Subtropical North Pacific Ocean Research Articles

Speciation and cycling of iodine in the subtropical North Pacific Ocean

Iodine intersects with the marine biogeochemical cycles of several major elements and can influence air quality through reactions with tropospheric ozone. Iodine is also an element of interest in paleoclimatology, whereby iodine-to-calcium ratios in marine carbonates are widely used as a proxy for past ocean redox state. While inorganic iodine in seawater is found predominantly in its reduced and oxidized anionic forms, iodide (I−) and iodate (IO3−), the rates, mechanisms and intermediate species by which iodine cycles between these inorganic pools are poorly understood. Here, we address these issues by characterizing the speciation, composition and cycling of iodine in the upper 1,000 m of the water column at Station ALOHA in the subtropical North Pacific Ocean. We first obtained high-precision profiles of iodine speciation using isotope dilution and anion exchange chromatography, with measurements performed using inductively coupled plasma mass spectrometry (ICP-MS). These profiles indicate an apparent iodine deficit in surface waters approaching 8% of the predicted total, which we ascribe partly to the existence of dissolved organic iodine that is not resolved during chromatography. To test this, we passed large volumes of seawater through solid phase extraction columns and analyzed the eluent using high-performance liquid chromatography ICP-MS. These analyses reveal a significant pool of dissolved organic iodine in open ocean seawater, the concentration and complexity of which diminish with increasing water depth. Finally, we analyzed the rates of IO3− formation using shipboard incubations of surface seawater amended with 129I−. These experiments suggest that intermediate iodine species oxidize to IO3− much faster than I− does, and that rates of IO3− formation are dependent on the presence of particles, but not light levels. Our study documents the dynamics of iodine cycling in the subtropical ocean, highlighting the critical role of intermediates in mediating redox transformations between the major inorganic iodine species.

Unraveling prokaryotic diversity distribution and functional pattern on nitrogen and methane cycling in the subtropical Western North Pacific Ocean

Prokaryotes play an important role in marine nitrogen and methane cycles. However, their community changes and metabolic modifications to the concurrent impact of ocean warming (OW), acidification (OA), deoxygenation (OD), and anthropogenic‑nitrogen-deposition (AND) from the surface to the deep ocean remains unknown. We examined here the amplicon sequencing approach across the surface (0–200 m; SL), intermediate (200–1000 m; IL), and deep layers (1000–2200 m; DL), and characterized the simultaneous impacts of OW, OA, OD, and AND on the Western North Pacific Ocean prokaryotic changes and their functional pattern in nitrogen and methane cycles. Results showed that SL possesses higher ammonium oxidation community/metabolic composition assumably the reason for excess nitrogen input from AND and modification of their kinetic properties to OW adaptation. Expanding OD at IL showed hypoxic conditions in the oxygen minimum layer, inducing higher microbial respiration that elevates the dimerization of nitrification genes for higher nitrous oxide production. The aerobic methane-oxidation composition was dominant in SL presumably the reason for adjustment in prokaryotic optimal temperature to OW, while anaerobic oxidation composition was dominant at IL due to the evolutionary changes coupling with higher nitrification. Our findings refocus on climate-change impacts on the open ocean ecosystem from the surface to the deep-environment integrating climate-drivers as key factors for higher nitrous-oxide and methane emissions.

Subtropical Mode Water south of Japan impacts typhoon intensity.

Subtropical Mode Water (STMW), characterized by vertically uniform temperature of ~17°C, is distributed horizontally over 5000 kilometers at the 100- to 500-meter depths in the subtropical North Pacific Ocean. Its formation and spreading fluctuate in relation to the Pacific Decadal Oscillation and the Kuroshio path variation, but the feedback from STMW on the sea surface temperature (SST) and the overlying atmosphere remains unclear. Using Argo profiling float data, we show that STMW south of Japan, whose thickness varies decadally, modulates the overlying thermal structure throughout the year by increasing isotherm uplift with increasing thickness. The STMW-induced decadal temperature change has a magnitude of up to ~1°C and is large in the warm season in the presence of the seasonal thermocline. Furthermore, 50-year observations, together with numerical simulation, show that SST, upper ocean heat content, and typhoon intensification rate have been significantly lower in years with thicker STMW and higher in years with thinner STMW.

East‐West Variabilities of N2 Fixation Activity in the Subtropical North Pacific Ocean in Summer: Potential Field Evidence of the Phosphorus and Iron Co‐Limitation in the Western Area

AbstractIn the subtropical North Pacific, the east‐west gradient of iron deposition as atmospheric Asian dust strongly affects the zonal distribution of biological N2 fixation activity in numerical models, but the in‐situ relationship at a basin‐scale is not well examined. We examined the trans‐Pacific zonal variation in N2 fixation activity on 23°N in summer along with environmental parameters that potentially influence diazotrophy. Dissolved inorganic iron (DFe) was consistently low (<0.4 nM) throughout the transect. The atmospheric dust iron (dust‐Fe) flux increased westward, whereas phosphate and labile phosphoric monoesters in the surface decreased westward. N2 fixation varied between 34.6 and 298 μmol‐N m−2 d−1 and was high (>200 μmol‐N m−2 d−1) in the central area (150–180°W). N2 fixation rates significantly increased with dust‐Fe input only in the western area (137–180°E), whereas the contribution of DFe diffused from below the euphotic zone was often larger in the eastern area (120–170°W). N2 fixation was considerably low in the phosphate‐depleted western area despite the excess amount of iron relative to phosphate, and it increased with the labile phosphoric monoesters stock. These indicated that N2 fixation was primarily limited by phosphorus in the western area, though this activity also increased with iron supply from dust, likely due to phosphorus‐iron co‐limitation. In contrast, in the phosphorus‐repleted eastern area, iron supplied from dust and below the euphotic zone appeared to limit N2 fixation. Overall, N2 fixation in the subtropical North Pacific was likely limited by zonally different factors relating to iron and phosphorus availability.

Distribution and Production of N2O in the Subtropical Western North Pacific Ocean During the Spring of 2020

Nitrous oxide (N2O) is an important greenhouse gas emitted in significant volumes by the Pacific Ocean. However, the relationship between N2O dynamics and environmental drivers in the subtropical western North Pacific Ocean (STWNPO) remains poorly understood. We investigated the distribution of N2O and its production as well as the related mechanisms at the surface (0–200 m), intermediate (200–1500 m), and deep (1500–5774 m) layers of the STWNPO, which were divided according to the distribution of water masses. We applied the transit time distribution (TTD) method to determine the ventilation times, and to estimate the N2O equilibrium concentration of water parcels last in contact with the atmosphere prior to being ventilated. In the surface layer, biologically derived N2O (ΔN2O) was positively correlated with the apparent oxygen utilization (AOU) (R2 = 0.48), suggesting that surface N2O may be produced by nitrification. In the intermediate layer, ΔN2O was positively correlated with AOU and NO3− (R2 = 0.92 and R2 = 0.91, respectively) and negatively correlated with nitrogen sinks (N*) (R2 = 0.60). Hence, the highest ΔN2O value in the oxygen minimum layer suggested N2O production through nitrification and potential denitrification (up to 51% and 25% of measured N2O, respectively). In contrast, the deep layer exhibited a positive correlation between ΔN2O and AOU (R2 = 0.92), suggesting that the N2O accumulation in this layer may be caused by nitrification. Our results demonstrate that the STWNPO serves as an apparent source of atmospheric N2O (mean air−sea flux 2.0 ± 0.3 μmol m-2 d-1), and that nitrification and potential denitrification may be the primary mechanisms of N2O production in the STWNPO. We predict that ongoing ocean warming, deoxygenation, acidification, and anthropogenic nitrogen deposition in the STWNPO may elevate N2O emissions in the future. Therefore, the results obtained here are important for elucidating the relationships between N2O dynamics and environmental changes in the STWNPO and the global ocean.

Overlooked and widespread pennate diatom-diazotroph symbioses in the sea

Persistent nitrogen depletion in sunlit open ocean waters provides a favorable ecological niche for nitrogen-fixing (diazotrophic) cyanobacteria, some of which associate symbiotically with eukaryotic algae. All known marine examples of these symbioses have involved either centric diatom or haptophyte hosts. We report here the discovery and characterization of two distinct marine pennate diatom-diazotroph symbioses, which until now had only been observed in freshwater environments. Rhopalodiaceae diatoms Epithemia pelagica sp. nov. and Epithemia catenata sp. nov. were isolated repeatedly from the subtropical North Pacific Ocean, and analysis of sequence libraries reveals a global distribution. These symbioses likely escaped attention because the endosymbionts lack fluorescent photopigments, have nifH gene sequences similar to those of free-living unicellular cyanobacteria, and are lost in nitrogen-replete medium. Marine Rhopalodiaceae-diazotroph symbioses are a previously overlooked but widespread source of bioavailable nitrogen in marine habitats and provide new, easily cultured model organisms for the study of organelle evolution.

Laysan albatross exhibit complex behavioral plasticity in the subtropical and subarctic North Pacific Ocean

Animals that regularly traverse habitat extremes between the subtropics and subarctic are expected to exhibit foraging behaviors that respond to changes in dynamic ocean habitats, and these behaviors may facilitate adaptations to novel and changing climates. During the chick-provisioning stage, Laysan albatross Phoebastria immutabilis parents regularly undertake short- and long-distance foraging trips throughout the vast central North Pacific Ocean. We examined GPS tracking data among chick-provisioning albatrosses in Hawai‘i to characterize habitats during short- and long-distance trips. The study period encompassed a marine heatwave (2014) and the cooling period after an extreme El Niño event (2016), enabling us to examine foraging habitats under novel and changing climates. First passage time and generalized additive mixed models indicated that during 183 short and 110 long trips (n = 32 birds), wind-assisted flight efficiency, proximity to productive areas, and moonlit-searching were important in both subtropical and subarctic habitats. Laysan albatross took foraging trips that had similar lengths and durations in 2014 and 2016 and visited similar areas, indicating that their foraging range did not expand in response to climatic variability. A strategy that uses similar foraging areas across years combined with reliance on environmental processes that enhance flight efficiency (wind) and that enable searching behaviors (moonlight) indicate that Laysan albatross exhibit complex behavioral plasticity that allows them to utilize subtropical and subarctic habitats affected by dynamic climate variability. This strategy may benefit their ability to respond to oceanographic and climatic change, including expanding warm water regions and changing atmospheric conditions influenced by global warming.

Light and depth dependency of nitrogen fixation by the non-photosynthetic, symbiotic cyanobacterium UCYN-A.

SummaryThe symbiotic cyanobacterium UCYN‐A is one of the most globally abundant marine dinitrogen (N2)‐fixers, but cultures have not been available and its biology and ecology are poorly understood. We used cultivation‐independent approaches to investigate how UCYN‐A single‐cell N2 fixation rates (NFRs) and nifH gene expression vary as a function of depth and photoperiod. Twelve‐hour day/night incubations showed that UCYN‐A only fixed N2 during the day. Experiments conducted using in situ arrays showed a light‐dependence of NFRs by the UCYN‐A symbiosis, with the highest rates in surface waters (5–45 m) and lower rates at depth (≥ 75 m). Analysis of NFRs versus in situ light intensity yielded a light saturation parameter (I k ) for UCYN‐A of 44 μmol quanta m−2 s−1. This is low compared with other marine diazotrophs, suggesting an ecological advantage for the UCYN‐A symbiosis under low‐light conditions. In contrast to cell‐specific NFRs, nifH gene‐specific expression levels did not vary with depth, indicating that light regulates N2 fixation by UCYN‐A through processes other than transcription, likely including host–symbiont interactions. These results offer new insights into the physiology of the UCYN‐A symbiosis in the subtropical North Pacific Ocean and provide clues to the environmental drivers of its global distributions.

Production and diversity of microorganisms associated with sinking particles in the subtropical North Pacific Ocean

AbstractSinking particulate organic matter controls the flux of carbon (C) from the surface ocean to the deep sea. Microorganisms actively colonize particles, but the extent to which microbial metabolism influences particle export remains uncertain. We conducted experiments to quantify rates of bacterial production (derived based on 3H‐leucine incorporation) and dark C‐fixation (based on 14C‐bicarbonate assimilation) associated with sinking particles collected from the base of the euphotic zone (175 m) in the subtropical North Pacific Ocean. Seawater was amended with sinking particles and rates of filter size‐fractionated (0.2, 2, and 20 μm) bacterial production and dark C‐fixation were measured. Sequencing of 16S ribosomal RNA (rRNA) gene amplicons revealed that microorganisms in the particle‐amended treatments differed from those in the unamended seawater controls, with the particle treatments enriched in putative copiotrophic bacteria. The addition of sinking particles increased rates of bacterial production (by 6‐ to 9‐fold), and to a lesser extent dark C‐fixation (by 1.7‐ to 4.6‐fold), relative to unamended controls, with most of the production associated with filter pore sizes < 20 μm. Normalizing production to concentrations of particulate C yielded rates that were statistically indistinguishable between particle‐amended treatments and unamended controls. We then examined possible relationships between sinking particulate C flux attenuation and its supply to the mesopelagic waters, revealing that flux attenuation was positively related to increases in particulate C supply. Together with results from our experiments, we suggest processes that contribute to sinking particle disaggregation both increase flux attenuation and favor microbial mineralization of particle‐derived organic matter.

Cross-basin differences in the nutrient assimilation characteristics of induced phytoplankton blooms in the subtropical Pacific waters

Abstract. To better understand the nutrient assimilation characteristics of subtropical phytoplankton, deep-water addition incubation experiments were carried out on surface waters collected at seven stations across the subtropical North and South Pacific Ocean. These deep-water additions induced phytoplankton blooms with nutrient drawdown at all stations. The drawdown ratios of dissolved inorganic nitrogen (DIN) to phosphate (PO4) varied from 14.1 to 30.7 at the PO4-replete stations in the central North Pacific (CNP) and eastern South Pacific (ESP). These ratios were similar to the range represented by the canonical Redfield ratio (16) through to typical particulate N:P ratios in the surface subtropical ocean (28). In contrast, lower DIN:PO4 drawdown ratios (7.7–13.3) were observed in induced blooms at the PO4-depleted stations in the western North Pacific (WNP). The DIN:PO4 drawdown ratios in the PO4-replete ESP were associated with eukaryote-dominated blooms, while those in the PO4-depleted WNP were associated with eukaryotic and cyanobacterial blooms. The surplus PO4 assimilation, relative to DIN, by phytoplankton in the WNP was not expected based on their typical cellular N:P ratio and was likely due to the high PO4 uptake capability as induced by low-PO4-adapted phytoplankton. The low- and high-P* (=PO4- DIN/16) regimes geographically corresponded to the low and high DIN:PO4 drawdown ratios in the WNP and the CNP or ESP, respectively. The basin-wide P* distribution in the oligotrophic Pacific surface waters showed a clear regional trend from low in the WNP (<50 nM) to high in the ESP (>100 nM). These results suggest that the subtropical phytoplankton blooms as observed in our experiments could be an important factor controlling P* as well as the commonly recognized dinitrogen fixation and denitrification characteristics.

Strengthened ocean-desert process in the North Pacific over the past two decades

North Pacific ocean desert (NPOD) refers to the subtropical North Pacific Ocean of low chlorophyll-a (Chl-a) concentrations, as the largest ocean desert globally. Studies have suggested a development of NPOD over recent decades based on limited evidences from in-field measurements and yet elusive mechanism. In this study, we characterize intensity, area and position of the NPOD from year 1998 to 2018, and investigate its control by the coherent climate processes, based on an available, longest satellite observations of Chl-a concentration. Our results suggested that NPOD oligotrophication and expansion processes were correlated with warming upper oceans in most part of the NPOD, except for the SW NPOD area where the Chl-a variations were linked with regional change in sea surface heights. Moreover, based on our analysis, insignificant shift but only NW-SE variability of the NPOD mean position was likely controlled by the Pacific decadal oscillation processes.

Wide-range detection of Cu-binding organic ligands in seawater using reverse titration

The toxicity and bioavailability of Cu in seawater were estimated using its chemical speciation. In this study, the concentrations and conditional stability constants (K'CuL) of Cu complexes with Cu-binding organic ligands (L) in seawater were determined using the reverse titration–competitive ligand exchange–adsorptive cathodic stripping voltammetry method. The concentrations of strong and weak ligands (L1 and L2, respectively) were determined using reverse titration via the addition of Cu to seawater samples to achieve ligand saturation. Our results revealed that the reverse titration method can be successfully used for the detection of high concentrations of L2. Using the reverse titration method, we detected L2 concentrations that exceeded 80 and 120 nM in seawater samples from the subtropical western North Pacific Ocean and Otsuchi Bay Japan, respectively. The K′CuL1 and K′CuL2 values obtained using the reverse titration method were comparable with those obtained using the forward titration method. Therefore, the reverse titration method can be used to determine the chemical speciation of Cu in the ocean and coastal regions as supplement to the more conventional forward titration method.

Role of the eastern subtropical North Pacific Ocean on the El Niño’s transition processes

We examine the transition processes of El Nino occurring in 1 year after using observations, a multi-century model simulation and atmospheric general circulation model (AGCM) experiments. One type is characterized by a quick transition from an El Nino event into a La Nina event in the following winter, and the other type involves a slow decay to an almost neutral state or a continuous El Nino event. While both the Indian and the Atlantic Oceans contribute to the El Nino’s transition processes, we further find that sea surface temperature (SST) conditions in the eastern subtropical North Pacific and the associated surface wind anomalies play important roles in determining the different types of El Nino’s transition processes through atmosphere–ocean coupled processes. In particular, northeasterly wind anomalies in the central subtropical North Pacific during the early spring, which is in consequence of atmosphere–ocean coupled processes, contribute to decreasing ocean heat content (OHC) anomalies and strengthening easterly wind anomalies in the central tropical Pacific. These anomalies lead up to a transition into a La Nina event in the following winter. On the contrary, weakening of the northeasterly wind and warm SST anomalies in the eastern subtropical North Pacific during the early spring are conducive to a neutral state or a continuous El Nino event in the subsequent winter. Similar transition processes are also found in a multi-century model simulation. By conducting idealized AGCM experiments, we also show that the anomalous SST in the eastern subtropical North Pacific during El Nino peak season may induce surface wind anomalies in the central tropical Pacific during El Nino onset season. These results provide potential precursors for predicting the occurrence of a La Nina event, a neutral state and an El Nino event 1 year after the occurrence of an El Nino event.

Biogeochemical Controls of Particulate Phosphorus Distribution Across the Oligotrophic Subtropical Pacific Ocean

AbstractWe investigated basin‐wide distributions of total particulate phosphorus (TPP) and associated biogeochemical parameters in the euphotic zone (EZ) of the subtropical North and South Pacific Ocean. TPP is primarily composed of living microorganisms, and its distribution is likely controlled by nitrogen (N) supply in typical N‐limited Pacific waters as well as phosphorus (P) supply in P‐limited western North Pacific. TPP concentrations showed a vertically uniform distribution (approximately 15 nM) within the EZ at most stations, but at several northern North Pacific stations, TPP peaks (>20 nM) were observed in the lower EZ where nitrate and nitrite concentrations were high. Relationships between TPP and biogeochemical parameters indicate that a possible factor controlling TPP in the upper EZ was dinitrogen fixation and that in the lower EZ was shoaling of the nitracline. Geographical distribution of TPP was relatively uniform compared to that of phosphate, which varied from <10 nM in the western North Pacific to >100 nM in the eastern North and South Pacific. The western North Pacific was a domain characterized by lower dissolved organic P (DOP) concentrations, higher alkaline phosphatase activities, and higher particulate polyphosphate (PpolyP):TPP ratios compared to other domains. These characteristics indicated that active DOP utilization and PpolyP accumulation could play important roles in the maintenance of TPP stocks as alternatives to phosphate utilization in this distinctive domain. Our results demonstrated that TPP distributions were vertically and geographically uniform across the subtropical Pacific Ocean, concealing that TPP stocks were likely underpinned by a range of nutrient supply mechanisms.

Reproductive traits and seasonal variations in the spawning activity of female albacore, Thunnus alalunga, in the subtropical western North Pacific Ocean

To clarify the reproductive activity of albacore, Thunnus alalunga, in the subtropical western North Pacific Ocean (WNPO) (13–25°N, 133°E–171°W), reproductive traits, such as the sex ratio, spawning season, batch fecundity, size at maturity and spawning fraction, were investigated. The sex ratio (5 cm intervals) was skewed towards females in each fork length class less than 95 cm and was skewed towards males in each fork length class larger than 95 cm. Spawning capable fish appeared throughout the year, except in January, and primarily occurred between April and September. The size at 50% maturity in females was estimated to be a fork length of 87.5 cm (approximately 5 years). The estimated total spawning fraction and spawning interval during the spawning season were 0.35 and 2.83 days, and the spawning fractions from May to August were larger than those in the other sampling months. The estimated batch fecundity and relative batch fecundity (mean ± SD) were 1.16 ± 0.67 million oocytes and 59.1 ± 31.5 oocytes/g, respectively (fork length range: 88.6–104.8 cm). There was a positive correlation between the fork length and batch fecundity (r2 = 0.27). These results suggest that albacore in the WNPO spawn year round, while peak spawning occurs between May and August (i.e., between boreal spring and summer).

Physical drivers of the summer 2019 North Pacific marine heatwave

Summer 2019 observations show a rapid resurgence of the Blob-like warm sea surface temperature (SST) anomalies that produced devastating marine impacts in the Northeast Pacific during winter 2013/2014. Unlike the original Blob, Blob 2.0 peaked in the summer, a season when little is known about the physical drivers of such events. We show that Blob 2.0 primarily results from a prolonged weakening of the North Pacific High-Pressure System. This reduces surface winds and decreases evaporative cooling and wind-driven upper ocean mixing. Warmer ocean conditions then reduce low-cloud fraction, reinforcing the marine heatwave through a positive low-cloud feedback. Using an atmospheric model forced with observed SSTs, we also find that remote SST forcing from the central equatorial and, surprisingly, the subtropical North Pacific Ocean contribute to the weakened North Pacific High. Our multi-faceted analysis sheds light on the physical drivers governing the intensity and longevity of summertime North Pacific marine heatwaves.

Role of climate variability in the potential predictability of tropical cyclone formation in tropical and subtropical western North Pacific Ocean

The out of phase tropical cyclone (TC) formation in the subtropical and tropical western North Pacific associated with local low-level wind vorticity anomaly, driven by the remote central and eastern equatorial Pacific warming/cooling, is investigated based on the reanalysis and observational data in the period of 1979−2017. TC frequencies in the subtropical and tropical western North Pacific appear to be connected to different remote heating/cooling sources and are linked to eastern and central Pacific warming/cooling, which are in turn related to canonical El Niño/Southern Oscillation (ENSO) and ENSO Modoki, respectively. TCs formed in subtropics (SfTC) are generally found to be associated with a dipole in wind vorticity anomaly, which is driven by the tropical eastern Pacific warming/cooling. Tropically formed TCs (TfTC) are seen to be triggered by the single-core of wind vorticity anomaly locally associated with the warming/cooling of central and eastern Pacific. The predicted ENSOs and ENSO Modokis, therefore, provide a potential source of seasonal predictability for SfTC and TfTC frequencies.

Arsenate and microbial dynamics in different phosphorus regimes of the subtropical Pacific Ocean

Biologically toxic arsenate is physicochemically similar to biologically essential phosphate. Because arsenate and phosphate are indiscriminately incorporated by microbes, their ambient concentration ratios can be an important factor controlling microbial growth and metabolism. This study investigated the spatial distributions of arsenate and phosphate and the associated biogeochemical dynamics in the subtropical North and South Pacific Ocean. Vertical arsenate and phosphate profiles (≤ 150 m) in most of the study areas showed a nutrient-type distribution where the concentrations increased below the euphotic zone. The arsenate and phosphate concentrations in the surface waters ranged from the detection limits (5 nM and 4 nM, respectively) to approximately 40 nM and 400 nM, respectively. The surface arsenate:phosphate ratios were typically lower than 1, but those in the western subtropical North Pacific (WSNP) were frequently higher than 1 due to phosphate depletion. In the WSNP surface waters, Prochlorococcus and Pelagibacter arsenic detoxification and phosphorus acquisition genes were abundant. Results of the onboard bioassays involving the addition of arsenate or phosphate to the surface water indicated that microbes throughout the study areas possessed arsenate resistance and those in the WSNP during summer were under serious phosphate limitation. Although phosphate limitation likely accelerates the relative cellular accumulation of toxic arsenate, the lowest particulate As:P ratios were observed in the summer WSNP, concurrent with the lowest dissolved organic P (DOP) concentrations and the highest alkaline phosphatase activities. These results imply that active As excretion and/or DOP utilization could alleviate As accumulation while maintaining the cellular P quota.

Tropical Cyclone Projections: Changing Climate Threats for Pacific Island Defense Installations

Abstract Potential changing climate threats in the tropical and subtropical North Pacific Ocean were assessed, using coupled ocean–atmosphere and atmosphere-only general circulation models, to explore their response to projected increasing greenhouse gas emissions. Tropical cyclone occurrence, described by frequency and intensity, near islands housing major U.S. defense installations was the primary focus. Four island regions—Guam and Kwajalein Atoll in the tropical northwestern Pacific, Okinawa in the subtropical northwestern Pacific, and Oahu in the tropical north-central Pacific—were considered, as they provide unique climate and geographical characteristics that either enhance or reduce the tropical cyclone risk. Guam experiences the most frequent and severe tropical cyclones, which often originate as weak systems close to the equator near Kwajalein and sometimes track far enough north to affect Okinawa, whereas intense storms are the least frequent around Oahu. From assessments of models that simulate well the tropical Pacific climate, it was determined that, with a projected warming climate, the number of tropical cyclones is likely to decrease for Guam and Kwajalein but remain about the same near Okinawa and Oahu; however, the maximum intensity of the strongest storms may increase in most regions. The likelihood of fewer but stronger storms will necessitate new localized assessments of the risk and vulnerabilities to tropical cyclones in the North Pacific.

The influence of abrupt increases in seawater pCO2 on plankton productivity in the subtropical North Pacific Ocean.

We conducted a series of experiments to examine short-term (2–5 days) effects of abrupt increases in the partial pressure of carbon dioxide (pCO2) in seawater on rates of primary and bacterial production at Station ALOHA (22°45’ N, 158° W) in the North Pacific Subtropical Gyre (NPSG). The majority of experiments (8 of 10 total) displayed no response in rates of primary production (measured by 14C-bicarbonate assimilation; 14C-PP) under elevated pCO2 (~1100 μatm) compared to ambient pCO2 (~387 μatm). In 2 of 10 experiments, rates of 14C-PP decreased significantly (~43%) under elevated pCO2 treatments relative to controls. Similarly, no significant differences between treatments were observed in 6 of 7 experiments where bacterial production was measured via incorporation of 3H-leucine (3H-Leu), while in 1 experiment, rates of 3H-Leu incorporation measured in the dark (3H-LeuDark) increased more than 2-fold under high pCO2 conditions. We also examined photoperiod-length, depth-dependent (0–125 m) responses in rates of 14C-PP and 3H-Leu incorporation to abrupt pCO2 increases (to ~750 μatm). In the majority of these depth-resolved experiments (4 of 5 total), rates of 14C-PP demonstrated no consistent response to elevated pCO2. In 2 of 5 depth-resolved experiments, rates of 3H-LeuDark incorporation were lower (10% to 15%) under elevated pCO2 compared to controls. Our results revealed that rates of 14C-PP and bacterial production in this persistently oligotrophic habitat generally demonstrated no or weak responses to abrupt changes in pCO2. We postulate that any effects caused by changes in pCO2 may be masked or outweighed by the role that nutrient availability and temperature play in controlling metabolism in this ecosystem.