Evidence of under-ice phytoplankton blooms in the Chukchi Sea from 1998 to 2012

The discovery in 2011 of a massive phytoplankton bloom underneath first-year sea ice in the western Arctic has prompted an investigation of the spatial and temporal distribution of under-ice phytoplankton blooms. Here, we explore the satellite record from years 1998 to 2012 for evidence of under-ice blooms on the Chukchi Sea shelf. Phytoplankton blooms were categorized as under-ice blooms, probable under-ice blooms, or marginal ice zone blooms, depending on bloom timing in relation to the timing of ice retreat. Annual bloom type maps reveal that under-ice phytoplankton blooms were present in every year of the satellite record. Averaged over all years, the combination of under-ice blooms and probable under-ice blooms covered a portion of the observable study area that was 2.5-fold higher than that of marginal ice zone blooms (71.5% and 28.5%, respectively). This finding strongly contradicts the traditional view that phytoplankton in seasonally ice-covered waters bloom only after ice retreat and instead indicates that blooms are initiated whenever light and nutrient availability is sufficient for photosynthesis, a condition often reached early in the season underneath first-year sea ice on nutrient-rich continental shelves. Spatial patterns in bloom type were distinguished relative to the date of ice retreat, with probable under-ice blooms dominating the nutrient-rich western Chukchi Sea and at higher latitudes where ice retreats later, while marginal ice zone blooms were most common in the southern and eastern Chukchi Sea where ice retreats earlier. Our results suggest that under-ice phytoplankton blooms are widespread in the Chukchi Sea and had been prevalent there for more than a decade prior to their discovery in 2011.