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

Greenhouse gas emissions from headwater streams are linked to multiple sources influenced by terrestrial land use and hydrology, yet partitioning these sources at catchment scales remains highly unexplored. To address this gap, we sampled year-long stable water isotopes (δ$^{18}$O and δ$^2$H) from 17 headwater streams differing in catchment agricultural areas. We calculated mean residence times (MRT) and young water fractions (YWF) based on the seasonality of δ$1^{18}$O signals and linked these hydrological measures to catchment characteristics, mean annual water physico-chemical variables, and GHG % saturations. The MRT and the YWF ranged from 0.25 to 4.77 years and 3 to 53%, respectively. The MRT of stream water was significantly negatively correlated with stream slope (r$^2$ = 0.58) but showed no relationship with the catchment area. Streams in agriculture-dominated catchments were annual hotspots of GHG oversaturation, which we attributed to precipitation-driven terrestrial inputs of dissolved GHGs for streams with shorter MRTs and nutrients and GHG inflows from groundwater for streams with longer MRTs. Based on our findings, future research should also consider water mean residence time estimates as indicators of integrated hydrological processes linking discharge and land use effects on annual GHG dynamics in headwater streams

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

Anthropogenic activities increase the contributions of inland waters to global greenhouse gas (GHG; CO$_2$, CH$_4$, and N$_2$O) budgets, yet the mechanisms driving these increases are still not well constrained. In this study, we quantified year-long GHG concentrations, fluxes, and water physico-chemical variables from 28 sites contrasted by land use across five headwater catchments in Germany. Based on linear mixed-effects models, we showed that land use was more significant than seasonality in controlling the intra-annual variability of the GHGs. Streams in agriculture-dominated catchments or with wastewater inflows had up to 10 times higher daily CO$_2$, CH$_4$, and N$_2$O emissions and were also more temporally variable (CV > 55 %) than forested streams. Our findings also suggested that nutrient, labile carbon, and dissolved GHG inputs from the agricultural and settlement areas may have supported these hotspots and hot-moments of fluvial GHG emissions. Overall, the annual emission from anthropogenic-influenced streams in CO$_2$ equivalents was up to 20 times higher (∼ 71 kg CO$_2$ m$^{−2}$ yr$^{−1}$) than from natural streams (∼ 3 kg CO$_2$ m$^{−2}$ yr$^{−1}$), with CO$_2$ accounting for up to 81 % of these annual emissions, while N$_2$O and CH$_4$ accounted for up to 18 % and 7 %, respectively. The positive influence of anthropogenic activities on fluvial GHG emissions also resulted in a breakdown of the expected declining trends of fluvial GHG emissions with stream size. Therefore, future studies should focus on anthropogenically perturbed streams, as their GHG emissions are much more variable in space and time and can potentially introduce the largest uncertainties to fluvial GHG estimates

Patterns and drivers of nitrous oxide concentrations and fluxes in headwater streams of the Mara river basin, Kenya

Headwater streams are important inland aquatic ecosystems when it comes to their contributions to the global N2O (a potent greenhouse gas) fluxes. It is estimated that they contribute approximately 10% to the global anthropogenic N2O emission rate. Despite this, flux estimates from them are still poorly constrained and with a lot of uncertainties. Factors, such as stream geomorphology and land use/cover, influence water quality. Variation in nutrient and carbon concentrations have also been studied as controls of these fluxes from the streams. However, processes that regulate their production rates are still not well understood. In Africa, few studies have been done to quantify these fluxes from inland waters and there has been very little focus on headwater streams. This research sought to quantify N2O concentrations and fluxes from the headwater streams of the Mara River in Kenya, access their spatial and temporal variation and determine their relationship with both physical and chemical drivers. A total of 52 sites were sampled in the upper Mara of which 47 were streams of orders 1- 6 across 3 land use types (agricultural, forest and livestock). Water samples collected with the gas samples were analysed for DIN, TDN and DOC. N2O concentrations and fluxes found ranged from 0.33 - 2.6 µg L-1 N2O-N and -11 -323 N2O-N µg m-2 h-1 respectively. Agricultural stream sites had the highest mean fluxes of 26.38 ± 5.37 N2O-N µg m-2h-1 while forest sites exhibited a lower mean of 14.75 ± 42.40 N2ON µg m-2h-1. Analysis was done using linear mixed models to identify relationships between N2O fluxes and concentrations with their physical and chemical drivers. Stream slope and velocity did not significantly explain variation in N2O concentration from the studied sites. However, discharge had a positive relationship to N2O concentration. In stream NO3-N and CO2-C (aq) concentrations were strongly positively correlated with N2O explaining > 50% of their variation. However, DO and C:N ratio showed a negative correlation with N2O concentration. From the results, I hypothesised that nitrification may be the main biogenic process controlling N2O production via autotrophic consumption of dissolved CO2 as an energy source. However, more research is required in order to test this hypothesis with much more accurate measurements of denitrification and nitrification rates

Data_Sheet_1_Interactive effects of catchment mean water residence time and agricultural area on water physico-chemical variables and GHG saturations in headwater streams.pdf

Greenhouse gas emissions from headwater streams are linked to multiple sources influenced by terrestrial land use and hydrology, yet partitioning these sources at catchment scales remains highly unexplored. To address this gap, we sampled year-long stable water isotopes (δ18O and δ2H) from 17 headwater streams differing in catchment agricultural areas. We calculated mean residence times (MRT) and young water fractions (YWF) based on the seasonality of δ18O signals and linked these hydrological measures to catchment characteristics, mean annual water physico-chemical variables, and GHG % saturations. The MRT and the YWF ranged from 0.25 to 4.77 years and 3 to 53%, respectively. The MRT of stream water was significantly negatively correlated with stream slope (r2 = 0.58) but showed no relationship with the catchment area. Streams in agriculture-dominated catchments were annual hotspots of GHG oversaturation, which we attributed to precipitation-driven terrestrial inputs of dissolved GHGs for streams with shorter MRTs and nutrients and GHG inflows from groundwater for streams with longer MRTs. Based on our findings, future research should also consider water mean residence time estimates as indicators of integrated hydrological processes linking discharge and land use effects on annual GHG dynamics in headwater streams.</p