Phytoplankton community and trophic status assessment of a newly impounded sub-tropical reservoir: case study of the Nam Theun 2 Reservoir (Lao PDR, Southeast Asia)

Abstract

Phytoplankton community and its relationships with the physico-chemical variables were studied in the newly impounded Nam Theun 2 sub-tropical Reservoir, located in Lao PDR (Southeast Asia). Samples were collected on a monthly basis from March 2009 to December 2012 within the euphotic zone of five sampling sites. One hundred and eighty taxa were identified during this study. A significant seasonal effect on the NT2 phytoplanktonic community was related to water temperature, precipitation and water level. Dinoflagellates and green algae mainly contributed to phytoplanktonic biomasses usually highest in June with a maximum of 3 x 10 3 µg C.L -1 . Both biomasses and Chl- a exhibited similar variation with significant relationship. Moreover, total phytoplankton biomass has also a significant correlation with the Secchi depth suggesting that water transparency might be dictated by phytoplankton biomass.The trophic status was assessed according to the nutrients concentrations, Secchi depth and Chlorophyll a (Chl a ) (OECD classification and AARL-PC scores) and the phytoplankton composition (AARL-PP scores). This study revealed the suitability of Chl- a , phytoplankton biomass and phytoplankton composition for assessing the trophic state of a young and sub-tropical reservoir. The results illustrated an oligotrophication of the NT2 Reservoir at the end of period of study. Consequently, this approach might be recommended to follow the evolution of the trophic status of a young and sub-tropical reservoir.