Stable isotopes as a tool for nitrogen source identification and cycling in the Gulf of Trieste (Northern Adriatic)

• Load weighted δ15NNO3δ15NNO3 in rivers were controlled by land-use in their catchments.
• Manure fertilizers and/or sewage inputs were a source of nitrate at the rivers mouths.
• δ15NNO3δ15NNO3 in the Gulf of Trieste was a result of source mixing and assimilation.
• Nitrification in the marine system was present in autumn and winter.

Nitrogen concentrations and isotopic composition were used to identify nitrate sources and processes in the catchments of two Slovenian coastal rivers, Rižana and Dragonja, and to investigate whether they have any notable influence on the Gulf of Trieste. The sampling was performed from May 2010 to March 2011. The data indicated that δ15NNO3δ15NNO3 values in rivers provided mainly information about sources of riverine nitrate derived from soil nitrification, sewage, and/or manure. The load weighted δ15NNO3δ15NNO3 values of 4.3‰ and 9.4‰ determined at the mouth of Rižana and Dragonja, respectively, were largely controlled by land-use in the riverine catchment. Phytoplankton nitrate assimilation was found to be significant mainly at the mouth of Dragonja. The combined use of salinity, nutrient concentrations, and nitrate, particulate nitrogen and carbon isotopic compositions revealed that the seawater surface was influenced by mixing with different sources including seawater, rivers and sewage effluent. The site influenced by sewage effluent is relatively spatially isolated, which suggests that NO3−NO3− is not widely distributed by this point source. Besides mixing, phytoplankton uptake was the main process controlling the distribution and isotopic composition of NO3−NO3− in the marine system and was more extensive in spring 2010, while in autumn NH4+NH4+, not NO3−NO3−, was the dominant source of N for phytoplankton. In addition, our results are consistent with the occurrence of nitrification in the water column in autumn and winter. The nitrification activity was higher in autumn, while in winter it occurred in parallel to phytoplankton uptake. This study demonstrates how stable isotopes of nitrogen in nitrate along with concentration measurements can be relevant when trying to identify the sources and key processes such as assimilation and nitrification in rivers and coastal areas.