Anthropogenic activities significantly increase annual greenhouse gas (GHG) fluxes from temperate headwater streams in Germany

Abstract

Abstract. Anthropogenic activities increase the contributions of inland waters to global greenhouse gas (GHG; CO2, CH4, and N2O) budgets, yet the mechanisms driving these increases are still not well constrained. In this study, we quantified year-long GHG concentrations and fluxes, as well as water physico-chemical variables from 23 streams, 3 ditches, and 2 wastewater inflow sites across five headwater catchments in Germany contrasted by land use. Using mixed-effects models, we determined the overall impact of land use and seasonality on the intra-annual variabilities of these parameters. We found that land use was more significant than seasonality in controlling the intra-annual variability of GHG concentrations and fluxes. Agricultural land use and wastewater inflows in settlement areas resulted in up to 10 times higher daily riverine CO2, CH4, and N2O emissions than forested areas, as substrate inputs by these sources appeared to favor in situ GHG production processes. Dissolved GHG inputs directly from agricultural runoff and waste-water inputs also contributed substantially to the annual emissions from these sites. Drainage ditches were hotspots for CO2 and CH4 fluxes due to high dissolved organic matter concentrations, which appeared to favor in situ production via respiration and methanogensis. Overall, the annual emission from anthropogenic-influenced streams in CO2-equivalents was up to 20 times higher (~71 kg CO2 m-2 yr-1) than from natural streams (~3 kg CO2 m-2 yr-1). Future studies aiming to estimate the contribution of lotic ecosystems to GHG emissions should therefore focus on anthropogenically perturbed streams, as their GHG emission are much more variable in space and time.

Interactive effects of catchment mean water residence time and agricultural area on water physico-chemical variables and GHG saturations in headwater streams

“…These trends also reflect findings in terrestrial ecosystems, which have found agricultural soils to be hot spots of GHGs (e.g., Raich and Tufekciogul, 2000;Wangari et al, 2022). The above findings suggest that the annual oversaturation of GHGs within agricultural streams, as found in several other studies (e.g., Borges et al, 2018;Herreid et al, 2021;Mwanake et al, 2022Mwanake et al, , 2023, could be supported either by younger precipitation interflows through oversaturated soils or older groundwater nutrients.…”

“…While GHG dynamics of headwater catchments in this study were previously discussed by Mwanake et al (2023), the results of this study have contributed to a better conceptualization of catchment-scale land use effects linked to hydrological processes. We found notable differences in mean annual water physicochemical properties and GHG saturations among streams of catchments differing in agricultural areas and MRTs.…”

“…The Schwingbach and Loisach catchments were sampled from June 2020 to June 2021, while the Goldersbach, Ammer, and Steinlach catchments were sampled from April 2021 to April 2022. The sites were distributed across catchments with differing percentages of agricultural land area (See Mwanake et al, 2023 for details) (Table 1).…”

“…Based on 2-to 3-week measuring intervals, annual means of water physico-chemical variables and GHG saturations were calculated from previously published work, where stable water isotope sampling in this study was conducted in tandem (see detailed methods in Mwanake et al, 2023). In brief, water .…”

“…Within lotic ecosystems, agricultural land use has been shown to increase riverine GHG concentrations by either supporting internal production through the supply of nutrients and labile carbon or as a direct external source of dissolved GHGs (e.g., Drake et al, 2019;Mwanake et al, 2022Mwanake et al, , 2023. Apart from describing water storage and transit processes within a catchment, the MRTs and YWFs of stream water could also help to explain better how catchments respond to anthropogenic nutrient and dissolved GHG inputs from agricultural areas (e.g., Ensign and Doyle, 2006) and their subsequent cycling that results in the in situ formation of riverine GHGs.…”