The Study Of Lower Trophic Level Ecosystem In The South China Sea: Observation And Modeling Investigation

The lower trophic levels of marine ecosystem are the foundation of organisms within the food webs. Phytoplankton are primary producers; they pull nutrients from the deep ocean and manufacture their own organism by photosynthesis, which transfer energy from the solar radiation that usually powers the base of the food chain. The South China Sea (SCS), one of the largest marginal seas in the northwestern Pacific Ocean, has complex marine environments and diverse ecosystems. The spatio-temporal variations of lower trophic level marine ecosystem, including their interaction with marine hydrodynamic environments, are very important for evaluation and protection of marine ecological environments and resources, as well as prediction and forecasting of marine ecosystem in the SCS.The present study analyzes seasonal distributions and interannual variations of lower trophic level marine ecosystem, and their relationship with marine environments in the SCS, using in-situ historical cruises observation data with statistical model, remote sensing data with phytoplankton primary production (PP) models, and a three-dimensional physical-biogeochemical model. Base on these advanced methods and technology, some new findings and experiments are put forward, such as vertical distribution of chlorophyll-α(Chl-α) based on surface Chl-α, the different features of Chl-αand PP in the seasonal upwelling regions, the response of Chl-αconcentration to cold and warm events in recently years, modeled results of nutrients and Chl-αin the SCS nested in the Pacific Ocean model by improving parameters, and simulation stability of marine ecosystem model, etc.1. The present study investigates samplings of Chl-αby several historical cruises for about 10 years in the SCS, including the northern SCS, Nansha Islands sea area and other sea areas in the SCS. The profiles of Chl-αare sorted by Chl-αconcentration within the surface layer, and used to develop a parameterization of the shape of vertical Chl-αprofiles according to a generalized Gaussian model (adding a linear decrease with a slope starting from the surface value). The modeled vertical Chl-αprofiles show a normal distribution with a decrease slope, and a close relationship with the oceanic environment in different sea areas of the SCS. Modeled Chl-αvalues fit well with in situ Chl-αvalue, and max value appeared in sub-surface layer, as the surface Chl-αconcentration is low, which represents deep-sea basin (Case I) waters; while the difference between modeled Chl-αvaule and in situ Chl-αvalues are apparent, and max value appears near surface layer, as the surface Chl-αconcentration is high, which represents coastal and estuary (CaseⅡ) waters. The present results made an exploratory and tentative contribution to the vertical distributions of Chl-αconcentration in the SCS based on surface Chl-α.2. The remote sensing data of Chl-α, as well as PP by VGPM (Vertically Generalized Production Model), indicate the different features of Chl-αand PP within seasonal upwelling regions. In the summer upwelling region offshore the east coast of Vietnam, the interannual variability of Chl-αresponds to climate changes, especially to the mid-latitude expression of the equatorial El Nino Southern Oscillation (ENSO) cycle. The low values of Chl-αunder the strong El Nino (1997-1998) event are noticed. Moreover, the interannual changes of Chl-αconcentration influenced by ENSO event are displayed in this study: (1) high values of Chl-αresponding to low SST during 1999-2000 by the effect of La Nina event; (2) declining Chl-αconcentration responding to increasing SST during 2002-2003 by the effect of moderate El Nino event; (3) increasing Chl-αconcentration responding to falling SST after warm event in 2005. Near the deep-sea basin and the Mekong River estuary, PP also reveal their response to interannual variations of marine hydrodynamic environments, such as SCS monsoon and strong mixing induced by wind stress.3. The Hybrid Coordinate Ocean Model (HYCOM) is coupled with the NORWegian ECOlogical Model (NORWECOM) and run in a nested system (the Pacific Ocean). Base on this three-dimensional physical-biogeochemical model, this study investigates temporal and spatial distribution of nutrients and Chl-αin the SCS (1992-1996), nested in the Pacific Ocean (1992-2004), for the purpose of understanding the responses of PP and nutrients to monsoon forcing, mesoscale eddies and other important physical processes. The modeled results of temperature, salinity and circulation pattern fit well with World Ocean Atlas 2001 (WOA01) data, which provides proper physical input of temperature and velocities field to marine biological model. The marine biological model consists of 10 compartments, including two phytoplankton classes, diatoms and flagellates, nitrate, silicate, phosphate, biogenic silica, detritus, oxygen inorganic suspended particulate matter (ISPM) and yellow substances. In this study, the modules of"light in the water column","primary production"and"sedimentation and resuspension"are improved mainly parameters for the SCS and the Pacific Ocean respectively, and run without ISPM and yellow substances. The nutrient fields were initialized with WOA01. The modeled nutrients and Chl-αcan almost reveal similar patterns as WOA01 and SeaWiFS. The numerical experiments make an exploratory and tentative contribution to the understanding of spatial and temporal patterns of phytoplankton and their interactions with physical-biological environments within this complex marine environment in the SCS.4. Marine ecosystem model with high resolution grid in the SCS, running in a nested system of the Pacific Ocean, gives advantages to depicting fine spatial and temporal patterns of nutrients and Chl-α, influenced by mesoscale eddies in the SCS. For example, there is a time lag of phytoplankton growth, responding to nutrient accumulation during the process of cold eddies activity.