Effect of nutrient level on phytoplankton community structure in different water bodies

Available online at www.sciencedirect.comJOURNAL OFENVIRONMENTALScienceDirectSCIENCESISSN 101-0702. JESJoumal of Erirnmental Sciences 2010, 2(1)32-39www.jesc.Bc.cnOLLLEffect of nutrient level on phytoplankton community structurein diferent water bodiesWei Zhu', Lei Wan2*, Lianfang Zhao'1. College of Emironmental Science and Engineering, Hohai University, Nanjing 210098, China E-mail: weizhu863@ 126.com2. College of Environmental Engineering, Xuzhou Instiute of Technology, Xuzhou 221008, ChinaReceived 11 February 2009; revised 25 May 2009; aceped 28 July 2009AbstractIncreasing levels of pollution within water bodies can cause eutrophication and an associated rapid growth in and reproduction ofphytoplankton. Although most frequently ocrring in bodies of water such as lakes and dams, in recent years an increasing number ofriver systems in China have sufered serious algal blooms. The community structure of phytoplankton may dffer, however, dependent onthe hydrodynamic conditions and nutrient levels within the water body.”The feld insigation results obtained from a stagnant river inSuzhou City and Taihu Lake, China, showed that in water with higher concentrations of nitrogen and phosphorus, Chlorophyta becamethe predominant species and in water with lower concentrations of nitrogen and phosphonus, Cyanobacteria became the predominantspecies. Growth experiments with competitive species, Microcystis aeruginosa Kutz and Scenedesmus quadricauda (Turp,), wereconducted at three different nutrient levels. The biomass of algae in pure and mixed cultures was measured under conditions ofdifferent NP ratios at oligotrophic, eutrophic and hypertrophic nutrient levels. The resuts indicated that the most suitable state forthe growth and reproduction of M. aeruginosa and S. quadricauda were eutrophic conditions in both pure and mixed cultures. Undercompetition, however, the lower medium nutrient levels favoured M. aenginosa, while the higher medium nutrient levels better suitedS. quadricauda. Under similar hydrodynamic conditions, the community stucture of plbytoplankton in the water body was determinedby the dominant species in competition for nutrients.Key words: plhytoplankton; community structure; nutrieats level; NP ratioDOI: 10.1016/S1001-0742(09)60071-1IntroductionBroad Lake Norfolk, UK) had a high correlation with theconcentration of P and N in spring, summer and autumn.As water pollution intensifes in various water bodies,The community structure of phytoplankton dependsthe problem of eutrophication becomes ever more seri- on the morphology of the water body, hydrodynamicous and leads to an increase in the frequency of algaecharacteristics, sunlight intensity, temperature, nutrientblooms. These problems have influenced the ecosystem, concentration and the biological characteristics of algaehuman health and local economy (Wang and Lu, 2004). In and zooplankton. Previously, in the proces of improvinganalysing the causes of algae blooms, we considered three water quality of city waterways, we. have found thattypes of factors: physical, chemical and biological. Recent- in some water bodies with low_ velocity and very highly, in an atempt to control algae blooms, researchers have nutrient levels, Cyanobacteria blooms do not occur. Inpredominately concentrated on studying chemical factors. contrast, Cyanobacteria blooms have been observed inIn particular, such research has focussed on establishingsome water bodies with very low nutrient levels. Moreover,the relationship between algae growth and concentrations after wastewater interception around some heavily pol-of nitrogen and phosphorus, and determining the limiting luted water bodies, Cyanobacteria bloom outbreaks havefactors of algae growth. For example, Liu et al. (2003) ocurred after water quality improvement. In relation toanalysed the distribution characteristics of nutrients in a these phenomena, we investigated the community structurewater body and the relationship of nutrient distrbutions of phytoplankton in a river with very low fow velocities inwith chlorophyll a. From the analyses, they concluded Suzhou City and from around Taihu Lake, China. Throughthat phosphorus was the main limiting factor for the con中国煤化工ively stagnant Suzhougrowth of phytoplankton in the Fuxian Lake in China. CityYHC N M H Gferent phytoplanktondifferent nutrient con-In addition, based on long-term observations, Lau and cenLane (2002) found that the phytoplankton biomass in the community structures. In order to identify the formationmechanism of predominant species under different nu-trient levels, we selected Microcystis aeruginosa Kitz●Coresponding author. E-mail: bjwanl@163.comNo.1Effect of nutrient level on phytoplankton community structure in difcrent water bodics33and Scenedesmus quadricauda (Tup.) to study the algaespecies competition relationships and interactions usingNanyuan Riverlaboratory experiments. Understanding the characteristicsand formation mechanisms of phytoplankton population isof vital signifcance to the development of measures forcontolling eutrophication.A number of studies about the specifc relationshipsbetween diferent species of algae, typically Prochloro-coccus and Synechococcus, in the coastal oceans have●been conducted. Results showed that Prochlorococcus; Neicheng Riverabundance was recorded in oceanic warm currents, whileWeicheng RiverSynechococcus was most abundant in the coastal areasassociated with high nutrient levels (Jiao et al, 2002,Fg 1 Sketch map of riverways in Archaic Zone, Suzhou City. Jiangsu2005). Fresh water studies on the competition relationshipProvience, Chinaof algae have also been conducted. Hyenstrand et al.(2000), for example, studied the competition between S.1.2 Sample analysisquadricauda and Synechococcus under different modesof inorganic nitrogen supply. This research, however,Surface water (0 0.5 m) was collected at each sitefocussed on the effects of inorganic nitrogen form onevery month, and temperature, water transparency (Trans),the competition relationship of algae, not the nutrientdissolved oxygen (DO) and pH were measured in the field.levespecifcally. Chen et al. (1999) studied the com-Chemical oxygen demand (COD), total nitrogen (TN),petition relationship of M. aeruginosa and S. obliquusammonium nitrogen (NH4*-N) and total phosphorus (TP)under specific nitrogen and phosphorus concentrations,were determined in the laboratory following the standardwhich itally promulgated the reason why Cyanobacteria methods of Ministry of Environmental Protection Admin-became the dominant species under diferent kinds of istration of China Chlorophyl-a (Chl-a) was measuredalgal competition conditions. Their research, however,after extraction in 90% acetone by a freeze-thaw method.only examined the relationship of competition under fixed1.3 Experimental methodsHGZ medium and 1/2HGZ medium and did not examinethe relationship of competition under different nitrogen andThe experiment was carried out using two species ofphosphorus concentrations. Additionally, their results werealgae, M. aeruginosa (obtained from the Institute of Hy-from laboratory experimets only, and did not include feld drobiology, Chinese Academy of Sciences, Wuhan, China)experiments in natural water bodies. Consequently, the aimand S. quadricauda (separated from samples collectedof our study was to determine why particular species offrom the Miaojia River in Suzhou). Of these two species,algae become dominant under certain nutrient conditionsM. aeruginosa is the most familiar water bloom algae in(specifically, nutrient levels and N/P ratios).China and S. quadricauda is the most commom algae inrivers in Suzhou. The algae were grown in the laboratory1 Materials and methodsusing BG11 medium, which is considered suitable forboth M. aeruginosa and s. quadricauda (Hu et al, 2004).A series of experiments were conducted with different1.1 Study area and sampling sitesconcentrations of nitrogen and phosphorus and N/P ratios.In order to disclose the succession rule and communityThe experiments were carried out in a 250-mL Er-structure of phytoplankton in a stagnant water body in lenmeyer fask with 100 mL BG11 medium. The algaeSuzhou City, Jiangsu Province, China. Water quality and samples were washed using 15 mg/L NaHCO3 solutionphytoplankton conditions were monitored from July 2004 and then separated by centrifuge (Jin and Tu, 1990). Into May 2005. Suzhou City is one of the most developed order to maintain a relatively constant nutrient level in thecities in China. Located in the downstream basin area of culture medium, the experiments used perfusion cultureYangtze River. The city has a relatively fat topographysuch that the culture was replaced half by half everywith a dense mesh of waterways and rivers. The conditions two to three days, which was similar to chemostat. Theof these waterways are complex and characterised by very algae number in the supermatant fluid could be ignored,low flow velocities. There are six main waterways in thesince it was less than 2% of the number in the flask. Thesoutheast ancient part of the city: three of them flow from experiment was carried out in an iluminated incubationnorth to south and the other three from east to west, box at (24土1)°C. A light intensity of 2500 -3000 lx wasperpendicular direction. There was one fixed sampling siteapplied on a 12-hr light-dark cycle. The flasks were shakenin every river, as shown in Fig. 1.rerimental condition,Taibu Lake is a good example of a eutrophic shallowtwo Pa中国煤化工ed.lake system (Bai et al, 2006), and has been studiedIn-3rowth and competi-extensively. The phytoplankton community structure intion of.CNM H Grma undr aeeTaihu I ake is described in following sections.nutrient conditions, representing the nutrient levels of34Wei Zhuet al.Vol. 22Table 1 Concentrations of N and P and N/P ratios used in the experimentsOligotrophice conditionEutrophic conditionHypertrophic conditionNPN (mgL)P (mg/L)N (mg/L)P(mg/L)00.010.250.1520Qiandao Lake (Zhejiang Province, China), Taihu Lake andExcept for summer, when the high temperature favouredSuzhou urban waterways respectively, we used differentthe Cyanophyta populations, the biomass of Chlorophytaconcentrations of nitrogen and phosphorus as listed inwas the highest throughout the year. For example, theTable 1. Water quality of Qiandao Lake is very goodmeasured biomass of Pandorina in the Miaojia River inand the nutrient level is low. It is considered to be aJuly reached 22.28 mg/L, which accounted for 78.9% oftypical oligotrophic water body, with healthy stands ofthe total biomass. The results obtained from this studymacrophytes, dominated by submerged specics (Hiton etshowed that the phytoplankton community was clearlyal.. 2006). Taihu Lake is a typical eutrophic water body,characterised by a mixed form of Chlorophyta and diatomswith concentrations of TN > 0.2 mg/L, TP > 0.02 mg/L(Wan et al, 2006). .in 1991, and mean values of TN = 1.89 mg/L, TP =2.2 Phytoplankton community characteristics of Taihu0.051 mg/L (Huang and Zhu, 1995). The Suzhou urbanLakewaterways are typical hypertrophic waters, and the concen-tration of nitrogen and phosphorus is significantly higherCyanophyta can be seen throughout the year, spreadingthan Taihu Lake. As there is currently no unified standard over the entire lake. A small-scale belt-shaped algal bloomfor hypertrophic levels, we referred to nutrient levels inbecame evident in May. The bloom continued to expandnatural water bodies and defined TP > 0.5 mg/L, TN >and reached its maximum extent in July and August, which6.0 mg/L as the hypertrophic level standard temporarily.lasted until November. In the peak period, the quantityof M. aeruginosa can be as high as 8 X 107 cll/L as2 Resultsreported by Sun and Huang (1993). Over such events, Yang(1996) have stated that M. aeruginosa and Microcystis flos-2.1 Characteristics of phytoplankton population at theaquae were the dominant species; and Anabaena spiroidesfield sites in Suzhou CityKlebahn, Anabaena flos-aquae and Ch. limneticus Lemm.also existed in a relatively large amount.Our measurements showed that the annual concentra-tions of TN in these rivers were in the range of 3.48 -12.972.3 Laboratory experiments on competition character-mg/L and TP in the range of 0.23- -1.01 mg/L. The mainistics of the two algae speciescomponent (65% -93%) of the total nitrogen was ammonia2.3.1 Competitions under oligotrophic conditionsand the N/P ratio varied between 14 and 23. All theseThe algac growth curves at different concentrations offigures signifcantly exceed the thresholds of local waternitrogen and phosphorus are shown in Fig. 2. Under thequality standards, puting the rivers into the fth categoryoligotrophic conditions, as NP ratio increased from 10 to(the worst quality of environmental quality standards for20, the maximum biomass of M. aeruaginosa increased bysurface water, China) of water bodies.Seventy-four species of phytoplankton belonging to sixa factor of 7.4. The growth of M. aeruginosa in the mixedphyla were detected in the waterways of Suzhou Cityculture behaved diferently depending on the N/P ratio. Forduring our monitoring from 2004 to 2005: 13 CyanophytaN/P ratios were 10 and 15, the maximum biomass of M.species, 2 Cryptophyta species, 4 Pyrrophyta species, 17aeruginosa in the mixed culture was found to be greaterBaillariophyta species, 4 Euglenophyta species and 34than that in the pure culture. In particular, when N/P =Chlorophyta species. The number of species and total10, the maximum biomass of M. aeruginosa in the mixedbiomass varied seasonally and such variability was char-culture was nearly five times larger than that in the pureacterised by a decreasing trend from summer to autumn toculture. However, as the ratio increased to N/P = 20, thespring and to winter. In spring, the phytoplankton popula-maximum biomass of M. aeruginosa in the mixed culturetion was dominated by Chlorophyta, while the populationbecame less than that in the pure culture. The maximumof Cyanobacteria (Cyanophyta) increased significantly inbiomass of S. quadricauda was higher in the mixed culturesummer. In autumn and winter, the populations of Chloro-for all N/P ratios, by factors of 1.62.6.The competition results were analysed further throughphyta again became the dominating species, along withthe inhibition rate calculated as follows:the diatoms. The overall population of phytoplankton wasgenerally low from October to February of the next yearR=中国煤化工(1)and reached a relatively high level in July. Seven Chloro-phyta including; Crucigenia, Coelastrum, S. quadricauda,when:MYHC N M H Gaximum biomass inCyclotella, Melosira, Navicula, Chroococcus were com-the mixed culture; Mp is maximum biomass in the puremonly observed. The average phytoplankton populationculture.nd biomass over the whole year were relatively low.As shown in Table 2, based on the calculated inhibitioNo. 1Effecet of nutrient level on phytoplankto community structure in difereat water bodies35- 0- Pure culture (NP = 10)Pure culture (- Pure culture (N/F干Mixed culture(NP-To)-▲ Mixed culture(N-i15)- F Mixe Culture(NP=20)a言80三120tf80 t0400t5678910Culture time (day)Culture tine (day)Fig2 CGrowth curve of M. aeruginosa (田) and s. quadricauda 6) under oligotrophic conditions.Table 2 Inhibition or stimulation rate (%) based on comparison of the growth in mixed and pure culturesM. aeruginosaS. quadricandaConditionNP=10N/P=15N/P=20 .AverageN/P=10N/P=20Oligotrophic+390.12+67.92-64.53+131.13+64.31+72.10+157.83+98.08Eutophice-11.50-45.78-43.47-30.25-8.28-15.79-25.40Hyertophic- 55.11-66.99- 62.60-34.25-26.62-43.65-34.84“+”means stimulation and“”means inhibition.ates,aeruginosa did not inhibit the growth of s.2.3.2 Competitions under eutrophic conditionsquadricauda under oligotrophic conditions. On the con-The algae growth curves under eutrophic conditions aretrary, M. aeruginosa stimulated the growth of s. quadri- shown in Fig. 3. This nutrient level is representative ofcauda. The stimulation efect became more profound as themany water bodies in China. The experiments were runN/P ratio increased. Conversely, S. quadricauda stimulatedfor a longer time to obtain a more complete trend of algalthe growth of M. aernuginosa when the N/P ratio was rela-growth. The results showed that under eutrophic condi-tively small. This stimulation efct, however, weakenedtions, the maximum biomass of M. aeruginosa in the pureas N/P increased and when N/P = 20, s. quadricaudaculture increased mildly with the N/P ratio. The oppositeactually became ihibiting for the growth of M. aerugi- trend was evident in the mixed culure. The maximumnosa. Overall, the stimulation efect of s. quadricauda on value merely increased by about 10% from the minimum.M. aeruginosa was found to be more signifcant than thatWhen N/P = 10, the population of S. quadricauda in theof M. aeruginosa on S. quadricauda under oligotrophicmixed culture was higher than that in the pure cultureconditions.during the early growth period. The population differencebetween the mixed culture and pure culture was reduced,Pure culture (NrP 10) -A- Pure culturte (NP 159)-0 Pure culture (N/P = 20)。首眉280500-b.言240400? 200300 t中国煤化工YHCNMHG2002468012468202242628800246810114168201242628Fg3 Growth curve of M. aenuginosa (a) and s. quadricauda (6) under eutophic conditions.36Wei Zbu et al.VoL. 22. Pure culture (NP= 10) - A Pure culture (NP = 15)、- o Pure culture (NP= 20)Mixed culture (N/P= 10)- Mixed culture (NP= 15)-●Mixed culture (NP- 20)60 8250- b;200j1508(100405258101214“024Culture time (day)Culure tine (day)Fig4 Growth curve of M. aeruginosa (a) and s. quadricauda (6) under hypertrophic conditions.however, after 18 days had elapsed. As the NP increasedmore significant than the efect of M. aernuginosa on S.further, the maximum biomass of S. quadricauda in thequadricauda regardless of the N/P ratio.mixed culture became less than that in the pure culture.The competition between the two species under eu-3 Discussiontrophication conditions was also examined based on theinhibition rate (calculated as before). The results are shown3.1 Characteristics of phytoplankton communitiesin Table 2. When N/P = 10, the inhibition effect ofPrevious research on phytoplankton in different waterM. aeruginosa on S. quadricauda was not obvious. Thebodies has revealed some common features of the waterinhibition efect was enhanced, however, by increasing N/Psystems, including that the phytoplankton community inratios and became the most profound when N/P = 15,after which, as N/P increased further, the inhibition effectlakes is mostly in a mixed form of Cyanobacteria anddiminished. Results showed, therefore, that the inhibitiondiatoms. In reservoirs, the phytoplankton community iseffect of M. aeruginosa and S. quadricauda on each othertypically in a form of diatoms and Chlorophyta underwas weak under these experimental conditions and thatthe condition of large flow exchange and switches tothe Cyanobacteria and diatoms form under low flow ex-these two species may co-exist for a long time.change rates. Most rivers have a mixed form of diatoms2.3.3 Competitions under bypertrophic conditionsand Chlorophyta. According to statistical results obtainedThe algae growth curves under hypertrophic conditionsduring the 1970s, the total quantity of Cyanobacteria andare shown in Fig. 4. Because other algae appeared in thediatoms comprised more than 65% of the annual averageM. aeruginosa incubator on day 14, the experiment wasbiomass in 20 of main lakes in China, including Dongtingstopped. The results show that M. aeruginosa achievedLake, Taihu Lake and Hongze Lake (Nanjing Institutethe maximum biomass within 10 days. After this time,of Geography and Limnology, 1989). In rivers such ashe appearance of mixed algae afected the growth of M. the Yangte River, Heilongiang, Yellow River and Haiheaeruginosa, which also explained why M. aeruginosa didRiver, diatoms accounted for the greatest quantity (Hongnot grow well at high nutrient concentrations. Based onand Chen, 2002). Such results indicated that the hydro-the data from the experimental period (data of the firstdynamic conditions of a water body are a critical factor10 days), the maximum biomass of M. aeruginosa in thefor phytoplankton communities. In general, the biomasspure culture was at the highest level when N/P = 15,of Cyanobacteria tends to be higher in lake systems thanwith the maximum biomass at N/P = 10 slightly largerin river systems. Conversely, however, the quantity ofthan that at N/P = 20. The population of M. aeruginosadiatoms is greater in rivers than lakes because of the spe-was significantly reduced in the mixed culture comparedcial physiological structure of river systems (Jan, 1994).with the pure culture results. In contrast, the growth of S.Some researchers also believe that the survival strategyquadricauda was better than that of M. aeruginosa when of phytoplankton in unstable systems difers from that inthe concentrations of nitrogen and phosphorus were high.stable systems. In particular, phytoplankton in a stableThe maximum biomasses of S. quadricauda for N/P = 10system is an r-strategist and a K-strategist in an unstableand 15 were similar. As the ratio increased to N/P = 20,sys298). An r sralegististhe maximum biomass became slightly larger (by a factor an e)中国煤化工urvival is contingentof 1.11) than those at NP= 10 and 15. The inhibitionuponCNMH Gr are opportunists andresults are shown in Table 2. When the concentrations ofcan.matic increase and anitrogen and phosphorus were high, the inhibition efectjust as sudden and dramatic decrease. The maintenance K-of S. quadricauda on M. aeruginosa was considerablystrategist depends upon a stable environment. In a sense,No.131they are conservative and when the survival environmentchanges, it is dificult for them to restore (Li, 2000).The condition of Suzhou City riverways is complexand characterised by very low flow velocities. Basedon the hydrodynamic conditions, one would assume that20μm10umCyanobacteria would be the predominant species. Themonitoring results obtained from this study, however,showed that Chlorophyta is generally the predominantspecies. Which other factors can signifcantly influencethe form of phytoplankton communities besides hydrody-10pm茧，namic conditions? At present, the nitrogen concentrationin most fresh water bodies around the world is largelybelow 2 mg/L and the phosphorus concentration is belowFig5 Photo of M. aeruginosa croded s. quadricauda.0.25 mg/L. In contrast, the TN concentration in waterwaysof Suzhou is mostly above 7 mg/L and TP above 0.5 inosa was found to parasitize in s. quadricauda undermg/L. The phytoplankton population is also dominated by oligotrophic conditions, such that s. quadricauda's biggerChlorophyta and diatoms. The result that the phytoplank- shell became M. aeruginosa's habitat. This phenomenonton community and outrient concentration are related.was not found under eutrophic or hypertrophic conditions.3.2 Infuence of nitrogen, phosphorus and N/P on theUnder oligotrophic conditions, algae might reduce demandfor nutrients through autoeciousness. Previous researchgrowth of M. aeruginosa and S. quadricaudahas shown similar symbiosis relationship: for example,Previous work has indicated that when Cyanobacteria microcystin could accelerate the growth of S. quadricauda,bloom outbreaks occurred, N/P in water was not fixed depending on the concentration of microcystin, category ofbut changed greatly (Chen, 2006). That is to say, the algac and density of specics (Hu et al, 2006).suitable N/P for growth of Cyanobacteria is not stable,Under oligotrophic and eutrophic conditions, the max-which conficted with the empirical formulaof algae. From imum biomass of M. aernginosa was close to that ofthe experimental results obtained in this study, we foundS. quadricauda and M. aeruginosa had some advantagethat under low nutrient conditions, the biomass of algae in the mixed culture. Under hypertrophic conditions, theincreased with increasing N/P. This tendency was more maximum biomass of S. quadricauda increased, whichpronounced for M. aeruginosa than for S. quadricauda. explains why s. quadricauda became the dominant speciesUnder eutrophic conditions, the maximum biomass of M. when concentrations of nitrogen and phosphorus wereaeruginosa was greater for smaller N/P. The maximumhigh.biomass of S. quadricauda was larger for a higher N/PThe competitive relationship between M. aeruginosain the pure culture. Under hypertrophic conditions, the and s. quadricauda can be explained by the resource-ratiomaximum biomasses of s. quadricauda and M. aerusinosa bypothesis. According to the resource ratio hypotbesisdid not show signifcant changewith the N/P ratio. The(Tilman, 1982), the resource regimes corresponding toresuts suggest that when the concentrations of nitrogen the decline, growth and steady state of Cyanobacteriaand phosphorus were very low, the NP ratio strongly and Chlorophyta under conditions of limited nitrogen andinfluenced the growth and reproduction of algae, anyphosphorus are ilustrated in Fig. 6. Generally speaking,nutrient element can stimulate the growth of algae. Oncethe Cyanobacteria is ftted to lower N/P and hence itsthe concentrations of nitrogen and phosphorus increasedto esource regime is leaning to left-up (Fig. 6a); the Chloro-a certain level, and thus nutients were very abundant, the phyte is ftted to higher N/P and thus its resource regimeinfuence of N/P on the growth and reproduction of algaeis leaning to right-down (Fig. 6b). In theory, when com-diminished.petition exists between two species, their resource regimes3.3 Symbiosis and competition between M. aeruginosamay be superimposed (Fig. 7a). However, the results fromthis study showed that in the presence of competition, theand s. quadricaudaresource regimes of the Cyanobacteria and ChlorophyteThe experimental resuts showed that under eutrophic changed. The demands for nitrogen and phosphorus byand hypertrophic conditions, the maximum biomasses ofthe Cyanobacteria and Chlorophyte varied, which resultedM. aeruginosa and S. quadricauda were lower in thein modifications of the resource regimes (Fig. 7b). Asmixed culture than in the pure culture. This indicates thatdemonstrated in Fig. 7b, the dashed line (1) denotes themutual competitin and inhibition might exist between resource regimes of the Cyanobacteria and dashed lineM. aeruginosa and S. quadricauda. Under low nutrient(2) denotes the resource regimes of the Chlorophyte inconditions, the maximum biomasses of M. aeruginosa andA nf in 7h the Chlorophyte has noS. quadricauda were higher in the mixed culture than chance中国煤化工eria, and vice versain the pure culture, suggesting that a symbiosis relation-in are:hlorophyte and theship existed between M. aeruginosa and S. quadricauda.CyanoTHCNMHCnded period of time.Such a symbiosis relationship was also implicated by the This explanation is only qualitative not quantitative.microscopic photo (Fig. 5). Physically smaller M. aerug-The experiments conducted in this study used the38Wei Zhu et al.Vol. 22CyanobacteriaChlorophytaNitogen concenrationNtrogen concentrationFg6 Resource regimne of Cyanobacteria (间) and Chlorophyta (b).|c.Line(1). Line (2)→→Nitrogen concentrationFig 7 Superimposed (a) and moved (b) resource regimes of Cyanobacteria and Chloroplhyta.perfusion cultivation method; and the effects of nutrientM. aeruginosa, after the initial improvement of watersupply frequency and pulse were not considered. In addi-quality when nitrogen and phosphorus concentrations aretion, given the difference between the experimental set-upreduced. For example, improvements in the hydrodynamicin the laboratory and field conditions, the results obtainedconditions and increases in the fuidity of a water body canhere may not be directly transferable to the field. Forcontrol the growth of Cyanobacteria.example, in the natural water body, M. aeruginosa canMost previous research on Cyanobacteria blooms havecapture light using vacuoles, which created an advantagefocussed on the relationship between the growth of M.to becoming the dominant species. Nevertheless, theseaeruginosa and N, P and/or the N/P ratio. The results ofexperiments simulated similar light conditions and aimedsuch single species studies have had limited implications.to examine the infuence of nutrients on the competitiveOur investigation has sbown that the optimal growthrelationship between two diferent algac species that dom-and reproduction of M. aeruginosa and S. quadricaudainated the waterways in Suzhou City.separately in pure culture occurs under medium level nu-3.4 Implications of the experimental results for solvingtrient conditions. However, under competitive conditions,lower medium nutrient levels are more suitable for M.the eutrophication problemaeruginosa while higher medium nutrient levels promoteThe community structure of phytoplankton is differentthe growth of S. quadricauda. In fact, the suitable growtha different eutrophic water bodies. Qin (1998, 2002)regime of N, P and/or N/P for algae is likely to change inpointed out that prior to eutrophication, the main speciesa complex fashion along with the competition relationshipfound in most water bodies are typically Pyrrophyta andamong diferent algae. In reality, the community structureChlorophyta. After the occurrence of eutrophication, the and dominant species of phytoplankton result from mutualpopulation of Cyanobacteria can increase rapidly. How-competition among various algae.ever, if the water pollution worsens with further elevatedconcentrations of nitrogen and phosphorus, the previously4 Conclusionscompetitive Cyanobacteria may gradually lose dominance.The experimental results presented in current study helpThis article introduced community structure of phy-如o determine the mechanisms of competitive processestoplankton in the waterways of Suzhou City and didin the formation of algae community structures. If onlylaboratory experiments on competition characteristics ofM. aeruginosa and S. quadricauda are considered, a M.M. aeruginosa and S. quadricauda . The main conclusionsaeruginosa bloom is more likely to occur under poorfrom this study are:nutrients conditions, while nutrient rich conditions favour中国煤化idyamic cnditionsa S. quadricauda bloom. This may explain why Cyanobac-the-