ABSTRACT: The German Bight contains waters of mixed origin: freshwater input from the rivers Elbe, Ems and Weser, Wadden Sea water that is rich in organic compounds, and highly saline water (>33 SU) from the central North Sea. In order to investigate the very complex biological and biogeochemical situation caused by the various types of fronts and different water bodies, a spring drift experiment was carried out in this area following a survey of 45 stations covering the German Bight. A typical springtime situation was found: diatom blooms (biomasses >500 ug C l^-1 in the mouth of the river Elbe) had decreased the silicate to around 0.3 uM; phosphate concentrations (<0.03 uM) were limiting primary production in the northeastern part of the investigated area, but, due to remineralization processes and riverine entrainment, they showed higher values in the south and in the river Elbe mouth (>0.3 uM). Nitrate ranged from 10 to 40 uM and was clearly dominated by the river Elbe input. From the southwest a beginning bloom of Phaeocystis globosa spread into the drift area and, later on, into the whole German Bight. Zooplankton biomass was highest along the North Frisian coast (>50 ug C l^-1), and was dominated by copepods at the northwestern border of the German Bight and in the drift area. During the drift experiment, 2 phases and a transition state could be distinguished: In the first phase, the total phytoplankton biomass was low and nutrient concentrations remained relatively constant. In the transition state, strong wind forces moved the drifter southwards into another water body. A significant drop in nitrate, nitrite and phosphate concentrations was observed, whereas silicate concentrations almost doubled. During the second phase of the experiment, a strong increase of total phytoplankton biomass (cell size >5 um) was recorded, dominated by P. globosa (>95%) and accompanied by rising concentrations of particulate compounds. Simultaneously, the nutrient concentrations declined, and the zooplankton biomass reached minimum values. At a depth of 10 m, ammonium and phosphate concentrations showed strong diurnal variations, indicating enhanced planktonic activity during the third phase. Summing up the results, the overall evaluation of the drift experiment as a method for studying in situ biological and biogeochemical processes within the same water body was positive.

KEY WORDS: German Bight · Phytoplankton · Zooplankton · Nutrients · Drift experiment

Published in MEPS Vol. 156 (1997) on September 25
ISSN: 0171-8630. Copyright © Inter-Research, Oldendorf/Luhe, 1997