Phytoplankton responses to the 2014–2016 warming anomaly in the northeast subarctic Pacific Ocean

Abstract

Significant changes in the abundance and composition of phytoplankton were observed along Line P in the northeast subarctic Pacific as a result of a rapid warming of surface waters in 2014–2015. This feature, labeled “the blob,” reached ~ 4°C above normal and restricted winter ocean–surface nutrient renewal due to increased stratification. As a result, surface nutrients were the lowest observed and nitrate depletion in summer extended farther offshore than in the last three decades. Within this nitrate-depleted region, there was unusually low phytoplankton biomass and a dramatic increase in the dominance of cyanobacteria, including Prochlorococcus, which had not been previously observed in this region. Farther offshore, in the iron-limited region, phytoplankton biomass and the abundance of haptophytes and chlorophytes increased during “the blob.” By 2016, surface nutrient and phytoplankton concentrations were still low, but at most of the stations, phytoplankton composition was similar to that observed before the warming occurred, except for an increase in diatoms farther offshore. These changes at the base of the food web could have ecosystem-wide implications.