Bringing backwaters to the foreground

Posted on December 28, 2013

Around the world, the desire to control floods and expand agriculture has led engineers to straighten river channels, construct embankments and separate rivers from their floodplains.  For example, in Europe the area of connected floodplain has been reduced by more than 90%.  Backwaters are sections of river that have lost their upstream connection with the main channel, but which connect again briefly during floods.  Despite a growing awareness of the ecological value of floodplain–river interactions and the significance of backwaters in providing aquatic organisms with opportunities for refuge, reproduction and growth, most biological assessments of large rivers have either been confined to the main river channel or have treated backwaters as isolated habitats.  To assess how floodplain connectivity contributes to the productivity of aquatic macrophytes in large rivers, transect sampling of backwaters and the main channels of the rivers Tay and Tummel in Perthshire, Scotland was carried out on five occasions during the summer months (May to September) of 2009–2011.  Compared with the main channel, backwaters supported significantly (three times) more submerged, floating-leaf and emergent plant species, and a plant biomass that was on average 150 times greater.  There were clear differences between the plant communities in backwaters and the main channel, 65% of plant species being found only in backwaters.  The significance of backwaters was highlighted by the fact that they accounted for almost 90% of the standing crop of vegetation but only 5% of the total area of aquatic habitat.  Although the species richness of backwaters didn’t vary much with water depth, the number of species was inversely related to the frequency of connection with the main channel, showing that standing water had a positive effect on plant colonisation and growth.  The findings showed that the main channels are not representative of overall river production.  Because of their importance of backwaters as habitats for aquatic vegetation, invertebrates, fish, amphibians, birds and mammals, it’s important to include them in plans to protect riverine environments.

Reference:  Keruzore, A.A., Willby, N.J. & Gilvear, D.J.  2013.  The role of lateral connectivity in the maintenance of macrophyte diversity and production in large rivers.  Aquatic Conservation: Marine and Freshwater Ecosystems 23, 301–315.