Tham khảo tài liệu 'studies on water management issuese part 10', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 170 Studies on Water Management Issues onwards after Alqueva dam completion. In view of decreasing trend in chlorophyll and overall phytoplankton abundance from 2003 to 2010 nitrogen availability appeared to play a preponderant role rather than light in this turbid estuary. This shift from light to nutrient limitation was probably the most determinant trend for phytoplankton observed after Alqueva Barbosa et al. 2010 . However since chroococcoid cyanobacteria have higher affinity for nutrients due to small size and most filamentous forms as well as some chroococcoid species have nitrogen fixing potential these photosynthetic prokaryotes should have been less affected by lower nitrogen availability than larger non-nitrogen fixing eukaryotic phytoplankton. Freshwater reservoirs created by dams do not retain only water but also suspended particulate material including planktonic microorganisms. In consequence not only are nutritional regimes affected downstream but also freshwater microbial populations with complex life cycles such as cyanobacteria. Filamentous cyanobacteria in response to environmental forcing can produce different cell types which are adapted to nitrogen fixation nutrient storage and reproductive strategies such as winter dormancy and dispersal. Thus freshwater reservoirs by retaining these morphotypes could seriously affect not only bloom formation but also species composition downstream. The lack of correlation between chlorophyll and cyanobacteria abundance could be simply explained by predominance of small chroococcoid cells with reduced chlorophyll content. Consequently poor or absent correlation between chlorophyll and microcystin concentrations should also be expected. As previously described by Galvão et al. 2008 microcystin concentration were generally not correlated with cyanobacteria abundance or biomass in natural waters freshwater reservoirs and Guadiana river South Portugal since different strains and or species could produce .
