Abstract
Riparian areas are recognized for their buffering capacity regarding phosphorus and nitrogen from agricultural and urban runoff. However, their role in attenuating nutrient loads of rivers receiving point source nutrient inputs (e.g., from wastewater treatment plants, WWTPs) is still little understood. Here, we investigated whether ammonium (NH4-N), nitrate (NO3-N), and soluble reactive phosphorus (SRP) retention were influenced by the riparian land use in three Brazilian rivers receiving WWTP effluents. We hypothesized that nutrient attenuation would be potentially influenced by the hydrological connectivity between the main channel and riparian areas with native vegetation. We estimated retention from longitudinal patterns of dilution-corrected nutrient concentrations below the WWTPs. We assessed nutrient retention during periods with high (i.e., the wet) and low connectivity (i.e., the dry season). Relationships between non-conservative (nutrients) and conservative (chloride) solutes in both seasons were used to identify potential changes in the river chemistry due to the hydrological connectivity with the riparian areas. We also evaluated the relationship between net uptake velocities (Vf-net) and the accumulated percent native vegetation cover in the 100-m buffer using linear regressions, comparing the response for each nutrient between seasons with Analysis of Covariance. Slopes of regressions between nutrients and chloride significantly differed between seasons for NO3-N and SRP but not for NH4-N. The relationships between Vf-net and accumulated native vegetation in the riparian buffer presented steeper slopes for SRP in the wet than in the dry season. No significant relationships between NO3-N Vf-net and native vegetation cover were observed in either season. In contrast, increases in Vf-net with increasing vegetation cover were observed for NH4-N in the dry season. In periods with expected higher connectivity, NO3-N and SRP concentrations tended to be lower relative to chloride concentrations, with a potential effect of native vegetation in the riparian area on SRP retention. Our results suggest that seasonal connectivity between nutrient-rich river water and riparian areas is likely to induce changes in the predominant nutrient transformation processes, thereby favoring either nutrient retention or export in such rivers.
Highlights
Anthropogenic impacts, such as land-use change, water pollution, hydraulic alteration, and geomorphologic simplification, degrade freshwaters and adversely affect ecosystem services provided by natural aquatic systems, such as water supply and nutrient abatement (Dodds et al, 2013)
This study aimed to analyze whether NH4-N, NO3-N, and soluble reactive phosphorus (SRP) declines were influenced by hydrological connectivity and riparian land use in three Brazilian 4th-order rivers receiving Wastewater treatment plants (WWTPs) inputs
We addressed the following research questions: 1) does the riparian land use along the studied reaches act as a potential control of nutrient abatement in tropical rivers receiving WWTP effluents? and 2) which nutrient abatement mechanisms are potentially related to the presence of native vegetation in riparian zones? Our study was based on the following hypotheses: 1) During periods of high connectivity between rivers and riparian zones, conservative and non-conservative solutes should show divergent development of concentrations due to biogeochemical nutrient retention occurring in the riparian areas
Summary
Anthropogenic impacts, such as land-use change, water pollution, hydraulic alteration, and geomorphologic simplification, degrade freshwaters and adversely affect ecosystem services provided by natural aquatic systems, such as water supply and nutrient abatement (Dodds et al, 2013). Depending on treatment technology and efficiency, treated effluents can still be a major source of several contaminants to the receiving aquatic systems (Meng et al, 2013; Mußmann et al, 2013; Carr et al, 2016; Aubertheau et al, 2017) Such inputs, for example, can induce a suite of changes in the water chemistry and ecosystem functioning in different ways, from shifts in nutrient availability shaping the biological community’s structure to alterations in biogeochemical cycles and nutrient export to downstream waters (Gücker et al, 2006, Gücker et al, 2011; Atashgahi et al, 2015; Rodriguez-Castillo et al, 2017; Bernal et al, 2020). Improving the understanding of the impacts of potential pollution point sources (e.g., WWTPs) on riverine functioning is crucial for better water resource management of urban and periurban areas
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