| Growth of phytoplankton in aquatic environments is frequently limited by theavailability of nutrients. A number of methods are available for determining nutrientlimitation in phytoplankton. However, many of them are complicated, timeconsuming and vary with species. There are problems extrapolating from laboratorystudies to an understanding of nutrient limitation on natural populations.At present, two approaches, i.e. criteria of nutrient concentrations and ratios, andnutrient enrichment bioassays using chlorophyll response as indicators, are commonlyused to assess nutrient limitation in the field. However, it is becoming increasinglyclear that the criteria of nutrient concentrations and ratios vary greatly between taxa,which introduce substantial difficulties when applying them to specific aquaticecosystems. In addition, bioassays may indicate the potential for nutrient limitation,but not necessarily show in situ nutrient status. Also, bioassays are time consumingand often require multiple days.Chlorophyll a fluorescence allows for rapid, sensitive, and non-invasiveassessment of the physiological state of phytoplankton. The fluorescence parametermost commonly used is the maximum photochemical efficiency of photosystem Ⅱ(F_v/F_m). Changes in F_v/F_mduring nutrient limitation have been observed in numerouslaboratory cultures, but the processes involved in recovery from nutrient limitationhave been less well studied, especially the fine-scale kinetics of repair of F_v/F_mafternutrient resupply. In addition, their usefulness in the field has yet to be determined. Anumber of factors, such as light, temperature and community structure may have influences on F_v/F_m.Here, the effects of nitrogen (N) and phosphorus (P) starvation and resupply onF_v/F_mwere examined in seven species of marine unicellular algae. F_v/F_mcoupled withnutrient enrichment experiment was used to determine nutrient limitation in thecoastal sea of Qingdao. In addition, spatial and temporal variations of F_v/F_mduringthe process of spring bloom in the adjacent area of Changjiang river estuary wereobserved. The main results are as follows:1、 In laboratory cultures, F_v/F_mdeclined markedly under nitrate and phosphatestarvation in all species examined and recoverd completely within18h-72h ofresupply of the limiting nutrient. After24h, resupply of the limiting nutrientsignificantly increased F_v/F_mof all species above the control treatment (P <0.05),while addition of non-limiting nutrient did not change F_v/F_mrelative to the controltreatment (P>0.05). Species differences were observed in the maximal values ofF_v/F_m.2、 F_v/F_mcoupled with nutrient enrichment experiments can be useful for rapidlydetecting N and P limitation of natural phytoplankton populations before majorchanges in community structure, and can provide information about the physiologicalstate of the community.3、 F_v/F_mand the two above mentioned methods, i.e. nutrient criteria and bioassaysusing chlorophyll response as indicators, were used to identify limiting nutrients inthe Qingdao Olympic sailing venue. In July of2009, low DIP concentrations andF_v/F_min the initial samples coupled with F_v/F_mand Chlorophyll a (Chl a) responses toP addition indicated that P was the limiting element. In August, P was still thelimiting nutrient. In the southwestern part of the Olympic sailing venue, only the N+Ptreatment caused marked increase of F_v/F_mover the control treatment and the Chl aconcentration was highest in the N+P treatment, suggesting that N and P wereco-limiting. In September, a lack of F_v/F_mand Chl a response following nutrientenrichments together with the relatively high nutrient concentrations andphotosynthetic efficiency observed at the start of experiment suggested that N and Pwere replete and phytoplankton community was healthy in the southwestern part of the Olympic sailing venue. In the northeastern part, the initial F_v/F_mandphytoplankton community structure were similar to those in the southwestern part,and F_v/F_mdid not increase after N and P additions, indicating that phytoplanktoncommunity was in a healthy state on the day of collection. F_v/F_mand Chl a in thecontrol and N treatments were lower than those in the P and N+P treatments duringthe incubation, coupled with the high N/P ratio at the start of experiment, suggestingthat P was the potential limiting nutrient, and would be exhausted first as growthproceeded. In July of2010, neither N nor P was deficient in the southwestern part ofthe Olympic sailing venue, which was different from the result in the same period of2009.4、The Characteristics of F_v/F_mduring the process of spring bloom in surface watersof the adjacent area of Changjiang river estuary(1) F_v/F_mwas between0.19and0.74from7to24May2010, and ranged from0.17to0.73in the frequent blooms occurrence area, south of the Changjiang river estuaryfrom29March to27May2011. Bloom occurrence areas were mainly located onthose areas where F_v/F_mis above0.6.(2) The primary factors affecting F_v/F_mwere temperature, turbidity and nutrients.Values of F_v/F_mgreater than0.60, were associated with healthy state of phytoplanktoncommunities. In addition, F_v/F_mchanged followed a day/night cycle with maximumvalues at night and a strong reduction at noon. Samples were dark adapted for1h atambient temperatures before measurement to avoid diel variations.(3) The Characteristics of F_v/F_mduring the process of diatom bloom were similar tothose within the dinoflagellate bloom. F_v/F_mincreased to more than0.60before bloom.Then Chl a concentration rocketed and the bloom broke out. F_v/F_mwas above0.6inthe retention stage of bloom and decreased to below0.6in the dissipation phase. ThusF_v/F_mcan help detecting and characterizing bloom events.(4) Dinoflagellate population can live in water mass characterized by low PO4-Pconcentration and high DIN/PO4-P ratio, but diatom population growth needs highPO4-P concentration and low DIN/PO4-P ratio. Maybe it is one of the reasons whydinoflagellate could grow better and cause bloom under lower PO4-P concentration after diatom bloom. |
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