Rising temperatures in the arctic ocean can lead to large die-offs of phytoplankton, which are autotrophs. what would most likely happen in an arctic ecosystem if the phytoplankton population decreased?

Phytoplankton refers to the microscopic marine species that thrive at the bottom of the food chain. They obtain their energy from carbon dioxide via photosynthesis, like plants and thus are very essential in carbon cycling.  

Phytoplankton forms the base of almost every ocean food web. Thus, it can be stated that they make the majority of other ocean life possible. They are the source of food for zooplankton and small fishes, as well as for larger animals like jellyfish and whales.  

Thus, if the population of phytoplankton gets declined, it will indirectly affect other species in the aquatic ecosystem. It can also be said that with the decrease in the population of phytoplankton, the population of other major species in the ecosystem also gets a decline.  

yes it is weird but true