Investigation of the potential use of the supersaturation ratio of N2 (ΔN2) for the estimation of the seasonal and spatial variability of denitrification and anammox in the water column of the Baltic Proper

Denitrification and anammox are the main nitrogen (N) removal pathways in seawaters. Both processes are important in regions, such as the Baltic Sea, which receive high nutrient loads, that enhance primary production and eutrophication. The Baltic Sea is also characterized by a strong vertical salinity gradient and the presence of a permanent halocline hampering mixing in the water column and ventilation of the deep water layers. Rare events of deep water renewal, together with high oxygen consumption, lead to suboxic and anoxic conditions in the Baltic Sea, which are favorable for denitrification and anammox – processes for which the end product is a non-reactive N₂. In seawater, the concentration of dissolved gases is controlled by biological and physical processes. The latter can be traced by measuring inert gases such as argon (Ar). Hence, the N₂/Ar ratio can be used to separate physical and biological effects influencing N₂ fields. This approach may suit especially to the stratified water bodies, where deep waters are separated from the surface water layer influenced by the gas exchange with the atmosphere.

The study aimed at investigating the potential use of the supersaturation ratio – ΔN₂ as a tracer of denitrification and anammox processes in the water column of the Baltic Proper. The ΔN₂ ratio was derived as an anomaly from the N₂/Ar ratio in seawater being at equilibrium with the atmosphere. The used technique was Membrane Inlet Mass Spectrometry, which allows performing high-precision measurements of dissolved N₂ and Ar in water (masses 28 and 40 were detected, respectively). Seawater samples were collected between 2017 and 2021 from nineteen stations, including Gdańsk, Gotland, and Bornholm Deeps.

The ΔN₂ indicated N₂ accumulation in the oxygen minimum zones below the halocline with the highest values found​​ in the bottom layers. This can be explained by both denitrification and possibly anammox in the water column and with N₂ release from sediments. ΔN₂ values ranged from 1.0 to 32.6 µmol L^-1. In autumn 2021 a significant difference in ΔN₂ (p = 0.0008) between the studied sites was observed. For example on station located in Gotland Deep ΔN₂ values were in the range from 17.6 to 32.6 µmol L^-1, while on station located in the Central Baltic Proper the maximum was 6.1 µmol L^-1. The seasonal ΔN₂ changes (autumn, spring, and winter) were investigated for two stations located in the Gdańsk Deep and indicated statistically significant variability (p=0.0077) with the highest ΔN₂ observed in winter. Additionally, ΔN₂ was negatively correlated with nitrate (R²=0.5469) and oxygen (R²=0.6382), positively with phosphate (R²=0.4382) and ammonium (R²=0.2898), while no clear dependency was observed for nitrite (R²=0.0388).

The presented study was the first attempt performed on such a large scale in the Baltic Proper. It demonstrates a high potential in the use of supersaturation ratio for identification of the active sites for denitrification and anammox processes.

 

The results were obtained within the framework of the statutory activities of the Institute of Oceanology Polish Academy of Sciences and the research project: 2019/34/E/ST10/00217 funded by the Polish National Science Centre.