N and Fe as Factors Controlling Phytoplankton Limitation in Fram Strait at 79°N, Arctic Ocean

The rise in surface ocean temperatures over recent decades has led to a severe reduction in the extent of Arctic sea ice cover. The subsequent relief from light limitation might have beneficial effects to phytoplankton communities, potentially raising the overall magnitude of primary production in the Fram Strait region. Yet, first hand evidence of factors controlling phytoplankton limitation is missing for the Arctic Ocean to date. In summer 2016, GEOTRACES cruise GN05 conducted a transect from Svalbard to the NE-Greenland Shelf and sampled for bioessential trace metals (Fe, Co) and macronutrients (N, Si, P) at 79°N. Additionally, incubation experiments were performed to characterize ‘in-situ’ phytoplankton responses to nutrient additions of Fe, N, Fe+N and volcanic dust in the prevailing water masses. In-situ and ex-situ measurements showed a west-to-east trend in the relative deficiency of N and Fe. Generally, increasing N and decreasing Fe deficiency was observed towards the Greenlandic coast. The combination of incubation experiments and nutrient ratios suggested primary N- and secondary Fe-limited phytoplankton in West Spitsbergen Current surface waters. We propose productivity in the study region to be dependent upon mixing of nutrients provided by the East Greenland Current (Fe, Si) and West Spitsbergen Current (N).