Oxygen minimum zones in the eastern tropical Atlantic and Pacific oceans

Within the eastern tropical oceans of the Atlantic and Pacific basin vast oxygen minimum zones (OMZ) exist in the depth range between 100 and 900 m. Minimum oxygen values are reached at 300–500 m depth which in the eastern Pacific become suboxic (dissolved oxygen content <4.5 μmol kg⁻¹) with dissolved oxygen concentration of less than 1 μmol kg⁻¹. The OMZ of the eastern Atlantic is not suboxic and has relatively high oxygen minimum values of about 17 μmol kg⁻¹ in the South Atlantic and more than 40 μmol kg⁻¹ in the North Atlantic. About 20 (40%) of the North Pacific volume is occupied by an OMZ when using 45 μmol kg⁻¹ (or 90 μmol kg⁻¹, respectively) as an upper bound for OMZ oxygen concentration for ocean densities lighter than σ_θ < 27.2 kg m⁻³. The relative volumes reduce to less than half for the South Pacific (7% and 13%, respectively). The abundance of OMZs are considerably smaller (1% and 7%) for the South Atlantic and only 0% and 5% for the North Atlantic. Thermal domes characterized by upward displacements of isotherms located in the northeastern Pacific and Atlantic and in the southeastern Atlantic are co-located with the centres of the OMZs. They seem not to be directly involved in the generation of the OMZs.OMZs are a consequence of a combination of weak ocean ventilation, which supplies oxygen, and respiration, which consumes oxygen. Oxygen consumption can be approximated by the apparent oxygen utilization (AOU). However, AOU scaled with an appropriate consumption rate (aOUR) gives a time, the oxygen age. Here we derive oxygen ages using climatological AOU data and an empirical estimate of aOUR. Averaging oxygen ages for main thermocline isopycnals of the Atlantic and Pacific Ocean exhibit an exponential increase with density without an obvious signature of the OMZs. Oxygen supply originates from a surface outcrop area and can also be approximated by the turn-over time, the ratio of ocean volume to ventilating flux. The turn-over time corresponds well to the average oxygen ages for the well ventilated waters. However, in the density ranges of the suboxic OMZs the turn-over time substantially increases. This indicates that reduced ventilation in the outcrop is directly related to the existence of suboxic OMZs, but they are not obviously related to enhanced consumption indicated by the oxygen ages. The turn-over time suggests that the lower thermocline of the North Atlantic would be suboxic but at present this is compensated by the import of water from the well ventilated South Atlantic. The turn-over time approach itself is independent of details of ocean transport pathways. Instead the geographical location of the OMZ is to first order determined by: (i) the patterns of upwelling, either through Ekman or equatorial divergence, (ii) the regions of general sluggish horizontal transport at the eastern boundaries, and (iii) to a lesser extent to regions with high productivity as indicated through ocean colour data.     

A review of eastern tropical Pacific oceanography: Summary

The collection of articles in this volume reviewing eastern tropical Pacific oceanography is briefly summarized, and updated references are given. The region is an unusual biological environment as a consequence of physical characteristics and patterns of forcing – including a strong and shallow thermocline, the ITCZ and coastal wind jets, equatorial upwelling, the Costa Rica Dome, eastern boundary and equatorial current systems, low iron input, inadequate ventilation of subthermocline waters, and dominance of ENSO-scale temporal variability. Remaining unanswered questions are presented.     

热带北太平洋深海小型底栖生物大尺度空间分布

通过对2001年热带北太平洋深海小型底栖生物大尺度空间的数量分布的初步调查,共发现了15个类群的小型底栖生物,其平均丰度为34.1个/10 cm2,东太平洋区的东小区小型底栖生物类群数明显高于其它区域,小型底栖生物量比例随着沉积物厚度的增加而降低,生物量比例为68%~87%的小型底栖生物集中在沉积物0~2 cm层(表层)内。食物是深海小型底栖生物分布的限制因子,沉积物类型和底层流对小型底栖生物空间分布也有一定的影响。     

Assessment of storage lipid accumulation patterns in eucalanoid copepods from the eastern tropical Pacific Ocean

Members of the copepod family Eucalanidae are widely distributed throughout the world׳s oceans and have been noted for their accumulation of storage lipids in high- and low-latitude environments. However, little is known about the lipid composition of eucalanoid copepods in low-latitude environments. The purpose of this study was to examine fatty acid and alcohol profiles in the storage lipids (wax esters and triacylglycerols) of Eucalanus inermis, Rhincalanus rostrifrons, R. nasutus, Pareucalanus attenuatus, and Subeucalanus subtenuis, collected primarily in the eastern tropical north Pacific near the Tehuantepec Bowl and Costa Rica Dome regions, noted for its oxygen minimum zone, during fall 2007 and winter 2008/2009. Adult copepods and particulate material were collected in the upper 50 m and from 200 to 300 m in the upper oxycline. Lipid profiles of particulate matter were generated to help ascertain information on ecological strategies of these species and on differential accumulation of dietary and modified fatty acids in the wax ester and triacylglycerol storage lipid components of these copepods in relation to their vertical distributions around the oxygen minimum zone. Additional data on phospholipid fatty acid and sterol/fatty alcohol fractions were also generated to obtain a comprehensive lipid data set for each sample. Rhincalanus spp. accumulated relatively large amounts of storage lipids (31–80% of dry mass (DM)), while E. inermis had moderate amounts (2–9% DM), and P. attenuatus and S. subtenuis had low quantities of storage lipid (0–1% DM). E. inermis and S. subtenuis primarily accumulated triacylglycerols (>90% of storage lipids), while P. attenuatus and Rhincalanus spp. primarily accumulated wax esters (>84% of storage lipids). Based on previously generated molecular phylogenies of the Eucalanidae family, these results appear to support genetic predisposition as a major factor explaining why a given species accumulates primarily triacylglycerols or wax esters, and also potentially dictating major fatty acid and alcohol accumulation patterns within the more highly modified wax ester fraction. Comparisons of fatty acid profiles between triacylglycerol and wax ester components in copepods with that in available prey suggested that copepod triacylglycerols were more reflective of dietary fatty acids, while wax esters contained a higher proportion of modified or de novo synthesized forms. Sterols and phospholipid fatty acids were similar between species, confirming high levels of regulation within these components. Similarities between triacylglycerol fatty acid profiles of E. inermis collected in surface waters and at >200 m depth indicate little to no feeding during their ontogenetic migration to deeper, low-oxygen waters.     

Eddies and Tropical Instability Waves in the eastern tropical Pacific: A review

Mesoscale eddies and tropical instability waves in the eastern tropical Pacific, first revealed by satellite infrared imagery, play an important role in the dynamics and biology of the region, and in the transfer of mass, energy, heat, and biological constituents from the shelf to the deep ocean and across the equatorial currents.From boreal late autumn to early spring, four to 18 cyclonic or anticyclonic eddies are formed off the coastal region between southern Mexico and Panama. The anticyclonic gyres, which tend to be larger and last longer than the cyclonic ones, are the best studied: they typically are 180–500 km in diameter, depress the pycnocline from 60 to 145 m at the eddy center, have swirl speeds in excess of 1 m s⁻¹, migrate west at velocities ranging from 11 to 19 cm s⁻¹ (with a slight southward component), and maintain a height signature of up to 30 cm. The primary generating agents for these eddies are the strong, intermittent wind jets that blow across the isthmus of Tehuantepec in Mexico, the lake district in Nicaragua and Costa Rica, and the Panama canal. Other proposed eddy-generating mechanisms are the conservation of vorticity as the North Equatorial Counter Current (NECC) turns north on reaching America, and the instability of coastally trapped waves/currents.Tropical Instability Waves (TIWs) are perturbations in the SST fronts on either side of the equatorial cold tongue. They produce SST variations on the order of 1–2 °C, have periods of 20–40 days, wavelengths of 1000–2000 km, phase speeds of around 0.5 m s⁻¹ and propagate westward both north and south of the Equator. The Tropical Instability Vortices (TIVs) are a train of westward-propagating anticyclonic eddies associated with the TIWs. They exhibit eddy currents exceeding 1.3 m s⁻¹, a westward phase propagation speed between 30 and 40 km d⁻¹, a signature above the pycnocline, and eastward energy propagation. Like the TIWs, they result from the latitudinal barotropically unstable shear between the South Equatorial Current (SEC) and the NECC with a potential secondary source of energy from baroclinic instability of the vertical shear with the Equatorial Undercurrent (EUC).This review of mesoscale processes is part of a comprehensive review of the oceanography of the eastern tropical Pacific Ocean.     

Primary production in the eastern tropical Pacific: A review

The eastern tropical Pacific includes 28 million km² of ocean between 23.5°N and S and Central/South America and 140°W, and contains the eastern and equatorial branches of the north and South Pacific subtropical gyres plus two equatorial and two coastal countercurrents. Spatial patterns of primary production are in general determined by supply of macronutrients (nitrate, phosphate) from below the thermocline. Where the thermocline is shallow and intersects the lighted euphotic zone, biological production is enhanced. In the eastern tropical Pacific thermocline depth is controlled by three interrelated processes: a basin-scale east/west thermocline tilt, a basin-scale thermocline shoaling at the gyre margins, and local wind-driven upwelling. These processes regulate supply of nutrient-rich subsurface waters to the euphotic zone, and on their basis we have divided the eastern tropical Pacific into seven main regions. Primary production and its physical and chemical controls are described for each.Enhanced rates of macronutrient supply maintains levels of primary production in the eastern tropical Pacific above those of the oligotrophic subtropical gyres to the north and south. On the other hand lack of the micronutrient iron limits phytoplankton growth (and nitrogen fixation) over large portions of the open-ocean eastern tropical Pacific, depressing rates of primary production and resulting in the so-called high nitrate-low chlorophyll condition. Very high rates of primary production can occur in those coastal areas where both macronutrients and iron are supplied in abundance to surface waters. In these eutrophic coastal areas large phytoplankton cells dominate; conversely, in the open-ocean small cells are dominant. In a ‘shadow zone’ between the subtropical gyres with limited subsurface ventilation, enough production sinks and decays to produce anoxic and denitrified waters which spread beneath very large parts of the eastern tropical Pacific.Seasonal cycles are weak over much of the open-ocean eastern tropical Pacific, although several eutrophic coastal areas do exhibit substantial seasonality. The ENSO fluctuation, however, is an exceedingly important source of interannual variability in this region. El Niño in general results in a depressed thermocline and thus reduced rates of macronutrient supply and primary production. The multi-decadal PDO is likely also an important source of variability, with the ‘El Viejo’ phase of the PDO resulting in warmer and lower nutrient and productivity conditions similar to El Niño.On average the eastern tropical Pacific is moderately productive and, relative to Pacific and global means, its productivity and area are roughly equivalent. For example, it occupies about 18% of the Pacific Ocean by area and accounts for 22–23% of its productivity. Similarly, it occupies about 9% of the global ocean and accounts for 10% of its productivity. While representative, these average values obscure very substantial spatial and temporal variability that characterizes the dynamics of this tropical ocean.     

Trophic ecology and vertical patterns of carbon and nitrogen stable isotopes in zooplankton from oxygen minimum zone regions

The unique physical and biogeochemical characteristics of oxygen minimum zones (OMZs) influence plankton ecology, including zooplankton trophic webs. Using carbon and nitrogen stable isotopes, this study examined zooplankton trophic webs in the Eastern Tropical North Pacific (ETNP) OMZ. δ¹³C values were used to indicate zooplankton food sources, and δ¹⁵N values were used to indicate zooplankton trophic position and nitrogen cycle pathways. Vertically stratified MOCNESS net tows collected zooplankton from 0 to 1000 m at two stations along a north-south transect in the ETNP during 2007 and 2008, the Tehuantepec Bowl and the Costa Rica Dome. Zooplankton samples were separated into four size fractions for stable isotope analyses. Particulate organic matter (POM), assumed to represent a primary food source for zooplankton, was collected with McLane large volume in situ pumps.The isotopic composition and trophic ecology of the ETNP zooplankton community had distinct spatial and vertical patterns influenced by OMZ structure. The most pronounced vertical isotope gradients occurred near the upper and lower OMZ oxyclines. Material with lower δ¹³C values was apparently produced in the upper oxycline, possibly by chemoautotrophic microbes, and was subsequently consumed by zooplankton. Between-station differences in δ¹⁵N values suggested that different nitrogen cycle processes were dominant at the two locations, which influenced the isotopic characteristics of the zooplankton community. A strong depth gradient in zooplankton δ¹⁵N values in the lower oxycline suggested an increase in trophic cycling just below the core of the OMZ. Shallow POM (0–110 m) was likely the most important food source for mixed layer, upper oxycline, and OMZ core zooplankton, while deep POM was an important food source for most lower oxycline zooplankton (except for samples dominated by the seasonally migrating copepod Eucalanus inermis). There was no consistent isotopic progression among the four zooplankton size classes for these bulk mixed assemblage samples, implying overlapping trophic webs within the total size range considered.     

Atmospheric forcing of the eastern tropical Pacific: A review

The increase in marine, land surface, atmospheric and satellite data during recent decades has led to an improved understanding of the air–sea interaction processes in the eastern tropical Pacific. This is also thanks to extensive diagnoses from conceptual and coupled ocean–atmosphere numerical models. In this paper, mean fields of atmospheric variables, such as incoming solar radiation, sea level pressure, winds, wind stress curl, precipitation, evaporation, and surface energy fluxes, are derived from global atmospheric data sets in order to examine the dominant features of the low level atmospheric circulations of the region. The seasonal march of the atmospheric circulations is presented to depict the role of radiative forcing on atmospheric perturbations, especially those dominating the atmosphere at low levels.In the tropics, the trade winds constitute an important north–south energy and moisture exchange mechanism (as part of the low level branch of the Hadley circulation), that determines to a large extent the precipitation distribution in the region, i.e., that associated with the Inter-Tropical Convergence Zone (ITCZ). Monsoonal circulations also play an important role in determining the warm season precipitation distribution over the eastern tropical Pacific through a large variety of air–sea–land interaction mechanisms. Westward traveling waves, tropical cyclones, low latitude cold air intrusions, and other synoptic and mesoscale perturbations associated with the ITCZ are also important elements that modulate the annual rainfall cycle. The low-level jets of the Gulf of California, the Intra-Americas Sea (Gulf of Mexico and Caribbean Sea) and Chocó, Colombia are prominent features of the eastern tropical Pacific low-level circulations related to sub-regional and regional scale precipitation patterns. Observations show that the Intra-Americas Low-Level Jet intensity varies with El Niño/Southern Oscillation (ENSO) phases, however its origin and role in the westward propagation and development of disturbances that may hit the eastern tropical Pacific, such as easterly waves and tropical cyclones, are still unclear. Changes in the intensity of the trade winds in the Caribbean Sea and the Gulf of Mexico (associated with eastern tropical Pacific wind jets) exert an important control on precipitation by means of wind–topography interactions. Gaps in the mountains of southern Mexico and Central America allow strong wind jets to pass over the continent imprinting a unique signal in sea surface temperatures and ocean dynamics of the eastern tropical Pacific.The warm pools of the Americas constitute an important source of moisture for the North American Monsoon System. The northeastern tropical Pacific is a region of intense cyclogenetic activity, just west of the coast of Mesoamerica. Over the oceanic regions, large-scale properties of key variables such as precipitation, moisture, surface energy fluxes and wind stress curl are still uncertain, which inhibits a more comprehensive view of the region and stresses the importance of regional field experiments. Progress has been substantial in the understanding of the ocean and atmospheric dynamics of the eastern tropical Pacific, however, recent observational evidence such as that of a shallow meridional circulation cell in that region, in contrast to the classic concept of the Hadley-type deep meridional circulation, suggests that more in situ observations to validate theories are still necessary.This paper is part of a comprehensive review of the oceanography of the eastern tropical Pacific Ocean.     

Simulating the impacts of fishing on central and eastern tropical Pacific ecosystem using multispecies size-spectrum model

The size-spectrum model has been considered a useful tool for understanding the structures of marine ecosystems and examining management implications for fisheries. Based on Chinese tuna longline observer data from the central and eastern tropical Pacific Ocean and published data, we developed and calibrated a multispecies size-spectrum model of twenty common and commercially important species in this area. We then use the model to project the status of the species from 2016 to 2050 under five c...     

Lipid and 13C signatures of submicron and suspended particulate organic matter in the Eastern Tropical North Pacific: Implications for the contribution of Bacteria

The contribution of bacterial biomass to total particulate organic matter (POM) in the ocean, including exported material, is poorly constrained. To examine potential signatures for the presence and export of bacterioplankton and their detrital remains, here we provide a detailed compound-specific ¹³C characterization of fatty acids from membrane polar lipids obtained from a water column profile in the Eastern Tropical North Pacific. POM of submicron size (0.2–0.7 μm; “X-POM”) was sampled and analyzed separately from the size class typically collected as “suspended” POM (0.7–53 μm; “L-POM”). The distributions of polar head group classes, specific fatty acid side chains, and natural ¹³C contents all vary, both between particle size classes and with depth in the water column. In general, the polar lipids in submicron material – and by inference, lipids of bacterial origin – have higher ¹³C content than polar lipids from larger POM and are equally abundant. Lipid signatures from the photic zone appear to be partially conserved in the suspended pool during transit down the water column. However, bacterial heterotrophy and possibly chemoautotrophy partially overprint these surface signatures. In addition, active metabolisms in the oxygen minimum zone (OMZ) appear to mediate the disaggregation of POM transported from the surface, thus adding complexity to the pathways of mid-water carbon flux and providing additional organic substrates to the OMZ and below. This “substrate injection” may provide important fuel for the denitrification and anammox reactions. Finally, examination of ¹³C content in polar lipids provides a basis for new interpretation of depth-related variations in δ¹³C values of bulk suspended POM.     

1980～1999年热带太平洋次表层盐度年际变化同化数据分析

通过统计方法利用一套海洋同化数据分析了热带太平洋次表层的盐度变化特征.结果表明次表层盐度的年际变化与ENSO相关,且次表层盐度信号区域呈东西方向“跷跷板”的分布.对影响这些次表层的盐度信号区域平均的纬向平流、经向平流、垂直运动和淡水通量异常等因素进行了分析,并且与影响表层盐度年际变化模态的影响因素差异进行了比较,结果表明,纬向平流的异常对表层盐度的异常变化影响较大,而对次表层盐度异常有较大影响的是海水的垂直运动异常.     

热带西太平洋砂壳纤毛虫的多样性

为了探明热带西太平洋海区砂壳纤毛虫的多样性、垂直分布及纬向变化,我们于2012年11-12月在该海区四个断面采集了0-200 m的水样。共检出砂壳纤毛虫39属124种,多数种类喜好生活在表层和次表层。砂壳纤毛虫种丰富度、丰度以及多样性指数的高值主要出现在比叶绿素浓度最大值稍浅的位置,与温度和叶绿素浓度显著正相关,与盐度和采样深度显著负相关。多数优势种与环境因子间的相关性不显著。热带西太平洋海区砂壳纤毛虫多样性极高,各站种丰富度在25-52之间。在从表层至75 m处的多数采样点香农指数均大于3。该海区砂壳纤毛虫冗余种的比例很高,占砂壳纤毛虫总种类数的87.90%,贡献了砂壳纤毛虫60.38%的丰度,表明热带西太平洋海区砂壳纤毛虫群落对饵料组成和摄食压力改变的应对能力很强,稳定性很高。     

Epi-benthic megafaunal zonation across an oxygen minimum zone at the Indian continental margin

The Arabian Sea oxygen minimum zone (OMZ) impinges upon the Indian continental margin at bathyal depths (150-1500 m) producing changes in ambient oxygen availability and sediment geochemistry across the seafloor. The influence of these environmental changes upon the epi-benthic megafaunal assemblage was investigated by video survey at six stations spanning the OMZ core (540 m), lower boundary (800-1100 m) and below the OMZ (2000 m), between September and November 2008. Structural changes in the megafaunal assemblage were observed across the six stations, through changes in both megafaunal abundance and lebensspuren (biogenic traces). Most megafauna were absent in the OMZ core (540 m), where the assemblage was characterised by low densities of fishes (0.02-0.03 m⁻²). In the lower OMZ boundary, megafaunal abundance peaked at 800 m, where higher densities of ophiuroids (0.20-0.44 m⁻²) and decapods (0.11-0.15 m⁻²) were present. Total abundance declined with depth between 800 and 2000 m, as the number of taxa increased. Changes in the megafaunal assemblage were predicted by changes in abundance of seven taxonomic groups, correlated to both oxygen availability and sediment organic matter quality. Lebensspuren densities were highest in the OMZ boundary (800-1100 m) but traces of large infauna (e.g., echiurans and enteropneusts) were only observed between 1100 and 2000 m station, where the influence of the OMZ was reduced. Thus, changes in the megafaunal assemblage across the Indian margin OMZ reflect the responses of specific taxa to food availability and oxygen limitation.     

基于延绳钓观察员数据的热带太平洋中东部食物网拓扑结构分析

热带太平洋是金枪鱼类、旗鱼类等渔业的主要作业海域,但目前针对此海域食物网的研究仍十分缺乏。本文根据2017年6月-11月中国金枪鱼科学观察员在热带太平洋中东部海域（13°09''N -34°27''S, 100°03''-176°17''W）采集的鱼类样品,结合胃含物及其他分析方法以35个物种间的摄食关系为基础对此海域的食物网结构进行了分析。拓扑学指标（节点强度,D;中心性指数,BC和CC;拓扑重要性指数,TI¹和TI³;关键性指数,K、K_t和K_b）和Key-Player算法（KPP-1和KPP-2）用来筛选关键种,并结合体长数据建立了基于关键种的简化食物网。肯德尔相关性分析和聚类分析结果表明,不同拓扑学指标筛选出的关键种是一致的。大多数关键种与太平洋其他海域的研究结果是相同的,如剑鱼（Xiphias gladius）、长鳍金枪鱼（Thunnus alalunga）、小吻四鳍旗鱼（Tetrapturus angustirostris）头足类和鲭属。移除5个关键种后,食物网将被离散为许多互不连接的部分（F=0.632,F^D=0.795,R^D=0.957）,表明这些物种在维持食物网结构和稳定性中的必要性。物种体长是本研究构建简化食物网时的决定性因素。研究结果有助于提高对热带太平洋食物网结构的认识,同时为进一步的生态系统动力学研究提供科学依据。     

Thermohaline intrusions in the thermocline of the western tropical Pacific Ocean

The existing high-resolution hydrographic data in the western tropical Pacilit; Ocean are used to explore the spatial distribution and primary characteristics of thermohaline intrusions in the thermocline. Statistics show that the vertical scales of intrusions are 20-40 m in the upper thermocline （22.0-26.0δ0） and 40-80 m in the lower thermocline （26.0-27.2δ0）. In the upper thermocline, the most intensive intrusions exist at the equatorial front （EF） where north/sonth Pacilic water masses converge, anti Ihe westward spreading of the north Pacilic tropical waler （NPTW） in the Philippines Sea also produces patches of intrusions surrounding its high-salinity tongue. In the lower thermocline, intrusions are also strong at the tropical front （TF） which is the boundary between the north Pacilic subtropical/tropical waters. At the bottom of the thermocline （at about 27.0δ0）, intrusions mainly exist near the western boundary, which are produced by intermediate water convergence through the advection of subthermocline western boundary Ilows. Most strikingly a ＂C＂-shape distribution of intrusions at around 26.4δ0 is revealed, covering the vicinity of the EF the TE and the Mindanao Current （MC）, i.e., tile western boundary pathway ol the norlh Pacilic subtrnpical cell （STC）. Synoptic section analysis reveals that intrusions are more prominent on the warm/sally flank ot the fronts, implying more cross-front tongues of cold/fresh water. Among the intrusions, those at the EF are of best lateral coherence which implies a unique driving mechanism involving near-inertial velocity perturbations near the equator.     

Vertical zonation and distributions of calanoid copepods through the lower oxycline of the Arabian Sea oxygen minimum zone

This paper provides the first comprehensive analysis of calanoid copepod vertical zonation and community structure at midwater depths (300–1000 m) through the lower oxygen gradient (oxycline) (0.02 to 0.3 ml/L) of an oxygen minimum zone (OMZ). Feeding ecology was also analyzed. Zooplankton were collected with a double 1 m² MOCNESS plankton net in day and night vertically-stratified oblique tows from 1000 m to the surface at six stations during four seasons as part of the 1995 US Joint Global Ocean Flux Study (JGOFS) Arabian Sea project. The geographic comparison between a eutrophic more oxygenated onshore station and an offshore station with a strong OMZ served as a natural experiment to elucidate the influence of depth, oxygen concentration, season, food resources, and predators on the copepod distributions.Copepod species and species assemblages of the Arabian Sea OMZ differed in their spatial and vertical distributions relative to environmental and ecological characteristics of the water column and region. The extent and intensity of the oxycline at the lower boundary of the OMZ, and its spatial and temporal variability over the year of sampling, was an important factor affecting distributional patterns. Calanoid copepod species showed vertical zonation through the lower OMZ oxycline. Clustering analyses defined sample groups with similar copepod assemblages and species groups with similar distributions. No apparent diel vertical migration for either calanoid or non-calanoid copepods at these midwater depths was observed, but some species had age-related differences in vertical distributions. Subzones of the OMZ, termed the OMZ Core, the Lower Oxycline, and the Sub-Oxycline, had different copepod communities and ecological interactions. Major distributional and ecological changes were associated with surprisingly small oxygen gradients at low oxygen concentrations. The calanoid copepod community was most diverse in the most oxygenated environments (oxygen >0.14 ml/L), but the rank order of abundance of species was similar in the Lower Oxycline and Sub-Oxycline. Some species were absent or much scarcer in the OMZ Core. Two copepod species common in the Lower Oxycline were primarily detritivorous but showed dietary differences suggesting feeding specialization. The copepod Spinocalanus antarcticus fed primarily on components of the vertical particulate flux and suspended material, a less versatile diet than the co-occurring copepod Lucicutia grandis. Vertical zonation of copepod species through the lower OMZ oxycline is probably a complex interplay between physiological limitation by low oxygen, potential predator control, and potential food resources. Pelagic OMZ and oxycline communities, and their ecological interactions in the water column and with the benthos, may become even more widespread and significant in the future ocean, if global warming increases the extent and intensity of OMZs as predicted.     

Spatial and temporal variability of heat content above the thermocline in the tropical Pacific Ocean

Abstract-Heat content of the upper layer above the 20℃ isotherm in the tropical Pacific Ocean isestimated by using the sea temperature data set with a resolution 2°latitude×5°longitude (1980～1993)for the water depths (every 10 m) from 0 m to 400 m, and its temporal and spatial variabilities are an-alyzed. (1) The temporal variability indicates that the total heat in the upper layer of the equatorial Pa-cific Ocean is charcterized by the interannual variability. The time series of the equatorial heat anomaly5 months lead that of the El Nino index at the best positive lag correlation between the two, and theformer 13 months lag behind the latter at their best negative lag correlation. Therefore the equatorialheat content can be used as a better predictor than the El Nino index for a warm or cold event. In addi-tion, it is also found that less heat anomaly in the equator corresponds to the stronger warm events inthe period (1980～1993) and much more heat was accumulated in the 4 years including 1992/1     

热带太平洋海平面低频变化

本文选取ECMWF ORAS4再分析数据对1959-2015年热带太平洋海平面的低频变化进行了分析。热带太平洋海平面年际变化第一模态反映了ENSO爆发阶段的海平面变化,热带东、西太平洋变化反相,其时间序列与Niño3.4指数高度相关。海平面第二模态则体现了El Niño爆发前后热带太平洋暖水的输运过程。El Niño爆发前热带西太平洋暖水聚集的位置,以及爆发后暖水向赤道外输运的位置在两类El Niño事件中均有所不同。此外,ENSO的周期在近半个世纪发生了显著的年代际变化,这一变化与热带太平洋的年代际变化有关。热带太平洋的年代际变化对海平面趋势变化也有着显著的影响。卫星高度计观测到的近20年海平面的快速上升（下降）正是由20世纪90年代后热带太平洋频繁的位相转换引起的。