Photothermal conditions and upwelling enhance very short-lived brominated halocarbons emissions in the western tropical Pacific Ocean

Sea-to-air emissions of very short-lived brominated halocarbons (VSLBrHs) are known to contribute to 30 % of stratospheric and tropospheric ozone depletion. However, empirical data on their occurrence in open ocean are scarce, which makes it difficult to estimate the significant contribution of open ocean releases to the global budget of halocarbons. This study was conducted in 2022 to explore the spatial variations of VSLBrHs and their controlling factors in the western tropical Pacific Ocean (WTPO). The findings highlighted that high biological productivity and the resulting dissolved organic matter (DOM) as well as upwelling dynamics significantly influenced the distribution and production of VSLBrHs in seawater, with atmospheric levels primarily governed by oceanic emissions. Based on the simultaneous observation of seawater and atmospheric concentrations, the mean sea-to-air fluxes of CH2Br2, CHBr3, CHBrCl2, and CHBr2Cl were estimated to be 1.01, 6.65, 9.31, and 7.25 nmol m-2 d-1, respectively. Sea-to-air fluxes of these gases in the upwelling regions were 9.0, 4.6, 2.9, and 6.8 times those in the non-upwelling regions, respectively. Additionally, in-situ incubation experiments revealed that the enzymatic mediated biosynthesis pathways of VSLBrHs were enhanced under temperature and light-induced stress and in waters rich in humus-like substances. Therefore, we tentatively concluded that abundant photothermal conditions and the existence of upwelling in the WTPO made it a potential hotspot for the emission of VSLBrHs. This study offers critical insights into the environmental dynamics of VSLBrHs emissions and underscores the importance of regional oceanic conditions in influencing atmospheric greenhouse gas compositions.

Tropical decadal variability in nutrient supply and phytoplankton community in the Central Equatorial Pacific during the late Holocene

We have reconstructed baseline δ15N and δ13C of export production at Kingman Reef in the Central Equatorial Pacific (CEP) at sub-decadal resolution, nearly continuously over the last 2000 years. The changes in δ15N reflects the strength of the North Equatorial Counter Current (NECC) relative to the South Equatorial Current (SEC), and to a lesser extent, the North Equatorial Current (NEC). Seasonal to multi-decadal variation in the strength of these currents, through the redistribution of heat, have global climate impacts and influence marine and terrestrial ecosystems. We use modern El Niño-La Nina dynamics and the Tropical Pacific Decadal Variability (TPDV) pattern, which is defined in the CEP, as a framework for analyzing the isotopic data. The CEP δ15N and δ13C records exhibit multi-decadal (50-60 year) variability consistent with TPDV. A large multi-centennial feature in the CEP δ15N data, within age-model uncertainties, is consistent with one of the prolonged dry-pluvial sequences in the American west at the end of the Medieval Climate Anomaly, where low TPDV is correlated with drier conditions. This unique record shows that the strength of the NECC, as reflected in baseline δ15N and δ13C, has at quasi-predictable intervals throughout the late Holocene, toggled the phytoplankton community between prokaryotes and picoplankton versus eukaryotes.

Thermal coupling of the Indo-Pacific warm pool and Southern Ocean over the past 30,000 years

The role of the tropical Pacific Ocean and its linkages to the southern hemisphere during the last deglacial warming remain highly controversial. Here we explore the evolution of Pacific horizontal and vertical thermal gradients over the past 30 kyr by compiling 340 sea surface and 7 subsurface temperature records, as well as one new ocean heat content record. Our records reveal that La Niña-like conditions dominated during the deglaciation as a result of the more intense warming in the western Pacific warm pool. Both the subsurface temperature and ocean heat content in the warm pool rose earlier than the sea surface temperature, and in phase with South Pacific subsurface temperature and orbital precession, implying that heat exchange between the tropical upper water column and the extratropical Southern Ocean facilitated faster warming in the western Pacific. Our study underscores the key role of the thermal coupling between the warm pool and the Southern Ocean and its relevance for future global warming.

The mechanism of the last deglacial global warming is key for future climate. Here, the authors shed light on the pivotal role of the thermal coupling between the western Pacific warm pool and the Southern Ocean.

A Maluku Sea intermediate western boundary current connecting Pacific Ocean circulation to the Indonesian Throughflow

The Indonesian Throughflow plays an important role in the global ocean circulation and climate. Existing studies of the Indonesian Throughflow have focused on the Makassar Strait and the exit straits, where the upper thermocline currents carry North Pacific waters to the Indian Ocean. Here we show, using mooring observations, that a previous unknown intermediate western boundary current (with the core at ~1000 m depth) exists in the Maluku Sea, which transports intermediate waters (primarily the Antarctic Intermediate Water) from the Pacific into the Seram-Banda Seas through the Lifamatola Passage above the bottom overflow. Our results suggest the importance of the western boundary current in global ocean intermediate circulation and overturn. We anticipate that our study is the beginning of more extensive investigations of the intermediate circulation of the Indo-Pacific ocean in global overturn, which shall improve our understanding of ocean heat and CO₂ storages significantly.

Here the authors use in situ mooring data in the Maluku Channel and the Lifamatola Passage of the Indonesian seas to show that a western boundary current transports Antarctic water from the South Pacific to the Indian Ocean.

Direct Measurements of Turbulence in the Upper Western Pacific North Equatorial Current over a 25-h Period

Measurements of the turbulent kinetic energy dissipation rate (ε) were conducted by a free-fall microstructure profiler in the western Pacific North Equatorial Current (WPNEC) during a continuous period of 25 h, from the sea surface to about 160 m depth. In the mixed layer (ML), ε values were typically on the order of 10⁻⁸∼10⁻⁷ W kg⁻¹, and an obvious diurnal cycle existed in the upper 40 m of the surface mixing layer. Below the ML, ε was reduced to 10⁻⁹∼10⁻⁸ W kg⁻¹ with some patches of high ε reaching 10^−7.5 W kg⁻¹. The barrier layer was identified in the nighttime with a maximum thickness of 20 m, and it was eroded by the advection of freshwater within the lower part of the isothermal layers associated with an anticyclonic eddy in the afternoon. A simple scaling relevant to shear (S²) instability and stratification (N²) that can predict turbulent dissipation rates in the transition layer, between the well-mixed layer and the thermocline below, was obtained through the scaling ε∼S−0.40N0.20. Besides turbulence, double-diffusive processes also contributed to the vertical mixing levels in the upper WPNEC.

Environmental influences on sinking rates and distributions of transparent exopolymer particles after a typhoon surge at the Western Pacific

A multidisciplinary approach was used to investigate the causes of the distributions and sinking rates of transparent exopolymer particles (TEPs) during the period of September–October (2017) in the Western Pacific Ocean (WPO); the study period was closely dated to a northwest typhoon surge. The present study discussed the impact of biogeophysical features on TEPs and their sinking rates (sTEP) at depths of 0–150 m. During the study, the concentration of TEPs was found to be higher in areas adjacent to the Kuroshio current and in the bottom water layer of the Mindanao upwelling zone due to the widespread distribution of cyanobacteria, i.e., Trichodesmium hildebrandti and T. theibauti. The positive significant regressions of TEP concentrations with Chl-a contents in eddy-driven areas (R² = 0.73, especially at 100 m (R² = 0.75)) support this hypothesis. However, low TEP concentrations and TEPs were observed at mixed layer depths (MLDs) in the upwelling zone (Mindanao). Conversely, high TEP concentrations and high sTEP were found at the bottom of the downwelling zone (Halmahera). The geophysical directions of eddies may have caused these conditions. In demonstrating these relations, the average interpretation showed the negative linearity of TEP concentrations with TEPs (R² = 0.41 ~ 0.65) at such eddies. Additionally, regression curves (R² = 0.78) indicated that atmospheric pressure played a key role in the changes in TEPs throughout the study area. Diatoms and cyanobacteria also curved the TEPs significantly (R² = 0.5, P < 0.05) at the surface of the WPO. This study also revealed that TEP concentration contributes less to the average particulate organic carbon in this oligotrophic WPO.

Structure and dynamics of the Pacific North Equatorial Subsurface Current

The North Equatorial Subsurface Current (NESC) has recently been found to flow westward below the North Equatorial Countercurrent in the subsurface layer across the Pacific Ocean. The structure, water mass properties, and the dynamics of the NESC are studied using Argo profiles and geostrophic currents, combined with moored current meter observations. The mean westward geostrophic currents of the NESC has been validated with moored current meter measurements at 4.7° N, 142° E in the far western tropical Pacific Ocean. Sizable seasonal-to-interannual variability of the NESC is indicated by the observations, with strong transports in boreal summer and during La Niña events, whereas weak transports in boreal winter and during El Niño events. The water masses of the NESC appear to be the mixture of the North and South Pacific intermediate waters, with the waters immediately below the thermocline closer to the North than to the South Pacific waters. A simulation using a linear continuously stratified model of ocean circulation suggests that the mean NESC is forced by wind curl through low baroclinic mode responses of the ocean.

Release of eDNA by different life history stages and during spawning activities of laboratory-reared Japanese eels for interpretation of oceanic survey data

To assist in detection of offshore spawning activities of the Japanese eel Anguilla japonica and facilitate interpretation of results of environmental DNA (eDNA) analysis in their spawning area, we examined the eDNA concentration released by each life history stage of artificially reared Japanese eels in the laboratory using quantitative real-time PCR (qPCR). We also compared eDNA concentrations between before and after artificially induced spawning activities. eDNA was not detected from three 30 L seawater tanks containing each single fertilized egg, but eDNA was found from other tanks each containing single individuals of larval stages (preleptocephalus and leptocephalus), juvenile stages (glass eel, elver and yellow eel) or adult stage (silver eel). The eDNA concentrations increased in the life history stages, showed a significant difference among all stages, and were positively correlated with the total length and wet weight. Moreover, the eDNA concentration after spawning was 10-200 times higher than that before spawning, which indicated that the spawning events in the ocean would produce relatively high eDNA concentration. These results in the laboratory suggested that eDNA analysis appears to be an effective method for assisting oceanic surveys to estimate the presence and spawning events of the Japanese eel in the spawning area.

Concentrations and distribution of phthalate esters in the seamount area of the Tropical Western Pacific Ocean

A total of 14 phthalate esters (PAEs) were analysed by gas chromatography-mass spectrometry (GC-MS) to better understand its occurrence and distribution in seawater samples of M2 seamount in the Tropical Western Pacific Ocean (TWPO). The concentrations of ΣPAEs in the seawater ranged from 12.13 ng L^-1 to 60.69 ng L^-1 (av. 28.86 ng L^-1), dominated by dibutyl phthalate (DBP), di(2‑ethylhexyl) phthalate (DEHP) and diisobutyl phthalate (DiBP). ΣPAEs concentrations in the southwest of the seamount were lower than those in the northeast, with the minima appearing above the seamount summit. Current-seamount interaction was reckoned to be the principal driving factors in the distribution of PAEs. DEHP posed a medium risk in seawater, suggesting that marine plastic pollution has become an urgent environmental issue that calls for more attention and actions. Microplastics leaching and atmospheric deposition might be the potential sources of PAEs.

Isopycnal mixing of interhemispheric intermediate waters by subthermocline eddies east of the Philippines

Both sporadic observations and modelling studies suggest that subthermocline eddies (SEs) exist east of the Philippines, where interhemispheric waters meet. However, effects of SEs on water mass mixing have never been observed. Here, using data from mooring and buoy deployed in the frontal region of the interhemispheric water masses, we show for the first time that the SEs act as an “underwater mixer” of intermediate waters from north and south Pacific oceans. The SEs have typical swirl speeds of 0.1~0.4 m s⁻¹ between 200 and 800 m depth with a dominant period of ~90 days. Variation in intermediate water salinity also had a period of ~90 days, lagging eddy speed by ~8 days. Horizontal eddy diffusivity representative of eddy mixing rate was quantified using a mixing-length framework. Horizontal eddy diffusivity had both surface and subthermocline maxima. The vertically varying eddy diffusivity can be used to improve parameterization of eddy stirring in the tropical Pacific by coarse-resolution ocean climate models. The effect of the SEs on mixing of intermediate water masses seems not resolved by available eddy-resolving ocean models typically used for this region.

Spatiotemporal variability in the δ18O-salinity relationship of seawater across the tropical Pacific Ocean

The relationship between salinity and the stable oxygen isotope ratio of seawater (δ¹⁸O_sw) is of utmost importance to the quantitative reconstruction of past changes in salinity from δ¹⁸O values of marine carbonates. This relationship is often considered to be uniform across water masses, but the constancy of the δ¹⁸O_sw-salinity relationship across space and time remains uncertain, as δ¹⁸O_sw responds to varying atmospheric vapor sources and pathways, while salinity does not. Here we present new δ¹⁸O_sw-salinity data from sites spanning the tropical Pacific Ocean. New data from Palau, Papua New Guinea, Kiritimati, and Galápagos show slopes ranging from 0.09 ‰/psu in the Galápagos to 0.32‰/psu in Palau. The slope of the δ¹⁸O_sw-salinity relationship is higher in the western tropical Pacific versus the eastern tropical Pacific in observations and in two isotope-enabled climate models. A comparison of δ¹⁸O_sw-salinity relationships derived from short-term spatial surveys and multi-year time series at Papua New Guinea and Galápagos suggests spatial relationships can be substituted for temporal relationships at these sites, at least within the time period of the investigation. However, the δ¹⁸O_sw-salinity relationship varied temporally at Palau, likely in response to water mass changes associated with interannual El Niño-Southern Oscillation (ENSO) variability, suggesting nonstationarity in this local δ¹⁸O_sw-salinity relationship. Applying local δ¹⁸O_sw-salinity relationships in a coral δ¹⁸O forward model shows that using a constant, basin-wide δ¹⁸O_sw-salinity slope can both overestimate and underestimate the contribution of δ¹⁸O_sw to carbonate δ¹⁸O variance at individual sites in the western tropical Pacific.

Structure and Variability of the North Equatorial Current/Undercurrent from Mooring Measurements at 130°E in the Western Pacific

The mean structure and variability of the North Equatorial Current/Undercurrent (NEC/NEUC) are investigated with one-year Acoustic Doppler Current Profilers measurements from 4 subsurface moorings deployed at 10.5°N, 13°N, 15.5°N, and 18°N along 130°E in the western Pacific. The strong westward flowing NEC ranges from the sea surface down to 400 m, and the mean zonal velocity of the NEC at 10.5°N is around −30 cm/s at the depth of 70 m. Eastward flowing NEUC jets are detected below the NEC at 10.5°N and 13°N, and the depth of the NEUC could reach at least 900 m. The mean velocity of the NEUC is around 4.2 cm/s at 800 m. No eastward undercurrents are observed at 15°N and 18°N. The mooring measurements also reveal a strong intraseasonal variability of the currents at all 4 mooring sites, and the period is around 70–120 days. The vertical structure of this intraseasonal variability varies at different latitudes. The variability of the NEUC jets at 10.5°N and 13°N appears to be dominated by subthermocline signals, while the variability of the currents at 15.5°N and 18°N is dominated by surface-intensified signals.

Genetic differentiation and phylogeography of partially sympatric species complex Rhizophora mucronata Lam. and R. stylosa Griff. using SSR markers

Background

Mangrove forests are ecologically important but globally threatened intertidal plant communities. Effective mangrove conservation requires the determination of species identity, management units, and genetic structure. Here, we investigate the genetic distinctiveness and genetic structure of an iconic but yet taxonomically confusing species complex Rhizophora mucronata and R. stylosa across their distributional range, by employing a suite of 20 informative nuclear SSR markers.

Results

Our results demonstrated the general genetic distinctiveness of R. mucronata and R. stylosa, and potential hybridization or introgression between them. We investigated the population genetics of each species without the putative hybrids, and found strong genetic structure between oceanic regions in both R. mucronata and R. stylosa. In R. mucronata, a strong divergence was detected between populations from the Indian Ocean region (Indian Ocean and Andaman Sea) and the Pacific Ocean region (Malacca Strait, South China Sea and Northwest Pacific Ocean). In R. stylosa, the genetic break was located more eastward, between populations from South and East China Sea and populations from the Southwest Pacific Ocean. The location of these genetic breaks coincided with the boundaries of oceanic currents, thus suggesting that oceanic circulation patterns might have acted as a cryptic barrier to gene flow.

Conclusions

Our findings have important implications on the conservation of mangroves, especially relating to replanting efforts and the definition of evolutionary significant units in Rhizophora species. We outlined the genetic structure and identified geographical areas that require further investigations for both R. mucronata and R. stylosa. These results serve as the foundation for the conservation genetics of R. mucronata and R. stylosa and highlighted the need to recognize the genetic distinctiveness of closely-related species, determine their respective genetic structure, and avoid artificially promoting hybridization in mangrove restoration programmes.

Electronic supplementary material

The online version of this article (doi:10.1186/s12862-015-0331-3) contains supplementary material, which is available to authorized users.

Optimum interpolation analysis of basin-scale ¹³⁷Cs transport in surface seawater in the North Pacific Ocean

¹³⁷Cs is one of the conservative tracers applied to the study of oceanic circulation processes on decadal time scales. To investigate the spatial distribution and the temporal variation of ¹³⁷Cs concentrations in surface seawater in the North Pacific Ocean after 1957, a technique for optimum interpolation (OI) was applied to understand the behaviour of ¹³⁷Cs that revealed the basin-scale circulation of Cs ¹³⁷Cs in surface seawater in the North Pacific Ocean: ¹³⁷Cs deposited in the western North Pacific Ocean from global fallout (late 1950s and early 1960s) and from local fallout (transported from the Bikini and Enewetak Atolls during the late 1950s) was further transported eastward with the Kuroshio and North Pacific Currents within several years of deposition and was accumulated in the eastern North Pacific Ocean until 1967. Subsequently, ¹³⁷Cs concentrations in the eastern North Pacific Ocean decreased due to southward transport. Less radioactively contaminated seawater was also transported northward, upstream of the North Equatorial Current in the western North Pacific Ocean in the 1970s, indicating seawater re-circulation in the North Pacific Gyre.

Observations of CFCs and SF6 as ocean tracers

An advantage of using chlorofluorocarbons (CFCs) and sulfur hexafluoride (SF6) as tracers of ocean circulation is that the time-dependent source functions permit calculation of rates for ocean processes. These compounds are also sensitive indicators highlighting interior ocean regions where surface-derived anomalies can be transported on timescales of decades. Significant applications for CFCs have been for the deep limb of the Atlantic meridional overturning circulation, upper ocean ventilation, and biogeochemical rates, including apparent oxygen utilization rates and anthropogenic CO2 inventories. Although CFCs have started to decrease in the atmosphere, SF6 continues to increase. There are benefits to measuring both CFCs and SF6: A large global CFC data set exists; CFCs are still increasing in older waters; SF6 expands estimates of age; and calculations of anthropogenic CO2 inventory are enhanced. Thus, the outlook for using CFCs as tracers for oceanic processes, and in particular in concert with SF6, remains very positive.

Abrupt shift in subsurface temperatures in the tropical pacific associated with changes in El Nino

Radiocarbon (14C) content of surface waters inferred from a coral record from the Galapagos Islands increased abruptly during the upwelling season (July through September) after the El Nino event of 1976. Sea-surface temperatures (SSTs) associated with the upwelling season also shifted after 1976. The synchroneity of the shift in both 14C and SST implies that the vertical thermal structure of the eastern tropical Pacific changed in 1976. This change may be responsible for the increase in frequency and intensity of El Nino events since 1976.