Some Ecological Characteristics Of Phytoplankton In Northern Beibu Gulf

As the main component of primary producers in marine ecosystems, phytoplankton is the starting point of the entire marine food chain and a vital link in the biogeochemical cycle. Phytoplankton community structure and dynamics directly reflect the stability of the ecosystem, which has a profound impact on the marine environment and global climate.Two cruises were carried out in northern Beibu Gulf (20.06°N～21.46°N,108.25°E～109.88°E) in April (spring) and August (summer)2011. In this study, the phytoplankton community was investigated, which were mainly composed of species composition, abundance, biodiversity, community classification, spatial niche and the horizontal distribution of Chlorophyll a and primary productivity. The relationship between phytoplankton community and environmental factors was also explored. The main results are as follows:(1) Species composition of phytoplanktonA total of313species (with varietas and metatype) were identified, including198species were identified in spring and273species were identified in summer. They belong to84genera, the5phyla, i.e. Bacillariophyta, Pyrrophyta, Cyanophyta, Chrysophyta and Euglenophyta. The first dominant specie in spring was Thalassiosira subtilis and the absolute dominant specie in summer was Thalassiosira sp.. The species composition changed with obvious seasonal succession.(2) Quantitative distribution of phytoplanktonThe average abundance of981.86×104cells/m3and23,952.48×104cells/m3in spring and summer, respectively. Horizontal patterns of phytoplankton cell abundance were controlled by that of diatoms, which has the trend of decreasing from inshore to offshore.(3) Phytoplankton DiversityThe Shannon-wiener diversity index, Pielou evenness index, and Margalef richness index were respectively2.27,0.41,2.17in spring and2.38,0.37,3.40in summer. Considered with K-dominant curve, the overall performance of spring phytoplankton community diversity was greater than summer. The distribution of diversity showed obvious regional difference of change, which may be related with seasonal variation of longshore currents.(4) Phytoplankton community divisionPhytoplankton communities in northern Beibu Gulf in spring and summer were examined with cluster analysis, NMDS analysis and CCA analysis, based on data of species composition and environmental factors. In spring, two communities were distinguished:the northern community of Weizhou Island (Community SPI) and the southern community of Weizhou Island (Community SPII). And in summer, three communities were distinguished:the northern community of Weizhou Island (Community SUI), the southwestern far-shore community of Weizhou Island (Community SUII) and the western off-shore community of Leizhou Peninsula (Community SUIII). The seasonal fluctuation between coastal fresh water, South China Sea waters and the western coastal current of Guangdong Province was the main factor for the community division.(5) Phytoplankton spatial nicheThe niche breadth, niche overlap, spatial differentiation and environmental effects on the abundance and spatial distribution of phytoplankton were studied with qualitative and quantitative methods. The niche breadths and their overlaps of phytoplankton were measured using the formulas proposed by Levins Niche Breadth index and Pianka Niche Overlap index, and the spatial niche differentiation was studied by using CCA analysis. The niche breadth and niche overlap of the dominant species differed in different periods, and different dominant species had different adaptive capacity to the environmental factors, which were related to the total amount of environmental resources. The results showed that the zooplankton dominant species in northern Beibu Gulf were classified in to three groups, i.e., wide niche breadth groups, middle niche breadth groups, narrow niche breadth groups based on the niche breadth value. The species had a wider niche breadth had a greater opportunity of niche overlap with other species. The high niche overlaps were often occurred in the specie-pairs which had similar or opposite ecological adaptation. Nutrition and depth had more important influence on phytoplankton spatial distribution than temperature and salinity.(6) Phytoplankton chlorophyll a and primary productivityThe chlorophyll a of surface layer, total chlorophyll a and primary productivity in water column were respectively0.84mg/m3,16.10mg/m2,365.44mg·C/(m2·d) in spring and0.42mg/m3,10.14mg/m2,422.67mg·C/(m2·d) in summer. Although chlorophyll a and primary productivity had opposite seasonal variation, there was no link between their distribution trends in each season. The biomass of phytoplankton was mainly contributed by nano phytoplankton in spring and summer, which reached to48.63%and59.28%. The increasing of the total chlorophyll a concentration is mainly controlled by nano phytoplankton, followed by micro phytoplankton, pico phytoplankton at last. The growth of phytoplankton was restricted by many environmental factors and phosphorus was the main factor in spring and summer.