Twenty years of daily metabolism show riverine recovery following sewage abatement

Human activities have altered the environmental controls on river metabolism, i.e., gross primary production (GPP) and ecosystem respiration (ER). These coupled processes affect water quality, CO2 emissions, and biodiversity. Efforts to mitigate these impacts often lack long-term, high-resolution data needed to assess their success. Here, we estimated a 20-yr time series of daily metabolism from dissolved oxygen (DO) data following pollution abatement (sewage treatment) in the Oria River, Spain. Ammonium concentration declined immediately after sewage abatement, whereas dissolved organic matter absorbance (proxy for concentration) declined slowly. Summer GPP declined with ammonium concentration. ER declined more strongly but more gradually than GPP and was driven by lower rates of autotrophic respiration (AR) and heterotrophic respiration following sewage treatment. Reduced AR reflected a decrease in the fraction of daily GPP being respired by algae themselves. Decline in ER caused net ecosystem production to increase from highly negative to approximately zero. Diel DO swings also declined and episodes of hypoxia nearly disappeared following reduced metabolic rates, and were paralleled by lower benthic chlorophyll a, increased macroinvertebrate indices, and a recovery of fish assemblages. Overall, wastewater treatment plants installation constrained river metabolism, greatly improved water quality, and enabled invertebrates and fish to recolonize the river. However, most variables responded gradually and these changes would not have been evident with short-term DO data. These gradual responses revealed the need of long-term, high-resolution data to gain insight into long-temporal scale responses superimposed on short-temporal scale metabolic variation, especially in the current context of climate and land-use change.