Nitrogen fixation in the shallow waters off a coastal wetland with outcropping peat

Biological nitrogen (N) fixation is the microbial transformation of atmospheric N₂ to ammonia, which is carried out by various groups of microorganisms and in all environments. The organisms, called diazotrophs, do not rely on bioavailable combined N such as nitrate or ammonium, which are often limiting ecosystem productivity. On the other hand, their activity provides nutrients to the otherwise N-limited ocean. In the central Baltic Sea high nitrogen fixation occurs each summer in surface waters introducing up to 792 000 t N per year, but was also identified in the deep and anoxic waters. In coastal waters the heterotrophic and autotrophic N₂ fixation is not well studied and even less is known about the annual cycle and its regulation by the environment. Since coastal environments are considered to act as a filter for nutrients and organic matter, knowledge on an additional N source through N₂ fixation is of great importance.

Here, we present N₂ fixation rates for bulk water and sediment slurries (upper 5 cm), incubated for 24 hours in the dark and during a daily light cycle. We selected three stations near a peatland with outcropping peat layers and sandy sediments. Monthly sampling over the course of one year was done together with in-situ measurements of temperature, salinity, pH, nutrient concentrations and dissolved organic substances. Incubations were spiked with ¹⁵N₂ gas and incubated in the lab. The fixation rates ranged from our detection limit up to 285 nmol N L^-1 d^-1 in water and 2 nmol N g_dw^-1 d^-1 in sediments with a mean fixation rate of 11.2 nmol N L^-1 d^-1 and 0.1 nmol N g_dw^-1 d^-1 for water and sediment, respectively. We could not find significant difference between stations and overall, the rates were much lower than in the surface waters of the central Baltic Sea. Though the rates in the water observed in June 2022 agree well with the rates of a cyanobacterial bloom in late summer (4.3 – 7.8 µmol N m^-3 h^-1). The rates for the water as for the sediment showed significant positive correlation (Spearman, sig. level 0.05) with variables affected by the seasonal change as temperature, daylength, pH and oxygen saturation. during winter and spring, the rates in the water were low to non-detectable and highest in summer. Also, in the sediment the lowest rates were found during winter and highest rates in spring. In general, the light cycle treatment showed higher rates than the dark incubation, with the exception of spring where the dark incubated sediments had higher rates than the ones in a daily light cycle. The outcropping peat layer seemed to induce some variability in N₂ fixation rates, reflecting the heterogeneity of substrate which was sometimes covered with sand layers of different thickness.

Even though the rates in this study are comparably low for both water and sediment, a seasonal pattern became visible. Sediments and shallow waters clearly deserve more attention to better understand the process and the potential role as food and nitrogen source.