Total Maximum Allocated Loads on Stoichiometry of Nitrogen and Identification of Critical Form in Jiaozhou Bay, China

Total pollutant load control management for total dissolved nitrogen (TDN) is an urgent task required to gain a good water quality status in Jiaozhou Bay (JZB), China. In this paper, the stoichiometry of multiform TDN on land-ocean interactions associated with marine biogeochemical reaction (LOIMBR) was studied by modeling the load-response relationship based on a three-dimensional water quality model of nitrogen in JZB. The results showed that the stoichiometry on LOIMBR of dissolved organic nitrogen (DON), NO3-N and NH4-N was 3:1:1, with one-third of the contribution on the concentration of dissolved inorganic nitrogen (DIN) in JZB for the land-based DON loads to DIN loads. Based on the stoichiometric relationship of nitrogen forms, the total maximum allocated load (TMAL) of equivalent TDN (ETDN) was approximately 5300 t a−1 in JZB, equivalent to the TMAL of 5700, 5800 and 15600 t a−1 for NH4-N, NO3-N and DON, respectively. According to the loads of ETDN, there were four outfalls overloaded in JZB in 2015, which lie in the head of the bay. In the four overloaded outfalls, besides NO3-N, NH4-N was the critical nitrogen control form for Moshui River, while DON for Dagu River and Haibo River. The results of numerical experiments further showed that JZB will achieve good water quality after 7 years by implementation of the ‘different emission reduction’ based on TMAL of ETDN, which is significantly better than ‘equal percent removal’.