Future Arctic Ocean primary productivity from CMIP5 simulations: Uncertain outcome, but consistent mechanisms

Abstract

[1] Net Arctic Ocean primary production (PP) is expected to increase over this century, due to less perennial sea ice and more available light, but could decrease depending on changes in nitrate (NO 3 ) supply. Here Coupled Model Intercomparison Project Phase 5 simulations performed with 11 Earth System Models are analyzed in terms of PP, surface NO 3 , and sea ice coverage over 1900-2100. Whereas the mean model simulates reasonably well Arctic-integrated PP (511 TgC/yr, 1998(511 TgC/yr, -2005 and projects a mild 58 TgC/yr increase by 2080-2099 for the strongest climate change scenario, models do not agree on the sign of future PP change. However, similar mechanisms operate in all models. The perennial ice loss-driven increase in PP is in most models NO 3 -limited. The Arctic surface NO 3 is decreasing over the 21st century (-2.3˙1 mmol/m 3 ), associated with shoaling mixed layer and with decreasing NO 3 in the nearby North Atlantic and Pacific waters. However, the intermodel spread in the degree of NO 3 limitation is initially high, resulting from >1000 year spin-up simulations. This initial NO 3 spread, combined with the trend, causes a large variation in the timing of oligotrophy onset-which directly controls the sign of future PP change. Virtually all models agree in the open ocean zones on more spatially integrated PP and less PP per unit area. The source of model uncertainty is located in the sea ice zone, where a subtle balance between light and nutrient limitations determines the PP change. Hence, it is argued that reducing uncertainty on present Arctic NO 3 in the sea ice zone would render Arctic PP projections much more consistent.

Net primary productivity estimates and environmental variables in the Arctic Ocean: An assessment of coupled physical-biogeochemical models

“…Five BOGCMs manifested similar levels of light limitation owing to a general agreement on the simulated sea ice distribution [Popova et al 2012]. On the other hand, the ESMs participating in the Vancoppenolle et al [2013] study showed that Arctic phytoplankton growth was more limited by light related to sea ice than by nutrient availability for the period of 1980-1999. Furthermore, with the large uncertainties on present-day nitrate in the sea ice zone, ESMs used in the framework of the CMIP5 exercise were inconsistent in their future projections of primary production and oligotrophic condition in the Arctic Ocean [Vancoppenolle et al, 2013].…”

“…This observation is not necessarily new [Popova et al, 2012;Vancoppenolle et al, 2013], but one should be cautious when interpreting this result because these three simulated parameters were positively biased (i.e., means usually overestimated) (Figures 8 and 9). For example, the simulated, mean surface NO 3 was vastly overestimated, especially in the lowproductivity, nutrient-depleted interior regions of the Arctic Ocean, i.e., the Beaufort Sea and the central Arctic Basin where model and in situ iNPP agreed best (Figures 6b and 6e; Table 4).…”

“…Indeed, the future Arctic Ocean may produce less NPP per unit area due to less surface NO 3 [e.g., Vancoppenolle et al, 2013] resulting from enhanced freshening , as also hypothesized by McLaughlin and Carmack [2010]. Unlike surface NO 3 , deep NO 3 was more realistically reproduced pan-Arctic wide in terms of the multimodel ensemble mean values at 100 km grid scales (average symbols in Figure 7).…”

Resistance of Arctic phytoplankton to ocean acidification and enhanced irradiance

“…In this region, future trends in primary production are critically dependent on the relative importance of different environmental drivers: beneficial effects of increased irradiances and potentially detrimental effects of decreased nutrient input, provided that the effects of enhanced stratification dominate over those of increased wind-driven mixing (Arrigo and van Dijken 2011;Vancoppenolle et al 2013;Ardyna et al 2014;Tremblay et al 2015). Observational data are indispensable to estimate the potential effects of climate change on Arctic phytoplankton assemblages.…”

Towards the Marine Arctic Component of the Pan-Eurasian Experiment

“…In typical Arctic ecosystems the most important primary consumers are large-sized herbivorous copepods, which have lifecycles synchronized with the seasonal algae bloom (Kosobokova, 2012). Among the key questions is what would be the joint effect of Arctic warming, ocean freshening, pollution load and acidification on the Arctic Ocean ecosystem (Janout et al, 2016), primary production (Vancoppenolle et al, 2013;Arrigo and van Dijken, 2015) 15 and carbon cycle (Christensen et al, 2017). Considering the Arctic marginal seas, however, high volumes of additional GPP seem highly unlikely.…”