The demand for organic certified seaweed is growing in European countries, but only few studies have investigated how nutrient sources approved for organic seaweed farming affect the growth and development of macroalgae. Through a series of four experiments, this study documented the effects of nitrogen (N) source and concentration on the key hatchery processes of meiospore germination, gametogenesis and initial sporophyte growth of Saccharina latissima. The nutrient sources were Provasoli Enriched Seawater (PES) with NO
3-N or NH
4-N and six potential organic certified growth media with N predominantly as NH
4-N, all with N concentrations up to 150 μM N. The potential organic certified N sources were: Degassed manure, protamylasse, three commercial liquid fertilizers and mussel excreted ammonium. In contrast to gametogenesis and initial sporophyte growth, the germination of S. latissima meiospores was generally not affected by neither N source nor concentration. Gametogenesis was the process being most sensitive to N source and concentration: In PES media, the N concentration had an overall positive effect on gametogenesis, albeit at concentrations of 150 μM NH
4, gametogenesis was impaired. The degassed manure, one liquid fertilizer and mussel excreted ammonium supported successful gametogenesis and sporophyte growth, but resulted in a 4.7, 20 and 1.3 fold lower sporophyte density after 23 days of growth, respectively, as compared to cultures growing with NO
3-PES. Optimum N concentrations were 50–100 μM N. We conclude that degassed manure and mussel excreted ammonium in concentrations <100 μM NH
4-N, are useful, but suboptimal N sources, for an organic hatchery production of S. latissima.
