Effect of morphodynamic processes on benthic invertebrates in large rivers and some morphological adaptations to flow: case study on the Paraná River (Argentina).

BLETTLER M.; AMSLER M; EZCURRA DE DRAGO I.

San Pietro, Italia

Simposio; Symposium on Fluvial Hydraulics, Morphodynamics and Ecology. In frame of the program “An environmental fluid dynamics laboratory in the field: studies and numerical modeling of hydro- dynamics, morpho- dynamics and ecology in river meanders invertebrate”; 2009

Leibniz-Institute of Freshwater Ecology and Inland Fisheries Berlin, Germany (IGB); and TU Delft University of Technology, Netherlands

The ecohydrology discipline, specifically the ecohydraulic sub discipline, implies research in the limit between the hydraulic and ecological sciences. This relatively new discipline is still poorly explored despite the capability to explain properly the fauna distribution. A study aimed to investigate the incidence of flow structure prevailing along mobile dunes recorded in the main channel of the Middle Paraná River on the spatial distribution of the benthic macroinvertebrates living in those bed areas, is presented herein. The research involved measurements of vertical velocity profiles along large migrating dunes which provided the bed flow characteristics associated later to the benthic distribution. The following conclusions have arisen: i) dunes in the active channel of the Paraná River could be considered as “hydraulic biotopes at a mesohabitat scale” since the studied dunes within the thalweg of the river  experienced high flows and served as habitats for relatively few organisms; on the other hand, dunes in shallower slower flows were home to a large population of organisms; ii) differences in benthic densities were also found at “microhabitat scales”, i.e. within-dune scales; hydraulic conditions vary between the crests, troughs and stoss sides of dunes with more organisms found in the dune troughs where bed shear stresses were the lowest and, minimum densities recorded in the vicinity of flow reattachment at the beginning of the stoss side of dunes, where bed shear stresses are generally highest (this is also the region where other researchers suggest turbulent fluctuations are large, leading to greater disturbance of bed particles); iii) superimposed dunes on larger dunes may be consider as another microhabitat of still smaller dimensions though its influence on the benthic fauna remains to be explored. Considering the highly stressful environment where the organisms live, an interesting question emerges:  how do invertebrates cope with the swift currents prevailing in the active channel? At least two morphological adaptations to flow were recorded out of direct microscopic observations: a smaller body size of species living in the active channel compared to those on the banks, and an increment of the organism weight with ballast of ingested sand grains or by sticking them on the body surface.