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 Gdansk Deep during the first ten days of June 2001. The mean concentrations of nutrients in the euphotic zone were: N-N (NO2-, NO3-, NH4+) - 1.93 mu mol dm(-3), PO43- -0.12 mu mol dm(-3) and Fe-tot - 0.11 mu mol 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 minimurn (0.03 mu mol dm(-3)) between 04:00 and 10:00 hrs, while total iron dropped to 0.01 mu mol dm(-3) between 10:00 and 16:00 hrs. Both levels were below the limiting values for phosphorus and iron. At night, concentrations of N03 and PO43- rose by 25%, those of NH4+ and Fe-tot by 35%. The mean molar ratios of N-N:PO43- and Fe-tot:PO43- in the surface layer were subject to significant daily fluctuations. The molar NNPO43- 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.