Temperature dependence of CO2-enhanced primary production in the European Arctic Ocean

Abstract

The Arctic Ocean is warming at two to three times the global rate 1 and is perceived to be a bellwether for ocean acidification 2,3 . Increased CO 2 concentrations are expected to have a fertilization e ect on marine autotrophs 4 , and higher temperatures should lead to increased rates of planktonic primary production 5 . Yet, simultaneous assessment of warming and increased CO 2 on primary production in the Arctic has not been conducted. Here we test the expectation that CO 2 -enhanced gross primary production (GPP) may be temperature dependent, using data from several oceanographic cruises and experiments from both spring and summer in the European sector of the Arctic Ocean. Results confirm that CO 2 enhances GPP (by a factor of up to ten) over a range of 145-2,099 µatm; however, the greatest e ects are observed only at lower temperatures and are constrained by nutrient and light availability to the spring period. The temperature dependence of CO 2 -enhanced primary production has significant implications for metabolic balance in a warmer, CO 2 -enriched Arctic Ocean in the future. In particular, it indicates that a twofold increase in primary production during the spring is likely in the Arctic.Primary production in the Arctic Ocean supports significant fisheries 6 and renders it an important sink for anthropogenic carbon 2 ; however, climate change has the potential to alter these capacities. Accelerated ice loss is opening surface area across the Arctic, resulting in observations of increased rates of primary production 7 . The reduced salinity caused by melting ice, combined with increasing temperatures, however, increases stratification, restricting turbulent nutrient supply to surface layers 8 . Ice loss also increases surface area for air-sea CO 2 exchange, causing an uptake from the atmosphere into surface waters with already low p CO 2 (ref. 9), and ice melt introduces freshwater with low alkalinity and dissolved inorganic carbon, further lowering the carbon content of surface waters 10 . The surface waters of the Arctic Ocean are largely undersaturated with respect to CO 2 throughout spring and summer 2 . In the European sector of the Arctic Ocean (BarentsGreenland Sea/Fram Strait), p CO 2 varies seasonally by more than 200 µatm, with values as low as 100 µatm in spring months 11 owing to strong net community production associated with the spring bloom of ice algae followed by that of planktonic algae

Resistance of Arctic phytoplankton to ocean acidification and enhanced irradiance

“…Another possible reason for the lack of pCO 2 effects is the relatively high incubation temperature. High temperatures have been shown to shift phytoplankton pCO 2 optima to higher levels and could thus dampen OA effects on phytoplankton assemblages (Sett et al 2014;Holding et al 2015;Wolf et al 2017). Although the incubation temperatures were higher than in situ, we argue that our experimental set-up nonetheless reflects a realistic scenario for the current, and certainly the future Arctic Ocean.…”

“…Results from recent field studies have shown a potential for positive OAresponses in primary production and phytoplankton growth (Engel et al 2013;Coello-Camba et al 2014;Holding et al 2015). Increasing temperatures, however, seem to reduce potential benefits of increased CO 2 concentrations (Holding et al 2015), which could be due to a shift in CO 2 optima to higher levels (Sett et al 2014).…”

“…OA is studied widely because of its multiple effects on the physiology, ecology, and biogeochemistry of marine ecosystems (Pörtner et al 2014). While some studies show a high potential for a stimulation of highlatitude NPP under elevated pCO 2 (e.g., Tortell et al 2008;Hoppe et al 2013;Holding et al 2015), other studies have demonstrated little to no stimulation of growth of Table 4 Photo-physiological characteristics of phytoplankton assemblages immediately before the dilution and at the end of the experiment (n = 3; mean ±1 SD) Time point…”

“…Results from recent field studies have shown a potential for positive OAresponses in primary production and phytoplankton growth (Engel et al 2013;Coello-Camba et al 2014;Holding et al 2015). Increasing temperatures, however, seem to reduce potential benefits of increased CO 2 concentrations (Holding et al 2015), which could be due to a shift in CO 2 optima to higher levels (Sett et al 2014). Beyond changes in phytoplankton productivity, OA has been shown to influence Arctic phytoplankton community structure-i.e., the relative abundance of various taxonomic and functional groups (Newbold et al 2012;Brussaard et al 2013).…”

“…While some studies have begun to examine the interactive effects of warming and OA in the Arctic (CoelloCamba et al 2014;Holding et al 2015;Pančić et al 2015), little is known about the interactive effects of OA and increasing light levels in the Arctic (Hoppe Clara et al 2017). Given the expected changes in both light availability and carbonate chemistry over the coming decades, it is important to investigate their interactive effects on Arctic primary producers.…”