Determination Method and Accurate Determination of Nitrogen and Phosphorus in Coastal Water

Accurate determination of nitrogen and phosphorus in coastal zone plays a vital role in land-sea coordination and coastal zone management. The purpose of this study is to explore the influence of salinity (S) on test accuracy for different forms of nitrogen and phosphorus and to clarify the basis for the selection of nitrogen and phosphorus test methods in salinity transition zone of estuarine and coastal waters. The mass concentration of different forms of nitrogen and phosphorus in both simulated seawater samples and real seawater samples were analyzed by the national standard fresh water and seawater methods. The results indicated that the salinity had a certain influence on the test results using fresh water and seawater methods. To determine the ammonia nitrogen，when the sample salinity was less than 4.0 using both fresh water and seawater methods can accurately determine the concentration of ammonia nitrogen in the coastal water. However, when the salinity was more than 4.0，to adjust water samples pH was an indispensable step for using the seawater method. And for determining the nitrite, there was no obvious difference between using fresh water method or seawater method. To determine the total nitrogen, a salinity of 5.0 was the threshold for selecting either fresh water method or seawater method. To determine the active phosphate in coastal waters, the influence of salinity was negligible under the low salinity and low phosphorus (S≤35.0 and ρpo3-≤0.2 mg·L-1) ) conditions, but for water samples with high salinity and high phosphorus(S> 35.0 and Pp03->0.2 mg·L-1))，it was more accurate to choose more sensitive seawater method. To determine the total phosphorus, the seawater method was of higher accuracy. This study provided a scientific basis for the accurate determination and method selection for different forms of nitrogen and phosphorus in coastal waters.