4.2.1. Water Temperature

During the investigation period, water temperature rose steadily from winter to summer (Figure 5a). The increase in water temperature corresponds to the temporal development of air temperature, which only collapsed shortly in May 2019 due to a thunderstorm (see Supplementary Materials) [72]. During winter (December 2018–March 2019), the maximum water temperatures indicated good ecological status. The threshold for poor ecological status (20 ◦C) was reached in July and (locally) during the following months.

In the Antrift reservoir, maximum water temperatures ranged between 24.4 ◦C (26 June 2019) and 21.6 ◦C (5 June 2019), whereas the temperatures in its tributaries were ca. 6.3 ◦C lower on these dates (Figure 6a). Since February 2019, the water temperature has been higher in the Antrift reservoir than in the headwaters and the main river. During this time, median water temperatures ranged below the threshold for good ecological quality. Single values, mainly recorded at station AB-2 and AB-3 inside the Antrift reservoir, exceeded the threshold during summer.

#### 4.2.2. pH

Our investigation period started with low surface water pH (6.8–6.9; 5 November 2018; Figures 5b and 6b). All our pH data of the first measurement date, as well as single outliers in the following periods, were within the range for good ecological status [46]. Since April 2019, the pH values inside the Antrift reservoir have been exceeding the threshold for good ecological quality until the end of the investigation period.

With incipient precipitation in late November 2018, pH increased all over the catchment and remained between pH 7.5–8.5 until March 2019. Since March 2019, pH was significantly higher (mean: + 1.5 pH) in the Antrift reservoir than in the main river and headwaters. The lowest pH was recorded in the headwaters, ranging by about 1.9 pH units under the reservoir pH.

**Figure 5.** Time series of water quality data (n = 8 per date). Dashed lines represent thresholds for good ecological status according to OGewV (2016): (**a**) water temperature, winter maximum (blue line), and summer maximum (red line); (**b**) pH, minimum (blue line) and maximum (red line); (**c**) dissolved oxygen (DO), minimum; (**d**) total phosphorus (TP), threshold between good (<0.03 mg TP/l) and moderate ecological status (>0.03 mg TP/l; blue line), threshold between moderate and poor ecological status (>0.1 mg TP/l; red line).

**Figure 6.** Temporal trends of water quality parameters during the investigation period; means according to types of water bodies (headwaters (n = 2): GOE, OCH; main river (n = 3): AT-1, AT-2, AT-3; reservoir (n = 3): AB-1, AB-2, AB-3). Dashed lines represent thresholds for good ecological status according to Bundesministerium der Justiz und für Verbraucherschutz (2016): (**a**) water temperature, summer maximum; (**b**) pH, maximum; (**c**) dissolved oxygen (DO), minimum; (**d**) total phosphorus (TP), threshold between moderate and poor ecological status (0.1 mg TP/l).

#### 4.2.3. Dissolved Oxygen

Our DO data show no clear trend (Figures 5c and 6c). Generally, DO concentrations increased after a minimum in November 2018 (mean: 4.8 ppm) to stay above the threshold for good ecological status (8 ppm DO). Between July and August 2019, DO concentrations clearly decreased in the headwaters and the main river (80.3% and by 61.0%, respectively), compared to DO concentrations in June. The minimum DO concentrations in August 2019 ranged between 2.57 ppm in the headwaters and 4.52 ppm inside the reservoir. No significant spatial variability of DO concentrations was found for the Antrift catchment (Figure 6c). However, headwaters had slightly higher DO concentrations at single measurement dates.

#### 4.2.4. Total Phosphorus

The TP concentrations were relatively high, varied strongly, and indicated bad ecological status in the investigated water courses (Figure 5d). During November and December 2018, TP levels remained high (>0.10 mg TP/l). From February 2019 to August 2019, lower TP concentrations were recorded, ranging between the lower and upper thresholds of eutrophic nutritional status. However, outliers occurred with >0.25 mg TP/l. The largest outliers were found at station AB-2 (reservoir), with 0.30 mg TP/l (18 February 2019) and 0.25 mg TP/l (3 March 2019). In May 2019, a single increase was detectable to TP concentrations of >0.20 mg TP/l. We found no systematic spatial trend of TP concentrations in the catchment despite slight differences between the hydrological water body types (Figures 6d and 7).

**Figure 7.** Total phosphorus (TP) concentration during the investigation period, according to sampling stations (n = 14 per station). Dashed lines represent thresholds of nutritional status according to OGewV (2016): orange line = threshold between good (<0.03 mg TP/l) and moderate ecological status (>0.03 mg TP/l); blue line = threshold between moderate and poor ecological status (>0.1 mg TP/l).

#### *4.3. Correlation of Total Phosphorus Concentrations and Precipitation Events*

The cross correlation between the time series of precipitation and discharge data shows a significant (*p* ≤ 0.05) mutual influence for a period from 0 to ± 2 days (see Supplementary Materials). Within this two-day period, the results of the auto correlation function (ACF) were above the 0.1 ACF threshold. At point zero (i.e., precipitation and discharge event on the same day), the highest ACF value (0.416) is reached, followed by a continuous decrease until point +2 (i.e., discharge event two days after precipitation event).

Based on this result, Spearman correlation analyses (Figure 8) show a strong and significant (*p* ≤ 0.05) positive correlation between the number of days since the last high, maximum, or heavy precipitation event (>10 mm/day) and the maximal drainage for each measurement period.


**Figure 8.** Spearman correlation coefficients of TP concentrations in the water and the number of days between each precipitation event and the next sampling date. (Significance levels: *p* ≤ 0.001 (\*\*\*); *p* ≤ 0.01 (\*\*); *p* ≤ 0.05 (\*)).

Comparing the average TP concentrations with the number of days since the last maximal drainage or maximal and heavy precipitation events, a strong significant (*p* ≤ 0.01) negative correlation was found. Moreover, a moderate positive correlation was observed between average TP and the number of days since the last higher precipitation event. Instead, no significant correlation was found for TP and the number of days since the last higher drainage event.

Beside these results of the correlation analyses, the absolute number of days since an event was mostly larger than two days for all the precipitation events during our investigation period (see Supplementary Materials). Only in August 2019 did a maximal precipitation event coincide with our sampling date. For higher precipitation, six of 14 measurement periods showed an event within two days. No heavy precipitation event occurred within the two-day interval.

#### *4.4. Sediment Analyses*
