Seafloor primary production in a changing Arctic Ocean

Phytoplankton and sea ice algae are traditionally considered to be the main primary producers in the Arctic Ocean. In this Perspective, we explore the importance of benthic primary producers (BPPs) encompassing microalgae, macroalgae, and seagrasses, which represent a poorly quantified source of Arctic marine primary production. Despite scarce observations, models predict that BPPs are widespread, colonising ⁓3 million km2 of the extensive Arctic coastal and shelf seas. Using a synthesis of published data and a novel model we estimate that BPPs currently contribute ⁓77 Tg C yr-1 of primary production to the Arctic, equivalent to ~20-35% of annual phytoplankton production. Macroalgae contribute ⁓43 Tg C yr-1, seagrasses contribute ⁓23 Tg C yr-1, and microalgae-dominated shelf habitats contribute ⁓11-16 Tg C yr-1. Since 2003, the Arctic seafloor area exposed to sunlight has increased by ~47000 km2 yr-1, expanding the realm of BPPs in a warming Arctic. Increased macrophyte abundance and productivity are expected along Arctic coastlines with continued ocean warming and sea ice loss. However, microalgal benthic primary production has increased in only a few shelf regions despite substantial sea ice loss over the past 20 years, as higher solar irradiance in the ice-free ocean is counterbalanced by reduced water transparency. This suggests the complex impacts of climate change on Arctic light availability and marine primary production. Despite significant knowledge gaps on Arctic BPPs, their widespread presence and obvious contribution to coastal and shelf ecosystem production call for further investigation and for their inclusion in Arctic ecosystem models and carbon budgets.