Comparison of POC flux vertical attenuation between the subarctic and subtropical regions in the western North Pacific

In order to verify the biological carbon pump and its future change by multiple stressors such as warming, acidification and hypoxia in the western North Pacific, comparative study of biogeochemistry between the subarctic-eutrophic and subtropical-oligotrophic regions (K2S1 project: Honda et al. 2017) was conducted between 2010 and 2014. Seasonal or time-series observation by using research vessels and sediment trap mooring system, satellite data analysis and mathematical simulation revealed that primary productivity and particulate organic carbon (POC) flux upper 200 m at subtropical station S1 were comparable to or slightly higher than those at subarctic station K2. However, POC flux at deep sea (~ 5000 m) of K2 was ~ 2 times higher than that at S1. These observations resulted in that POC flux vertical attenuation at K2 was smaller than that at S1. In other words, POC in the subarctic Pacific was transported to deep more efficiently than the subtropical Pacific. Major chemical component of sinking particle was biogenic opal at K2 while CaCO₃ was major component at S1. Multiple linier regression analysis indicated that correlation coefficient between biogenic opal and POC at not only K2 but also S1 was the highest among other ballasts (CaCO₃ and lithogenic materials). Thus, biogenic opal might play an important role in effective POC vertical transport in the western North Pacific. In addition, from the view point of metabolism, lower water temperature and dissolved oxygen concentration in the twilight zone at K2 might also support smaller POC flux vertical attenuation. On the other hand, based on seasonal onboard observation, zooplankton / prokaryote carbon demand (CD) in the water column at K2 was ~ 2.5 / 1.5 times higher than those at S1. This observation is contradictory to smaller POC attenuation. Moreover, these CD was higher than carbon supply suspected from POC flux and “active” carbon flux by zooplankton, especially at K2. In order to explain this mismatch, beside gravitational POC flux, other carbon supply mechanism such as Particulate Injection Pumps (PIPs: Boyd et al. 2019) should be taken into account.