Transformation Of Sewage Nutrients In West Coast Of Shenzhen And Its Impact On The Growth Of Marine Phytoplankton

In recent years, the seawater quality decline and pollution is serious increasingly with the rapid economic development of Shenzhen, caused by a large amount of domestic sewage, industrial wastewater and aquaculture wastewater discharged into the sea, especially in west sea area of Shenzhen. The growth of phytoplankton was promoted by excessive amounts of inorganic nitrogen and active phosphate runoff input, which increases the frequency of red tides. All of those may weaken the sustainable use of marine resources and the development of marine economy in Shenzhen. The investigation on the conversion kinetics of nitrogen and phosphorus with domestic sewage, industrial wastewater and aquaculture wastewater, which was discharged into the west sea areas of Shenzhen was carrid out in this study. And the effects of waste water on the phytoplankton growth were also investigated by enclosure ecosystems experiments. The results of the research are as follows:1. According to the results of enclosure experiments in two times, higher the initial concentration of nutrient salt was, faster nutrient salts transformed. After 4-5 days, the concentrations of dissolved nutrients (inorganic nitrogen and phosphate) changed little during the rest experimental period. Based on the evaluation of restrictive facter reported by Redfield, Justic and Dortch for determining the limiting factor of phytoplankton growth, the active phosphate was the limiting factor to the growth of phytoplankton in our enclosure ecosystem.2. Preliminary study on the transformation kinetics of dissolved inorganic nitrogen (DIN) and dissolved reactive phosphate (DIP) showed that the content of DIN and DIP in polluted seawater in western coastal areas of Shenzhen basically followed the first-order kinetics. The result of mesocosm experiment showed that the input of 3 kinds of land-sourced sewage including waste water, sewage, and industrial wastewater would influence the degradation of DIN and DIP, and the regularity of degradation viaries as season changed. In May, the addidtion of some sewage could promote the degradation of DIN and DIP. In January, the wastewater input inhibited the degradation of DIN and the inhibitory efficiency was in an order of wastewater> wastewater> sewage. Certain amount of industrial wastewater and domestic sewage could promote the degradation ofDIP in seawater. While agriculture waste water inhibited the degradation of DIP in seawater. The natural degradation coefficient of DIN and DIP was calculated in western estuary of Shenzhen (KDIN=0.055d-1, KDIP=0.061 d-1) based on the first-order kinetic equations.3. To study the absorption of nutrients during the growth of phytoplankton in different conditions, content changes of different nutrients (nitrogen, phosphorus, silicon) and phytoplankton cell density in its breeding period was analized, which showed that the nutrients absorption of phytoplankton was depending on different kinds of waste water and seasons in Shenzhen coastal seawater. In May, nitrates and ammonium nitrate were absorbed mainly by marine phytoplanktonin seawater-sewage compared with the prefer absorbtion on nitrate in natural seawater, and more the sewage input, better the active phosphate and silicate phytoplankton absorbed within a certain range. The results in January showed that ammonium nitrate-based (nitrate-based in natural seawater) but not nitrate or nitrite in seawater-sewage was absorbed by marine phytoplankton compared with natural seawater.4. The input of seawater-sewage affected the absorption of nutrient salts by the phytoplankton. In the view of the absorption of DIP by phytoplankton, the absorption amount followed the sequence of farming wastewater> industrial wastewater> natural seawater> sewage, but as for silicate absorption by phytoplankton, the absorption sequence was of farming sewage> sewage enter> industrial wastewater> natural seawater. Moreover, all of domestic sewage, industrial wastewater and aquaculture wastewater could obviously promote the growth of phytoplankton. The biomass of phytoplankton was found to be related closely to the content and composition of the nutrient salt in sewage (domestic sewage> industrial wastewater> aquaculture wastewater). The impacts on the growth of phytoplankton due to the addition of sewage water were variational in different seasons. Sewage promoted mainly the breeding process of cyanobacteria and diatoms (cyanobacteria> diatoms) on January, and the breeding process of cyanobacteria and algae (algae> cyanobacteria) on May.