Environmental correlates of fish distribution
Interpretation of eDNA-based results is not straightforward as eDNA
dynamics, including deposition, accumulation, transport, and fate, and
fish distribution are influenced by biological and environmental
variables (Barnes et al., 2014; de Souza et al., 2016; Mehner et al.,
2005). In heterogeneous environments, hydrodynamics affecting water
movements should be taken into account. Flow in lotic environments, such
as rivers or brooks, combines eDNA of local residents with eDNA being
transported from upstream (Jane et al., 2015; Pont et al., 2018). Flow
is also present in reservoirs and most likely explains the combined
detection of taxa associated with running water and upstream standing
waters as well as the high diversity found in the main inflows of the
study reservoirs. For example, detection of common carp and topmouth
gudgeon in running water may have been due to eDNA transport from
commercial ponds in the catchment, although escaped fish from these
locations could also explain the eDNA signal in rivers. Downstream eDNA
transport of eDNA was further supported by the negative correlation with
normalised water age for some taxa. In contrast, a positive correlation
with normalised water age reflects species’ preference for a particular
habitat and lentic conditions, e.g. prussian carp and European
bitterling (Kottelat & Freyhof, 2007).
Wind and wave action can also create currents in lentic environments
that transport eDNA (Moyer et al., 2014). This effect is more pronounced
in summer, with strong thermal stratification separating layers of
different temperatures and densities. In this study, species associated
with reservoir bottoms, such as European eel, stone loach and barbel
(Barbus barbus ), were detected only in littoral habitats. One
exception is ruffe, the most common species associated with the benthos
in the studied reservoirs that has a pelagic juvenile phase before
settlement on the bottom (Jůza et al., 2015). Ruffe detection in both
habitats can be explained by potential horizontal currents as well as
eDNA shed by juveniles. Nonetheless, horizontal eDNA transport could
prove useful to detect different ontogenetic phases of a species with
eDNA metabarcoding if they utilise distinct habitats (Zhang et al.,
2019).
Despite eDNA transport facilitated by flow, wind, and wave action in
reservoirs, eDNA signals for different taxa were spatially structured
and reflected species’ ecological niches. Spatial distribution of fishes
in heterogeneous freshwater bodies can result from various environmental
factors individually or combined, but the most important factor is
temperature. Higher temperature stimulates metabolic and behavioural
activity of most poikilotherm fish (Stoner, 2004), which can also
increase eDNA release and deposition by fish into environment as they
exhibit greater rates of movement, feeding, digestion, and excretion (Jo
et al., 2019; Robson et al., 2016). More active fish may have higher
eDNA detection probabilities; however, warmer water temperature is also
associated with greater UV irradiation, greater microbial activity
(including feeding on eDNA as a dissolved organic phosphorus substrate),
and reduced oxygen solubility (Barnes et al., 2014; Collins et al.,
2018; Scharfenberger et al., 2019). All of these factors contribute to
eDNA degradation and subsequently lower detection probabilities (Jo et
al., 2019; Tsuji et al., 2017).
Species classed as cold water were detected in late autumn, mostly in
tributary parts of the reservoirs. Specifically, coldwater salmonids
were detected in running waters as expected due to higher water
temperature at the surface and low DO concentrations in the colder,
lower layers of standing water. eDNA accumulation in deep hypolimnetic
water was not supported by our data. Species typically associated with
deep open water, such as Coregonus spp., are naturally missing in
artificial water bodies and where stocked, their presence is often
restricted by lack of DO during summer stratification (Kottelat &
Freyhof, 2007). The effects of DO and locality are further masked by
tight correlations with temperature, trophic status and water age. For
robust statistical analyses, more seasons and an equal number of
habitats should be sampled. We selected two contrasting seasons and the
number of habitats was derived by each reservoir´s morphology to achieve
a reasonable compromise between investigator effort and information
gained.
In our study, fish communities in canyon-shaped reservoirs were heavily
driven by trophic status along the longitudinal gradient. The proxy of
total ChlA concentration was positively correlated with temperature, and
species associated with warmer water were detected in samples with
higher trophic status. European eel was the only species strongly
associated with trophic status independent of temperature. This species
is an ambush predator, often with nocturnal activity (Trancart et al.,
2019), and so may benefit from the dense water blooms and artificial
darkness created by high trophic status. Community composition and
distribution of particular species were found to change in response to
trophic status in freshwater (Lawson Handley et al., 2018) and marine
(Djurhuus et al., 2020) systems using eDNA metabarcoding. These findings
and our own provide evidence that eDNA metabarcoding can be used to
understand shifts in communities in response to environmental gradients.