New estimates of primary productivity and its limiting factors in the Central Arctic

The Arctic Ocean is changing dramatically due to the rapid sea ice decrease. The reduction in ice thickness and cover leads to an increase in the amount of light penetrating through the ice. Thus changes in primary production are expected. Nevertheless, our knowledge about how other limiting factors such as nutrients might affect budgets and ratios of sea ice algae and phytoplankton productivity is still limited, especially in the central basins. This study presents recent results from the Central Arctic Ocean collected during late summer and early autumn 2011 and 2012. Primary productivity was measured using the 14C method in the water column, the sea ice and in the melt ponds through a wide variety of ice types and water masses. Additionally PI curves and nutrient enrichment experiments were performed at selected stations in order to infer primary productivity´s limiting factors at the end of the season. Aggregates of sea ice algae found in melt ponds and directly under the ice showed hardly any photoinhibition and presented the highest productivity on small scales . Nevertheless their contribution to the total primary production remains difficult to assess due to their patchiness. Regarding the different ice types, multi-year ice contained more algal biomass than first year ice at the end of the summer, but due to its much larger spatial extent and higher light transmission throughout the season, first year ice is likely to provide a higher proportion of overall sea ice algal productivity in a changing Arctic. The ice-covered water column had in general very low NPP rates probably due to both light and nutrient limitation, and the depth of the mixed layer in the Eurasian basins was limited to 15 m in 2012. Therefore in a summerly ice free Arctic Ocean, the proportional contribution of sea ice and sub ice algae compared to phytoplankton in highly stratified, nitrate limited surface waters is still difficult to predict, and will depend on nutrient transport and mixing processes