This data was collected from a mesocosm experiment to assess the influence of silicic acid on Arctic phytoplankton, incubating the spring bloom community of Kongsfjorden, Svalbard, for 3 weeks (May 4 - May 26) under a gradient of Si concentrations. For phytoplankton community composition via light microscopy, a 50 mL subsample was taken from each mesocosm once per week and preserved with hexamine-buffered formaldehyde (1-2% final concentration)- Microscopic analyses were conducted with an inverted light microscope (Zeiss Axio Observer D1, Oberkochen, Germany) by settling between 3 mL and 10 mL of sample in an Utermöhl chamber (Utermöhl, 1958). For sampling days 0 and 22 a different inverted light microscope was used (Zeiss Axiovert 135, Oberkochen, Germany). Identification to the lowest possible taxonomic level was based on available literature (Tomas et al., 1997; Throndsen, Hasle and Tangen, 2007; Hoppenrath, Elbrächter and Drebes, 2009; Kraberg, Baumann and Dürselen, 2010).