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).