Abstract
Mediterranean rivers in intensive agricultural watersheds usually display outgrowths of macrophytes – notably alien species – due to a combination of high concentrations of nutrients in the water runoff and low flows resulting from water abstraction for irrigation. Standard mechanical and chemical control is used to mitigate the problems associated with excessive growth of plant biomass: mainly less drainage capacity and higher flood risk. However, such control measures are cost and labor-intensive and do not present long-term efficiency. Although the high sensitivity of aquatic vegetation to instream hydraulic conditions is well known, management approaches based on flow management remain relatively unexplored. The aim of our study was therefore to apply physical habitat simulation techniques promoted by the Instream Flow Incremental Method (IFIM) to aquatic macrophytes – the first time it has been applied in this context – in order to model shifts in habitat suitability under different flow scenarios in the Sorraia river in central Portugal. We used this approach to test whether the risk of invasion and channel encroachment by nuisance species can be controlled by setting minimum annual flows. We used 960 randomly distributed survey points to analyze the habitat suitability for the most important aquatic species (including the invasive Brazilian milfoil Myriophyllum aquaticum, Sparganium erectum, and Potamogeton crispus) in regard to the physical parameters ‘flow velocity,’ ‘water depth,’ and ‘substrate size’. We chose the lowest discharge period of the year in order to assess the hydraulic conditions while disturbances were at a low-point, thus allowing aquatic vegetation establishment and subsistence. We then used the two-dimensional hydraulic River2D software to model the potential habitat availability for different flow conditions based on the site-specific habitat suitability index for each physical parameter and species. Our results show that the growth and distribution of macrophytes in the hydrologically stable vegetation period is primarily a function of the local physical instream condition. Using site-specific preference curves and a two-dimensional hydraulic model, it was possible to determine minimum annual flows that might prevent the excessive growth and channel encroachment caused by Myriophyllum aquaticum.
Highlights
Aquatic macrophytes play an important role in riverine ecosystems, providing habitats for many organisms and affecting the hydraulic and chemical instream condition (Carpenter and Lodge, 1986)
The main aim of this study was, for the first time, to apply and validate the hydraulic habitat modeling techniques promoted by the Instream Flow Incremental Method (IFIM) for the assessment of annual minimum flows with the ability to reduce the risk of channel encroachment and invasion by the alien Myriophyllum aquaticum in a heavily regulated Mediterranean river
The habitat sampling resulted in 224 M. aquaticum, 135 P. crispus, and 85 S. erectum presences in a total of 961 habitat samples
Summary
Aquatic macrophytes play an important role in riverine ecosystems, providing habitats for many organisms and affecting the hydraulic and chemical instream condition (Carpenter and Lodge, 1986). Whereas loss processes are caused by increased drag forces during high flood events that cause stem breakage and uprooting of plants, biomass gain processes happen while disturbances are absent during medium to low flow conditions (Riis et al, 2008) In these stable interflood periods, macrophyte growth is controlled by several physical and chemical factors, including flow velocity and depth (Chambers et al, 1991; Riis and Biggs, 2003), light availability (Carr et al, 1997; Köhler et al, 2010), water temperature (Barko et al, 1986; Carr et al, 1997), and riverbed grain size (Baattrup-Pedersen and Riis, 1999), as well as the nutrient content of the riverbed and water (Barko et al, 1986; Demars and Edwards, 2009). Management of aquatic macrophytes by mechanical (cutting) or chemical (herbicides) means is common practice in many rivers worldwide (Madsen, 2000; Hussner et al, 2017)
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