Impact of ship-borne nitrogen deposition on the Gulf of Finland ecosystem: an evaluation*

The degree of inter-annual variability in spring and summer phytoplankton blooms and nitrogen fixation in response to the deposition of oxidized nitrogen originating from ship emissions (hereafter nitrogen deposition) was evaluated in the Gulf of Finland (Baltic Sea) based on 10-year (1997–2006) simulation results using a coupled hydrodynamic (GETM) and ecological (ERGOM) model. Ship emissions were generated for 2008 using the Ship Traffic Emission Assessment Model, and ship nitrogen deposition was calculated using the Hilatar chemistry-transport model over the Baltic Sea. The annual ship nitrogen deposition in the Gulf of Finland was 1.6 kt N, about 12% of the annual atmospheric deposition, and increased in summer, up to 30% compared to the monthly atmospheric deposition. Ship nitrogen deposition caused an increase in spring and post-bloom primary production in two functional groups (diatoms and flagellates), at the same time reducing phosphate resources in the upper layer. Nitrogen fixation due to ship nitrogen deposition decreased by 1–1.6 kt N year−1 (2–6%). The effect of ship nitrogen deposition on nitrogen fixation was greater in the western and central Gulf of Finland. The additional ship nitrogen deposition to the Gulf was practically compensated for by a decrease in nitrogen fixation.