Spatial distribution and assessment of nutrients in marine ranching in Zhelin Bay-Nanao Island in summer

The marine ranching in Zhelin Bay-Nanao Island is divided into four different function areas: cage culture area, shellfish farming area, seaweed culture area and artificial reef area. Based on the survey data in that marine ranching in August of 2011, we analyzed the spatial distribution characteristics of nutrients in the surface seawater and evaluated the nutritional status. The results show that the contents of dissolved inorganic nitrogen (DIN) and active phosphate (PO4-P) decreased from the northwest of Zhelin Bay to the southeast of the investigated area. Among the four different function areas, the high-valueof DIN and PO4-P occurred in the cage culturearea, while the maximum content of silicate (SiO3-Si) wasoccurred in the shellfish framing area. Nutrient salts levels in the artificial reef area were relatively low. We assessed the sea water quality in the marine ranching by using single factor index, share rate of pollutants and comprehensive pollution index. The results suggest that the nutrient pollution trend caused by DIN and PO4-P in the marine ranching declined from the northwest to the southeast and from inshore to offshore waters. Moreover, the pollution and eutrophication state in the cage culture area was most serious, followed by the shellfish farming area, while the artificial reef area and control area were found for less pollution.