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Effects of light and iron availability on the photophysiology of phytoplankton in the western subarctic Pacific

The western subarctic Pacific is known as one of the major High Nutrient Low Chlorophyll (HNLC) regions This region is, on the other hand, the largest CO 2 sink by biological carbon pump activity These characteristics can be related to the active photosynthesis by phytoplankton Nishioka suggested that intermediate water transport processes supply sedimentary iron (Fe) from the continental shelf of the Sea of Okhotsk through the Kuril straits to the Pacific. Yoshimura on the hand, that deep intertidal mixing in the straits induces Fe limitation due to Fe-light co-limitation In this study, effects of Fe supply on the photophysiology and community composition of phytoplankton in the western subarctic Pacific, near the Kuril Islands and the eastern Kamchatka Peninsula, were investigated by conducting on board Fe-manipulated incubation experiments during the R/V Professor Multanovskiy expedition in summer in 2014. Photophysiology of phytoplankton was assessed by measuring photosynthesis-irradiance (P-E) curves and variable chlorophyll (chl) a fluorescence. Community composition was investigated with pigment analysis with an ultra-high performance liquid chromatography (UHPLC) (Suzuki et al., 2015) and scanning electron microscopy (SEM). Additionally, at stations with the deepest (an offshore station near the eastern Kamchatka Peninsula) and the second shallowest (an offshore station near the Urup and the Bussol’ Strait) mixed layer depths (MLD), on board Fe-manipulated bottle incubation experiments were carried out with the following four treatments: Initial (Time-zero); Control (without Fe addition); +Fe (10 nM Fe added); and +DFB (1 μM Fe chelator DFB added). During the expedition, chl a concentrations were > 5 mg m –3 at stations in the Sea of Okhotsk and in the Bussol’ Strait at the beginning of the expedition, whereas low chl a concentrations (< 1 mg m –3 ) were found at the other stations. Interestingly, considerable amounts of chl a derivatives (pheophorbides and chlorophyllide a) were observed at stations near the Kamchatka Peninsula and the Kruzenshtern Strait, suggesting that the high abundance of senescent algal cells and/or high activity of zooplankton grazing. These indicated that the expedition was conducted in the latter half of the bloom. Diatoms accounted for 40–96% in the chlorophyll biomass, and the SEM suggested that the genus Chaetoceros was exclusively dominant with some resting spores in the diatom community throughout the expedition. Although in situ Fe concentrations did not show any correlation with photophysiological parameters (p>0.05, Spearman-Rank correlation), primary productivity was positively correlated with euphotic depths (i.e., 1% light depths) and E k (light saturation indices of the P-E