Data_Sheet_1_Episodic Arctic CO2 Limitation in the West Svalbard Shelf.docx

<p>The European Sector of the Arctic Ocean is characterized by low CO₂ concentrations in seawater during spring and summer, largely due to strong biological uptake driven by extensive plankton blooms in spring. The spring plankton bloom is eventually terminated by nutrient depletion and grazing. However, low CO₂ concentrations in seawater and low atmospheric resupply of CO₂ can cause episodes during which the phytoplankton growth is limited by CO₂. Here, we show that gross primary production (GPP) of Arctic plankton communities increases from 32 to 72% on average with CO₂ additions in spring. Enhanced GPP with CO₂ additions occur during episodes of high productivity, low CO₂ concentration and in the presence of dissolved inorganic nutrients. However, during summer the addition of CO₂ supresses planktonic Arctic GPP. Events of CO₂ limitation in spring may contribute to the termination of the Arctic spring plankton blooms. The stimulation of GPP by CO₂ during the spring bloom provides a biotic feedback loop that might influence the global role played by the Arctic Ocean as a CO₂ sink in the future.</p