Gudenå or Gudenåen is Denmark’s longest river and came intoexistence 15,000 years ago towards the end of the last ice age. The 160 kmriver was transformed by the melting ice and glacial streams, the riverharbours many species of animals and plants, parts of it are protected underthe regulations of Natura 2000. River Gudenå has a total catchment area of2,650 km2 and is located in the eastern part of Jutland dominated bysub-glacial loamy sand moraine till deposits and patches of low-gradientlandscape with sandy soils (Nørrevang andLundø, 1980; Pedersen et al.
, 2004). Severe anthropogenic disturbancesassociated with channelizing, dredging and continuous weed cutting hasphysically degraded and severely reduced the substrata for heterogeneoushabitats leading to loss of natural dynamic physical structure. The biologicalcommunities (macro invertebrates, macro vertebrates and macrophytes) are highlydependent on the physical structures and stability of the stream conditions.Successful species of macrophytes in Danish streams share a range of traitsthat provide them competitive advantages in the disturbed habitats (Cavalli etal., 2014).
Danish streams are invaluable in terms of biodiversity andecosystem services. River Gudenå collects and transports organic matter,chemicals, nutrients and anthropogenic substances through extensive networksystems which are in turn influenced by the physical processes like substratetype, flow conditions and turbidity. Organicmatter 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; Turneret al., 2014; Taberlet et al.
, 2012). In recent years, the eDNA detection techniques(Shogren et al., 2017) have become a powerful tool for discovery, surveillanceand monitoring of endangered (Goldberg et al., 2011), rare (Machler et al.,2014; Jerde et al.
, 2011), and invasive species(Simmons et al., 2016). eDNAstudies have been used to estimate the population abundance of certain targetspecies (Takahara et al., 2012; 2013) in lentic systems like ponds and lakes.
But in lotic system (eg. Rivers) eDNA studiesare limited due to linking of various environmental processes. The transport of eDNA studies in rivers andstreams is challenging.
The challenging issues with regard to transportation andconcentration of eDNA in streams are dependent on processes like retention,resuspension, residence time of water, substrate type and turbidity (Shogren etal., 2017). The important aspect of eDNA studies in rivers involvesunderstanding the physical and biological variables that influence theretention, resuspension and transportation of substances. Another importantaspect 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) dueto variably sized particles from various sources increases the complexity ofeDNA studies in rivers. In previous studies eDNA has been used as qualitativedetection tool only for confirming the presence or absence of a species.
Recently, the mechanistic of eDNAtransportation has emerged by eDNA releases into experimental streams (Shogrenet al., 2016, 2017) to note the effect of hyporheic exchange and flowvelocities on eDNA.