LAUSR

Abstract The inflow of Alaska Coastal Water (ACW) acts as a source of heat and freshwater for the Arctic Ocean, and contributes to ecosystem succession as well as phytoplankton change. To explore the mechanisms and patterns of the phytoplankton composition in response to the ACW, diagnostic pigments and environmental parameters were analyzed along three similar transects from shelf to basin in the Arctic Ocean during summer in 2008–2016. The results showed that the incursion of the ACW reduced the chlorophyll a (Chl a) concentration in the shelf areas, but favored phytoplankton growth in basin areas, especially that of small prasinophytes and chrysophytes. A possible explanation is the change in nutrient status, because during its northward flow, the ACW dissipates heat and receives nutrients from the surrounding cold water. The concentrations of Prasino and Chl b (the diagnostic pigments of prasinophytes) increase from barely detected in 2008 to higher than 15 ng/dm3 (Prasino) and 65 ng/dm3 (Chl b) respectively in 2014 and 2016, which occurred synchronously with presence of ACW and an increase in subsurface nitrate reservoirs. These findings indicate that the incursion of the ACW brings nutrients into subsurface basin layers, which could favor the growth of small phytoplankton in the western Arctic Ocean.