Preliminary Study On The Relationship Between Spring Phytoplankton Bloom And Physical Environment In The Central Southern Yellow Sea

Phytoplankton bloom is the process that phytoplankton biomass and production accumulate rapidly. In mid-latitude ocean, phytoplankton bloom occurring twice a year is of significance extremely in sustaining marine food web and marine ecology. It is beneficial to knowing more about the key process of China's coastal ocean ecosystem food production to study the spring phytoplankton bloom process and factors which affect it. After long years of investigation, it illustrates that spring phytoplankton bloom in central Southern Yellow Sea arises before seasonal stratification has formed. This is different from the common sense——enhanced stratification and surface mixed layer, which is shallower than the critical depth of phytoplankton bloom's growth, is the prerequisite for phytoplankton bloom to arise.In 2007 and 2009 spring, the group attended multidiscipline investigation of phytoplankton bloom and background field in Southern Yellow Sea organized by the National Project 973 "Key Processes and sustainable mechanisms of ecosystem food production in the coastal ocean of China". Meantime, we found the development of the phytoplankton bloom tightly connected to weather conditions and the spot where bloom happened matched with the weather situation. We proposed the hypothesis that in spring, wind speed declines and SST rises rapidly, then the water stability goes up, which is the essential factor of phytoplankton bloom in central Southern Yellow Sea. This paper plans to prove this hypothesis by numerical model.First, by processing and analyzing the RBR-CTD data of the cruising stations, bloom tracing stations and two continuous stations from phytoplankton bloom voyage in South Yellow Sea in 2009 spring, the temperature, salinity, density, chl-a concentration and so on in this region is studied. And the Thorpe Scale, PEA (potential energy anomaly) and floating frequency is calculated. Then, the characteristics of turbulence mixing in Southern Yellow Sea in 2009 spring are discussed. The result shows that, salinity controls the South Yellow Sea in 2009 spring before SST rises. The high value region of chlorophyll concentration covers coastal region and the southern region of the Yellow Sea. Along with the conversion of the monsoon, wind speed reduces and SST rises at the same time, then the stratification is strengthened.Then, by analyzing SeaWiFS/MODIS chlorophyll concentration, QuikSCAT scatterometer wind field, NCEP/NCAR net sea surface heat flux reanalysis data and AVHRR SST data from January to May,2003 to 2010, several physical factors affecting the trigger of the spring phytoplankton blooms in the central South Yellow Sea are studied, and an index of spring primary production is developed. The result shows that:absolute water temperature has little effect on the trigger of the spring phytoplankton blooms in the central South Yellow Sea, while sea surface wind and net heat flux can affect it significantly; Chlorophyll concentration can attain a peak within 3-5 days if wind speed is less than 5 m/s and net heat flux remains positive. The longer the wind speed keeps less then 5 m/s, the higher the peak of the chlorophyll concentration and the spring primary production will be.Finally, temperature and salinity field and circulation of Bohai Sea and Yellow Sea are simulated by FVCOM model driven by atmospheric forcing, and an analysis is executed. PEA is calculated from temperature and salinity profiles, and we find its contour of 20 J/m3 can be seemed as the index of phytoplankton bloom in central Southern Yellow Sea in spring. Futhermore, through another two simulating experiments under changed atmospheric forcing, it is proved that the turbulence mixing in subsurface layer declines which results from decreasing wind speed and rising SST, and decreasing wind speed is more remarkable. This paper proves the hypothesis that the rising water stability resulted from decreasing wind speed and rapidly rising SST in spring is the main factor which leads to the trigger of spring phytoplankton bloom in central Southern Yellow Sea.