Responses of Gulf of Alaska plankton communities to a marine heat wave

Time series of phytoplankton and zooplankton collected from the shelf and oceanic northern Gulf of Alaska from 2000 to 2018 are examined to describe changes in abundance and composition that occurred during the 2014–2016 marine heat wave (MHW). Zooplankton abundances were very high on the shelf during the MHW, particularly copepods and pteropods, while large diatoms were very low. Community Temperature Indices (CTI) were derived and showed significant, positive correlations with temperature for both trophic levels on the shelf and in the deep ocean. While no common taxa disappeared from the communities, there were changes in relative abundance that contributed to the increase in CTI. Additionally, some rarer taxa were not found during or after the MHW, and fewer new taxa appeared with its onset. There is thus evidence for a change in ecosystem functioning in the lower trophic levels with the northeast Pacific MHW bringing; lower plankton taxonomic richness, a bias towards species that prefer warm conditions, increased effects down the food chain, likely exerted by changes in forage fish, and uncertainty in data from 2017 to 2018 as to whether plankton metrics had, or would, return to pre-MHW values. • Evidence of top-down effects on Gulf of Alaska plankton during the Marine Heat Wave of 2014–2016. • Community Temperature Indices for phytoplankton and zooplankton increased during MHW years. • Taxonomic richness decreased during and immediately after the MHW compared to before in most cases.