Studies Of Eutrophication Features And Eutrophication-HABs Relationship In The Changjing Estuary And Its Adjacent Area During The Past 30 Years And Strategies On Controlling Eutrophication

Eutrophication and its ecological effects in Changjiang Estuary and its adjacent area have been widely interested. Although the eutrophication-HABs ( harmful algal blooms) relationship has been reported in a lot of papers, there were still gaps to learn the long term trends of eutrophication and eutrophication-HABs relationship in Changjing River Estuary and its adjacent area. To fill the gaps, the data of concentrations of DIN, PO4-P and COD since 1950s especially of 1980s was collected, and the eutrophication index ( EI ) was used to analyze the eutrophication status in Changjing River Estuary and its adjacent area . Based on those data and the data of field surveys during the period of 2002 to 2005, considering the seasonal changes, the linear regression analysis of annual mean value as the chief method was used to explain the relationships between EI or EI' and the occurrence scale and times of HABs. Similarly, the relationships among biomass, COD and DO were analyzed too. At last, the marine environmental capacity of DIN, PO4-P and COD in the area within 31 salinity range of East China Sea was estimated for the first time. The main conclusions are as follows:1. In the Changjiang Estuary and its adjacent area, the annually mean value of DIN concentration showed an undulatory increase, and PO4-P's remained practically unchanged, but COD's appeared a fluctuant downtrend in the last 50 years. And the monthly concentration of DIN and PO4-P exhibited a bimodal curve of which peaks were in the mid autumn and the end of winter (or early spring), the bottom was in the mid-summer. As for COD, its monthly concentration increased during high-water periods while decreased during low-water periods. The horizontal distribution of DIN, PO4-P and COD generally took on a clear pattern of decreaseing from inshore to offshore area. Thus, the temporal and spatial changing trend of EI was summarized. EI's value has been increasing annually. Its monthly change suggested a bimodal distribution of which peaks were showed in autumn and spring and the bottom in summer, and its horizontal distribution showed a trend of higher inshore and lower offshore clearly. 2. The occurrence of HABs was controlled by DIN coordinated with PO4-P.The relationships between the three factors (eg. DIN, PO4-P and COD )and the characters of HABs including occurrence scales , frequency, outbreak time and area were studied in the Changjiang Estuary and its adjacent area. An appreciable liner relationship was observed between the annual mean concentration of DIN and PO4-P with the frequency or scales of HABs, while the pertinency for COD is not quite noticeable. The HABs mainly appeared in May and June, lagging behind the peaks of DIN and PO4-P in late winter or early spring, but coinciding with the high value period of COD or even taking place ahead of time a bit. Obviously, the occurrence of HABs and COD concentration didn't associate tightly. Thus, the effect of COD should not be taken into account in the calculation of EI. According to this principle, a new computational formula of EI (EI') was generated. By comparing the linear coefficient of the four factors (such as EI, EI', DIN and PO4-P) with the occurrence scale or times of HABs annually, EI' showed the tightest correlativity with the HABs. So, the occurrences of HABs in the researched area is controlled by DIN and PO4-P synergistically.3. The eruption of HABs brought exceptional rise of COD's concentration, but abiotic factor was still the main source of COD that led to the uncertain relationship between EI' and COD. Furthermore, the rise of EI' could ultimately worsen the depletion of oxygen in the bottom waters.Based on field surveys, when HABs occurred, the organic matter produced by phytoplankton accounted for 49~68% of COD, becoming the leading source of COD in the HABs area. While in the non-red-tide area and the whole investigated area, the organic matter produced by phytoplankton accounted for only 13~17% and 21~27% of COD respectively. The results indicated that abiotic factor was the main source of COD. Additionally, seasonal variation existed when it came to the contribution of phytoplankton to the COD concentration. Generally, phytoplankton produced the most large portion of COD in summer, less in autumn, much less in spring, and least in winter. Even in summer, COD produced by phytoplankton accounted for only 24 percentage of the total amount. It is suggested that the annual mean concentration of COD was mainly attributed by the abiotic source such as terrigenous, which led to the obscure correlativity between the annual mean value of EI' and COD since 1980s. Furthermore, the rise of EI' would increase the hypoxia area and depress the minimal value of dissolved oxygen. The formation time of Changjiang Estuary hypoxic zone basically coincided with the high value period of COD and the peak outbreak period of HABs, but lagged behind the peak period of EI' for one or two months.4. Under the First Grade Standard of National Sea Water Quality Requirement, the environmental capacity of DIN,PO4-P and COD was 1.7×106 t·a-1 , 8.2×104 t·a-1 and 9.7×107 t·a-1 respectively. Thus, 1.7×106 t·a-1 of DIN and 6.0×104 t·a-1 of PO4-P, amounts to 60% and 80% of present total discharge fluxes respectively, must be reduced to decrease the occurrence of HABs.