Recent Changes of Phytoplankton Bloom Phenology in the Northern High-Latitude Oceans (2003-2020)

Eighteen years (2003-2020) of data on remotely-sensed ocean color in high northern latitudes show a significant increase (0.56% yr(-1)) in double blooms of phytoplankton. Starting in 2015 the rate of increase rose to 1.10% yr(-1). Double blooms have become especially prevalent in regions north of 50 degrees N in the North Atlantic, Gulf of Alaska and southern Chukchi Sea. Our correlation analysis suggests that double bloom occurrences are dependent on latitude and seasonal changes in both sea surface temperature (SST) and mixed layer depth (MLD), particularly in the subpolar North Pacific (SNP) and polar oceans (65 degrees-75 degrees N), although we cannot attribute the increased double blooms to specific causes. Single blooms indicated by Chlorophyll a (Chl a) dominate in subpolar-polar waters and occur close to the timing of the Particulate Inorganic Carbon (PIC) peak (i.e., June-August). In the SNP single bloom, Chl a peaks occur earlier than PIC peaks. This interval was shorter during 2016-2020, which is associated with warming anomalies. In the southern part of the Chukchi Sea, however, we found that PIC peaks occur earlier than Chl a peaks, suggesting a potentially different succession pattern between calcifiers and other functional groups. We speculate that in the near future double blooms of phytoplankton will become a prevalent feature in high-latitude oceans with concurrent changes in the timing of blooms and seasonal phytoplankton succession in the surface ocean due to climate change.