| Four cruises were carried out to study primary productivity in Beibu Gulf in July (summer),December(winter) 2006,April(spring) and October(autumn) 2007.By investigation,to understand the temporal and spatial distribution patterns of chlorophyll a concentration and primary productivity,and the size-fractioned structure of phytoplankton and primary productivity.In the meantime,we compartmentalized ecorgions of Beibu Gulf via clustering analysis.The main results were as follows:1.The chlorophyll a(Chl a)concentration of Beibu Gulf ranged from 0.16 mg·m-3 to 6.72mg·m-3,1.42mg·m-3 in average,with a seasonal variation pattern as winter＞autumn＞summer＞spring.It showed that the Chl a high-value areas located to the west of Leizhou Peninsula in the north Gulf,and the Chl a low-value areas located in offshore deep waters of the south Gulf.The abundance of Chl a decreased gradually from east to west,from inshore to offshore,and from north to south.The vertical distribution of Chl a concentration in inshore shallow waters was comparatively uniform,while obvious different in all sampling layers of deep waters. As a whole,the notable characteristic of phytoplankton in size-fractioned structure was that nanophytoplankton occupied comparatively significant portion in Beibu Gulf. Nanophytoplankton has the largest contribution to the gross chlorophyll a in average, and microphytoplankton was the secondary contributor(except spring),while picophytoplankton was the least important(except in spring).The contribution of picophytoplankton in offshore deep waters was obviously more than that in inshore shallow waters.2.In Beibu Gulf,the range of primary productivity was 17.14mgC/(m2·d)～2034.82mgC/(m2·d),276.76mgC/(m2·d) in average.The average value of primary productivity in spring,summer,autumn and winter was 178.64±167.59mgC/(m2·d), 186.00±125.85mgC/(m2·d)),669.36±536.16mgC/(m2·d),73.04±45.32mgC/(m2·d), and respectively.The seasonal fluctuation pattern of primary productivity was autumn＞summer＞spring＞winter.Significant difference of the primary productivity between surface layer and the bottom of euphotic zone was found,and primary productivity sharply decreased from surface layer to bottom of the euphotic zoon.In most station,primary productivity in the bottom of euphotic zone was less than 1mgC/(m3·d).As a whole,the notable characteristic of primary productivity in size-fractioned structure was that nanoplankton occupied comparatively significant advantage in Beibu Gulf.Nanoplankton has the largest contribution to gross primary productivity,and picoplankton was the secondary contributor,while microplankton the least.The contribution of microplankton for primary productivity in the north Gulf was more than that in the other waters.Nanoplankton and picoplankton contribute more to the gross primary productivity in offshore deep waters than in inshore shallow waters3.The Beibu Gulf can be divided into three ecoregions:Region-â… is the ecoregion in inshore shallow waters of the north Gulf.In average,the water depth is 18m,DIN is 1.88μmol/L,DIP is 0.20μmol/L,N:P is 9.4:1,dissolved silicate is 5.17μmol/L,the Chl a conentration in surface layer is 2.27mg·m-3,the assimilation index in surface layer is 3.80mg/(mg·h),the primary productivity is 198.78mgC/(m2·d), and potential fishery production is estimated to be 0.24gC/(m2·a) according to the primary productivity.The ecoregion is mainly affected by the northern coastal water systems,and may be fit for aquaculture;Region-â…¡is the ecoregion in offshore deep waters of the north Gulf and the coastal shallow waters to the west Hainan Island.In average,the water depth is 35m,DIN is 2.01μmol/L,DIP is 0.18μmol/L,N:P is 11.2:1,disovled silicate is 4.23μmol/L,the chlorophyll a of surface layer concentration is 1.45mg·m-3,the assimilation index of surface layer is 4.12 mg/(mg·h),the primary productivity is 276.60mgC/(m2·d),and the estimated potential fishery production is 0.34gC/(m2·a) according to the primary productivity. The ecoregion was mainly influenced by ocean current from the South China Sea, rivers in the west coast of Hainan Island and the water from Qiongzhou Strait.It may be fit for aquaculture and fishery;Region-â…¢is the ecoregion in offshore deep waters of the mid and south Gulf.In average,the water depth is 75m,DIN is 0.77μmol/L, DIP is 0.15μmol/L,N:P is 5.1:1,disovled silicate is 3.05μmol/L,the chlorophyll a of surface layer concentration is 0.70mg·m-3,the assimilation index of surface layer averaged is 3.69mg/(mg·h),the primary productivity is 350.89mgC/(m2·d),and the estimated potential fishery production is 0.43gC/(m2·a) according to the primary productivity.The ecoregion was mainly affected by the circulation inside Beibu Gulf, and may be fit for fishery.
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