Seasonal variation of nitrogen transformations in the pelagial of selected nearshore waters of the Baltic Sea with emphasis on the particulate pool

Abstract

Physical and chemical conditions, particulate matter and N-uptake were characterized at two sampling sites at the eastern German coast of the Baltic Sea (Pomeranian Bay) over the annual period of 1997 (February–November). The inshore sampling sites (5 m water depth) differed with respect to the potential influences of river run-off and salt water exchange (mean values of salinity: 7.05 and 8.72 PSU), respectively. The mean org-C_diss/org-C_part-ratios (4.9 and 12.6) fell in the same order of magnitude (1.0–12.6) as values of neighboring inshore waters, and increasing values reflect an enhancement of the trophic level. Beside differences of nitrogen concentrations (dissolved inorganic nitrogen: 1.8–23.8 and 0.9–9.9 μmol l⁻¹), particulate nitrogen (4.30–41.01 and 2.69–9.08 μmol l⁻¹) and absolute uptake of N-nutrients (mean sum of NH₄ ⁺, urea, NO₃ ⁻ uptake rates: 0.141 and 0.087 μmol l⁻¹ h⁻¹), specific uptake of ¹⁵N-labelled nutrients (NH₄ ⁺, urea, NO₃ ⁻) as well as the relationships between the measured variables characterize distinguishable inshore systems. The high variability at the shallow sampling sites prevents, however a simple resolution of the seasonal courses. Light dose could be identified as a potential key in order to describe long-term variations of N-uptake at the station with higher organic matter concentration (station KW), but phytoplankton development is better reflected in the seasonal course of N-uptake at the other station. Specific nitrogen uptake rates (NH₄ ⁺: 0.0009–0.0353 h⁻¹, urea: 0.0001–0.0137 h⁻¹, NO₃ ⁻: 0.000004–0.0009 h⁻¹) and relative nitrogen preferences indicate extraordinary importance of reduced nitrogenous nutrients (NH₄ ⁺, urea) at both stations throughout the year.