio river for in situ measurements, water sampling, and epilithic algae collection in different seasons from 2007 to 2018. Water pH increased along the length of the river, the average increase being highest in summer (from 2.5 to 7.0, acidity to neutrality) and lowest in spring (from 2.8 to 4.0, acidity to acidity). Conductivity and nutrient and ion concentrations decreased along the length of the river, showing a seasonal pattern of variation mainly influenced by the hydrological cycle. Epilithic algal biomass varied seasonally along the river, generally increasing in summer and decreasing in spring, with seasonal maximum values at different locations mainly associated with the dynamics of iron precipitation and nutrient availability. Species diversity and numbers were low at the most acidic site (regardless of season), but in the remaining sites these attributes tended to increase during summer–autumn and decrease during winter–spring. Statistical analysis showed that differences between sites responded mainly to the chemical gradient of the river and seasonality. Therefore, seasonal changes in the longitudinal chemical gradient of the river imposed differential, changing conditions on the epilithic algae present in each site, allowing identification of characteristic zones and their respective epilithic algal attributes. In the context of proposed models of ecosystem response to stress, similarities with systems acidified by anthropic activities are discussed.