Study On Phytoplankton Biomass In Yangtze River Estuary Of Eutrophication

Ten multi-discipline cruises were undertaken from August 2004 to November 2006 in Yangtze River Estuary and its adjacent waters, where 39 stations at eight transect were visited. Samples for phytoplankton, salinity, temperature and nutrients were collected at standard water layers, and water samples were filtered for size-fractionated chlorophyll a on board. According to the data of chlorophyll a and environmental factors, the spatial and temporal distribution of phytoplankton biomass and the size composition of phytoplankton are reported, and the effects of environmental factors and physical processes on phytoplankton biomass are preliminarily studied in this thesis as well.Among the average phytoplankton biomass in four seasons, the maximum value was observed in spring, while the minimum value in winter, when the low temperature, weak solar radiation and low silicate restrained the growth of phytoplankton. Generally, chlorophyll a in summer was a little higher than that in autumn. The variation of growth rate of phytoplankton and grazing rate of micro-zooplankton on phytoplankton was in the similar pattern to the phytoplankton biomass.Phytoplankton biomass at surface was much lower inside than outside of the river mouth, where the distribution of chlorophyll a had different patterns in four seasons. In summer, two regions of highly concentrated chlorophyll a were located in the northeast and southeast of the river mouth respectively. In autumn, mean value and distribution of chlorophyll a were rather variable among three autumnal cruises. In winter, the high chlorophyll a region was observed close to the river mouth, and chlorophyll a was much lower in the east of 122.25? E. In spring, chlorophyll a was extremely high in the east of 122.5? E, where algal blooms were observed during two spring cruises. To some extent, surface phytoplankton biomass correlated with the salinity. High chlorophyll a concentration was most appeared in water with salinity 10– 20 in summer and winter, while 25– 30 in autumn and spring, and chlorophyll a concentration was low in water with salinity >30, where the growth of phytoplankton was restrained by the low concentrated nitrate and silicate. In dilution zone, nitrate and silicate were seemingly not the key controlling factors of phytoplankton growth; however, considerable amount phosphate was consumed in spring. The area with algal bloom was characteristic of low salinity, high temperature and comparatively low nutrients resulted from the depletion by phytoplankton.Phytoplankton biomass was much high at shallow water layer, especially in spring and summer, whereas, high value could appear near the bottom in autumn and winter as well. In the northeastern area, where the vertical exchange of water was sufficient, chlorophyll a concentration kept constant with the depth. In the dilution zone, the peak chlorophyll a concentration appeared at surface. In winter the peak concentration was observed below the surface, even at bottom, in both two regions. It is obviously that the solar radiation affected the vertical profile of chlorophyll a and the salinity cline enhanced the stratification of chlorophyll a.The vertical profile of chlorophyll a in two diluted transects showed that high value appeared above 20m and the region with high value was generally located in the middle of survey area, though the distance between the river mouth and it was variable seasonally. The dilution water strongly affected the chlorophyll a profile, and Yellow Sea coastal current affected the chlorophyll a profile of transectⅠremarkably in autumn and winter, and Taiwan warm current induced the water stratification which resulted in the aggregation of phytoplankton at surface water.Pico- and nano- phytoplankton were the main component of surface phytoplankton community, and contributed 55.8%,83.3%,68.0% and 84.6% of total phytoplankton biomass respectively in summer, autumn, winter and spring. In summer, the distributions of two size fractions were similar to that of total chlorophyll a, and the region with high chlorophyll a <20μm was much close to the river mouth. In autumn, chlorophyll a <20μm was highly concentrated in the southeast, while chlorophyll a >20μm was highly concentrated in the northeast. In winter, both the horizontal and the vertical distribution of chlorophyll a <20μm were similar to that of chlorophyll a >20μm. In spring, two size-fractions were generally highly concentrated in 122.5– 123.0? E, and the difference between two size fractions decreased with the depth.Generally, the phytoplankton biomass of spring and winter fluctuated faintly during the survey period of 28 months, but phytoplankton biomass of summer and autumn are much different among three cruises. It is observed that the phytoplankton biomass in summer, autumn and winter of 2005 was much lower than that in 2004 and 2006. Comparing with historical data, the phytoplankton biomass was at similar level in summer, while at much higher level in winter and spring. The horizontal distribution and seasonal patter of surface chlorophyll a was parallel with that reported in other papers, while the region with high chlorophyll a concentration was much more close to the river mouth.