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Nitrogen incorporation into total particulate suspended matter, hydrolysable amino acids and bacterial biomarker d-Alanine was assessed seasonally in the coastal North Sea using 15N-labeled ammonium, nitrate, nitrite and 15N- and 13C-labeled urea, glycine, leucine, phenylalanine, and two complex pools of dissolved organic matter (DOM) derived from algal and bacterial cultures (A-DOM, B-DOM). We investigated: 1) uptake rates for the various substrates and their contribution to total N uptake; 2) microbial preferences for the different N sources; 3) the coupling of C and N uptake from organic substrates; 4) the contribution of bacteria to the total microbial uptake of these substrates, and 5) the role of a complex pool of organic matter for plankton nutrition. Seasonality in the preferences for N substrates was observed, with A-DOM and B-DOM being preferred in autumn and winter whereas NH 4+ was preferentially taken up in spring and summer. C and N uptake was coupled for all the organic substrates, except urea that was mainly used as a nitrogen source in summer and spring. Bacterial contribution to the uptake of A-DOM and B-DOM was, on an annual average, the lowest among the N-substrates. This suggests an important role for phytoplankton in the incorporation of complex organic matter and the importance of DOM for phytoplankton nutrition. [ABSTRACT FROM AUTHOR]