Impact of mesoscale eddies on carbon cycle: In the Western Subarctic North Pacific
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When phytoplankton photosynthesize in the ocean surface layer, the CO2 in the ocean surface layer becomes organic carbon, which then sink. The CO2 in the ocean surface layer then decreases, and CO2 is dissolved from the atmosphere into the ocean. This sequence of events is known as the biological pump. It is known that organic carbon is efficiently transported to the deep ocean in the western subarctic North Pacific (inside the western subarctic gyre in Figure 1) (Honda, 2003), which is the target area of this study, and that microbial decomposition of organic carbon in the mid-depth layers is low (Honda 2020). Therefore, it is likely that when biological production occurs in the surface layers of the area, much of it is transported to the deep layers. In this study, we investigated the impact of the oceanic mesoscale eddies, which are thought to influence biological production, on the carbon cycle in the western subarctic North Pacific. The oceanic mesoscale eddies are explained in detail in a separate course, "Oceanic Mesoscale Eddies: High and Low Pressures in the Ocean".
Figure 1: Location of station K2 (47°N, 160°E) (★)
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The mean organic carbon flux at station K2 for the entire period was 4.2 ± 4.8 mg m-2 day-1, with seasonal variation, high in spring to fall and low in winter (red line in Figure 2). This seasonal variation corresponded to the spring and fall phytoplankton blooms in the ocean surface layer. In addition, 12 maxima, which were clearly higher than the seasonal variability, were observed (blue circle in Figure 2). The relationship between organic carbon fluxes and sea surface chlorophyll concentrations 1-2 months prior to the fluxes suggests that 11 of the 12 observed organic carbon flux maxima were influenced by high magnitude phytoplankton blooms in the vicinity of K2.
Figure 2: Time-series of organic carbon flux measured at K2 (black bars, unit: mg m−2 day−1). Red line represent seasonal variation. Shaded areas indicate the absence of sediment trap data. Blue circles and numbers represent data points considered as peaks in this study.
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We investigated how phytoplankton blooms, the main cause of the organic carbon flux maxima, occurred. Based on the distribution of oceanic mesoscale eddies and sea surface chlorophyll concentrations captured by satellite observations, we found that a total of 9 out of 12 organic carbon flux maxima were affected by horizontal transport by mesoscale eddies (Figures 2 and 3). As introduced in "Oceanic Mesoscale Eddies: Oceanic Highs and Low Pressures”, it is indicated that mesoscale eddies enhanced biological production in the open ocean by transporting nutrients and phytoplankton from the coastal regions to the open ocean through horizontal transport. It is also suggested that a large amount of organic carbon was transported to the deep layer as a result. These results indicate that the mesoscale eddies had a significant influence on the interannual variation of organic carbon fluxes in K2.
Figure 3. Images of the monthly Chl-a concentration (colors, units: mg m−3) and absolute dynamic topography (thin black contours, interval: 5 cm). White areas indicate the absence of chlorophyll measurements due to cloud cover. Thick black (red) lines indicate the peripheries of anticyclonic (cyclonic) eddies. White arrows indicate the eddies that were considered to affect the Chl-a concentration around K2 in this study. Star indicates the location of K2. Numbers in the lower right of each figure correspond to the organic carbon flux maxima in Figure 2.
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