SNOOP

1. SNOOP The effect of ship Nox deposition on cyanobacteria blooms Marine Systems Institute at Tallinn University of Technology Urmas Raudsepp

2. Aim Additional atmospheric deposition of nitrogen caused by shipping likely result in changes of phytoplankton spring bloom and summer nitrogen-fixing cyanobacteria bloom dynamics. The task was to assess the degree of inter-annual variability of spring and summer phytoplankton blooms and nitrogen fixation in response to a shipnitrogen deposition on the background of natural variability of environmental conditions in the Gulf of Finland. Two 10-year runs with coupled3D physical and biological-chemical model were performed using atmospheric deposition of NOx without ship deposition and including ship deposition (year 2008).

3. 3D physical (GETM) and biological-chemical model (ERGOM) Model domain was the whole Baltic Sea Model domain Gulf of Finland Blue line B along longitude 23º30’E marks the western boundary of the Gulf of Finland basin in the present study

4. Model validation (monitoring station LL7) D is mean of observations M is mean of simulated data SD is standard deviation CFis cost function Cost function definition (Eilola et al. 2009): CF =|(M–D)/SD| SD is standard deviation of observations Cost function values: 0–1 indicate good result, 1–2 indicate reasonable result and >2 indicate poor result. The 10-year mean, standard deviation and cost function values of temperature (T, ºC), salinity (S), dissolved oxygen (O, ml l–1), nitrate (N, mmol m–3), phosphate (P, mmol m–3) for the surface (s) and near-bottom (b) layers and chlorophyll (Chl, mg m–3).

5. Atmospheric and ship NOx deposition to the Gulf in 2008 Monthly atmospheric NOx deposition (t N month–1) without ship deposition (black bars) and ship NOx deposition (t N month–1) (shaded bars) to the Gulf of Finland area in 2008. Annual atmospheric NOx deposition was 13.7 kton and ship NOx deposition 1.6 kton to the Gulf of Finland area.Annual ships NOx deposition is about 12% from total atmospheric deposition.Ship NOx deposition is highestin May, June, July and August(20-30%).

6. Annual ship NOx deposition to the Gulf of Finland area (year 2008). Distribution of annual ship NOx deposition (kg N km–2 y–1) to the Gulf of Finland area. The annual ship NOx deposition was larger in the western and central Gulf (up to 70 kg N km–2 y–1) compared to the eastern part (10-40 kg N km–2 y–1).

7. Winter nutrient conditions in the Gulf of Finlandupper layer Mean winter ratio of dissolved inorganic nitrogen to phosphorus (DIN:DIP) in the upper 10-m water column. DIN:DIP ratio was low, varying inter-annually from 8.5 to 11.5. The increase of DIN:DIP ratio caused by ship NOx deposition was minor (about 1%) compared with natural variability. Mean winter excess phosphate (eP=DIP–DIN/16) in the upper 10-m layer. Black bars show excess phosphate without ship NOx deposition and shaded bars show excess phosphate ship NOx deposition included. The addition of ship NOx deposition reduced excess phosphate from 1 to 2.5% depending on DIN and DIP resources in the upper layer.

8. Annual phytoplankton primary production and nitrogen fixation by cyanobacteria (in nitrogen units). phytoplankton cyanobacteria Annual primary production of phytoplankton over the Gulf area. Green bars indicate phytoplankton production and blue bars nitrogen fixation by cyanobacteria. Simulations performed without ship NOx deposition. Note different scales.

9. Impact of ship NOx deposition on the phytoplankton primary production and nitrogen fixationby cyanobacteria phytoplankton cyanobacteria Changes of the annual phytoplankton primary production (green bars) and cyanobacteria nitrogen fixation (blue bars) caused by ship NOx deposition in the Gulf water column. Changes are calculated: primary production with ships−primary production without ships nitrogen fixation with ships − nitrogen fixation without ships.

10. 10-year mean decrease of nitrogen fixation rate in the Gulf Distribution of 10-year mean decreaseof nitrogen fixation rate (ship NOx deposition included–without ship NOx deposition) in the Gulf of Finland Annual averaged nitrogen fixation over the Gulf area was 1300 kg N km-2 y-1

11. Conclusions • The annual ship NOx deposition was about 1.6 kt N y–1 i.e. 12% from annual atmospheric deposition. • Ship NOx deposition was larger during summer months when the relative importance is about 20-30%. • Ship NOx deposition caused the increase of spring bloom and post bloom biomass phytoplankton biomass (~1%) reducing at the same time phosphate resources. • Ship NOx deposition causes the decrease of nitrogen fixation about 2-6%. Thus the increase of the Gulf nitrogen pool caused by the additional ship NOx deposition to the Gulf was partly compensated by the decrease of nitrogen fixation. • The effect of ship NOx deposition on cyanobacteria bloom was larger in the westernand centralGulf of Finland. • No trend in phytoplankton primary production and nitrogen fixation due to accumulation of ship NOx deposition in the Gulf water column was observed. The 10-year simulation with coupled hydrodynamic and ecological model forced by ship NOx deposition showed changes in the dynamics of phytoplankton spring bloom and summer N-fixing cyanobacteria bloom. Due to the phytoplankton composition (green algae+cyanobacteria) the annualtotal biomass in the Gulf is actually limited by phosphorus, i.e. the increase of green algae primary production is practically compensated by the decrease of nitrogen fixation by cyanobacteria.