Iron Regulation of North Atlantic Eddy Phytoplankton Productivity.

Cyclonic ocean eddies drive upwelling of deep waters enhanced in nutrients, which can elevate phytoplankton productivity. At mid‐latitudes in the North Atlantic, satellite images show enhanced chlorophyll‐a associated with eddies. However, surface macronutrient concentrations are often not fully depleted in this region, implying enhanced macronutrient supply is not the primary control. We conducted high resolution sampling through two mid‐latitude Atlantic eddies in late spring, located 800 and 350 km east of the Newfoundland Grand Banks. Waters outside of both eddies had unused residual macronutrients, low dissolved iron, and iron‐stressed phytoplankton. Inside both eddies, plankton biomass was higher and macronutrient concentrations lower. However, full macronutrient drawdown and an absence of iron stress were only present in the eddy nearer the continental shelf. From these two examples, iron supply and proximity to shelf iron sources appear to be important factors regulating productivity and macronutrient utilization in mid‐latitude North Atlantic cyclonic eddies. Plain Language Summary: Satellites show that circulating currents of water, eddies, in the mid‐high latitude North Atlantic often have elevated amounts of phytoplankton. This has been ascribed to enhanced nutrient and light availability. However, the background field (that is, outside eddies) in this region have elevated light in late spring and summer, and macronutrient concentrations (nitrate, phosphate, silicate) are not particularly depleted. To investigate this in more detail, we sailed through two North Atlantic eddies in late spring making measurements of phytoplankton, nutrients, and trace elements. Our results indicated that the background field was depleted in the micronutrient iron relative to other nutrients and additional experimental tests confirmed that phytoplankton were iron‐stressed. We therefore hypothesize that elevated iron supply to sunlit surface waters, via eddy‐driven upwelling, is a key factor enhancing phytoplankton in the region. This was supported by concentrations of manganese, a tracer of sedimentary iron, which were elevated within the eddy where Fe stress was fully relieved. Our results suggest enhanced iron supply to surface waters in eddies is an important factor regulating the distribution of phytoplankton in this region; this in turn would be an important regulator of higher trophic levels, including fish and seabird stocks. Key Points: Drivers of enhanced productivity in two mid‐latitude North Atlantic cyclonic eddies were investigatedDetailed biogeochemical measurements and experiments suggested enhanced iron was a key driverEnhanced sediment iron supply to the eddy near continental shelf is proposed to have driven full within‐eddy macronutrient utilization [ABSTRACT FROM AUTHOR]