Eutrophication, Phytoplankton Assemblage And Blue-green Algal Blooms In Zhuhai Reservoirs

In order to guarantee the water supply to the Zhuhai City and Macau,many middle and small sized reservoirs have been built and connected by pipes and pumping station to the Pearl Rivers.These pumped storage reservoirs are different from those normally formed by damming rivers.Pumping water from polluted lowland rivers results in eutrophication and algal blooming.On the basis of investigating eutrophication,we analyzed response of phytoplankton community to eutrophication and water pumping.SRC model of phytoplankton survivorship was applied to the adaptive strategies of phytoplankton dominant species in the special reservoir ecosystems.Canonical Correspondence Analysis(CCA) was used to identify the main environmental factors affecting the abundance of dominant phytoplankton.A simple dynamic modeling system was established with Stellao-â…¡to simulate the accumulations of cyanobacterial cells under several cases with varying temperature and water retention time.The risk of cyanobacteria bloom was evaluated.Among the 19 investigated reservoirs,9 of them were eutrophic nearly in the whole year,and the others are mesotrophic.More reservoirs are eutrophicated and at the high risk of cyanobacteria blooming in April just before the wet season is coming. Fewer reservoirs were eutrophic in December.This seasonal variation was regulated by both water pumping and water temperature.In general,the trophic state indices are higher in the pumped storage reservoirs than in the normal reservoirs.A total of 143 species of phytoplankton from 95 genera in 7 phyla were observed,66 species in Chlorophyta,34 species in Bacillariophyta,21 species in Cyanophyta,5 species in Euglenophyta,4 species of Pyrrophyta and Chrysophyta and 2 species of Cryptophyta.As pumping water from the river enhanced the mixing and nutrient concentration and reduced water transparency,filamentous blue-green algae (Pseudanabaena,Cylindrospermopsis and Limnothrix etc.) and Peridinium in Pyrrophyta were favored in these reservoirs.Synedra and Cyclotella usually dominant in winter and spring.Microcystis blooms occasionally in the warm and low mixing stages.Medium-frequent pumping and flushing could promote the dominant of filamentous blue green algae with R survival strategy,and decrease Microcystis blooming.Very high frequent pumping and flushing could favor the dominant of C survival strategy species(representive as flagellated green algae).The eutrophicated reservoirs are in two states:Cyanobacteria dominant state and flagellated C-strategy dominant state(so called Holotrophic).The frequency of pumping is the key factor triggering the shift between the two states.Most of non-pumped storage reservoirs are mesotrophic;their phytoplankton was dominant by Peridinium(dinophyta) yearly.Large sized species of green algae such as Staurastrum spp.,Botrycoccus sp.and Cosmarium sp.with S strategy were dominant in the dry season,while diatoms(Cyclotella spp.,Melosira sp., Rhizosolenia sp.) dominated in the flood season.Precipitation determines the shift between two states of communities.CCA analysis showed that water retention time,pH,water level and phosphate concentration are major environmental variables influencing the structure of phytoplankton community,following by Secchi disk depth and precipitation.There is a markedly positive correlation between high phosphorus concentration and cyanobacteria abundance in April in Shedikeng Reservoir.The abundance of diatom in Zhuxiandong reservoir was correlated with water volume and Secchi Disk depth. Diatom abundance had a negative relationship with water residence time in Nanping reservoir,while the abundance of dinoflagellate peridinium minutum was positively correlated to precipitation.The dominant phytoplankton species such as Chlamydomonas sp.,Planctonema lauterbornii and Cryptomonas sp.were more sensitive to the variation of environmental variables than the other species in Zhuxiandong reservoir.Their relative abundance increased in April with relatively low water temperature,low water level and longer water residence time.Another group- the small unicellular chlorophytes showed positive correlation with high phosphorus concentration and water level in June and August.The large sized species such as Melosira spp.,Nitzschia spp.and Euglena acus prefer to high Secchi depth and precipitation,they have high relative abundance in June,August and December. In Zhuxiandong reservoir,a eutrophic water body,its phytoplankton was characterized with fewer phytoplankton species and but relatively high abundance of green algae,being different from that in most of eutrophic reservoirs in the same region.Flagellated green algae,filamentous green algae and Cyclotella meneghiniana (diatom) were the major contributors for phytoplankton biomass in the reservoirs mentioned above.The structure and dynamics of phytoplankton community was primarily controlled by short water residence time in Zhuxiandong and Nanping reservoir.In simulation with Stella-â…¡based model,the Fenghuangshan Reservoir has the highest risk of Microcystis blooming for its long water retention time Zhuxiandong reservoir has the lowest risk of Microcystis bloom for its short water retention time.