The role of lateral connectivity in the maintenance of macrophyte diversity and production in large rivers

Abstract

1. Large European river floodplains have been intensively reduced through human activities over several millennia. Ecological assessments of natural large river systems remain rare and potentially flawed since they typically omit either the main channel of the river or, more commonly, aquatic habitats that occur naturally in the floodplain of dynamic systems.2. Surveys were conducted over a longitudinal and lateral gradient to assess distribution, richness and production of macrophytes along two little-disturbed large upland rivers in Scotland.3. Lateral dynamics, through the creation of backwaters, underpinned the occurrence, abundance and production of macrophytes in these rivers. Indeed, backwaters, despite representing only 5% of the total area of aquatic habitat, supported a significantly higher concentration of species (65% of species recorded at the riverscape scale were unique to backwaters) than the main channel.4. The frequency with which backwaters were connected to the main channel during flood flows influenced their species richness. Highest species richness in backwaters was typically found at low connectivity. Standing crop in backwaters was 150 times higher per unit area than in the main channel, while at the riverscape scale backwaters accounted for an average 89% of aquatic plant biomass. The highest plant biomass was found at low and medium connectivity with the main channel.5. Backwaters appear to be crucial habitats in maintaining macrophyte diversity and production in large river ecosystems. These results emphasize the importance of river hydrodynamics and lateral connectivity in maintaining macrophyte community diversity along large rivers. The results illustrate also the potentially very significant role of backwaters as source habitats supplying propagules and organic matter to downstream reaches.6. It is argued that the entire riverscape (floodplain plus main channel) must be considered in the holistic assessment of such systems, while protection of this resource requires greater recognition of linkages within and across habitats, both aquatic and terrestrial, to be effective.