Intensive mixing along an island chain controls oceanic biogeochemical cycles

Abstract

The subarctic Pacific is a high-nutrient low-chlorophyll (HNLC) region in which phytoplankton growth is broadly limited by iron (Fe) availability. However, even with Fe limitation, the western subarctic Pacific (WSP) has significant phytoplankton growth and greater seasonal variability in lower trophic levels than the eastern subarctic Pacific. Therefore, differences in Fe supply must explain the west-to-east decrease in seasonal phytoplankton growth. The Fe flux to the euphotic zone in the WSP occurs at a moderate value, in that it is significantly higher than its value on the eastern side, yet it is not sufficient enough to cause widespread macronutrient depletion, that is, HNLC status is maintained. Although we recognize several Fe supply processes in the WSP, the mechanisms that account for this moderate value of Fe supply have not previously been explained. Here we demonstrate the pivotal role of tidal mixing in the Kuril Islands chain (KIC) for determining the moderate value. A basin-scale meridional Fe section shows that Fe derived from sediments in the Sea of Okhotsk is discharged through the KIC into the intermediate water masses (similar to 800m) of the western North Pacific. The redistribution of this Fe-rich intermediate water by intensive mixing as it crosses the KIC is the predominant process determining the ratio of micronutrient (Fe) to macronutrients (e.g., nitrate) in subsurface waters. This ratio can quantitatively explain the differences in surface macronutrient consumption between the western and eastern subarctic, as well as the general formation and biogeochemistry of HNLC waters of the subarctic North Pacific.Copyright 2013 American Geophysical Union

The contiguous domains of Arctic Ocean advection: Trails of life and death

“…A similar situation may also be found in the northern Bering Sea region with C. glacialis from the remote, ice-covered Sea of Okhotsk representing a "relict" population (Safranov, 1984, Musaeva andKolosova, 1995). In fact, genetic results support this idea as C. glacialis from the Sea of Okhotsk have primarily the "arctic" haplotype of the 16S ribosomal RNA gene (Nelson et al, 2014) while Nakatsuka et al (2002) and Nishioka et al (2013) depicted physical mechanisms that appear to inject Sea of Okhotsk C.…”

Global view of sea-ice production in polynyas and its linkage to dense/bottom water formation

“…Thus, the overturning can contribute to the material cycle and subsequent biological productivity through the supply of nutrients such as iron. It has been recently revealed in the Sea of Okhotsk that when DSW formed by the polynya ice production sinks to the intermediate layer, re-suspended iron from sediment over the continental shelf is also brought to this layer (Nishioka et al 2013). The iron is considered to originate from the land through the Amur River.…”

“…The iron is considered to originate from the land through the Amur River. One hypothesis says that the iron from DSW is also supplied to the western area of the North Pacific and supports high biological productivity there (Nishioka et al 2013). As such, the land or the Amur River possibly feeds the ocean through the supply of iron in the western North Pacific.…”

The Oyashio Nutrient Stream and Its Nutrient Transport to the Mixed Water Region

“…The significance of subarctic thermocline nutrient export has been addressed in previous studies (Nishioka et al, 2013;Sarmiento et al, 2004). Whitney et al (2013) suggested that subarctic nitrate export along with NPIW formation had a value of 9.5 ± 3.2 kmol/s, which was derived from the product of Oyashio volume transport (Talley et al, 1995;Yasuda, 2004) associated with NPIW formation (3 Sv) and the difference in the nitrate concentration between the subarctic and subtropical gyres.…”

“…The cool, fresh Oyashio water mixes with the warm, saline Kuroshio water (based on the water property, its intermediate water is referred to as "Old" NPIW), forms the intermediate salinity minimum centered at 26.8 σ θ (or "New" NPIW), and circulates within the subtropical gyre. From its origin, the nutrients are transported from the abyssal North Pacific (Sarmiento et al, 2004) and Okhotsk Sea (Nishioka et al, 2013) to the nutrientdepleted subtropical North Pacific and replenishes the nutrient reservoir of the intermediate layer. From its origin, the nutrients are transported from the abyssal North Pacific (Sarmiento et al, 2004) and Okhotsk Sea (Nishioka et al, 2013) to the nutrientdepleted subtropical North Pacific and replenishes the nutrient reservoir of the intermediate layer.…”