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

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 (NO_2-, NO_3-, NH_4+) - 1.93 žmol dm^-3, PO_4^3- - 0.12 žmol dm^-3 and Fetot - 0.11 žmol 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 žmol dm^-3) between 04:00 and 10:00 hrs, while total iron dropped to 0.01 žmol dm^-3 between 10:00 and 16:00 hrs. Both levels were below the limiting values for phosphorus and iron. At night, concentrations of NO_3- and PO_4^3- rose by 25%, those of NH_4+ and Fetot by 35%. The mean molar ratios of N_N:PO_4^3- and Fetot:PO_4^3- in the surface layer were subject to significant daily fluctuations. The molar N_N:PO_4^3- 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.