Abstract – Phytoplankton community and its relationships with the physico-chemical variables
were studied in the newly impound 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 103 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 statuswasassessed 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.
Abstract – Phytoplankton community and its relationships with the physico-chemical variableswere studied in the newly impound Nam Theun 2 sub-tropical Reservoir, located inLao PDR (Southeast Asia). Samples were collected on a monthly basis from March 2009 toDecember 2012 within the euphotic zone of five sampling sites. One hundred and eighty taxawere identified during this study. A significant seasonal effect on the NT2 phytoplanktoniccommunity was related to water temperature, precipitation and water level. Dinoflagellatesand green algae mainly contributed to phytoplanktonic biomasses usually highest in Junewith a maximum of 3 x 103 μg C.L-1. Both biomasses and Chl-a exhibited similar variation withsignificant relationship. Moreover, total phytoplankton biomass has also a significant correlationwith the Secchi depth suggesting that water transparency might be dictated by phytoplanktonbiomass.The trophic statuswasassessed according to the nutrients concentrations,Secchi depth and Chlorophyll a (Chl a) (OECD classification and AARL-PC scores) and thephytoplankton composition (AARL-PP scores). This study revealed the suitability of Chl-a,phytoplankton biomass and phytoplankton composition for assessing the trophic state of ayoung and sub-tropical reservoir. The results illustrated an oligotrophication of the NT2Reservoir at the end of period of study. Consequently, this approach might be recommendedto follow the evolution of the trophic status of a young and sub-tropical reservoir.
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