Abstract
Abstract. The Sea of Okhotsk is known as one of the most biologically productive regions among the world's oceans, and its productivity is supported in part by the discharge of iron (Fe)-rich water from the Amur River. However, little is known about the effect of riverine-derived Fe input on the physiology of the large diatoms which often flourish in surface waters of the productive continental shelf region. We conducted diatom-specific immunochemical ferredoxin (Fd) and flavodoxin (Fld) assays in order to investigate the spatial variability of Fe nutritional status in the microplankton-sized (20–200 μm; hereafter micro-sized) diatoms. The Fd index, defined as the proportion of Fd to the sum of Fd plus Fld accumulations in the cells, was used to assess their Fe nutritional status. Additionally, active chlorophyll fluorescence measurements using pulse–amplitude-modulated (PAM) fluorometry were carried out to obtain the maximum photochemical quantum efficiency (Fv/Fm) of photosystem II for the total micro-sized phytoplankton assemblages including diatoms. During our observations in the summer of 2006, the micro-sized diatoms were relatively abundant (> 10 μg C L−1) in the neritic region, and formed a massive bloom in Sakhalin Bay near the mouth of the Amur River. Values of the Fd index and Fv/Fm were high (>0.9 and >0.65, respectively) near the river mouth, indicating that Fe was sufficient for growth of the diatoms. However, in oceanic waters of the Sea of Okhotsk, the diatom Fd index declined as cellular Fld accumulation increased. These results suggest that there was a distinct gradient in Fe nutritional status in the micro-sized diatoms from near the Amur River mouth to open waters in the Sea of Okhotsk. A significant correlation between dissolved Fe (D-Fe) concentration and the Fd index was found in waters off Sakhalin Island, indicating that D-Fe was a key factor for the photophysiology of this diatom size class. In the vicinity of the Kuril Islands between the Sea of Okhotsk and the Pacific Ocean, micro-sized diatoms only accumulated Fld (i.e., Fd index = 0), despite strong vertical mixing consistent with elevated surface D-Fe levels (>0.4 nM). Since higher Fe quotas are generally required for diatoms growing under low-light conditions, the micro-sized diatoms off the Kuril Islands possibly encountered Fe and light co-limitations. The differential expressions of Fd and Fld in micro-sized diatoms helped us to understand how these organisms respond to Fe availability in the Sea of Okhotsk in connection with the Amur River discharge.
Highlights
Iron (Fe) plays an important role in metabolic processes such as photosynthesis, respiration, and nitrogen assimilation for marine phytoplankton (Twining and Baines, 2013; Behrenfeld and Milligan, 2013)
In terms of the geographic (Fig. 1) and hydrographic conditions (Table 1, Fig. 2), the study area was divided into two realms: north and east of Sakhalin Island (Stns B1– G16) and the vicinity of the Kuril Islands (Stns UrupW-A4)
The lowest salinity was observed at Stn G9 with the influence of fresh water reaching to Stns C1–C9 (< 32 in salinity), consistent with high discharge of fresh water from the Amur River and distributed along the East Sakhalin coast through the southward-flowing East Sakhalin Current (Ohshima et al, 2002)
Summary
Iron (Fe) plays an important role in metabolic processes such as photosynthesis, respiration, and nitrogen assimilation for marine phytoplankton (Twining and Baines, 2013; Behrenfeld and Milligan, 2013). It has become evident that Fe limitation for phytoplankton growth can occur in offshore waters, and in coastal upwelling regions and marginal seas (e.g., Bruland et al, 2001; Hutchins et al, 2002; Aguilar-Islas et al, 2007; Sedwick et al, 2011; Gerringa et al, 2012). In coastal waters, both riverine and sedimentary Fe can cause pronounced near-shore to offshore gradients in dissolved Fe concentrations. No literature has been published on spatiotemporal variability in the Fe nutritional status of large diatoms in coastal or marginal seas
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