Sedimentary nitrogen removal processes across coastal, lagoon and open waters of the Baltic Sea

Denitrification and anammox (anaerobic ammonium oxidation) are the main nitrogen removal pathways. Denitrification is a microbial process in which NO₃^- is sequentially reduced to dinitrogen gas (N₂) while anammox is the anaerobic microbiological process in which NO₂^- and NH₄⁺ are converted to N₂ under anoxic conditions. Both processes are critical in regulating nitrogen (N) availability in marine ecosystems, particularly in the stratified and oxygen-depleted aquifers such as Baltic Sea. The Baltic Sea, highly complex and semi-enclosed marine ecosystem that contains brackish water due to high freshwater discharge and limited water exchange with the North Sea. The sedimentary nitrogen cycling was studied extensively in the Baltic Sea but still, understanding the nitrogen loss process, especially in the coastal area is challenging. Additionally, studies usually use different methods to assess the N removal rates which disables the comparison of obtained rates and limits the overall understanding of the N cycle. The main aim of the study was to quantify denitrification and anammox rates in surface sediments from various locations in the Baltic Sea. Three coastal sites MP2 (Puck Bay), PB3 (Puck Bay under submarine groundwater discharge (SGD) impact),  lagoon MS2 (Szczecin lagoon) and two open-sea sites IDEAL, P1 (Baltic Proper) were selected for this study. Slurry incubation experiments were conducted at each site with the addition of labelled substrates ¹⁵NO₂⁻ and ¹⁵NH₄⁺ to measure denitrification and anammox rates. The addition of ¹⁵NO₂⁻ produced ¹⁴N¹⁴N, ¹⁴N¹⁵N, and ¹⁵N¹⁵N for denitrification, while ¹⁵NH₄⁺ produced ¹⁴N¹⁴N and ¹⁴N¹⁵N for anammox. The denitrification rate in the coastal sites ranged from 1440.82 to 7.21 nM N L^-1 d^-1, for the open sea sites (IDEAL) 533.42 nM N L^-1 d^-1  and at P1 consumption of N₂ was observed. Apart from MP2, anammox activity was detected at PB3 (32.67 nM N L^-1 d^-1), MS2 (0.41 nM N L^-1 d^-1), IDEAL (0.46 nM N L^-1 d^-1), and P1 (0.67 nM N L^-1 d^-1). The anammox rates were lower than denitrification at all sites, indicating its minor role in nitrogen removal in the surface sediments of Baltic Sea. However, the presence of anammox highlights the contribution of a diverse microbial community that can increase with the future expansion of anoxic areas in the Baltic Sea. The observed spatial variability in N removal rates is likely influenced by site-specific factors such as organic matter availability, nutrient discharge, and oxygen conditions. However, hypoxic submarine groundwater discharge (SGD), enriched in nutrients and dissolved organic carbon appears to be a key driver of nitrogen removal. Further studies employing similar methodological approaches are essential to better understand nitrogen cycling in marine ecosystems, particularly in coastal areas.

Acknowledgments

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