The Optical Properties Of Chromophoric Dissolved Organic Matter In Xiamen Bay And The Application In The Monitoring Of Coastal Eutrophication

In recent years, with the rapid development of modern industrialization and the dramatic increase of population in the coastal region, a large number of industrial waste water rich in nitrogen and phosphorus as well as domestic sewage are discharged into the sea, which leads to the increasingly prominent water eutrophication problem and even probably triggers ecological disasters like red tide phenomenon and proliferation of jellyfish. At present, China’s offshore eutrophication status often evauated by the first generation model, which uses parameters that require laborious artificial sampling and laboratory analysis, and the long period cannot realize the real-time fast testing and evaluation. For the second generation of eutrophication evaluation, it’s also unable to realize the real-time online monitoring due to the support of in-situ data. Based on above, this paper introduced the parameters that can characterize the optical properties of the CDOM to establishe the empirical algorithms of COD, TP, and TN, so as to use buoy to real-time online monitor the eutrophication status where are significantly affected by the runoff.Based on the survey data of colored dissolved organic matter（CDOM） and the main environmental factors in the Xiamen bay during the February, May, August, October and November of 2009, this paper discussed the possibility of using fluorescent components of CDOM and optical absorption coefficient to replace the regular eutrophication evaluation parameters such as TN、TP and COD, and then furthermore established an evaluation model that can real-time fast evaluate the coastal eutrophication status.The main research results were as follows:（1） There was obvious seasonal difference of CDOM distribution in Xiamen bay in 2009. The CDOM absorption coefficient was highest in the autumn with average coefficient of 4.15 m^-1and lowest in the winter with average coefficient of 2.72 m^-1. Over all, the seasonal variation trend of CDOM abundance in the surface waters of Xiamen bay was autumn >spring>summer>winter. As for the spectral slope, it was highest in the spring with average slope of 18.2×10^-3nm^-1 and lowest in the winter with average slope of 16.7×10^-3 nm^-1. The seasonal variation trend of spectral slope was spring>autumn >summer>winter. According to the PARAFAC model, the Xiamen bay had five fluorescent components during the four seasons of 2009, among which C1（250,345/454nm）, C2（230,310/374nm）, and C5（265,425/478nm） were Humic-like fluorescent components, while C3（230/342nm） and C4（230,275/322nm） were protein-like fluorescent components.In Xiamen Bay, the CDOM was significantly negative correlated to salinity and positively correlated to chlorophyll-a, indicating that land runoff input and phytoplankton were the main sources of CDOM. Affected by the runoff, the Xiamen bay is significantly semi-closed, where the CDOM abundance can be fitted by the contents of chlorophyll-a and salinity. The relative fitting error was only 11.1%, and this accuracy was almost the same as other domestic scholars’ results.（2） The maximum TN content appeared in autumn in Xiamen bay, whose variation range was 0.31⁴.14 mg/L and mean value was 1.41 mg/L; the minimum TN content appeared in winter with the variation range 0.13⁰.74 mg/L and mean value 0.37 mg/L. In different seasons, the average TN content sorted from high to low was autumn > spring > summer > winter. For TP, the maximum value appeared also in autumn with the variation range 0.043⁰.18 mg/L and mean value 0.083 mg/L; the minimum TP content also appeared in winter, whose variation range was 0.015⁰.052 mg/L and mean value was 0.024 mg/L. From high to low, the rank of seasonal mean value was autumn> summer > spring > winter. However, COD content reached its highest value in summer with variation range 0.54³.82 mg/L and mean value 1.54 mg/L; and reached its lowest value in autumn with variation range 0.16².81 mg/L and mean value 1.05 mg/L. The average seasonal value differed as summer> spring > winter > autumn.（3）This paper showed that we can use CDOM fluorescent components and absorption coefficient to estimate the concentration of TN、TP and COD. The multivariate regression equations of TN、TP and COD were showed below: TN=0.40×α₂₈₀+0.45×C1-1.17×C4-0.36（R=0.96） TP=0.017×α₂₈₀-0.0089×C2-0.033×C4-0.0031（R=0.78） COD=0.38×α₂₈₀-0.80×C1+9.41×C5-0.26（R=0.89）...