| Eutrophication of surface waters can induce algal blooms and in salt water or estuarine waters, red tide can occur frequently, which can cause impairment of both the use of the water body and the local fishery. Therefore, eutrophication is an important ecological problem, and the study of how to deal with this problem has attracted extensive attention.Prawn aquaculture is the pillar industry of the national marine-based economy. In China, the area of prawn aquaculture exceeded6.67×104hm2, with an annual output of more than120×104t, which accounts for38%of the global output, making China the highest prawn yielding country. Eutrophication is the major pollution issue in prawn culture, the second problem is heavy metal pollution. Nitrogen compounds discharged from aquaculture waste water are the primary sources of nitrogen pollution and the resultant eutrophication in the near-shore marine environment. There are two kinds of dissolved nitrogen in prawn aquatic system. One is dissolved inorganic nitrogen in the form of ammonia nitrogen (NH4+-N), nitrite (NO2--N), nitrate (NO3--N).. The other form is dissolved organic nitrogen as Urea-N, Methyl-N and protein. Inorganic forms, NH4+-N and NO2-N, are the primary nitrogen pollutants which are harmful to prawn. The technique for maintaining healthy prawn aquaculture is to improve the culture environment by regulating the water quality through select organisms in the prawn pond. Planktonic microalgae can take up dissolved nitrogen and heavy metal ions, and can be an important biological factor in a prawn culture system. Thus, microalgae can transform dissolved nitrogen and reduce heavy metal loads. To improve prawn pond water quality, we can enhance the pollutant removal capacity of planktonic microalgae by assembling an optimum planktonic microalgae colony through enhancing the community structure of certain planktonic microalgal species. The studies on the Planktonic microalgae in the prawn high level pond and the absorption of dissolved nitrogen and heavy metal ions adsorption by them have not been reported. In this study, we chose high elevation prawn pond to study the pollution caused by prawn breeding, planktonic microalgal assemblages in prawn ponds, the ability and regulatory mechanism for planktonic microalgal uptake of dissolved nitrogen and heavy metals and the control of the prawn pond environment. This will provide a scientific basis for optimizing the community structure of planktonic microalgae and improving water quality in prawn ponds and additionally providing a theoretical and technical basis for controlling eutrophication.The content of the major research and the results are as follows:(1) Through the study of dissolved nitrogen from prawn culture, we found the amount of feed showed a positive correlation with chemical oxygen demand (COD) and was negative correlated with dissolved oxygen. The dissolved nitrogen came primarily from the artificial breeding diet, with the contents of dissolved inorganic-nitrogen, dissolved organic nitrogen and the total dissolved nitrogen being0.470mg·L-1,0.325mg·L-1,0.795mg·L-1respectively. In addition, the concentrations from the late breeding stage were higher than in the early breeding stage, and the threshold of eutrophication. The content of dissolved inorganic nitrogen was40.9%of the total nitrogen, which was the major source of the dissolved nitrogen in the prawn pond during the breeding period. Over the course of the breeding period, the mean ratio of inorganic nitrogen to inorganic phosphorus (IN/IP) was5.98. It was<5at48%of time and never exceeded30. The average chlorophyll-a concentration in the pond was91.6ug·L-1, which was positively correlated with the total dissolved nitrogen. The mean value of the Carlson trophic state index (TSI) was73.0, indicating that the pond was in a state of serious eutrophication. The concentrations of NH4+-N and NO2--N during the early breeding were0.21mg·L-1and0.02mg·L-1respectively, and their concentrations in the late breeding stage were0.42mg·L-1and0.03mg-L"1. Both nitrogen species were higher than the threshold for healthy prawn breeding. Prawn immunity was reduced when NO2--N achieved a concentration of0.04mg·L-1.(2) In the Guangdong and Hainan provinces, the major species of planktonic microalgae belong to five phyla, including12species found during the middle and late stages of prawn breeding, with a the degree of dominance from0.12to0.99. During the early breeding stages, diatoms dominated, but with eutrophication during middle and late breeding stage becoming serious, green algae and blue-green algae became dominant. The average cell number of planktonic microalgae was19.71×107cell/L-1and the mean diversity index was0.56, indicating that the system was suffering from serious contamination. The period with Oocystis borgei as the dominant algae was relatively stable, remaining dominant for40d-50d. The period when the dominant population was diatoms proved to be unstable and only lasted for5d～15d. The cell number of Phormidium tenue was2160×107cell/·L-1, which potentially is a source of red tide. The additional stable period occurred when the dominant population was green algae, which was a benefit for the growth of the prawn. Oocystis borgei was widely spread, adaptable, with the characteristics of a stable species persisting for a long duration.(3) Using a stable isotope labeling technique, the uptake rate for nitrogen of planktonic microalgae in shrimp ponds was studied under different ecological conditions. The most suitable conditions for the uptake of NH4+-N, NO2--N, NO3--N, Urea-N and Met-N by microalgae in prawn pond were a temperature of25℃～30℃, a salinity of20～30, light illumination at45μmol·m-2·s-1～126μmol·m-2·s-1, and an algal density of3.222×108cell·L-1～4.784×108cell·L-1. When the nitrogen concentration was14.3mg·L-1, the uptake rate of NH4+-N, NO2--N and NO3--N by microalgae was highest. The uptake rate of Urea-N and Met-N was the highest when the Urea-N and Met-N concentrations were48.4mg-L"1and20.9mg-L"1respectively. High concentrations of dissolved nitrogen apparently inhibit the uptake of nitrogen. The inhibition of NH4+-N uptake occurred when the concentration of NH4-N was high.These results demonstrated the uptake rate of nitrogen by planktonic microalgae in prawn ponds was closely related to temperature, salinity, light illumination, algal concentration and nitrogen concentration, with these specific conditions commonly in found in coastal areas in subtropical regions. The "saturation effect" for nitrogen uptake by algae would exist under high dissolved nitrogen conditions.(4) The effect of salinity and temperature on relative preference indices (RPI) for planktonic microalgae under four dissolved inorganic nitrogen concentrations was studied using a stable isotope labeling method. The dominant factor which affected the nitrogen uptake rate of planktonic microalgae was also studied using an orthogonal experiment. The results showed that the RPI of NH4+-N of planktonic microalgae was greater than that of NO2--N, Urea-N and NO3--N. Planktonic microalgae took up NH4+-N initially and NO2--N followed, the next being Urea-N and finally NO3- -N. The orthogonal experiment showed that microalgae had a higher uptake rate for the four nitrogen sources when the temperature was20℃～30℃, the light illumination was81μmol·m-2·s-1, the salinity was15～30, the pH was7.5, and the algae density was4.5×108cell·L-1～5.5×108cell·L-1. Microalgae density and salinity were the main factors that affected the uptake rate of NH4+-N and NO2--N, while light illumination was the main factor that affected the uptake rate of NO2--N and Urea-N.(5) Predicted values and measured values of an uptake rate model for Oocystis bergei were examined using a t-test. The test results showed that there was not a statistically significant difference between the predicted values and the measured values, as well as the population mean, which indicated that the degree of model fit was high.(6) The uptake of Cu2+and Zn2+by Oocystis borgei could be divided into three steps:the first step was to complete the uptake process within30min, with an uptake rate of more than70%; in the second step, the rate decreased and the uptake process was completed within8h; in the third step, the uptake process reached equilibrium. The optimum uptake of Cu+and Zn2+by planktonic microalgae Oocystis borgei was when the temperature was25～30℃, the light illumination was greater than54.00μmol·m-2·s-1, and the salinity was10～30. When the Oocystis borgei concentration was2.9×108cell·L-1, the adsorption rate and capacity of Cu2+were52.5%and9.5mg·g-1respectively; when the algae concentration was2.3×108cell·L-1, the uptake rate and capacity of Zn2+were81.4%and2.9mg·g-1respectively; for Cyclotella striata when its concentration was2.5×107cell·L-1, the uptake rate and capacity of Cu2+were63.0%and9.3mg·g-1respectively; when its concentration was1.8×107cell·L-1, the uptake rate and capacity of Zn2+were60.5%and20.1mg·g-1respectively. Under these algae concentrations, the growth of Oocystis borgei and Cyclotella striata would not be affected.(7) Introducing Oocystis borgei into prawn farming systems not only improved the content of dissolved oxygen, regulated pH, reduced COD, but also effectively fixed dissolved nitrogen. Due to the introduction, NH4+-N concentration was reduced by51.7%～37.8%, NO--N concentration reduced by30.2%～26.4%, and the disease-resistance of the prawn was improved significantly (P<0.05). In the treated prawn pond, average biomass of Oocystis borgei accounted for95.86%to45.12%, The dominance is between0.27to0.58and was the dominant algae during the breeding period. As the dominant species, it persisted for about77days. Concentrations of NH4+-N and NO2- -N were reduced by37.0%and81.2%respectively. Oocystis borgei can potentially alleviate the effect of stocking-density constraints, significantly improve prawn growth, and increase aquaculture production by83.8%. With a water temperature of27℃～32℃, a salinity of20～28, Oocystis borgei showed logistic growth with a K (carrying capacity) value of6899.959×105, r (instantaneous rate of increase) approximate value of0.002, and the maximum sustainable yield (MSY) of3.45×105·L-1·d-1. Removal of3.45·105cell·L-1of algal cells every day from the shrimp pond waters can keep Oocystis borgei population stable. |
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