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. Throughout the study light and temperature were monitored inside each mesocosm with a HOBO pendant logger (Hobo, Massachusetts, US). The loggers were weighed down to ensure a vertical position in the water column and submerged ~20 cm below the water surface. To compare light conditions inside and outside the mesocosms an additional logger was placed next to mesocosm 5.