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
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.