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Nitrates (3D CEMBS model)
The concentration of nitrates (NO3) calculated using the Baltic Sea ecosystem model 3D CEMBS (3D Coupled EcosystemModel of the Baltic Sea). The SO SatBałtyk service provides distributions of nitrate concentrations at the surface and different depths in milligrams per cubic metre [mg/m3], four times a day, in the form of maps with a spatial resolution of 1 km.
Methodology for determining nitrate concentrations using the 3D CEMBS model
The concentration of nitrates is calculated using the 3D CEMBS prognostic ecohydrodynamic model [1] [2]. The boundary conditions at the sea surface are atmospheric data from the UM prognostic model (ICM University of Warsaw). The model has an open boundary with the North Sea for the better mapping of changes resulting from inflows of North Sea water. The 3D CEMBS model is equipped with a river inflow module, which supplies information on the amount of fresh water entering the Baltic from 72 rivers and the amounts of nutrients (including nitrates) it carries. In addition, the atmospheric module supplies information on nutrients entering the Baltic via precipitation. KPP parameterization has been applied in order to ensure that mapping of mixing in the vertical is as accurate as possible. The model results have a horizontal resolution of ca 2 km. In the vertical the model is divided into 21 layers. The first four layers are each 5 m thick. The thickness of the other layers increases with depth. The results supplied to the SO SatBałtyk System have been interpolated on a grid with a resolution of 1 km. The model has a module for assimilating remote sensing data (sea surface temperature and chlorophyll a concentration) obtained from the MODIS sensor deployed on the Aqua (EOS PM) satellite. Cressman’s [3] method is used for this assimilation. This involves calculating, for every point on the model grid, temperatures and chlorophyll a concentrations on the basis of data from the previous model forecast and satellite data weighted according to the distance between these data and the accuracy of the results. The algorithm ignores overcast areas and remote sensing data deviating widely from the model values. Since the distribution of chlorophyll a is a logarithmically normal one, all the processes in its assimilation are performed for logged values.
Validation (assessment of accuracy) The accuracy of the model values was assessed by comparing them with available measurements made during ICES monitoring cruises in 2010-2013. The statistical error, expressed as the standard deviation of differences, was estimated at 36.14 mg/m3. The systematic error (mean difference) was 21.57 mg/m3.
Eutrophication
Eutrophication, that is, the enhanced inflow of nutrients into waters, causes the productivity of aquatic ecosystems to increase. In consequence, water quality deteriorates, leading among other things to less light penetrating the water, oxygen deficits and the death of fish. At first, eutrophication was common only in freshwater ecosystems, but it has become a global problem, affecting sea water areas, including the Baltic Sea. Silicates are nutrients, and high levels of them give rise to eutrophication, contributing to “water blooms”, in other words, the intensive growth of phytoplankton biomass. Nitrates enter the Baltic mainly with river waters. In this way about 75% of total nitrate load is entered to the Baltic Sea.Concentration distribution maps show that the highest levels are measured around river mouths and in coastal waters (Figure). The remainder of the nitrate load gets into the sea from scattered inflows, point discharges from sewage treatment plants and atmospheric precipitation; nitrates are also released from bottom sediments. Silicates have the greatest effect on the growth of one group of phytoplankton
Concentration distribution maps show that the highest levels are measured around river mouths and in coastal waters (Figure). The remainder of the silicate load gets into the sea from scattered inflows, point discharges from sewage treatment plants and atmospheric precipitation; silicates are also released from bottom sediments. Nitrates are the most important component limiting phytoplankton biomass.
The seasonal nutrient cycle
A characteristic feature of nutrients is their cyclic presence in Baltic Sea waters, directly linked to phytoplankton blooms [7]. The stages in phytoplankton growth are the same all over the Baltic. In spring there is an intensive but short-lived bloom, and then another, longer one from mid-summer extending all the way into autumn. During the spring bloom, nutrients are consumed, which means that their levels immediately afterwards are very low. Weak vertical mixing maintains low levels of nutrients until the autumn, when they are raised from deeper water layers and replenish the euphotic zone. This process is sufficiently strong to ensure that the nutrient supply is not exhausted, even though the autumnal bloom is not as intensive as the one in spring. The cessation of primary production in winter enables the full regeneration of nutrient resources in the euphotic zone (Figure).
Nitrates (3DCEMBS model) (for registered users)
[4] Ministerstwo Środowiska, 2010, Wstępny Krajowy Program Wdrażania Bałtyckiego Planu Działań
[5] Council Directive 91/271/EEC of 21 May 1991 concerning urban waste-water treatment
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