Spatial and temporal variation of benthic macroinvertebrates in the Nam Gnom Basin receiving discharged waters from the Nam Theun 2 Reservoir (Lao PDR)

Abstract

-In order to assess the impact of water release in the downstream area of the Nam Theun 2 Reservoir (Lao PDR), the spatial and temporal variation of benthic macroinvertebrates was explored. Between 2010 and 2012, five sites were monitored in the Nam Gnom Basin, a tributary of the Xe Bangfai River, receiving the turbinated waters. Repeatedtime collections revealed the presence of 109 taxa mainly identified to the family level, attesting to a rich biota. Preliminary results did not show shifts in diversity, population composition, and feeding metrics. Redundancy Analysis indicated that the altitudinal gradient (i.e. altitude and altitude-related variables) remained the major environmental factor influencing the macroinvertebrates distribution before water discharge. The presence of a tributary downstream of the release point, and the respect of the natural river inflow can explain the preservation of natural conditions. More samples and a strengthened ecological knowledge of Southeast Asian macroinvertebrates are still required to confirm these preliminary results.Key words -freshwater invertebrates, Southeast Asia, biomonitoring, hydropower, downstream section Résumé -Afin d'évaluer l'impact du relâcher des eaux dans la zone aval du Réservoir Nam Theun 2 (RDP du Laos), la variabilité spatio-temporelle des invertébrés aquatiques a été étudiée. Entre 2010 et 2012, cinq sites ont été échantillonnés sur le bassin de la Nam Gnom, un affluent de la rivière Xe Bangfai, qui reçoit les eaux turbinées. Les prélèvements issus du suivi ont permis de mettre en évidence la présence de 109 taxa principalement