Nutrient co-limitation at the boundary of an oceanic gyre

Nutrient amendment experiments at the boundary of the South Atlantic gyre reveal extensive regions in which nitrogen and iron are co-limiting, with other micronutrients also approaching co-deficiency; such limitations potentially increase phytoplankton community diversity. Moderating marine micronutrients Whether micronutrient co-limitation is pervasive in the ocean and whether it affects phytoplankton growth and productivity is poorly understood. Through a series of large-scale nutrient amendment experiments at the eastern boundary of the South Atlantic gyre, Thomas Browning and colleagues report extensive regions in which nitrogen and iron are co-limiting, with near co-deficiency of the trace metal cobalt and vitamin B 12 . These findings suggest that nitrogen–iron co-limitation is pervasive in the ocean and potentially increases phytoplankton community diversity. Nutrient limitation of oceanic primary production exerts a fundamental control on marine food webs and the flux of carbon into the deep ocean 1 . The extensive boundaries of the oligotrophic sub-tropical gyres collectively define the most extreme transition in ocean productivity, but little is known about nutrient limitation in these zones 1 , 2 , 3 , 4 . Here we present the results of full-factorial nutrient amendment experiments conducted at the eastern boundary of the South Atlantic gyre. We find extensive regions in which the addition of nitrogen or iron individually resulted in no significant phytoplankton growth over 48 hours. However, the addition of both nitrogen and iron increased concentrations of chlorophyll a by up to approximately 40-fold, led to diatom proliferation, and reduced community diversity. Once nitrogen–iron co-limitation had been alleviated, the addition of cobalt or cobalt-containing vitamin B 12 could further enhance chlorophyll a yields by up to threefold. Our results suggest that nitrogen–iron co-limitation is pervasive in the ocean, with other micronutrients also approaching co-deficiency. Such multi-nutrient limitations potentially increase phytoplankton community diversity.