Anomalously Low New Production in the Northeast Atlantic During Spring 2024: A Consequence of Weaker Winter Mixing?

Abstract In 2024 an anomalous region of low chlorophyll water covering ∼721,000 km 2 or 1.7% of North Atlantic surface area dominated the Northeast Atlantic. This feature formed during spring, remained identifiable as a region of low chlorophyll throughout the summer months in temperate and subpolar waters and, due to low opal ballasting potential of newly formed biomass, likely impacted ecosystem processes and carbon export fluxes across a wide sector of the Northeast Atlantic. In situ sampling along the southern edge of this region for 15 days in May–June encountered an unusually deep euphotic layer that shoaled rapidly from >80 to ∼40 m over subsequent days and low surface chlorophyll concentrations (<0.3 mg m −3 ) despite non‐limiting nitrate and phosphate availability, though silicate was exhausted. Integrated net primary production rates within this feature ranged between 0.5 and 0.6 g C m −2 d −1 , NO 3 − uptake rates between 0.8 and 1.3 mmol N m −2 d −1 , and new production rates between 0.08 and 0.13 g C m −2 d −1 ; rates that on average were 45%–79% lower than rates outside of this feature. Integrated concentrations of particulate organic carbon (POC), nitrogen (PON) and phosphorous (POP) were up to 44%–63% lower than surrounding waters. We hypothesize that this region of low productivity may be the consequence of prolonged and anomalous warming of the wider North Atlantic throughout 2023–2024 leading to weaker mixing and preconditioning of the surface ocean during winter 2024 with implications for the resident phytoplankton community.