Ocean Deoxygenation Conference | Kiel 2018

Upwelling associated with eastern boundary currents transports nutrients to sunlit surface waters, fuelling phytoplankton productivity. Phytoplankton growth rapidly depletes supplied nutrients such than one or more become limiting; the supply flux of the limiting nutrient(s) is therefore the proximal control on productivity. In this presentation the results of direct experimental tests in surface waters overlying and offshore of two oxygen minimum zones (Peru and Benguela) will be presented, which map the identity of nutrients that are proximally/serially/co-limiting to phytoplankton communities. We find that in the current ocean the relative supply of iron and fixed nitrogen are the most critical proximal controls on productivity in these zones, with cobalt playing a secondary role. The results of these experiments can be interpreted in terms of the ratios of ambient dissolved seawater nutrient concentrations at experimental sites, and by inference, nutrient supply: demand ratios and the processes that control these.