Riparian Land Use and Hydrological Connectivity Influence Nutrient Retention in Tropical Rivers Receiving Wastewater Treatment Plant Discharge

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.

Limnology and Oceanography Bulletin Volume 31 Number 1 November 2021 1–29

“…Another study led by some of the authors of the present paper revealed the influences of riparian vegetation on nutrient attenuation in sub/tropical streams receiving inputs from wastewater treatment plants (WWTPs; for more details, see Finkler et al 2021). Although riparian areas are well recognized for their buffering capacity regarding pollutants from agricultural and urban runoff (e.g., Daniels and Gilliam 1996), their role in attenuating nutrient loads in streams and rivers receiving point source nutrient inputs (e.g., from WWTPs) remains poorly understood.…”

“…Nutrient retention during the dry (low hydrological connectivity) and wet (high hydrological connectivity) seasons was assessed. Finkler et al (2021) hypothesized that the presence of riparian vegetation in a 100-m buffer would induce mechanisms responsible for either attenuation or mineralization of nutrients due to lateral hydrological connectivity (via overbank flooding). The results indicated that in periods with expected higher connectivity, nitrate and SRP concentrations tended to be lower downstream of the WWTP than in the dry period.…”

Assessing Restoration of Ecosystem Functioning in Brazilian Subtropical and Tropical Streams

“…Nutrient retention during the dry (low hydrological connectivity) and wet (high hydrological connectivity) seasons was assessed. Finkler et al (2021) hypothesized that the presence of riparian vegetation in a 100‐m buffer would induce mechanisms responsible for either attenuation or mineralization of nutrients due to lateral hydrological connectivity (via overbank flooding). The results indicated that in periods with expected higher connectivity, nitrate and SRP concentrations tended to be lower downstream of the WWTP than in the dry period.…”

Effects of deforestation on dissolved organic carbon and nitrate in catchment stream water revealed by wavelet analysis

“…This difference may be attributed to the small riparian zone (i.e., Gleysols, Histosols and Planosols region; Figure 1) in the reference catchment, which results in lower in-stream DOC and higher nitrate concentrations (Stockinger et al, 2014). Riparian zones play a role in nutrient retention as the physical and biogeochemical properties that occur in these ecotones control the fluctuation of water, nutrients and other substances between the hillslopes and the stream (Weigelhofer et al, 2012;Finkler et al, 2021). Thus, riparian zones play an important role in removing nitrate from subsurface flows due to vegetation uptake and increased denitrification (Hill et al, 2014;Hill, 2019) and in C production and storage (Rheinhardt et al, 2012).…”