Patterns of (micro)nutrient limitation across the South Pacific Ocean

Nutrient limitation regulates phytoplankton growth throughout much of the global ocean and its assessment is important for our understanding of future changes in ocean productivity. The South Pacific Ocean represents a vast region where limiting nutrients have so far been investigated with only a handful of experiments. Here we report the results of eleven nutrient addition bioassay experiments conducted across the South Pacific Ocean at ca. 30°S as part of the GEOTRACES GP21 expedition. Nitrogen addition alone stimulated chlorophyll-a accumulation at all sites, whilst increases were larger following supplementary addition of iron in the eastern and central basin, and phosphate in the western basin. Enhanced iron stress following nitrogen addition in the eastern and central basin further pointed towards ambient iron concentrations approaching co-limiting levels. The overall east-to-west shift in serial limiting nutrients from iron to phosphate was attributed to a broadscale east-west increase in iron availability relative to nitrogen. Nitrogen is a limiting nutrient across the South Pacific Ocean with iron of secondary importance towards the east and phosphorus of secondary importance to the west, according to nutrient addition bioassay experiments from the GEOTRACES GP21 expedition