Abstract
In the face of plans for increased construction of dams and reservoirs in the Mekong River Basin, it is critically important to better understand the primary-producer community of phytoplankton, especially the warm-water cyanobacteria. This is because these algae can serve as the primary source of carbon for higher trophic levels, including fishes, but can also form harmful blooms, threatening local fisheries and environmental and human health. We monitored the dynamics of three cyanobacteria—Synechococcus spp., Microcystis aeruginosa, and Dolichospermum spp.—for two years in nine large lakes and reservoirs in the Mekong River Basin. The densities of these algae were largely system-specific such that their abundance was uniquely determined within individual water bodies. However, after accounting for the system-specific effect, we found that cell densities of Synechococcus spp., M. aeruginosa, and Dolichospermum spp. varied in response to changes in photosynthetically active radiation (PAR), total nitrogen, and water level, respectively. Because both PAR and water level tend to fluctuate concordantly over a wide geographic area, Synechococcus spp., and to a lesser extent Dolichospermum spp., varied synchronously among the water bodies. Sustaining the production of pico-sized primary producers while preventing harmful algal blooms will be a key management goal for the proposed reservoirs in the Mekong Basin.
Highlights
The Mekong River, an international river flowing through six countries on the Indochina Peninsula of Southeast Asia, has been dammed primarily for hydropower generation and irrigation since the 1960s
The nine water bodies were distinctly separated along the first principal components (PCs), from deep, clear, oligotrophic waters (e.g., Nam Ngum (NN) and SI) to shallow, turbid, eutrophic waters (e.g., Huay Luang (HU) and Tonle Sap (TS))
It has been widely accepted that Synechococcus spp. and picophytoplankton in general are common in large, deep oligo–mesotrophic water bodies because of their very rapid nutrient uptake rates and low-light adaptation [5,6,39], and that their contribution to overall algal biomass declines with increasing trophic status [40]
Summary
The Mekong River, an international river flowing through six countries on the Indochina Peninsula of Southeast Asia, has been dammed primarily for hydropower generation and irrigation since the 1960s. Increasing demand for electricity has prompted nearly 100 proposals for additional dams throughout the basin [1]. The enormous fish production in the Mekong is undoubtedly attributable to massive primary production by algae and plants in the river system. Despite significant concerns over damming, one can argue that overall algal production might be enhanced if numerous dams and the resultant shallow water bodies (i.e., reservoirs) are constructed throughout the basin. To determine the validity of this argument, it is critically important to better understand the dynamics of primary producers in the existing water bodies of the Mekong River Basin
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