Comparison of land-based sources with ambient estuarine concentrations of total dissolved nitrogen in Jiaozhou Bay (China)

• Land-sea synchronous water sampling was conducted in Jiaozhou Bay and its watershed.
• Land-based DON is transformed into DIN in seawater throughout the year.
• Land-based inputs of both DIN and DON contribute to DIN concentrations in the bay.
• Reduction in land-based TDN discharges should be considered to manage eutrophication.

Seasonal, land-sea synchronous surveys were conducted from 2012 to 2013 to characterize the relationship between the composition of land-based total dissolved nitrogen (TDN) and the concentration of dissolved inorganic nitrogen (DIN) in Jiaozhou Bay (JZB). A total of 11 freshwater riverine sampling sites were selected at the river mouths and at waste water outfalls around JZB, while a total 23 Bay stations were established in JZB. Among them, 11 Bay stations were located near the 11 outfalls. Each land-sea sampling was conducted synchronously during a semi-tidal cycle. The contribution of NO3N, NO2N, NH4N, and dissolved organic nitrogen (DON) to TDN in land-based freshwater were similar to those in JZB seawater, while the contribution of the sum of NO3N and NO2N to TDN and the contribution of DON to TDN were about 3.2 and 4.1 times higher than the contribution of NH4N to TDN, respectively. These results showed that inputs of all land-based forms of nitrogen impact the DIN in seawater. Spatial distributions of DIN and DON, showing a gradual decrease from inner bay to the mouth of the bay, were negatively correlated with S in different seasons. In summer and winter, the ratio of DIN to DON in seawater (Rs) gradually decreased from the inner bay to the center of the bay, and the ratio of land-based DIN to DON (RL) was less than RS, indicating net transformation from land-based DON into marine DIN. However, in spring and autumn, the distribution of Rs was opposite to that in summer and winter, and RL was greater than RS, indicating net conversion from land-based DIN into marine DON. Throughout the whole year, net land-based DON was transformed into marine DIN. We provided direct evidence that the variation in DIN concentration in JZB was affected both by land-based TDN inputs and by their hydrodynamic transport and biogeochemical transformation processes.