Responses Of Phytoplankton To Phosphorus And Nutrient Dynamics In The Typical Marginal Seas

Nutrients are essential elements for phytoplankton growth, and could affectmarine primary production. In the recent years, phosphorus limitation happenedfrequently and led to various ecosystem problems in the Yellow Sea (YS) and EastChina Sea (ECS). In this thesis, the biogeochemical processes of nutrients (especiallyphosphorus) in the YS are discussed. Regulation on the blooms by nutrients duringdifferent bloom stages is also discussed from the view of supply and consumption ofnutrients, to understand the mechanism of the spring phytoplankton bloom in the YS.Then, the distributions and influencing factors of phytoplankton cell associatedphosphorus (including intracellular phosphorus and cell surface adsorbed phosphorus)are addressed, in order to preliminarily explain the utilization of phosphorus byphytoplankton in China marginal seas.The spring phytoplankton bloom is an important phenomenon in the YS. Boththe concentrations and distributions of nutrients in the YS varied significantly duringspring bloom. The nutrient concentrations had higher values in offshore area than innear shore area in winter and early spring. Abundant nutrients and fitting nutrientsratios in the central YS provided favorable conditions for phytoplankton bloom’soutbreak in April. While in early summer, dissolved inorganic nutrients were almostexhausted in the surface water in the central YS. Depletion of both nitrogen andphosphorus led to the termination of bloom in early summer. In the southern part ofthe Yellow Sea affected by the Changjiang Diluted Water, phosphorus would limit thephytoplankton growth. The Yellow Sea Cold Water was rich in nutrients, could beregarded as an important nutrients (especially phosphorus) source to the upper layerwaters of the YS. During the bloom, the upward nutrient fluxes from deep water to theeuphotic zone by means of diffusion and turbulent entrainment were important nutrients sources to sustain the bloom, which could match56%of N,56%of P and69%of Si for phytoplankton growth demand. The bioavailability of organic nutrients andnutrients regeneration in the euphotic zone during the bloom shouldn’t be ignored.57-76%of DIN and46-68%of phosphate in the upper layer waters could be rapidlyassimilated by phytoplankton to make blooms break out in about one week in thecentral YS if other physical factors (i.e. temperature, stability of water column, lightavailability) were optimal for phytoplankton growth.The temporal and spatial distributions of nutrients are influenced by manyenvironmental factors such as hydrographic properties and biological activities.Therefore, the nutrients distributions in the typical sections of the YS, ECS and SouthChina Sea (SCS) in autumn showed different characters. Low levels of both nitrogenand phosphorus in the upper layer waters of YS would limit phytoplankton growth,while the dissolved silicate concentration in the upper layer waters wouldn’t constraindiatom growth in the YS. The Yellow Sea Cold Water was rich in nutrients and couldbe considered as an important nutrient storage pool in the YS. The deep waters in thenorthern section of ECS were abundant in nutrients. Nutrient concentrations in theTaiwan Strait were lower than those in the northern section of ECS. The upper layerwaters with high temperature and high salinity in the SCS were oligotrophic water.Depletion of nitrogen and phosphorus would be the limiting factors of phytoplanktongrowth. Contrarily, the near bottom waters of the SCS with low temperature and highsalinity were rich in nutrients.Phytoplankton could adsorb phosphorus to the cell surface. Phytoplanktonsamples (20-200μm) in the surface waters of China costal seas in both spring andautumn were collected and the characters of phytoplankton cell associated phosphoruswere discussed. In spring, the phytoplankton total cellular phosphorus, intracellularphosphorus and cell surface adsorbed phosphorus were9.5-95.9,9.0-82.2,0.5-21.3μmol g-1in the YS and41.8-190.3,30.4-118.5,2.9-71.8μmol g-1in Zhejiang andFujian Province coastal waters, respectively. The contents of cell associatedphosphorus in Zhejiang and Fujian Province coastal waters showed no significantdifference between2007and2008. The contents of cell associated phosphorus were obviously higher in Zhejiang and Fujian Province coastal waters than in the YS,probably due to the difference in the phytoplankton compositions in these two regions.While the ratio of cell surface adsorbed phosphorus to total cellular phosphorusshowed no significant difference between these two regions. In autumn, the contentsof total cellular phosphorus, intracellular phosphorus and cell surface adsorbedphosphorus were20.8-66.0,18.5-41.8,1.1-21.2μmol g-1in the YS;44.5-101.7,31.7-80.0,1.0-40.2μmol g-1in the ECS;25.2-52.5,21.0-39.4,0.6-16.9μmol g-1in theSCS, respectively. Utilization of phosphorus by phytoplankton in the YS, ECS andSCS were strongly affected by the phosphorus concentration in the sea water. Thecontents of both total cellular phosphorus and intracellular phosphorus were obviouslyhigher in the ECS than in the YS and SCS. While cell surface adsorbed phosphoruscontents among the YS, ECS and SCS showed no significant differences. Thephytoplankton cell associated phosphorus contents in both spring and autumn alsoshowed no significant differences.Laboratory incubations indicated that the intracellular phosphorus and cellsurface adsorbed phosphorus of Skeletonema costatum and Prorocentrumdonghaiense contributed to64%-96%and4%-36%of total cellular phosphorus,respectively. The ratio of cell surface adsorbed phosphorus to total cellularphosphorus deceased with specific growth rate in the poor phosphorus conditions.When phosphorus was abundant in the environment, phytoplankton could luxuriouslyuptake phosphorus and store phosphorus in both interior of cell and cell surface.When the environment was depleted of phosphorus, phytoplankton could utilizephosphorus in both interior of cell and cell surface to sustain growth demand.