Effects of mining activities on fish communities and food web dynamics in a lowland river

Abstract

Fish communities of streams and rivers might be substantially subsidized by terrestrial insects that fall into the water. Although such animal-mediated fluxes are increasingly recognized, little is known about how anthropogenic perturbations may influence the strength of such exchanges. Intense land use, such as lignite mining, may impact a river ecosystem due to the flocculation of iron (III) oxides, thus altering food web dynamics. We compared sections of the Spree River in North-East Germany that were greatly influenced by iron oxides with sections located downstream of a dam where passive remediation technologies are applied. Compared to locations downstream of the dam, the abundance of benthic macroinvertebrates at locations of high iron concentrations upstream of the dam was significantly reduced. Similarly, catch per unit effort of all fish was significantly higher in locations downstream of the dam compared to locations upstream of the dam, and the condition of juvenile and adult piscivorous pike Esox lucius was significantly lower in sections of high iron concentrations. Using an estimate of short-term (i.e., metabarcoding of the gut content) as well as longer-term (i.e., hydrogen stable isotopes) resource use, we could demonstrate that the three most abundant fish species, perch Perca fluviatilis, roach Rutilus rutilus, and bleak Alburnus alburnus, received higher contributions of terrestrial insects to their diet at locations of high iron concentration. In summary, lotic food webs upstream and downstream of the dam greatly differed in the overall structure with respect to the energy available for the highest tropic levels and the contribution of terrestrial insects to the diet of omnivorous fish. Therefore, human-induced environmental perturbations, such as river damming and mining activities, represent strong pressures that can alter the flow of energy between aquatic and terrestrial systems, indicating a broad impact on the landscape level.

Keywords: acid mining drainage; aquatic‐terrestrial coupling; benthic invertebrates; browning; metabarcoding; δ2H.

FIGURE 1

Map of study area, including the location of sampling sites. Photos showing the Spremberg Reservoir with inflow of brown, iron‐rich water (upper photo), and the dam with outflow of clear water (lower photo). Red arrow depicts flow direction. Photos were taken in 2020 and 2021 and kindly supplied by the Lausitzer und Mitteldeutsche Bergbauverwaltungsgesellschaft mbH, Senftenberg/Germany.

FIGURE 2

Abundance of benthic macroinvertebrates (individuals m⁻²) collected at the three locations upstream (i.e., high iron concentrations) and the three locations downstream (i.e., low iron concentrations) of the dam. See Appendix S1 for a list of invertebrate taxa found at each site.

FIGURE 3

(a) Abundance (represented as catch per unit effort, CPUE, individuals 1000 m⁻¹) and (b) biomass of fish species (represented as biomass per unit effort, BPUE, g 1000 m⁻¹) collected at the three locations upstream (i.e., high iron concentrations) and the three locations downstream (i.e., low iron concentrations) of the dam. See Appendix S2 for a list of species abundances at each site.

FIGURE 4

Contribution of aquatic and terrestrial resources to the diet of the three dominant fish species (roach, perch, and bleak), as depicted by the terrestrial index of fishes. A higher index indicates higher contribution of terrestrial resources to the diet. Boxplots indicate medians, with whiskers expanding to the 25th and 75th percentile. Dots denote outliers and asterisks level of significance (***p ≤ .001; **p ≤ .01).