Temporal and Spatial Variation Characteristics of Water Nutrients and Eutrophication Evaluation in Aquaculture Water Areas of Sanggou Bay

Abstract:

In this study, based on the water nutrients data of Sanggou Bay surveyed in the autumn of 2014, and the spring, summer and winter of 2015, the seasonal variation and distribution of nutrients were analyzed and the nutrient levels were evaluated. The results showed that dissolved inorganic nitrogen (DIN) levels in the autumn of 2014 was the highest among four surveyed seasons. The average content of NO2-N and NO3-N was the highest during the summer and autumn, respectively. By comparison, the average contents of NH4-N and PO4-P were the highest during the spring and winter, respectively. Compared with the seawater quality standard, DIN contents in four seasons were lower than the levels in the first level of seawater standard. Active phosphate contents were lower than the first level of seawater standard in spring and summer, but were higher than first level of seawater standard in autumn and winter. From the perspective of horizontal distribution, the content of DIN of the inner bay was lower than that of the outer bay in spring, but was higher than those of the outer bay in summer and autumn. The content of DIN exhibited an increasing trend from the north to the south area during the winter. The content of PO4-P showed an increasing trend from the outer bay to the central area in spring and winter. Furthermore, the content of PO4-P showed a decreasing trend from the outer bay to the central area in summer, and the content of PO4-P of the inner bay was higher than that of the outer bay in autumn. The N/P ratio was the same as the Redfield ratio in autumn of 2014, and the N/P ratio was higher than the Redfield ratio in spring, summer and winter of 2015, with relatively low levels of inorganic phosphorus. According to the potential eutrophication assessment model, nutrition levels were relatively poor in the autumn of 2014, and the spring, summer and winter of 2015. Compared with the 1980s and 1990s data of Sanggou Bay, the main limiting factor for phytoplankton growth has changed from nitrogen to phosphorus limitation in spring and summer.