Spatial and temporal dynamics of macrophyte cover in a large regulated river

Abstract

The River Ebro basin is extensively dammed. Dams alter the geomorphological functioning of the river by altering its flow regime (e.g. reducing mean and maximum discharges), increasing bed stability (armouring) and decreasing turbidity (water clarity). These effects, together with an increase in nutrient concentrations and water temperature, have generated optimal conditions for the proliferation of aquatic macrophytes. In this paper, we analyse the temporal and spatial changes of macrophyte cover in the lowermost Ebro through a series of field campaigns carried out between 2009 and 2010. Special attention was paid to the spatial distribution of macrophytes in relation to flow hydraulics, channel geometry and bed sedimentology. Temporal changes in macrophyte cover were analysed in relation to the frequency and magnitude of both natural floods and flushing flows (artificial flow releases from dams with generally a magnitude that equates around a2-year flood in the river). Spatially, the proportion of macrophytes along the reaches showed a variable pattern, with a succession of areas with both high and low plant density, coinciding with the alternation of riffles and pools in the channel. The highest values of plant cover (>65%) occurred in riffles and in transition to riffle areas, while the lowest densities (1% or almost negligible) were observed in pools and transition to pool areas. Water depth and the grain-size distribution of the riverbed materials (i.e. D₈₄), are found to be the main factors controlling the degree of plant cover in the lower Ebro. Temporally, the macrophyte proportion varied during the hydrological year, with a clear increment from late spring to early autumn (i.e. vegetation cover reached 40%, on average, of the channel surface). Macrophyte coverage decreases immediately following a flushing flow but in the long term, vegetation re-occupied the area again, even slightly increasing in some sections; overall, the mean percentage of macrophyte cover was 19% higher at the end of the study period, despite the numerous flow events occurred on the meantime. This increase enhanced riverbed stability, which in turn reduced the possibility for bed-material entrainment. This study empirically confirms the necessity of improving the management options applied in the lower Ebro with complementary measures to help maximise the efficiency of flow releases (for instance, subject the macrophytes to a severe hydrological stress by decreasing discharge before a given flushing flow, undertake localise mechanical removal of plants in areas where density is high, and increase the frequency of floods in winter time when macrophyte stands are weaker).

Keywords: Bed stability; Dams; Flushing flows; Fluvial restoration; Macrophyte cover; Natural floods; River Ebro; Water clarity.