Size-fraction Structure, Species Component And Pigments Analyses Of Phytoplankton In The Bloom Zone Near Changjiang Estuary

Large-scale Dinoflagellate blooms occur in sea area adjacent to the ChangjiangRiver estuary every spring since2000. The frequent outbreak of dinoflagellate bloomsindicated potential changes of the phytoplankton communities, which might affect themarine structure and function of marine ecosystem and lead to ecological securityissues. In the current study, four seasonal cruises were carried out in2009in theChangjiang River estuary and its adjacent area (30°30′-32°N,122°20′-123°20′E) tostudy the composition and size-structure of the phytoplankton communities in the fourseasons, using methods of size-fractioned Chl a, species identification andenumeration, and pigments analyses. The long-term changes of the phytoplanktoncommunities and the potential driving forces are also discussed.Based on the result of size-fractioned Chl a, nanophytoplankton (2-20μm) wasthe most dominant in the survey area, with the average percentage of52%,55%,34%and57%in total Chl a in winter, spring, summer and autumn, respectively.Nanophytoplankton was mainly distributed in the plume zone, and had high value instratified upper water in spring. The percentages of pico-sized Chl a in total Chl a inthe four seasons were21%,34%,38%and16%. Picophytoplankton was moredominant with increasing salinity offshore, and took the advantage in the surfacewater in summer. The contribution of microphytoplankton (>20μm) to total Chl awas26%,11%,28%and27%in the four cruises. Most microphytoplankton appearednear the turbidity-maximum-zone, and decreased quickly in the sea area east to122.5°E. The total Chl a was much higher in spring and summer than those in autumnand winter.The abundance of phytoplankton was also high in spring and summer, while low in winter and autumn. Diatom and dinoflagellate were the most dominant classes inthe survey area. The diatom abundance took absolute advantage in winter, summerand autumn, while dinoflagellate dominated in spring. In spring, the algae bloomscaused by Prorocentrum donghaiense and Skeletonema spp. were the most prominentecologic phenomena. The bloom of Skeletonema spp. located in the plume area nearthe river mouth, with low salinity and high nutrients, while the blooms ofProrocentrum donghaiense distributed mainly in the plume and stratified waters, withrelatively higher salinity and lower turbidity. High abundance cells of Kareniamikimotoi were distributed mainly in the eastern part of the survey area. Results of thecluster analysis showed that the distribution pattern of algal blooms were influencedby the hydrological factors such as salinity and turbidity, and physical oceanographicprocesses, such as the formation of front and stratification, upwelling and so on.Contribution of different phytoplankton classes to the total Chl a was achieved bythe CHAMTAX analysis according to the analytical results of phytoplanktonpigments with a HPLC method. The results suggested that diatom, dinoflagellate,cryptophyta and chlorophyta were the major phytoplankton classes in the survey area,with little proportion of cyanophyta, haptophyta and prasinophyta. Diatoms contributethe most to Chl a in winter, summer and autumn, with the average percentage of51%,53%and43%, respectively. Dinoflagellates were dominant in spring with theproportion of41%in the total Chl a. Cryptophyta took an important proportion inwinter, spring and autumn, and chlorophyta in autumn and winter as well; both ofthem accounted for about20%in the total Chl a. Diatom, dinoflagellate, cryptophyta,prasinophyta, chlorophyta and haptophyta always distributed in the upper layer of thestratified water in the flume, while cyanophyta distributed mainly outside the plume.During the dinoflagellate blooms in spring, the distribution of cryptophyta,prasinophyta, and haptophyta had similar pattern with the blooms of P. donghaiense,while the distribution of cyanophyta is consistent with Karenia mikimotoi. It'ssuggested that dynamics of the dinoflagellate blooms could be closely linked to otherclasses of phytoplankton through the nutritional relationship. In conclusion,the phytoplankton biomass in the sea area adjacent to theChangjiang River estuary was high in spring and summer, and low in autumn andwinter. Diatom, dinoflagellate, cryptophyta and chlorophyta were the majorphytoplankton classes in the the survey area. The blooms of Prorocentrumdonghaiense and the high dominance of dinoflagellate were the most prominentecologic phenomena in this region. The blooms of diatom and dinoflagellatedistributed in differently patterns, which were affected by the hydrologic factors, thephysical oceanographic processes, and the distribution pattern of other classes ofphytoplankton as well. Picophytoplankton contributed significantly to the total Chl ain the southeast region of the survey area in spring and summer. It's suggested thatdinoflagellate, chlorophyta, haptophyta are the major component of thePicophytoplankton based on the pigment analytical results. The biologicalproduction of picophytoplankton in this eutrophic zone should be paid more attentionin the future studies.