Field sampling
All sampling was conducted in summer, between August 16th and 27th 2021. Sampling sites were located along a stretch of the Spree River of about 20 km, close to the town of Spremberg, Brandenburg, Germany, where the dam is located (Figure 1). We sampled at six locations in total, three located upstream of the dam (i.e., at total iron (TFe) >3.8 mg L-1; Zerre, Trattendorf, Wilhelmstal), and three downstream of the dam (i.e., at TFe < 0.3 mg L-1; Bräsinchen, Frauendorf, Madlow). Besides the water quality, sites were comparable with regards to habitat characteristics (Table 1). For three sites (Zerre, Wilhelmstal and Bräsinchen) data regarding turbidity, TFe, total phosphorus, and total nitrogen (TN) were obtained from the local authority (Landesamt für Umwelt Brandenburg) who collects this data monthly within the context of the European water framework directive. We collected benthic macroinvertebrates for abundance estimates and stable isotope analysis from river sediments using an Ekman grab. Samples were sieved through 500µm and stored in ethanol. In the lab at the Institute of Inland Fisheries Potsdam-Sacrow, they were sorted into broad taxon groups and individuals were counted. Abundances were expressed as individuals per m-2 and for every location an average of three samples was calculated for each taxon group. Seston samples for stable isotopes were collected using a 100µm plankton net that was lowered into the water. Samples included few and small zooplankton. We used Malaise traps without any collecting agent to sample flying terrestrial insects for the stable isotope analysis (i.e. Tineidea, Diptera, and Tipulidae), which we employed for 24 hours. Permit for sampling was received from the respective authority (Landesamt für Umwelt Brandenburg). In addition, single groups (i.e., Formicidae, and Orthoptera) were hand-collected from the meadows. Terrestrial insects were collected at two locations only: at Trattendorf representing insects upstream of the dam, and at Bräsinchen representing insects downstream of the dam (Figure 1). Insects were killed by freezing, sorted into groups and stored at -20°C prior to isotope analyses.
Electrofishing to sample the fish communities was conducted under the permit number 2-2021 approved by the local environmental agency (hunting and fisheries authority of the administrative district Spree-Neiße). Sampling was performed at both river banks during day-time using a generator-powered DC electrofishing unit (Type FEG 8000, 8 kW, EFKO Elektrofischfanggeräte GmbH, Leutkirch im Allgäu, Germany) equipped with one anodic handnet of 45 cm ring diameter and a 4 m long cupper litz wire as cathode. Length of electro fished river sections ranged between 300-640 m at the six locations. Each section was fished in both flow directions (i.e., upstream and downstream) in a standardized way according to the European standard protocol EN 14011:2000 and this procedure was repeated 3 times. Natural boundaries of each section (i.e. weirs or riffles) prevented fish to escape from electro fishing. At each section, captured fishes were identified to species level, counted, measured (up to 1 mm), weighted (up to 0.1 g), and placed in a holding net. Once completed, all captured fishes were released, unless they were taken for further analyses. We summed all fishes caught at one particular section and calculated catch per unit effort (CPUE, expressed as individuals 1000 m-1) and biomass per unit effort (BPUE, g 1000 m-1). Individuals of the most abundant fish species caught at Trattendorf (upstream of the dam) and Bräsinchen (downstream of the dam) were stored on ice and brought to the lab. Here, gut content was collected for metabarcoding and a small piece of dorsal muscle tissue was sampled for stable isotope analyses of hydrogen. Scales of pike were collected to allow age estimates via counting of annuli in this top predator. To compare the condition of the top predator, and thus, allow a comparison for the amount of energy that reaches the highest trophic levels, we calculated Fultons condition factor (K) according to the Htun-Han (1978) equation for pike of the different age cohorts:
\(K=\frac{W\times 100\ \ }{L^{3}}\),
where W =weight of fish (g) and L= total length of fish (g).