Abstract:

With the objective of supporting and guiding conservation of Xiangxi Bay (XXB),filed data were collected in Xiangxi Bay during the spring of 2012 to investigate the spatial and temporal distribution of permanganate index of oxygen demand(CODMn) and analyze the response of CODMn to chlorophyll a (Chl a),dissolved oxygen (DO),the vertical stability factor of water,total nitrogen (TN) and soluble silicates.The results show that the variation of CODMn concentration in XXB ranged from 140 to 536 mg/L in spring.The average concentration of CODMn was significantly higher in April than that in March and May.Both the value of Chl a and CODMn became markedly high with the outbreak of algal blooms.In April and May,significantly positive correlation was observed between CODMn and Chl a(rApril=072,rMay=074) ,whereas the correlation was weak in March.The concentration of CODMn increased significantly across downstream to upstream in the entire spring.Chl a and water stability factor were the main factors affecting the variation of CODMn when spring algal bloom occurred.The increase in water stability factor of top layers caused accumulation of algae and organic pollutants in surface water,leading to the increase of CODMn.On the other hand,the correlation between CODMn and TN was negative.In the estuary,silicate appeared with higher densities than in the upstream.Diatoms played a critical role in shaping the spatial distribution of soluble silicate in XXB.There was density flow which carried a large amount of nitrogen from Changjiang (CJ) mainstem into XXB,and much water with high CODMn from XXB into CJ,leading to decreased CODMn concentration in XXB.The water body was at high eutrophication level in the upper reach of XXB.Density flow diluted Chl a in the estuary but increased the risk of occurring algal bloom in the downstream.The weak correlation coefficient between CODMn and DO in March and May provided a good explanation for the special distribution of CODMn in XXB.The distribution of CODMn was influenced by many factors,including biochemical elements and hydrodynamic forces.In addition,further analysis indicated that exogenous pollution was still the main reason for the development of organic pollution.Because the sources of water pollution are different,a variety of pollution prevention is needed.Increased recognition of external pollution is finally highlighted from watershed management perspectives.The organic pollution in the upper reach of XXB gets worse in the spring,suggesting that reduce the discharge of the organic pollutions is an effective measure to improve the water quality in XXB