Seasonal variation in nitrification and nitrate- reduction pathways in coastal sediments in the Gulf of Finland, Baltic Sea

The Baltic Sea is one of the most eutrophic marine areas in the world. The role of nitrogen as a eutrophicating nutrient in the Baltic Sea has remained controversial owing to a lack of understanding of nitrogen cycling in the area. We investigated the seasonal variation in sediment nitrification, denitrification, anaerobic ammonium oxidation (anammox), and dissimilatory nitrate reduction to ammonium (DNRA) at 2 coastal sites in the Gulf of Finland. In addition to the estimation of in situ rates, we assessed the potential for nitrification and denitrification in different seasons. The nitrification and nitrogen removal processes were maximal during the warm summer months, when the sediment organic content was highest. In colder seasons, the rates of nitrification and nitrate reduction measured under in situ conditions decreased, but the potential for nitrification remained equal to, or higher than, that during the warm months. The rates of denitrification and nitrification were usually higher in the accumulation basin, where the organic content of the sediment was higher, but the transportation area, despite lower denitrification rates and potential, typically had a higher potential for nitrification than the accumulation basin. Anammox and DNRA were not important nitrate sinks in any of the seasons sampled. The results also show that the rates of denitrification in the coastal Gulf of Finland sediment have decreased, and that benthic denitrification might be a less important sink for fixed nitrogen than previously assumed.