Long term experiment using a semi-continuous batch cycle system with the phytoplankton species Chaetoceros affinis and Emiliania huxleyi each consisting of 9 genotypes (http://roscoff-culture-collection.org/; Emiliania huxleyi: GC22: RCC6383, C42: RCC6376, C41: RCC6375, C47: RCC4546, C96: RCC6381, C91: RCC6380, C35: RCC6374, C30: RCC6371, C48: RCC6377, Chaetoceros affinis: B63: RCC6347, B67: RCC6348, B68: RCC6349, B74: RCC6350). Cultivation of species separately in mono-cultures and together in mix-cultures at three different nutrient regimes (10N:1P, 20N:1P, and 30N:1P) with increasing nitrate concentration over 91 days. Nutrient concentrations were 8.60 ± 0.62, 19.08 ± 0.32, and 29.36 ± 0.47 µmol L−1 nitrate and 0.93 ± 0.09 phosphate. Silicate concentrations were aligned to reflect a 4:1 N:Si ratio. Transfer of part of the cells into bottles with new nutrients every 7 days at fixed batch cycle length and after 7,4, and 10 days in a recurring fashion at variable batch cycle length. Measurements of cell density and cell volume at the end of every batch cycle and cell density or fluorescence daily during the third batch cycle. Nitrate, phosphate, and silicate measurements in mix-cultures at variable batch cycle length at the end of the first (7 days), second (4 days), and third (10 days) batch cycle. Daily nutrient measurements in mix-cultures from day 4 to day 7 at batch cycle seven.