*3.1. Water Sampling and Water Quality Analyses in the Field*

To depict the temporal development of water quality in the Antrift catchment, we determined water quality parameters via field measurements and laboratory analyses of water samples. Our evaluation is based on threshold values and quantitative recommendations for a good ecological status of water bodies given in the "Oberflächengewässerverordnung", a national regulation implementing the guidelines of the EU Water Framework Directive (WFD) [46]. Water sampling and field measurements were performed in a three-week interval from 5 November 2018 until 6 August 2019 (i.e., 14 sampling/ measurement dates). Samples were taken from eight sampling stations in the Antrift catchment (Figure 1, Table 1). GOE and OCH represent the two main headwaters of the Antrift river and pass through forest areas. AT-1 represents the upper reaches of the Antrift river, and AT-2 was chosen because of its location downstream from the major settlement. AT-3 is situated downstream of the tributary Göringer Bach (GOE) and represents the central agricultural area of the catchment. Finally, we chose three sampling stations inside the Antrift reservoir, located at the inlet (AB-1), in the shallow water near the bank of the main reservoir (AB-2), and in the deepest section of the reservoir at the overflow construction of the dam (AB-3).

On each sampling date, three water samples were taken per sampling station from 10–30 cm below the water surface. For GOE, OCH, AT-1, AT2, and AT-3, samples were taken along the channel line. The stations within the Antrift reservoir (AB-1, AB-2, AB-3) were sampled at 3 m distance from the shoreline. Water samples were filled into PE-flasks and kept frozen until further analyses were performed [47].

The field measurements of water quality parameters were performed with an automatic water quality logging system during water sampling (portable logging multiparameter system, HI98290, Hanna Instruments Inc., Woonsocket, RI, USA). The system was operated with the following sensors and accuracies: temperature sensor (◦C, ± 0.15), HI7609829-1 sensor for pH (pH, ± 0.02), and HI7609829-2 sensor for DO (%, saturation and concentration, ± 1.5% and ± 0.10 ppm, respectively). The sensor setup was calibrated through autocalibration with standard solutions (Hanna Instruments Inc.) before each measurement date [48]. Temperature, pH, and DO were measured during the whole investigation period.
