Water Movement, Water Quality Investigation And The Strategies Towards Aquaculture Waste Water Discharge: A Case Study In Lianzhou Bay, Guangxi Province, China

By sampling several physical and biogeochemical parameters, such as waterdepth, current speed, and the temperature, salinity, pH, dissolved oxygen (DO),nutrients (nitrogen, phosphorous) in Lianzhou Bay, Guangxi Province, China, andcalculating some physical hydrodynamic parameters, e.g., Brunt-V is l frequency(N), Richardson number (Ri), Rossby radii (Lr), etc., tidal currents and mixingprocesses are determined in this coastal region. From gathering information of localliteratures, it is known that this bay is under strong influence of land-based runoff,Nanliu River flushes a considerable amount of sediments and nutrients into the sea.The results of these physical parameters show that Lianzhou Bay is under an efficientturbulent mixing progress, stratification here is rather weak. Plus the shallowness ofthe bay, a barotropic status is determined. Due to the characteristic length scale of thisbay is far shorter than its Rossby radii, earth rotation here can be safely neglected. Viacomparing the predicted tidal data and the in situ data obtained during the expeditions,associating with the calculation of resonant frequency, it is found that the tidal currentis equally distributed in Lianzhou Bay. Analyzing the mean water volume of LianzhouBay base on the2.55m’s datum shows that the water exchange rate is faster duringdiurnal tide compare to semi-diurnal tide. The residence time of the sea water inLianzhou Bay put a discern of which, faster current speed of the coastal current whichsweeps the bay mouth in winter causes shorter residence time. Shrimp farming as themajor mean of livelihood is abundant in this region. The related impact is studied, andthe major cultured species is Penaeus vanamei. By observation and interviewingamong the local farmers, it is confirmed that the shrimp culture here is individual, small scale activity. The interview also points out that, the shrimp ponds’ waste waterdischarge is supposed to be un-organized. Based on an aim to raise some suggestionstoward the shrimp farming effluents, the impact of shrimp farming is paid moreattention here. The nutrient data was analyzed in the lab after sampled in the bay andriver. The nutrient distribution maps show the tendency of negative impact of shrimpfarming waste water discharge. Along the branches of Nanliu River, the area whichaccepts the shrimp ponds effluents contains higher nutrients, especially NH4+. Thus,the concentration of dissolved oxygen is low due to highly active microbial activity.Through comparison, the water quality in the bay and river is investigated to beun-eligible according to the general Chinese water quality standards. Nutrients areindentified as the main pollutants. Interdiscipline studies, i.e., the researches whichare integrated with physical, chemical and management fields are rare around areas ofLianzhou Bay. In this study, by combining the results of water exchange rate andwater residence time with local conditions, some suggestions towards a better wastewater discharge strategy are raised, such as discharge the water after the peak ofdiurnal tide, and try the winter shrimp culture instead of increasing the shrimp densityof a single pond or increasing the number of ponds. For a better feasibility of thesesuggestions, perceptions of local people towards environment protection, which isclosely connected with their own interests need to be enhanced or even rebuilt. Somestrategies are listed.