Picoplankton Composition And Mechanisms Of Formation Of Subsurface Chlorophyll Maxima In The Northern South China Sea

Picoplankton composition and mechanisms of formation of subsurface chlorophyll maxima in the northern South China SeaAbstractIn July. 2000. samples were collected along the transect from Pearl River estuary to the southwest waters of Dongsha Island, one station in the upwelling regions of Southern Taiwan Bank sampled. Size-fractionated chlorophyll a biomass and primary production were determined. In addition, four categories of picoplankton were discriminated: Synechococcus spp. (SYN), Prochlorococcus spp. (PRO), eukaryotic picophytoplankton (PEUK) and heterotrophic bacteria (BAC). The number of each category in the samples was counted by flow cytometry. The carbon biomass concentration of each category in each sample was calculated. In the last chapter of this thesis, we discussed the mechanisms of formation of SCM. Main results are as follows:The horizontal distribution of chlorophyll a biomass showed the variable trends from the estuary to the coastal waters and the upwelling station, reaching the lowest values of chlorophyll a concentration offshore extended from SCS6 to SCS3A and with values of 0.086 ± 0.031 mg.m3 and 1.565 ~ 0.347 mg.m3.&' of average chlorophyll a concentration and photosynthetic rates in the surface waters, respectively. The column primary production varied from 306.29 mgC.m2.&' to 130.9 mgC.m*d', impressed with the distinct effect of thickness of euphotic layers. The vertical profiles of biomass and primary productivity displayed the maximum in the subsurface layers. PICO dominated the column chlorophyll a biomass and primary production at offshore stations, with the average contribution of 75.0±7.1% and 52.2±14.6%, respectively. During the 2-day time series at SCS2. the variation of 24h period was observed.The distribution of picoplankton populations were determined by flow cytometric analysis. The autotrophic categories were dominated by SYN and PEUK in the Pearl River estuary. with higher abundant than in the offshore waters. PRO was more abundant than SYN and PEUK offshore but was lower than BAC. The distribution of BAC indicated its close relationship with the photosynthetic populations and was controlled by the level of primary productivity. Four categories, especially PRO and PEUK were dominant in the subsurface layers, usually associated with the SCM, which may be greatly due to their ability to the low light intensity andnutrient dynamics. According to the composition of carbon biomass, three distinct cases of our surveyed waters were roughly classified. The first case, observed in the estuary, was dominated by SYN and PEUK; The second case was characterized by the oligotrophic status, with the highest contribution of PRO in the photosynthetic population; the last case may relate to the cold eddy and was co-dominated by PRO and PEUK. The diel variation indicated that PRO and PEUK reached the peak of abundance in the late night, which may be resulted from the mutual effects of the tidal currents, cell division and grazing.As for the characteristics of SCM in the northern South China Sea, results show that depth and thickness of SCM ranged from 24 m to 90 m and from 7 m to 38 m, respectively. In addition, SCM was located in the bottom of euphotic zone, where average of the percent irradiance is about 3.3% of the surface one and the average temperature is about 23.82 0C. In SCM layer, contribution of primary production to the column values can reach between 35.45 and 63.53%. Contribution of PICO, NANO and MICRO fractions can reach 64.67%, 43.96% and 30.89%, respectively.The mechanisms of formation of SCM in the northern South China Sea can be concluded as follows: Formation of SCM was greatly related with the presence of thermocline and nitracline. But no remarkable relationship was found between the concentration of nitrate and that of Chla in SCM layer, which suggested that flux of nitrate may be associated with the intensity of SCM. The biological elements resulting in formation of SCM mainly include the specific composition of phytoplankton (dom...