The study area is located at the Cuenca River basin situated in the southern province of Azuay in the Andes of Ecuador. The basin is composed of five main tributaries, i.e. Cuenca, Tarqui, Yanuncay, Tomebamba, and Machangara Rivers. The study area is 223 km2, representing 13% of the Cuenca River basin. The city of Cuenca has a population of approx. 401.000 inhabitants in 2019. Two natural reserves are also located upstream from the Cuenca River basin: Cajas National Park and the Machangara-Tomebamba protected forest. Both are water sources for the Tomebamba, Yanuncay and Machangara Rivers (Jerves-Cobo et al., 2018b). The mean altitude of the study area is 2655 m a.s.l. The annual average air temperature is 16.3 °C and the average rainfall is about 879 mm per year (Jerves-Cobo et al., 2018a). The rainy season starts from the middle of February until the beginning of July and from the second half of September until the first two weeks of November, while the rest of the year constitutes the dry season (Jerves-Cobo et al., 2020b). The area of Cuenca, Machangara, Tarqui, Tomebamba, and Yanuncay is 95.92, 111.19, 138.98, 113.03, 113.81 km2, respectively.
Besides assessing the dissolved concentrations of CO2, CH4, and N2O, we also gathered physiochemical, hydro-morphological, and meteorological data. Specifically, water temperature, pH, dissolved oxygen (DO), turbidity, total dissolved solid (TDS), and chlorophyll a were determined by a handheld multiprobe (Aquaread-AP5000 version 4.07). Calibration was performed prior to sampling and supplemented with a regular check after sampling.
Water samples from all sampling sites were collected and stored in cool and dark containers and then preserved in a refrigerator before being analyzed for other variables in the Water and Soil Quality Analysis Laboratory at Cuenca University. Particularly, ammonium (NH4+), nitrite (NO2-), nitrate (NO3-) and orthophosphate (PO43-) were determined spectrophotometrically (low-range Hach test kits with Hach DR3900). Moreover, water samples were kept frozen until shipment to Belgium for further analyses, i.e. biochemical oxygen demand (BOD5), chemical oxygen demand (COD), total nitrogen (TN), and total phosphorus (TP). Hydro-morphological information of the sites and their surroundings were collected, including land use, macrophytes, riparian vegetation, channel types, and sediment accumulation, via a modified field protocol. Note that land-use types surrounding the sampling sites were assessed using the modified field protocol based on the Australian River Assessment System physical assessment protocol (Parsons et al., 2002) and the United Kingdom and the Isle of Man River Habitat Survey (Raven et al., 1997). In total, 17 variables were measured following different categories (SI S2). River depth and velocity were measured at three points at each sampling site, two close to the riverbanks and one in the middle of the river. Meteorological data, including air temperature, solar radiation, rainfall, and wind speed, were obtained from the meteorological station of the University of Cuenca (-2.9050372°, -79.0124267°), located 7.8 km away from the Ucubamba WWTP and 0.7 km away from the city center.
Dissolved GHG concentrations (C_aq) were measured using the headspace equilibration technique. Before the field campaign, 12 mL vials with airtight septa (Exetainer®, Labco Ltd, High Wycombe, UK) were pre-conditioned with 50L of 50% ZnCl before capping and flushing with high purity N2 (Alphagaz 2, Carbagas, Gümlingen, Switzerland). At each sampling, 6 mL of water was pushed into the vials using a syringe after carefully removing air bubbles from the sample creating a headspace pressure inside the vial of ca. 2 atm. The headspace was analyzed for concentrations of CO2, CH4, and N2O using gas chromatography (Bruker, GC-456, Scion Instruments, Livingston, UK) equipped with a thermal conductivity detector, flame ionization detector, and electron capture detector. The instrument was calibrated for each gas using several sets of standards within each measurement run. Dissolved gas concentrations (µmol L-1) were calculated by applying Henry's law, taking into account the vial volume and headspace.
C_aq= p_a×k_h
where k_h is Henry’s constant adjusted for lab temperature (mol m-3 Pa-1) and p_a is the partial pressure of the gas in the headspace (Pa-1).
