The effects of the herbicide atrazine on marine phytoplankton typical of the German Bight (North Sea) were investigated by means of four mesocosm (tank) experiments. Three tanks were spiked with atrazine (120, 560, and 5,800 ng/L, respectively). These concentrations cover the range found in the German Bight, in coastal waters, and in the estuary of the river Elbe. The lowest atrazine concentration applied was of the same order of magnitude as the background concentration of triazine herbicides in North Sea water near the island of Heligoland. A relationship between concentration of atrazine (dosis) and concentration of excreted dissolved organic phosphorus (DOP) as well as dissolved organic nitrogen (DON) (effect) can be inferred, where high concentrations of triazines lead to high concentrations of DON and DOP. Furthermore, the decrease in the pH-value from 9.2 to about 8.7–8.9 in the three tanks spiked with atrazine during the stationary growth phase in comparison with the fourth tank (control, i.e., background levels) implies that photosynthesis is reduced. This effect is accompanied by lower chlorophyll concentrations, reduced primary production, and higher concentrations of amino acids in the water.