Long-term Dynamics Of Cyanobacteria Related To Environment Factors In Lake Taihu

In the present study we chose Taihu as the representative large shallow eutrophic laketo systematically investigate biotic and abiotic factors, which affect the bloom-formingcyanobacteria. The gross primary production of phytoplankton was measured in order toanalyse inorganic carbon quantitatively and predict tendency for bloom formation in LakeTaihu. The main contents included:(1) The history data of phytoplankton were analyzedto determine species of bloom-forming cyanobacteria in Lake Taihu;(2) The multivariatestatistical analysis methods were employed to investigate the effects ofmulti-environmental factors on the growth of cyanobacteria populations in Lake Taihu;(3)By analysing the long-term monitoring data of phytoplankton, the spatial and temporaldistribution of non-dominated phytoplankton and the relationship between dominatedspecies and non-dominated species were explored in Lake Taihu;(4) By measuring thegross primary production of algae, the inorganic carbon was calculated quantitatively inLake Taihu. In general, main observations and conclusions can be summed up as follows:(1) Microcystis, Anabaena and Aphanizomenon were included in bloom-formingcyanobacteria. Microcystis was the dominated species in Lake Taihu. There were totallysix to seven kinds of Microcystis spp., such as Microcystis aeruginosa, Microcystisflos-aquae and Microcystis wesenbergii, which their mean biomass accounted to85.7%oftotal cyanobacaterial biomass. Microcystis formed blooms almost every summer-autumnduring the year of1992-2002in the west-north part of Lake Taihu. Anabaena andMicrocystis were the co-dominated species on June1993and June1994in Meiliang Bay.In contrast to Microcystis, Aphanizomenon biomass was significantly higher on June1997in Meiliang Bay.(2) A multivariate statistical analysis, canonical correspondence analysis (CCA),revealed a negative correlation between the Microcystis and TN: TP, and a positivecorrelation between the Microcystis and NH4-N: NOX-N, water temperature and pH. Warmwater temperature was the principal force driving Microcystis blooms, which werepreceded declining concentrations of nitrogen compounds. Microcystis tended to dominate(Microcystis contributed above50%to total algal mass) in the north part of Lake Taihu during summer when the TN: TP mass ratio was less than30, NH4-N: NOX-N was below1,and a critical water temperature ranged from25℃to30℃, respectively. Meanwhile,suspended solids (SS) concentrations exceeded10mg/L and pH exceeded8.0duringblooms. Overall, this study advances our understanding of nutrient enrichment and highambient temperature influences on Microcystis biomass. The effects of global warminginclude the direct effects of higher water temperature are beneficial to cyanobacteria inLake Taihu.(3) The most abundant planktonic diatom was Aulacoseira sp. in addition to mainlyunicellular diatoms as Cyclotella sp. and Synedra sp. At the lake Mouth, diatoms occurredmore frequently and appeared at higher densities than in the Meiliang Bay and the LakeCentre. The higher water flowing where the river enters the lake seemed to maintain ahigh abundance of diatoms at the Lake Mouth. In contrast, nutrients as PO4-P and NH4-Nplayed the critical role shaping the spatial distribution of diatoms in Lake Taihu. Watertemperature appeared to be important in the timing of the annual onset of diatom growth.Our results showed the poor SRSi contents environment, the dominated cyanobacteriawhich is superior competitor for high temperature and nutrients, and secondarymetabolites and microcystins produced by cyanobacteria, all of which limited the growthof planktonic diatoms in Lake Taihu.(4) The mean biomass of the cryptophytes in Zhushan Bay (1.89mg/L) was muchhigher than that of Meiliang Bay (0.87mg/L) and Gonghu Bay (0.43mg/L). The averagebiomass value of cryptophytes during2008-2009(2.12mg/L) was much higher than thatof2005-2007(0.28mg/L). Principally, cryptophytes were most prevalent in Zhushan Baywhich was marked by higher nutrient levels, higher organic matter and SS concentrations.The succession of cryptophytes was mainly affected by the grazing of zooplankton inGonghu Bay. Furthermore, we found that cryptophytes and cyanobacteria abundanceswere inversely correlated, temporally, in Lake Taihu. Higher nutrient levels, higherorganic matter and SS concentrations environment, coupled with exclusive combination ofphotosynthetic pigments and, mixotrophically exploit available inorganic as well asorganic nutrients, all of which make cryptophytes a unique group able to potentiallyout-compete cyanobacteria in Lake Taihu. Our results proved the effects of increased grazing pressure and cyanobacteria competition pressure on cryptophytes which arecommon in cooler water. It is also said that cyanobacterial blooms are likely to dominatein Lake Taihu for longer time periods than they present do.(5) The significant positive correlations between gross primary production (GPP) andplankton community respiration (PCR) noted in Lake Taihu (PCR=1.22GPP+0.46,r2=0.80). Form the GPP and PCR regression equation, the value of PCR was0.46gO2m-2d-1, which was1.22times as much as GPP. The conditions here may be related tolimitation of phytoplankton photosynthesis by the poor underwater light climate (due toelevated SS and nutrients originating in the catchment) and the preferential enhancementof respiration by high water temperatures. The results indicated that plankton communitywas net heterotrophic. Partial pressure estimated for CO2also indicated that planktoncommunity was a net source of CO2in Lake Taihu.