Gudenå part of Jutland dominated by sub-glacial loamy

Gudenå or Gudenåen is Denmark’s longest river and came into
existence 15,000 years ago towards the end of the last ice age. The 160 km
river was transformed by the melting ice and glacial streams, the river
harbours many species of animals and plants, parts of it are protected under
the regulations of Natura 2000. River Gudenå has a total catchment area of
2,650 km2 and is located in the eastern part of Jutland dominated by
sub-glacial loamy sand moraine till deposits and patches of low-gradient
landscape with sandy soils (Nørrevang and
Lundø, 1980; Pedersen et al., 2004). Severe anthropogenic disturbances
associated with channelizing, dredging and continuous weed cutting has
physically degraded and severely reduced the substrata for heterogeneous
habitats leading to loss of natural dynamic physical structure. The biological
communities (macro invertebrates, macro vertebrates and macrophytes) are highly
dependent on the physical structures and stability of the stream conditions.
Successful species of macrophytes in Danish streams share a range of traits
that provide them competitive advantages in the disturbed habitats (Cavalli et
al., 2014). Danish streams are invaluable in terms of biodiversity and
ecosystem services. River Gudenå collects and transports organic matter,
chemicals, nutrients and anthropogenic substances through extensive network
systems which are in turn influenced by the physical processes like substrate
type, flow conditions and turbidity.  

matter in the form of free DNA produced from organisms via cells, tissue,
gametes, organelles and faecal matter is also transported through the rivers,
they are termed as environmental DNA (eDNA) (Deiner and Altermatt, 2014; Turner
et al., 2014; Taberlet et al., 2012). In recent years, the eDNA detection techniques
(Shogren et al., 2017) have become a powerful tool for discovery, surveillance
and monitoring of endangered (Goldberg et al., 2011), rare (Machler et al.,
2014; Jerde et al., 2011), and invasive species(Simmons et al., 2016). eDNA
studies have been used to estimate the population abundance of certain target
species (Takahara et al., 2012; 2013) in lentic systems  like ponds and lakes.  But in lotic system (eg. Rivers) eDNA studies
are limited due to linking of various environmental processes.  The transport of eDNA studies in rivers and
streams is challenging. The challenging issues with regard to transportation and
concentration of eDNA in streams are dependent on processes like retention,
resuspension, residence time of water, substrate type and turbidity (Shogren et
al., 2017). The important aspect of eDNA studies in rivers involves
understanding the physical and biological variables that influence the
retention, resuspension and transportation of substances. Another important
aspect is the physio-biogeochemical properties of the eDNA being transported,
like persistence, degradation and detection (Barnes and Turner, 2016). Since,
eDNA is known to be polydisperse (Turner et al., 2014; Wilcox et al., 2015) due
to variably sized particles from various sources increases the complexity of
eDNA studies in rivers. In previous studies eDNA has been used as qualitative
detection tool only for confirming the presence or absence of a species.  Recently, the mechanistic of eDNA
transportation has emerged by eDNA releases into experimental streams (Shogren
et al., 2016, 2017) to note the effect of hyporheic exchange and flow
velocities on eDNA.   

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