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Variations in concentrations of dissolved free amino acids (DFA4) were investigated during a phytoplankton bloom in the northern North Sea (Fladen Ground Area). Carbon to nitrogen ratios ( U N ) were calculated from the distribution patterns of the individual amino acids and related to biological and chemical developments. The ratio shifted from winter conditions of about 2.9 to values of more than 3.6 during the steady state phase and showed significant correlations only with the distribution of diatoms, mainly Chaetoceros species in the euphotic zone. Here phytoflagellates as well as zooplankton showed positive correlations with ornithine and lysine (zooplankton also with aspartic acid). Peridineans were negatively correlated with threonine. During the early exponential diatom growth phase a DFAA maximum occurred with a C/N ratio of 3.16 in the water column above the thermocline, while below there was a maximum with a ratio of 3.71 in the late exponential phase. With the help of similar results from an enclosed diatom culture of Thalassiosira rotula an attempt was made to relate the temporal and spatial C/N distribut~ons to the physiological state of the diatoms.

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