Conversion of nutrients in the Elbe river plume during drift experiments in the German Bight during spring 1995 and summer 1994

SummaryDuring spring 1995 and summer 1994, drift experiments lasting 10 and 8.5 days, respectively, were performed in the framework of the interdisciplinary project KUSTOS to quantify the net turnover in selected defined water masses of the propagating Elbe river plume in the German Bight, North Sea. Besides sampling near a drifter with a drag sail at about 3 m depth, surface salinity was measured along east-west transects crossing the actual drift position in order to estimate the representativity of the sampling at the drifter. Vertical profiles of total N and P as well as their different fractions (inorganic nutrients, dissolved organic and particulate matter) are presented from the central drift position. Decreases of nutrients near the surface were partly compensated by increases of biomass. High contributions of dissolved organic phosphorus in the mixed layer indicated a close relationship to biomass production which was not observed for dissolved organic nitrogen. Due to stratification, with oscillating densiclines at about 10 m depth and diverse advection in the different layers, conversion processes were only calculated for the upper 10 m of the mixed layer. During spring, high freshwater discharges of 1000-1400 m3/s caused strong vertical salinity gradients. During both spring and summer, the temperature differences between surface and bottom water were about 5 ‡C. The highest nutrient consumption (2.5 ΜM N/d, 0.05 pM P/d) and biomass formation (1.8 ΜM N/d, 0.03 ΜM P/d) was calculated for the upper 10 m during spring. During summer, with low river discharges (500 m3/s) and low nutrient fluxes, only moderate net uptake (0.04 ΜM N/d) and biomass increase (0.16 ΜM N/d) were found. Phosphate showed a small increase during summer due to maximum seasonal net remineralisation and remobilisation. In comparison with the conversion rates estimated from incubation experiments, these rates only had the same order of magnitude in spring and exceeded those in summer by far. This indicates the high turnover and exchange processes in the shallow ecosystem of the German Bight, especially during summer.