4:00 PM - 4:15 PM
[AOS18-03] Impact of sea ice meltwater on biogeochemical condition and spring phytoplankton bloom in the southern Sea of Okhotsk
Keywords:Sea ice meltwater, Iron, Spring bloom, Sea of Okhotsk
The Sea of Okhotsk is a marginal sea located in the subarctic North Pacific and is the southern limit of seasonal sea ice extent in the Northern Hemisphere. In the southern part of Sea of Okhotsk, extensive spring phytoplankton bloom occurs during the sea ice melt season, but it is not clear what causes the bloom to be larger than that in surrounding waters. In the previous study, the heat and salt fluxes were calculated based on satellite observation data, taking into account the processes of sea ice formation and melting, and indicated that the spatial distribution of the bloom roughly coincided with the melting region of sea ice (Nihashi et al., 2012). Kanna et al. (2018) evaluated the winter iron supply processes that promote the bloom through field observations and culture experiments conducted during November-February, suggesting that iron supply by sea ice plays an essential role in the occurrence of the bloom. Kishi et al. (2021) reported that the time series of the bloom period using oxygen sensor-equipped profiling floats suggested not only enhancement of stratification but also element supply from sea ice influenced the magnitude of phytoplankton production. Due to the lack of shipboard observation, information such as the light condition, concentrations of nutrients and iron, and species composition of phytoplankton during the bloom have not been observed yet. This study reveals the impact of sea ice meltwater on hydrography and biogeochemical condition of the surface layer then discusses influence to the spring bloom.
Observations were conducted in the southern Sea of Okhotsk from April 11 to May 1, 2021, by the Shinsei-Maru (KS-21-6; JAMSTEC). Vertical profiles of various parameters were obtained at 56 stations using CTD sensors, and seawater samples were taken at 23 stations using acid-cleaned Teflon-coated NiskinX bottles (12 L). In addition, continuous clean water sampling was conducted at the ocean surface (1.5~3 m) using a tow fish clean sampling system. Salinity, temperature, and density data obtained by the CTD sensor and nutrients (NO3 + NO2, NO2, NH4, PO4, Si), iron, chlorophyll-a (Chl-a), dissolved oxygen, and δ18O in seawater samples were measured and used for analysis. Iron concentrations were analyzed for total iron (TDFe; unfiltered) and dissolved iron (DFe; <0.2 µm) by using a flow-injection analysis chemiluminescence detection system (Obata et al., 1997; 1993).
In the surface mixed layer, low-salinity water (ave. 32.1) was observed in our studied area. Below the surface mixed layer, there was a low-temperature water near the freezing point (<0°C). Based on previous studies, the low-temperature water corresponded to the East Sakhalin Current (ESC) (Itoh and Ohshima, 2000). The low salinity surface water on the ESC indicates the input of sea ice meltwater. Estimates of the contribution of sea ice meltwater to the surface mixed layer by the mass balance equation using the observed value of δ18O and salinity from this and previous studies (Yamamoto et al., 2001; Toyota et al., 2007) showed that sea ice meltwater accounted for 1.0-3.6 % (ave. 2.2 %) of seawater. Using the meltwater contribution calculated from this study and Fe concentration data cited from Kanna et al. (2014), we estimated results of iron supply from sea ice showed that DFe only increased the concentration of ESC water by 0.02-0.08 nM (average 0.05 nM), while TDFe increased by 7.0-25.1 nM (ave. 15.0 nM). The Chl-a concentration was extremely high (2.6-19.0 mg m-3, average 9.0 mg m-3) in the surface layer under the influence of sea ice meltwater. It shows that our observation captured a significant spring phytoplankton bloom. The low concentrations of nutrients and DFe were also observed in the surface mixed layer, indicating that phytoplankton may have taken up these elements during the bloom.
Observations were conducted in the southern Sea of Okhotsk from April 11 to May 1, 2021, by the Shinsei-Maru (KS-21-6; JAMSTEC). Vertical profiles of various parameters were obtained at 56 stations using CTD sensors, and seawater samples were taken at 23 stations using acid-cleaned Teflon-coated NiskinX bottles (12 L). In addition, continuous clean water sampling was conducted at the ocean surface (1.5~3 m) using a tow fish clean sampling system. Salinity, temperature, and density data obtained by the CTD sensor and nutrients (NO3 + NO2, NO2, NH4, PO4, Si), iron, chlorophyll-a (Chl-a), dissolved oxygen, and δ18O in seawater samples were measured and used for analysis. Iron concentrations were analyzed for total iron (TDFe; unfiltered) and dissolved iron (DFe; <0.2 µm) by using a flow-injection analysis chemiluminescence detection system (Obata et al., 1997; 1993).
In the surface mixed layer, low-salinity water (ave. 32.1) was observed in our studied area. Below the surface mixed layer, there was a low-temperature water near the freezing point (<0°C). Based on previous studies, the low-temperature water corresponded to the East Sakhalin Current (ESC) (Itoh and Ohshima, 2000). The low salinity surface water on the ESC indicates the input of sea ice meltwater. Estimates of the contribution of sea ice meltwater to the surface mixed layer by the mass balance equation using the observed value of δ18O and salinity from this and previous studies (Yamamoto et al., 2001; Toyota et al., 2007) showed that sea ice meltwater accounted for 1.0-3.6 % (ave. 2.2 %) of seawater. Using the meltwater contribution calculated from this study and Fe concentration data cited from Kanna et al. (2014), we estimated results of iron supply from sea ice showed that DFe only increased the concentration of ESC water by 0.02-0.08 nM (average 0.05 nM), while TDFe increased by 7.0-25.1 nM (ave. 15.0 nM). The Chl-a concentration was extremely high (2.6-19.0 mg m-3, average 9.0 mg m-3) in the surface layer under the influence of sea ice meltwater. It shows that our observation captured a significant spring phytoplankton bloom. The low concentrations of nutrients and DFe were also observed in the surface mixed layer, indicating that phytoplankton may have taken up these elements during the bloom.