[A-OS12] Marine ecosystems and biogeochemical cycles: theory, observation and modeling

[AOS12-07] Variability of the euphotic depth in the Russian Arctic seas; an indirect biogeochemical environment effect on phytoplankton growth seen through light availability

*Takafumi Hirata¹ (1.Arctic Research Center, Hokkaido University)

Keywords:Euphotic Depth, Arctic , Phytoplankton, Remote Sensing

Growth of phytoplankton in the Arctic ocean is often limited due to insufficient light necessary for photosynthesis. Thus, understanding underwater light field plays an essential role on predicting phytoplankton dynamics in the Arctic ocean.However, in situ underwater observation in the Arctic is not readily feasible, due to its remote geography, and satellite remote sensing is indispensable for such a situation as already proven by sea ice observation. Towards a better understanding of the phytoplankton dynamics and underwater light field in the Arctic ocean, the euphotic depth, which is defined by a water depth where Photosynthetically Active Radiation (PAR) at the sea surface attenuates down to its 1% in intensity, is investigated to characterize the underwater light environment, using ocean color remote sensing data. Results show that the euphotic depth estimated by the remote sensing is relatively larger (i.e., more light penetrates into water column) at lower latitudes and smaller in higher latitudes within the Arctic ocean. It is in line with the theory that the euphotic depth is regulated largely by solar altitude. However, the euphotic depth in the Russian Arctic seas is strongly influenced by water quality, too, showing a distinct variability contrasting to that in other Artic seas. We find that concentrations of colored dissolved organic matter (CDOM) and suspended hydrosols in the Russian Arctic are very high, and that the euphotic depth in the Russian Arctic is anti-correlated with these biogeochemical quantities. Apart from nutrient supply which generally acts to increase phytoplankton growth, our results imply that local biogeochemistry off Russia can also act to limit phytoplankton growth by reducing light availability, and that the present primary production found in the Russian Arctic is a result of counterbalancing of the multiple biogeochemical effects.