In-situ Experimental Studies On Nutrient Limitation And Microzooplankton Grazing Impact On Phytoplankton In Typical Areas Of China Seas

In order to investigate nutrient limitation and grazing pressure on phytoplankton in natural marine ecosystems, in situ nutrient enrichment experiments with nitrate, animonium, phosphate, iron and silicate and dilution experiments were conducted in the Jiaozhou Bay, the East China Sea and the South China Sea during 1997 to 2000. Following are the main results. 1 Coastal Water ?Jiaozhou Bay Jiaozhou Bay, surrounded by the Qingdao city and with area of 390km2 and mean depth of 7m, is an eutrophic coastal water. The concentrations of nitrate, ammoniuin and phosphate were 1.98, 8.01 and 1.38jiM for the 1990s. Those of Si were 2.511iM for 1991-1994 and 1.54i.tM for 1997-1998. DIN:P molar ratio was almost always over 16 (the Redfield ratio, also called 揼eochemically balanced?ratio) in all seasons. There are 25 nutrient enrichment experiments which were conducted to investigate nutrient limitation on phytoplankton and 12 dilution experiments which were performed to investigate microzooplankton grazing on phytoplankton from autumn, 1997 to summer,2000. Nanophytoplankton (nano-, 2-20p.m) was the No.1 biomass contributor in terms of chlorophyll-a biomass, followed by netphytoplankton(net-, 20-200p.m) on year-round scale in the field. This was also true in the incubation bottles throughout the incubation time course. According to the experimental results, there were almost no nutrient limitation in phytoplankton growth in autumn, however, in winter and spring, there were very high possibilities for silicate to be a limiting factor for diatoms, while in summer limitation by all kinds of nutrients including nitrate, ammonium, phosphate and silicate but iron could take place. On large time and spatial scale( year-round and bay-wide), Si was the primary limiting nutrient, and P or N could also be limiting factors occasionally. Size-fractionated bioassess experiments showed that net- and nano- experienced serious Si-limitation in winter and spring, and N 條imitation, at less extent, in summer. While picophytoplankton (pico-, <2pm) growth was basically P-limitation regulated. Bottom-up (nutrient limitation ) was more efficient in controlling net-population dynamics than top-down effect (grazing impact). On the contrast, pico- population was regulated by top-down effect, The whole phytoplankton community(<200j.im) standing stock was mainly controlled by top-down effect in year-round level. 2. Continental Shelf Area ?the East China Sea The East China Sea is famous for its large continental shelf and great inland water input from the Yangtze River as well as front areas among the Kuroshio Currents in the east, Yellow Sea Could water mass in the north, Taiwan Current in the south and the coastal water currents in the west. In-situ enrichment experiment and dilution experiments were conducted at Sta.402 and 418 in sumnier,1998 and Sta.E064 and Sta.Y136 in spring, 1999. Except for St.402 near the Yangtze River estuary, all the other experimental sites,St.418~ St.Y13...