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Diurnal variations in nitrogen, phosphorus and iron compounds in the southern Baltic Sea

Author(s): Gabriela Sikorowicz | Lucyna Falkowska | Dorota Burska | Dominika Dunajska | Dorota Pryputniewicz | Rafał Magulski | Anita Lewandowska

Journal: Oceanologia ISSN 0078-3234
Volume: 47; Issue: 2; Start page: 243; Date: 2005;
Original page

Keywords: Nutrients | Iron | Oxygen conditions | Density stratification | Gdańsk Deep

ABSTRACT
In order to assess their short-term variability, nutrient concentrations were measured at standard depths at 2 h intervals in the deepest region of the Gdańsk Deep during the first ten days of June 2001. The mean concentrations of nutrients in the euphotic zone were: NN (NO2-, NO3-, NH4+) - 1.93 µmol dm-3, PO43- - 0.12 µmol dm-3 and Fetot - 0.11 µmol dm-3. During daylight hours, when the rate of assimilation was fastest, concentrations of nitrogen, phosphorus and iron compounds were very low. The phosphate concentration fell to a minimum (0.03 µmol dm-3) between 04:00 and 10:00 hrs, while total iron dropped to 0.01 µmol dm-3 between 10:00 and 16:00 hrs. Both levels were below the limiting values for phosphorus and iron. At night, concentrations of NO3- and PO43- rose by 25%, those of NH4+ and Fetot by 35%. The mean molar ratios of NN:PO43- and Fetot:PO43- in the surface layer were subject to significant daily fluctuations. The molar NN:PO43- ratio was higher than the optimum value established for the Baltic Sea. Below the halocline, the concentrations of dissolved iron and phosphorus rose as a result of diffusion from sediments in response to changing redox conditions.