Mesopelagic particulate nitrogen dynamics in the subarctic and subtropical regions of the western North Pacific

Recently, new spatiotemporal-scale particle observations by autonomous profiling floats equipped with bio-optical sensors have revealed that, in addition to gravitational particle sinking, the downward transport of surface particles by physical mixing events, which has been overlooked, contributes to particulate organic carbon export. However, the subsequent behavior of these exported particles in the mesopelagic zone (e.g., particle fragmentation and degradation) remains unclear, although it may influence the efficiency of carbon transport to further depths. This study successfully depicted the new annual mean mesopelagic particulate nitrogen (PN) dynamics with multi-layer, steady-state suspended PN pools by reanalyzing seasonal data on the stable nitrogen isotopic compositions of both suspended and sinking particles, each with different profiles, from subarctic station K2 and subtropical station S1 in the North Pacific, which are both CO2 sinks but in different oceanic settings. As analytical conditions, we assumed that the net loss of sinking PN was entirely due to abiotic fragmentation of particle aggregates to non-sinking particles and that the apparent 15N enrichment associated with heterotrophic degradation in the suspended PN pools was vertically constant. The 15N mass balance for the PN supply to the uppermost mesopelagic pool, derived from such constraints, allowed estimating the PN export by the mixed-layer pump, which was 1.6 times greater at K2 than at S1. However, its contribution to the total export (including gravitational PN sinking) from the surface layer was approximately 20% at both stations. Moreover, the ratio of PN supplied to the uppermost pool by the mixed-layer pump and by the fragmentation of particle aggregates was also similar at both stations, approximately 1:1. Using these ratios, together with separate observations of the mixed-layer pump-driven flux, it may be possible to estimate the efficiency of the particulate organic carbon transport due to the biological gravitational pump responsible for carbon sequestration in the deep sea

Seasonal variations in the nitrogen isotopic composition of settling particles at station K2 in the western subarctic North Pacific

Intensive observations using hydrographical cruises and moored sediment trap deployments during 2010 and 2012 at station K2 in the North Pacific western subarctic gyre (WSG) revealed seasonal changes in δ15N of both suspended and settling particles. Suspended particles (SUS) were collected from depths between the surface and 200 m; settling particles by drifting traps (DST; 100-200 m) and moored traps (MST; 200 and 500 m). All particles showed higher δ15N values in winter and lower in summer, contrary to the expected by isotopic fractionation during phytoplankton nitrate consumption. We suggest that these observed isotopic patterns are due to ammonium consumption via light-controlled nitrification, which could induce variations in δ15N(SUS) of 0.4-3.1 ‰ in the euphotic zone (EZ). The δ15N(SUS) signature was reflected by δ15 N(DST) despite modifications during biogenic transformation from suspended particles in the EZ. δ15 N enrichment (average: 3.6 ‰) and the increase in C:N ratio (by 1.6) in settling particles suggests year-round contributions of metabolites from herbivorous zooplankton as well as TEPs produced by diatoms. Accordingly, seasonal δ15 N(DST) variations of 2.4-7.0 ‰ showed a significant correlation with primary productivity (PP) at K2. By applying the observed δ15 N(DST) vs. PP regression to δ15 N(MST) of 1.9-8.0 ‰, we constructed the first annual time-series of PP changes in the WSG. Moreover, the monthly export ratio at 500 m was calculated using both estimated PP and measured organic carbon fluxes. Results suggest a 1.6 to 1.8 times more efficient transport of photosynthetically-fixed carbon to the intermediate layers occurs in summer/autumn rather than winter/spring

Temporal variation of particulate organic carbon flux at the mouth of Tokyo Bay

A sediment trap experiment was conducted at a depth of 750 m at the mouth of Tokyo Bay to clarify the quantity and transport process of particles from the bay to the open ocean. The high total mass flux (8.7 ± 4.5 g m(–2) d(–1)) suggests that the particles not only originate in the surface layer right above the trap, but are also focused in Uraga Channel and discharged into the bay mouth. The organic carbon and nitrogen isotope ratios (δ(13)C(org), δ(15)N) of the trapped particles were like those of the surface sediment in the bay, that is, a mixture of particles in rivers and suspended particles in the surface layer of the bay. Compared with the results of the experiment conducted in 1995–2002, the average total mass flux was reduced by 70% and organic carbon content was reduced by 50%. The δ(13)C(org) values of trapped particles were also lower than those observed in the previous experiment, indicating a lower contribution from surface-suspended particles with high δ(13)C(org) values in the bay. These results could partly reflect a decrease of the concentration of the suspended particulate carbon in the bay by half over 20 years. Another factor contributing to the decrease of the flux at the bay mouth would be that the intrusion of Kuroshio coastal water into the bay, which pushes particles out to the bay mouth, has not occurred in recent years

DataSheet1_Mesopelagic particulate nitrogen dynamics in the subarctic and subtropical regions of the western North Pacific.docx

Recently, new spatiotemporal-scale particle observations by autonomous profiling floats equipped with bio-optical sensors have revealed that, in addition to gravitational particle sinking, the downward transport of surface particles by physical mixing events, which has been overlooked, contributes to particulate organic carbon export. However, the subsequent behavior of these exported particles in the mesopelagic zone (e.g., particle fragmentation and degradation) remains unclear, although it may influence the efficiency of carbon transport to further depths. This study successfully depicted the new annual mean mesopelagic particulate nitrogen (PN) dynamics with multi-layer, steady-state suspended PN pools by reanalyzing seasonal data on the stable nitrogen isotopic compositions of both suspended and sinking particles, each with different profiles, from subarctic station K2 and subtropical station S1 in the North Pacific, which are both CO2 sinks but in different oceanic settings. As analytical conditions, we assumed that the net loss of sinking PN was entirely due to abiotic fragmentation of particle aggregates to non-sinking particles and that the apparent 15N enrichment associated with heterotrophic degradation in the suspended PN pools was vertically constant. The 15N mass balance for the PN supply to the uppermost mesopelagic pool, derived from such constraints, allowed estimating the PN export by the mixed-layer pump, which was 1.6 times greater at K2 than at S1. However, its contribution to the total export (including gravitational PN sinking) from the surface layer was approximately 20% at both stations. Moreover, the ratio of PN supplied to the uppermost pool by the mixed-layer pump and by the fragmentation of particle aggregates was also similar at both stations, approximately 1:1. Using these ratios, together with separate observations of the mixed-layer pump-driven flux, it may be possible to estimate the efficiency of the particulate organic carbon transport due to the biological gravitational pump responsible for carbon sequestration in the deep sea.</p

Examining the impact of rainfall on primary productivity in the oligotrophic subtropical ocean

http://www.godac.jamstec.go.jp/darwin/cruise/mirai/mr21-01/

Temporal changes of Fukushima-derived radiocaesium in pelagic oceanic zooplankton in the western North Pacific.

Temporal changes of contamination with the Fukushima-derived radiocesium in oceanic zooplankton were investigated from one month to three years after the Fukushima Dai-ichi nuclear power plant accident. Sampling sites were located in subtropical and subarctic areas (900 and 1900 km away from the nuclear power plant, respectively) of the western North Pacific. 134Cs and 137Cs were detected from all of the time-series samples with the maximum activity concentrations, 70.6 and 71.5 Bq kg-dry–1 for 134Cs and 137Cs, respectively, recorded in the subtropical site on May 2011. In the subtropical zooplankton, activities of 137Cs rapidly decreased by 87–92% from May to July 2011, thereafter activities were maintained one order of magnitude higher than the pre-accident level (180 mBq/kg-dry) through the study period. In the subarctic zooplankton, 137Cs activities with two order of magnitude higher than the pre-accident level were observed until July 2011, and the activities were gently decreased to the pre-accident level on July 2014. The different decreasing rates of radiocesium in zooplankton until first summer after the accident are probably due to different life spans of zooplankton (short in subtropical while long in subarctic). Radiocesium activities in seawater derived by atmospheric deposition rapidly decrease by dispersion in the oceanic area. So, most of the zooplankton that took radiocesium just after the accident might be replaced by newly hatched zooplankton until first summer in the subtropical site, while subarctic zooplankton highly contaminated with radiocesium probably still alive until first summer. Unexpectedly, activities of radiocesium in surface zooplankton (collected from surface mixed layer) were lower than those in subsurface zooplankton (between thermocline to 200 m) in the subarctic site although opposite relationship were observed in activities of radiocesium in seawater. Trophic levels of zooplankton, which were estimated from δ15N of primary producers and zooplankton, suggests that carnivore species are more dominant in the subsurface community. Thus, the difference of activity concentrations of radiocesium between surface and subsurface zooplankton are thought to be influenced by bioaccumulation. Since the first winter after the accident, 137Cs activities in zooplankton were maintained to be high in the subtropical site while those decreased to the pre-accident level in the subarctic site. The former was probably influenced by secondary supply of radiocesium into subtropical site due to advection of the subtropical mode water highly contaminated with the Fukushima-derived radiocesium. We can conclude that radiocesium activity concentrations in zooplankton and their temporal changes are influenced by not only supply of radiocesium into environment but also characters of zooplankton community. Comparing with the previous studies, activity concentrations of radiocesium in our oceanic zooplankton were lower than those in coastal zooplankton collected vicinity to the nuclear power plant, also lower than those in the Baltic Sea after the Chernobyl accident, and comparable to those in the Adriatic and Mediterranean Seas after the Chernobyl accident