Nutrient status and grade evaluation of seawater in Zhelin Bay-Nanao Island marine ranching

The Zhelin Bay-Nanao Island marine ranching consisted of four different functional areas including the cage culturing area, shellfish culturing area, seaweed culturing area and artificial reef area. Based on the surveying data in spring,summer, autumn and winter from 2011 to 2012, the spatial distributions of nutrients in different functional areas were analyzed, the nutrient status and the nutritional level were assessed by using N/P value and the potential eutrophication evaluation method to understand the nutrient status and its change trend during the construction of Zhelin Bay-Nanao Island marine ranching. The results showed that: (1) The distribution trends of dissolved inorganic nitrogen (DIN) and the active phosphate (PO4-P) decreased from the northwest to the southeast, as well as from the inshore water to the offshore water. (2)NO3-N was the main form of DIN, and the highest value of NO3-N/DIN occured in the cage culturing area in Zhelin Bay, and the contents of DIN and PO4-P in this area were much higher than those in other areas with the annual average of 0.726 mg/L and 0.036 mg/L, respectively. (3) According to the grade evaluation, the cage culturing area and shellfish culturing area were in the moderate phosphorus limited potential eutrophication status, and the seaweed culturing area was in the phosphorus limited moderate nutrient status, while the artificial reef area was in the moderate nutrient status. So it is possible to infer that the construction of marine ranching has a regulatory effect on the marine environment.