Morpho- hydrodynamic conditions along meander bends control the distribution of benthic fauna in a large river (Paraguay River, Argentina-Paraguay)

BLETTLER, MARTIN; AMSLER, MARIO; EZCURRA DE DRAGO, INÉS

Konstanz

Congreso; 5th International Limnoecological Congress; 2011

Universität Konstanz - Technische Universität Braunschweig

The Paraguay River is a large South-American river (mean discharge= 4000 m s, drainage basin area= 1095000 km). It has a meandering channel and drains areas of the tropical Brazilian Shield with Tertiary-Quaternary sediments. Investigations about morphological features and flow characteristics of meanders in large rivers are limited both in number and in detail of measurements because of inherent difficulties involved in obtaining quality data. Changes in morpho- and hydrodynamic conditions directly or indirectly influence benthic invertebrates at the scale of individual organisms or assemblages because they modify the specific food resources and refuge. Considering the above explanation, it is promising to study the links between morphological features and sedimentological characteristics, closely related to hydraulic conditions, and the ecological patterns of benthic invertebrates along meanders. It would be a pioneering contribution on this topic of science. We have selected two meanders bends of the Paraguay River. One of them is located The Paraguay River is a large South-American river (mean discharge= 4000 m s, drainage basin area= 1095000 km). It has a meandering channel and drains areas of the tropical Brazilian Shield with Tertiary-Quaternary sediments. Investigations about morphological features and flow characteristics of meanders in large rivers are limited both in number and in detail of measurements because of inherent difficulties involved in obtaining quality data. Changes in morpho- and hydrodynamic conditions directly or indirectly influence benthic invertebrates at the scale of individual organisms or assemblages because they modify the specific food resources and refuge. Considering the above explanation, it is promising to study the links between morphological features and sedimentological characteristics, closely related to hydraulic conditions, and the ecological patterns of benthic invertebrates along meanders. It would be a pioneering contribution on this topic of science. We have selected two meanders bends of the Paraguay River. One of them is located downstream of the Bermejo River, a tributary which supplies a huge amount of fine sediments (mainly in suspension) to the Paraguay. These particles in suspension are Precambrian and Paleozoic sediments transported from the Andean headwaters. The other meander is located upstream of this tributary and, consequently, the sediment load is significantly lower and coarser. The results reveal that at meso-habitat scale (i.e. meander scale) mean bed hydraulic conditions are most intense in the downstream meander (shear stress= 0.18 kg m, mobility number= 0.92) and the concentration of suspended sediments is higher (125 mg l), leading to low mean density of invertebrates (960 ind. m). Moreover, the upstream meander presents lower hydraulic stresses (shear stress= 0.04 kg m, mobility number= 0.13) and suspended sediments (56.5 mg l), prompting larger benthic densities (15600 ind. m). Besides of the bed hydraulic conditions, the suspended fine sediments would have strong ecological implications. A fraction of these suspended sediment particles cover the interstitial spaces between sand bed grains, i.e. the places where the invertebrates live, impeding their development. At micro-habitat scale (i.e. pool-riffle scale) the density of organisms decreases significantly on the scour hole (pool area) of the upstream meander, where the bed shear stresses are higher, in comparison with the riffle area. Nevertheless, this relation was not clear on the downstream meander. This situation could be attributed to the high sediment input from the Bermejo River, which would reduce the importance of the hydrodynamic role compared with the fine sediment one on the invertebrate densities and composition. This study suggests that benthic invertebrates prefer the benefits from an environment with lower hydrodynamic stress but without fine sediments on the interstitial spaces, providing refuge and probably food. The Paraguay River is a large South-American river (mean discharge= 4000 m s, drainage basin area= 1095000 km). It has a meandering channel and drains areas of the tropical Brazilian Shield with Tertiary-Quaternary sediments. Investigations about morphological features and flow characteristics of meanders in large rivers are limited both in number and in detail of measurements because of inherent difficulties involved in obtaining quality data. Changes in morpho- and hydrodynamic conditions directly or indirectly influence benthic invertebrates at the scale of individual organisms or assemblages because they modify the specific food resources and refuge. Considering the above explanation, it is promising to study the links between morphological features and sedimentological characteristics, closely related to hydraulic conditions, and the ecological patterns of benthic invertebrates along meanders. It would be a pioneering contribution on this topic of science. We have selected two meanders bends of the Paraguay River. One of them is located downstream of the Bermejo River, a tributary which supplies a huge amount of fine sediments (mainly in suspension) to the Paraguay. These particles in suspension are Precambrian and Paleozoic sediments transported from the Andean headwaters. The other meander is located upstream of this tributary and, consequently, the sediment load is significantly lower and coarser. The results reveal that at meso-habitat scale (i.e. meander scale) mean bed hydraulic conditions are most intense in the downstream meander (shear stress= 0.18 kg m, mobility number= 0.92) and the concentration of suspended sediments is higher (125 mg l), leading to low mean density of invertebrates (960 ind. m). Moreover, the upstream meander presents lower hydraulic stresses (shear stress= 0.04 kg m, mobility number= 0.13) and suspended sediments (56.5 mg l), prompting larger benthic densities (15600 ind. m). Besides of the bed hydraulic conditions, the suspended fine sediments would have strong ecological implications. A fraction of these suspended sediment particles cover the interstitial spaces between sand bed grains, i.e. the places where the invertebrates live, impeding their development. At micro-habitat scale (i.e. pool-riffle scale) the density of organisms decreases significantly on the scour hole (pool area) of the upstream meander, where the bed shear stresses are higher, in comparison with the riffle area. Nevertheless, this relation was not clear on the downstream meander. This situation could be attributed to the high sediment input from the Bermejo River, which would reduce the importance of the hydrodynamic role compared with the fine sediment one on the invertebrate densities and composition. This study suggests that benthic invertebrates prefer the benefits from an environment with lower hydrodynamic stress but without fine sediments on the interstitial spaces, providing refuge and probably food.