Introduction
In the current era of the Anthropocene, river ecosystems worldwide are
extensively modified by human activities (Cid et al. , Dudgeon et al.
2006). Land-use practices are one of the main threads that leads to the
loss of freshwater biodiversity (Dudgeon 2019). Besides the direct
negative effect on overall abundance and richness of organisms, indirect
effect may co-occur by altering the linkages among habitats and
communities (Foley et al. 2005). Rivers and their adjacent terrestrial
zones are tightly linked via multiple fluxes and pathways (Polis and
Hurd 1996, Baxter et al. 2005, Bartels et al. 2012). In this study, we
focus on the animal-mediated flux from land-to-water, which is the one
of terrestrial invertebrates falling into water bodies and being eaten
by fish. This flux can be pronounced and may provide up to 50% of the
annual energy budget of fish individuals (Nakano and Murakami 2001,
Baxter et al. 2005). While it has been shown that even the nutrient pool
of a lake can substantially be subsidized by terrestrial phosphorus
excreted by fish feeding on surface insects (Mehner et al. 2005), these
fluxes are generally assumed to be strongest in aquatic systems with
extended riparian zones, especially in streams with a high canopy cover
(Edwards and Huryn 1996, Kawaguchi and Nakano 2001). Although this
animal-mediated flux between adjacent ecosystems is increasingly
recognized, little is known on how anthropogenic perturbations may
influence the strength of such exchanges (but see Larsen et al. 2016).
For example, the degradation of riparian zones, such as from livestock
grazing (Saunders and Fausch 2012) or deforestation (Kawaguchi and
Nakano 2001), significantly decreased the reliance of fish on
terrestrial insects. However, another aspect of land use with a strong
impact on our river ecosystems and the potential to alter food web
dynamics is the one from mining activities.
Lignite (brown coal) mining activities on aquatic systems is influencing
the chemical, physical, biological, and ecological properties of the
system (Gray 1997, Byrne et al. 2012), and iron is one of the key
contaminants involved. Due to the oxidation of ferrous sulphide metals
(e.g. pyrite and marcasite) during mining or related to the progressive
groundwater resurgence in mine reclamation areas, large amounts of iron
contaminate groundwater and adjacent aquatic systems. The
well-oxygenated and neutral pH conditions in lotic systems will evoke a
flocculation of iron (III) oxides, leading to browning and a turbid
environment (Gray 1997, Byrne et al. 2012; Figure 1). Besides potential
toxic effects via direct metal uptake (Vuori 1995), river organisms
suffer from the formation of precipitates covering gills and eggs
(Andersson and Nyberg 1984, Gerhardt 1992). However, most dominantly,
iron precipitates alter the habitat availability for stream organisms as
the fine iron sediments bury hard substrates, periphyton, organisms, and
clog interstitial spaces of the benthic habitat (Letterman and Mitsch
1978, McKnight and Feder 1984). The communities of benthic invertebrates
in river sections characterized by high iron oxide concentrations are
therefore typically found to be of lower abundance, species biomass, and
diversity (Rasmussen and Lindegaard 1988, Vuori 1995, Maret et al. 2003,
Cadmus et al. 2016). In turn, this could have consequences for the food
availability of fish inhabiting the river sections of high iron
concentrations, potentially limiting the trophic transfer and the energy
available for fish production. Furthermore, we suggest that iron has the
potential to restructure the lotic food webs and rearrange the
connections to the terrestrial systems. It can be expected that fish
inhabiting river sections where benthic invertebrates as food is scarce
rely to a higher degree on terrestrial invertebrate food resources.
A straight-forward approach to estimate the contribution of terrestrial
insects to the diet of fish is to simply look at the gut content (Manko
2016). However, the identification of different prey items can be
problematic in cyprinid fishes that have pharyngeal teeth that crush the
food. Here, dietary studies using DNA metabarcoding can provide a high
resolution of resource use (Amundsen and Sánchez-Hernández 2019).
Nonetheless, DNA metabarcoding of gut content analyses only allows a
snap-shot of the recently ingested prey. In addition, stable isotopes
can provide an integrated estimate of resource use over longer time
periods (Grey 2006, Boecklen et al. 2011). In the context of identifying
the contribution of terrestrial insects to the diet of fish, stable
isotopes of hydrogen (δ2H) are particularly useful due
to the strong divergence between aquatic and terrestrial endmembers
(Doucett et al. 2007, Vander Zanden et al. 2016).
Here, we studied the cumulative effects of iron (III) oxides on a lotic
food web and the linkages to the terrestrial surroundings. Specifically,
we predict that in river sections of high iron concentrations (i.e.
upstream of the dam), less benthic invertebrates are present and fishes
feed on terrestrial insects to a higher degree compared to fish
individuals found in river sections of lower iron concentrations (i.e.
downstream of the dam, following water remediation). This will be
indicated by a higher number of terrestrial species found in the gut
content (identified by their DNA-sequences using metabarcoding
techniques), and further, by a higher terrestrial index calculated from
hydrogen stable isotope values demonstrating the longer-term resource
use. In addition, we predict that low abundance of benthic invertebrates
upstream of the dam will decrease the general fish abundance, and hence
the condition factor of the top predator (piscivorous pike Esox
lucius ).