North Pacific Subtropical Gyre Research Articles

Assessing N2 fixation flux and its controlling factors in the (sub)tropical western North Pacific through high‐resolution observations

AbstractThe (sub)tropical western North Pacific is potentially an area of intense nitrogen (N2) fixation in the global ocean, despite limited understanding of the flux and controlling factors. We conducted high‐resolution observations from 2016 to 2021 in this region and used machine learning algorithms to simulate N2 fixation flux. Models estimated an N2 fixation flux from 5.72 to 6.45 Tg N yr−1, with strong seasonal variation and peak rates in summer. The western North Pacific Subtropical Gyre and the Kuroshio Current contributed more to N2 fixation flux than did the adjacent areas. Models suggested that sea surface temperature, photosynthetically available radiation, and nutrient supply were most strongly correlated with seasonal and spatial variations in N2 fixation. This study provides an improved estimation of N2 fixation in the western North Pacific and advances our understanding of its role in ocean productivity.

Mixing Dynamics Within the Kuroshio Area Are Reflected in Dissolved Inorganic Radiocarbon Values

AbstractKuroshio is an important western boundary current system in the North Pacific subtropical gyre. Mesoscale eddies play an important role in Kuroshio path variations, which significantly affect fisheries, marine navigation, and climate in regions along the Kuroshio path. However, the direct physical impacts of the Kuroshio variabilities on mixing dynamics within the water columns off the southern coast of Japan remain unclear due to the lack of observational records. The radiocarbon (Δ14C) in dissolved inorganic carbon (DIC) of seawater has been used as a reliable tracer of water circulation and mixing processes. Here, we present the high‐resolution dissolved inorganic radiocarbon (DIC ∆14C) measurements for water samples collected during the Kuroshio large‐meander (LM) period in March 2022 to observe the mixing dynamics within the Kuroshio area and elucidate the controls of mixing processes. In the present study, horizontal variations (12–175‰) in DIC Δ14C values were observed between 400 and 1,000 m and were attributed mainly to changes in the depth of isopycnal surfaces associated with mesoscale eddies and the position of the Kuroshio axis. Furthermore, fluctuations in DIC Δ14C values (10–25‰) were observed on the same isopycnal surfaces ( 25.5–27.0), suggesting the occurrence of diapycnal mixing. By comparing this newly obtained data with values previously reported at similar locations, variations in penetration depths of high bomb 14C signals from past observations are found to be mainly caused by changes in the depth of isopycnal surfaces associated with mesoscale eddies and Kuroshio path variations.

Relationships Between Plankton Size Spectra, Net Primary Production, and the Biological Carbon Pump

AbstractPhotosynthesis in the surface ocean and subsequent export of a fraction of this fixed carbon leads to carbon dioxide sequestration in the deep ocean. Ecological relationships among plankton functional groups and theoretical relationships between particle size and sinking rate suggest that carbon export from the euphotic zone is more efficient when communities are dominated by large organisms. However, this hypothesis has never been tested against measured size spectra spanning the >5 orders of magnitude found in plankton communities. Using data from five ocean regions (California Current Ecosystem, North Pacific subtropical gyre, Costa Rica Dome, Gulf of Mexico, and Southern Ocean subtropical front), we quantified carbon‐based plankton size spectra from heterotrophic bacteria to metazoan zooplankton (size class cutoffs varied slightly between regions) and their relationship to net primary production and sinking particle flux. Slopes of the normalized biomass size spectra (NBSS) varied from −1.6 to −1.2 (median slope of −1.4 equates to large 1–10 mm organisms having a biomass equal to only 7.6% of the biomass in small 1–10 μm organisms). Net primary production was positively correlated with the NBSS slope, with a particularly strong relationship in the microbial portion of the size spectra. While organic carbon export co‐varied with NBSS slope, we found only weak evidence that export efficiency is related to plankton community size spectra. Multi‐variate statistical analysis suggested that properties of the NBSS added no explanatory power over chlorophyll, primary production, and temperature. Rather, the results suggest that both plankton size spectra and carbon export increase with increasing system productivity.

Biodegradation of polyethylene by the marine fungus Parengyodontium album

Plastic pollution in the marine realm is a severe environmental problem. Nevertheless, plastic may also serve as a potential carbon and energy source for microbes, yet the contribution of marine microbes, especially marine fungi to plastic degradation is not well constrained. We isolated the fungus Parengyodontium album from floating plastic debris in the North Pacific Subtropical Gyre and measured fungal-mediated mineralization rates (conversion to CO2) of polyethylene (PE) by applying stable isotope probing assays with 13C-PE over 9 days of incubation. When the PE was pretreated with UV light, the biodegradation rate of the initially added PE was 0.044 %/day. Furthermore, we traced the incorporation of PE-derived 13C‑carbon into P. album biomass using nanoSIMS and fatty acid analysis. Despite the high mineralization rate of the UV-treated 13C-PE, incorporation of PE-derived 13C into fungal cells was minor, and 13C incorporation was not detectable for the non-treated PE. Together, our results reveal the potential of P. album to degrade PE in the marine environment and to mineralize it to CO2. However, the initial photodegradation of PE is crucial for P. album to metabolize the PE-derived carbon.

Growth and mortality of aerobic anoxygenic phototrophs in the North Pacific Subtropical Gyre.

Aerobic anoxygenic phototrophic (AAP) bacteria harvest light energy using bacteriochlorophyll-containing reaction centers to supplement their mostly heterotrophic metabolism. While their abundance and growth have been intensively studied in coastal environments, much less is known about their activity in oligotrophic open ocean regions. Therefore, we combined in situ sampling in the North Pacific Subtropical Gyre, north of O'ahu island, Hawaii, with two manipulation experiments. Infra-red epifluorescence microscopy documented that AAP bacteria represented approximately 2% of total bacteria in the euphotic zone with the maximum abundance in the upper 50 m. They conducted active photosynthetic electron transport with maximum rates up to 50 electrons per reaction center per second. The in situ decline of bacteriochlorophyll concentration over the daylight period, an estimate of loss rates due to predation, indicated that the AAP bacteria in the upper 50 m of the water column turned over at rates of 0.75-0.90 d-1. This corresponded well with the specific growth rate determined in dilution experiments where AAP bacteria grew at a rate 1.05 ± 0.09 d-1. An amendment of inorganic nitrogen to obtain N:P = 32 resulted in a more than 10 times increase in AAP abundance over 6 days. The presented data document that AAP bacteria are an active part of the bacterioplankton community in the oligotrophic North Pacific Subtropical Gyre and that their growth was mostly controlled by nitrogen availability and grazing pressure.IMPORTANCEMarine bacteria represent a complex assembly of species with different physiology, metabolism, and substrate preferences. We focus on a specific functional group of marine bacteria called aerobic anoxygenic phototrophs. These photoheterotrophic organisms require organic carbon substrates for growth, but they can also supplement their metabolic needs with light energy captured by bacteriochlorophyll. These bacteria have been intensively studied in coastal regions, but rather less is known about their distribution, growth, and mortality in the oligotrophic open ocean. Therefore, we conducted a suite of measurements in the North Pacific Subtropical Gyre to determine the distribution of these organisms in the water column and their growth and mortality rates. A nutrient amendment experiment showed that aerobic anoxygenic phototrophs were limited by inorganic nitrogen. Despite this, they grew more rapidly than average heterotrophic bacteria, but their growth was balanced by intense grazing pressure.

Dissolved rare earth elements in the North Pacific Subtropical Gyre: Lithogenic sources and water mass mixing control

In the North Pacific Subtropical Gyre, which is one of the largest oligotrophic regions, there is a lack of information regarding the sources and transport of trace metals through water mass mixing. Rare earth elements (REEs) are essential for tracing lithogenic sources and water mass transport. In this study, we present dissolved REE concentrations and the factors controlling their distributions in the northwest Pacific during a GEOTRACES cruise (GP09). In the surface water along 11°N, we observed input signals from the Philippine and the Hawaiian Islands, characterized by positive Eu anomalies and slightly elevated REE concentrations. By incorporating our data and published REE data from the northwest Pacific to the southeast Pacific (40°N–40°S), we demonstrated that the REE concentrations and Yb/Nd ratios can distinguish North Pacific Intermediate Water (NPIW), Antarctic Intermediate Water (AAIW), and modified AAIW. By estimating the ratio of water mass mixing, we suggest that heavy REEs are predominantly contributed by water mass mixing (e.g., 93 % ± 4% for Yb) and can be used as semi-conservative tracers to quantify the mixing of NPIW and modified AAIW at the potential density of 27.2 kg/m3. At ∼800 m depth at stations K12, K13, and K14, weak negative Ce anomalies (>0.1) were observed, indicating the lateral transport of water masses imprinted with sediment signals from the Philippine Islands. In the deep waters (>1500 m) at the Luzon Strait (station K1), we propose that the weak Ce negative anomalies (>0.08) and low Yb/Nd ratios (<5), combined with the low beam transmission, are the result of particle resuspension and release. In deep water (>2000 m), combining high-resolution REE measurements with water mass analysis, our research reveals that dissolved REEs (except Ce) are dominantly controlled by water mass mixing (e.g., 80 %∼100 % for Yb and 70 %∼100 % for Nd). For the non-conservative behavior, which is not explained by water mass mixing, the residual fraction of heavy REEs (<20 % for Yb) originates from release of sinking particles (organic matter and siliceous particles), while the remaining concentrations of light REEs (<30 % for Nd) may be influenced by a combination of particle release and scavenging processes. These findings highlight valuable information about lithogenic sources and the proportions of REE distributions that are controlled by physical and biogeochemical processes. Moreover, it emphasizes the applicability of heavy REEs as effective tracers for understanding basin-scale water mass mixing in the northwest Pacific.

A Lagrangian coherent eddy atlas for biogeochemical applications in the North Pacific Subtropical Gyre

Abstract. Mesoscale eddies affect phytoplankton in several ways, including the horizontal dispersal and mixing of populations. Coherent eddies trap and contain fluid masses, whereas other eddies mix more freely with surrounding waters. To evaluate the role of lateral dispersal and trapping on the biogeochemical properties of eddies, we must accurately characterize their coherency. We employed a Lagrangian approach to identify materially coherent structures in remote sensing observations and developed a methodology to track them over their entire individual lifetimes. We provide an atlas of two decades of coherent eddies with an 8 d resolution in an intensely studied region of the North Pacific Subtropical Gyre (Jones-Kellett, 2023a, https://doi.org/10.5281/zenodo.8139149). The atlas was specifically designed to facilitate biogeochemical applications and was synchronized with available ocean color products. We identified coherent features using backward Lagrangian trajectories because the recent history of coherency (rather than the future coherency) will be most valuable for interpreting associated biogeochemical signatures. We compared the atlas of Lagrangian coherent eddies with an atlas of Eulerian eddies identified using the more conventionally used Sea Level Anomaly method. Although 65 % of tracked Sea Level Anomaly eddies are classified as coherent at some point in their lifetime, only 54 % contain a Lagrangian coherent structure at any given time. We found similar variations in the temporal and spatial distributions of coherent and Sea Level Anomaly eddies. However, strictly coherent eddies have a clearer relationship between size and longevity and form distinct regional regimes based on polarity. We illustrate the potential of the coherent eddy atlas for biogeochemical applications by examining the relationship between bloom development and eddy evolution in a case study of a Hawaiian Lee cyclone.

The combined effects of predation, fishing, and ocean productivity on salmon species targeted by marine mammals in the northeast Pacific.

Along the northeast Pacific coast, the salmon-eating southern resident killer whale population (SRKW, Orcinus orca) have been at very low levels since the 1970s. Previous research have suggested that reduction in food availability, especially of Chinook salmon (Oncorhynchus tshawytscha), could be the main limiting factor for the SRKW population. Using the ecosystem modelling platform Ecopath with Ecosim (EwE), this study evaluated if the decline of the Pacific salmon populations between 1979 and 2020 may have been impacted by a combination of factors, including marine mammal predation, fishing activities, and climatic patterns. We found that the total mortality of most Chinook salmon populations has been relatively stable for all mature returning fish despite strong reduction in fishing mortality since the 1990s. This mortality pattern was mainly driven by pinnipeds, with increases in predation between 1979 and 2020 mortality ranging by factors of 1.8 to 8.5 across the different Chinook salmon population groups. The predation mortality on fall-run Chinook salmon smolts originating from the Salish Sea increased 4.6 times from 1979 to 2020, whereas the predation mortality on coho salmon (Oncorhynchus kisutch) smolts increased by a factor of 7.3. The model also revealed that the north Pacific gyre oscillation (NPGO) was the most important large-scale climatic index affecting the stock productivity of Chinook salmon populations from California to northern British Columbia. Overall, the model provided evidence that multiple factors may have affected Chinook salmon populations between 1979 and 2020, and suggested that predation mortality by marine mammals could be an important driver of salmon population declines during that time.

Disentangling top-down drivers of mortality underlying diel population dynamics of Prochlorococcus in the North Pacific Subtropical Gyre

Photosynthesis fuels primary production at the base of marine food webs. Yet, in many surface ocean ecosystems, diel-driven primary production is tightly coupled to daily loss. This tight coupling raises the question: which top-down drivers predominate in maintaining persistently stable picocyanobacterial populations over longer time scales? Motivated by high-frequency surface water measurements taken in the North Pacific Subtropical Gyre (NPSG), we developed multitrophic models to investigate bottom-up and top-down mechanisms underlying the balanced control of Prochlorococcus populations. We find that incorporating photosynthetic growth with viral- and predator-induced mortality is sufficient to recapitulate daily oscillations of Prochlorococcus abundances with baseline community abundances. In doing so, we infer that grazers in this environment function as the predominant top-down factor despite high standing viral particle densities. The model-data fits also reveal the ecological relevance of light-dependent viral traits and non-canonical factors to cellular loss. Finally, we leverage sensitivity analyses to demonstrate how variation in life history traits across distinct oceanic contexts, including variation in viral adsorption and grazer clearance rates, can transform the quantitative and even qualitative importance of top-down controls in shaping Prochlorococcus population dynamics.

Ubiquitous filter feeders shape open ocean microbial community structure and function.

The mechanism of mortality plays a large role in how microorganisms in the open ocean contribute to global energy and nutrient cycling. Salps are ubiquitous pelagic tunicates that are a well-known mortality source for large phototrophic microorganisms in coastal and high-latitude systems, but their impact on the immense populations of smaller prokaryotes in the tropical and subtropical open ocean gyres is not well quantified. We used robustly quantitative techniques to measure salp clearance and enrichment of specific microbial functional groups in the North Pacific Subtropical Gyre, one of the largest ecosystems on Earth. We discovered that salps are a previously unknown predator of the globally abundant nitrogen fixer Crocosphaera; thus, salps restrain new nitrogen delivery to the marine ecosystem. We show that the ocean's two numerically dominant cells, Prochlorococcus and SAR11, are not consumed by salps, which offers a new explanation for the dominance of small cells in open ocean systems. We also identified a double bonus for Prochlorococcus, wherein it not only escapes salp predation but the salps also remove one of its major mixotrophic predators, the prymnesiophyte Chrysochromulina. When we modeled the interaction between salp mesh and particles, we found that cell size alone could not account for these prey selection patterns. Instead, the results suggest that alternative mechanisms, such as surface property, shape, nutritional quality, or even prey behavior, determine which microbial cells are consumed by salps. Together, these results identify salps as a major factor in shaping the structure, function, and ecology of open ocean microbial communities.

Hotspots of Floating Plastic Particles across the North Pacific Ocean.

The pollution of the marine environment with plastic debris is expected to increase, where ocean currents and winds cause their accumulation in convergence zones like the North Pacific Subtropical Gyre (NPSG). Surface-floating plastic (>330 μm) was collected in the North Pacific Ocean between Vancouver (Canada) and Singapore using a neuston catamaran and identified by Fourier-transform infrared spectroscopy (FT-IR). Baseline concentrations of 41,600-102,700 items km-2 were found, dominated by polyethylene and polypropylene. Higher concentrations (factors 4-10) of plastic items occurred not only in the NPSG (452,800 items km-2) but also in a second area, the Papaha̅naumokua̅kea Marine National Monument (PMNM, 285,200 items km-2). This second maximum was neither reported previously nor predicted by the applied ocean current model. Visual observations of floating debris (>5 cm; 8-2565 items km-2 and 34-4941 items km-2 including smaller "white bits") yielded similar patterns of baseline pollution (34-3265 items km-2) and elevated concentrations of plastic debris in the NPSG (67-4941 items km-2) and the PMNM (295-3748 items km-2). These findings suggest that ocean currents are not the only factor provoking plastic debris accumulation in the ocean. Visual observations may be useful to increase our knowledge of large-scale (micro)plastic pollution in the global oceans.

The California current system off Baja California Sur

Based on hydrographic data from two cruises (June 2010 and May 2012) off Baja California Sur, Mexico, historical in situ measurements from the World Ocean Database 2018, California Cooperative Oceanic Fisheries Investigations, and Investigaciones Mexicanas de la Corriente de California cruise programs, as well as satellite images and data from the Global Reanalysis, this study describes the California Current System off Baja California Sur. The California Current System is characterized by the interaction of four near-surface currents. Far from the coast, the California Current flows within the North Pacific Subtropical Gyre but does not reach the Pacific Tropical-Subtropical Convergence off Mexico. From December to June, an equatorward flow known as the Tropical Branch of California Current emerges along the coast. This flow intensifies from March to June and is closely related to the mass flux induced by coastal upwelling. In July, a new branch of the Tropical Branch of the California Current turns poleward along the Baja California Sur coast. This flow is often referred to as the California Surface Countercurrent because it flows in the opposite direction to the California Current. Additionally, the California Undercurrent is detected, influencing the water column from 100 to 900 m, with a maximum poleward flow between 200 m and 300 m, decreasing toward the surface. The California Undercurrent persists throughout the year over the continental slope, displaying a significant semiannual component around the Gulf of California entrance. The California Current System off Baja California Sur plays a crucial role in the formation of Transitional Waters within the Pacific Tropical-Subtropical Convergence off Mexico. The importance of equatorward flows by the Tropical Branch of California Current in ventilating the Oxygen Minimum Zone waters in the Central Pacific off Mexico is emphasized.

Revealing the Marine Cycles of Volatile Sulfur Compounds and Their Biogeochemical Controls: A Case of the Western North Pacific.

Volatile sulfur compounds, such as dimethyl sulfide (DMS), carbonyl sulfide (OCS), and carbon disulfide (CS2), have significant implications for both atmospheric chemistry and climate change. Despite the crucial role of oceans in regulating their atmospheric budgets, our comprehension of their cycles in seawater remains insufficient. To address this gap, a field investigation was conducted in the western North Pacific to clarify the sources, sinks, and biogeochemical controls of these gases in two different marine environments, including relatively eutrophic Kuroshio-Oyashio extension (KOE) and oligotrophic North Pacific subtropical gyre. Our findings revealed higher concentrations of these gases in both seawater and the atmosphere in the KOE compared to the subtropical gyre. In the KOE, nutrient-rich upwelling stimulated rapid DMS biological production, while reduced seawater temperatures hindered the removal of OCS and CS2, leading to their accumulation. Furthermore, we have quantitatively evaluated the relative contribution of each pathway to the source and sink of DMS, OCS, and CS2 within the mixed layer and identified vertical exchange as a potential sink in most cases, transporting substantial amounts of these gases from the mixed layer to deeper waters. This research advances our understanding of sulfur gas source-sink dynamics in seawater, contributing to the assessment of their marine emissions and atmospheric budgets.

Remote Estimates of Sea Surface Nitrate and Its Trends From Ocean Color in the Northwest Pacific

AbstractSea surface nitrate (SSN) plays an important role in assessing new production and phytoplankton growth in the ocean, yet it has been challenging to estimate SSN from satellites due to its complex and varying relationship with different environmental proxies. The different SSN trends in the northwest Pacific reported in previous studies call for more detailed research to examine the interannual variabilities in SSN. We addressed this problem by developing a stacking‐random‐forest based algorithm for Moderate Resolution Imaging Spectroradiometer (MODIS). It allows estimating SSN from daily sea surface temperature (SST) and Chlorophyll‐a concentration (Chl) at a spatial resolution of 4 km. For SSN ranging between 0.0005 and 25.88 μmol/kg (N = 3,452), the model had a root mean square difference of 1.34 μmol/kg (5.3%) and coefficient of determination of 0.92. Further independent validation and sensitivity tests demonstrated the validity of the algorithm in retrieving SSN. Using this novel data record, for the first time, we investigated the SSN interannual variabilities and trends from MODIS. Overall, the SSN showed a weak decreasing trend of −0.01 ± 0.007 μmol kg−1 yr−1 (p &lt; 0.05) in the northwest Pacific in 2002–2020, associated with an increasing trend in SST (0.03 ± 0.01˚C yr−1 at p &lt; 0.05) and insignificant trend (0.001 ± 0.001 mg m−3 yr−1 at p &gt; 0.05) in Chl. The interannual variabilities of SSN were significantly correlated with the environmental proxies (SST, Chl) and the climate indices (Pacific Decadal Oscillation and North Pacific Gyre Oscillation). The SSN trends can be further restricted with more data available.

Impact of climate variability on moonfish (Mene maculata) catch rate in the waters off southwestern Taiwan

AbstractA commercially significant small coastal forage fish, moonfish (Mene maculata), accounts for almost 98% of Taiwanese purse seine capture, primarily in the southwestern Taiwan region. Research indicates that climate indices affect coastal fish catch and dispersal, while the link between moonfish and climatic variability in this region is still unclear. This study found that the delayed period of Pacific Decadal Oscillation (PDO), North Pacific Oscillation (NPO) and North Pacific Gyre Oscillation (NPGO) affect moonfish distribution and catch rates off southwestern Taiwan. Understudied are the environmental factors connected to these oscillations and their delayed consequences on moonfish catch rates. We focussed on Taiwan purse seiner capture rates (catch per unit effort or CPUE) of moonfish in southwestern Taiwan from 2014 to 2020 and delayed oscillation occurrences up to 5 years to better understand these processes. CPUE demonstrated a 3 to 4‐year positive connection (r &gt; 0.5) with NPGO, NPO and PDO. The region's moonfish catch rates were most affected by NPGO with a 4‐year lag, followed by a 3‐year lag of PDO and lastly a 4‐year lag of NPO, according to the results of generalized additive models (GAMs). All the three oscillations had the greatest impact on moonfish catchability when a lag of &gt;2 years was present. Between 2014 and 2020, moonfish CPUE fluctuated, peaking in 2019. The climatic parameters that affect moonfish in southwestern Taiwan and the Taiwanese purse seine fisheries catches are shown by this study.

Modeling of swordtip squid (Uroteuthis edulis) monthly habitat preference using remote sensing environmental data and climate indices

Understanding the spatial arrangement of species in maritime settings necessitates the study of oceanography. Hence, doing a study on the correlation between oceanography and species dispersion is imperative, considering the impacts of global climate change. The study used a generalized additive modeling approach to analyze the influence of oceanographic conditions on the distribution of swordtip squid in northeastern Taiwan, integrating fishing data, climatic oscillation and oceanography. Among seven oceanographic characteristics, bottom sea temperature (SSTB), sea surface height (SSH), sea surface chlorophyll (SSC), and sea surface temperature (SST) showed significant influence in generalized additive model (GAM) analysis (combined deviance explained: 40.30%). The monthly catch rate of swordtip squid is influenced by six climatic oscillations, with the Pacific Decadal Oscillation having the most significant impact, accounting for 31% of the distribution, followed by the North Pacific Gyre Oscillation at 10.8% and the Western Pacific Oscillation at 6.05%. From 2015 to 2019, the main areas for squid fishing were situated in the northeastern waters of Taiwan, precisely within the geographical coordinates of 25°N to 28°N and 121.5°E to 125°E. This study provides crucial insights for managing swordtip squid fisheries in Taiwan's northwest waters, highlighting the importance of incorporating oceanographic conditions relating to climate change information into decision-making to protect global ocean fisheries and their dependent communities.

Efficient biological carbon export to the mesopelagic ocean induced by submesoscale fronts

Oceanic submesoscale processes are ubiquitous in the North Pacific Subtropical Gyre (NPSG), where the biological carbon pump is generally ineffective. Due to difficulties in collecting continuous observations, however, it remains uncertain whether episodic submesoscale processes can drive significant changes in particulate organic carbon (POC) export into the mesopelagic ocean. Here we present observations from high-frequency Biogeochemical-Argo floats in the NPSG, which captured the enhanced POC export fluxes during the intensifying stages of a submesoscale front and a cyclonic eddy compared to their other life stages. A higher percentage of POC export flux was found to be transferred to the base of mesopelagic layer at the front compared to that at the intensifying eddy and the mean of previous studies (37% vs. ~10%), suggesting that the POC export efficiency was significantly strengthened by submesoscale dynamics. Such findings highlight the importance of submesoscale fronts for carbon export and sequestration in subtropical gyres.

Geographical patterns of mesozooplankton functional diversity in the northwestern Pacific

To better understand how communities response to environmental change and how biodiversity affects ecosystem function, it is essential to quantify the functional diversity of communities. However, the current understanding of zooplankton functional traits and functional diversity in the northwestern Pacific (NWP) is inadequate. In this study, we analyzed the latitudinal changes in functional traits, diversities, and functional space of zooplankton in five biogeographic regions of the tropical and subtropical NWP. Zooplankton species richness varied unimodally over latitude, with the North Pacific Subtropical Gyre (NPSG) hosting the greatest number of species (367 taxa). The NWP was dominated by zooplankton of small and medium size (<2 mm; 72 %), current-feeding (47 %), omnivore–herbivore (71 %), and broadcast spawner (53 %). The functional trait composition of zooplankton varied significantly; for instance, omnivore–detritivores were prevalent in the Kuroshio Extension zone but few in the oligotrophic NPSG and North Equatorial Current regions. Protozoans with binary fission (primarily foraminifers) declined from the equator to high latitudes. The latitudinal variations of functional evenness, functional dispersion, and hypervolume of trait space of the zooplankton community were unimodal, with the maximum values occurring in the NPSG. This would suggest that the zooplankton community in the NPSG was more efficient in utilizing resources than in other regions, and the competition across functional groups was mild. Stochastic processes were crucial in zooplankton community assembly in tropical and subtropical NWP. This study enhanced our understanding of zooplankton community assembly and offered new insights into the functional traits of zooplankton communities in the NWP.

Radiative Impacts of Californian Marine Low Clouds on North Pacific Climate in a Global Climate Model

Abstract The northeastern Pacific climate system features an extensive low-cloud deck off California on the southeastern flank of the subtropical high that accompanies intense northeasterly trades and relatively low sea surface temperatures (SSTs). This study assesses climatological impacts of the low-cloud deck and their seasonal differences by regionally turning on and off the low-cloud radiative effect in a fully coupled atmosphere–ocean model. The simulations demonstrate that the cloud radiative effect causes a local SST decrease of up to 3°C on an annual average with the response extending southwestward with intensified trade winds, indicative of the wind–evaporation–SST (WES) feedback. This nonlocal wind response is strong in summer, when the SST decrease peaks due to increased shortwave cooling, and persists into autumn. In these seasons when the background SST is high, the lowered SST suppresses deep-convective precipitation that would otherwise occur in the absence of the low-cloud deck. The resultant anomalous diabatic cooling induces a surface anticyclonic response with the intensified trades that promote the WES feedback. Such seasonal enhancement of the atmospheric response does not occur without air–sea couplings. The enhanced trades accompany intensified upper-tropospheric westerlies, strengthening the vertical wind shear that, together with the lowered SST, acts to shield Hawaii from powerful hurricanes. On the basin scale, the anticyclonic surface wind response accelerates the North Pacific subtropical ocean gyre to speed up the Kuroshio by as much as 30%. SST thereby increases along the Kuroshio and its extension, intensifying upward turbulent heat fluxes from the ocean to increase precipitation.

Protistan community composition and metabolism in the North Pacific Subtropical Gyre: Influences of mesoscale eddies and depth.

Marine protists and their metabolic activities are intricately tied to the cycling of nutrients and the flow of energy through microbial food webs. Physiochemical changes in the environment, such as those that result from mesoscale eddies, may impact protistan communities, but the effects that such changes have on protists are poorly known. A metatranscriptomic study was conducted to investigate how eddies affected protists at adjacent cyclonic and anticyclonic eddy sites in the oligotrophic ocean at four depths from 25 to 250 m. Eddy polarity impacted protists at all depths sampled, although the effects of eddy polarity were secondary to the impact of depth across the depth range. Eddy-induced vertical shifts in the water column yielded differences in the cyclonic and anticyclonic eddy protistan communities, and these differences were the most pronounced at and just below the deep chlorophyll maximum. An analysis of transcripts associated with protistan nutritional physiology at 150 m revealed that cyclonic eddies may support a more heterotrophic community, while anticyclonic eddies promote a more phototrophic community. The results of this study indicate that eddies alter the metabolism of protists particularly in the lower euphotic zone and may therefore impact carbon export from the euphotic zone.