Nutrient Dynamics Along Coastal Ecosystems Of East Hainan

The coastal ecosystems usually have high primary productivity, diverse and complex community, therefore, nutrient biogeochemical cycles have become the cone theme in ocean science. Four cruises were carried out along coastal ecosystems of Eastern Hainan(such as rivers, estuaries, lagoons and near-shore waters) to understand the level, distribution and seasonal variations of nutrients, in order to examine biogeochemical processes that influence nutrients composition. Some important conclusions are drawn as follows.1. Nutrient levels in the tropical Wenchang/Wenjiao rivers show wide range of seasonal variations. DIN concentrations in these two rivers are generally at the levels between the average global conditions to polluted waters except that Wenjiao river one cruise in 2008 are at clean waters levels. Ammonium contribution to DIN concentration is relatively up to 37-44% in the August 2007 cruise related to the the application of inorganic N fertilizer and wastewater discharge. DIP concentrations in the rivers are at the levels between pristine level and clean waters level compared to the global river data. So DIN/DIP ratios show wide range of variation(60-413). DON was the major part of TDN in winter and summer, DIP was the main part of TDP in spring. DON accounted for 23% of TDN in the Wenchang river and 55% in the Wenjiao river. Dissolved silicate levels in Wenchang and Wenjiao rivers are lower than average levels of tropical rivers, it's lower than the level of Wanquan river.Wenchang/Wenjiao river estuaries significantly affected by tidal action and had obvious seasonal variation. Nitrate behaved conservatively subjected to a simple estuarine dilution in the winter 2006; underwent regeneration in the spring 2009; has denitrification in both summer 2007 and 2008. This indicated that dissolved inorganic and organic nitrogen regenerated from degradation of organic matter and denitrification processes existed in the estuary in the summer. Phosphorus behaved desorption/adsorption from suspended particles along the salinity or degradation of organic matter in the estuary. Dissolved silicate behaved conservatively in the winter 2006 and spring 2009.Nutrients showed obvious tidal effect with low values at flood tide in Gaolong bay.2. DIN concentrations in the Wanquan river were higher than tropical river that were not polluted in the world, such as Amion and Zaire rivers, but lower than temperate rivers which significantly affected by human activities, such as Changjiang, Huanghe and Pearl rivers. DIP concentration in the river is at the levels between pristine level and clean waters level compared to the global river data. Dissolved silicate level in the Wanquan river is not only higher than average global conditions, but also at higher to general levels in tropical systems.NO3-N/DIN is the main part of DIN. Nitrate and dissolved silicate had a negative correlation with salinity in the estuary, indicating largely subjecting to a simple estuarine dilution, while nitrite and ammonium were non-conservative; phosphate undergo desorption/adsorption from onto suspended particles along the salinity gradient and/or degradation of organic matter in the estuary and has regeneration in 2007 when salanity below 2.5. In addition, phosphorus maybe the potential limit element for phytoplankton growth with high N/P concentration ratio (averagely, more than 61).3. Nutrient concentrations in surface waters of the Xiaohai and Laoyehai lagoon were determined during surveys in both August 2008 and April 2009. The results showed that:the overall salinity of the Xiaohai lagoon is low, and summer higher than spring. The concentrations of nutrients were higher in summer than spring except nitrate. DON accounted for 79% and 36% of TDN in summer and winter, respectively; DOP concentrations represented-49% of TDP in summer and 47% in spring, but the concentrations of DOP increased by 2.3 fold in summer 2008 than in spring 2009. Nutrient ratios showed obvious seasonal variation, indicating that limiting nutrients were nitrogen in summer and phosphorus might be the potential limiting element for phytoplankton growth in spring.The salinity in the inner bay of Laoyehai lagoon is lower than the mouth of the lagoon, and the average salinity was higher than that that in Xiaohai lagoon. Nutrient concentrations showed an obvious tidal effect with low values in the mouth of the lagoon.4. Nutrients level are low in eastern coastal of Hainan. Shallow coastal waters were significantly affected by terrestrial input, but deep water was not obvious. Nutrient concentrations increased from surface to bottom waters. Phosphorus was the limiting factor for phytoplankton.5. One cruise was carried out in the northern South China Sea(SCS). The result showed that oligotrophy is a significant characteristic in upper water column of the northern SCS. The concentration increased with obvious stratification from surface to bottom. According to nutrient concentrations, the limiting nutrient for phytoplankton growth was phosphotus in this region.6. Nutrient budgets demonstrate that in Wenchang/Wenjiao estuary, riverine input is a major source of NO2-N, NH4-N, DSi, DON, DOP, while groundwater discharge is the major source of NO3-N, and the estuary is the sink of NO3-N,DOP, source of NO2-N, NH4-N, DSi and DIP. Wanquan estuary behaves as a sink of NO3-N, DIP, DSi and DOP, source of NO2-N, NH4-N and DON. Xiaohai lagoon is a sink of all nutrients except for DON. Laiyehao lagoon behaves as a sink of NH4-N, DIP and DOP, source of DSi and DON. Wanquan river estuary transports more nutrients to coastal waters than Wenchang/Wenjiao estuary, Xiaohai lagoon and Laoyehai lagoon.