Eastern Tropical North Pacific Research Articles

Distribution of glycerol ether lipids in the oxygen minimum zone of the Eastern Tropical North Pacific Ocean

The distributions of microbial glycerol ether lipids in suspended particulate matter in the oxygen minimum zone (OMZ) of the Eastern Tropical North Pacific Ocean (ETNP) were investigated. Nine groups of glycerol ether core lipids were detected and quantified: isoprenoid glycerol dialkyl glycerol tetraethers (GDGTs) (iso-GDGTs), isoprenoid glycerol dialkanol diethers (iso-GDDs) and hydroxylated isoprenoid GDGTs and GDDs (OH-GDGTs and OH-GDDs) of archaeal origin; branched glycerol dialkyl glycerol tetraethers (br-GDGTs) of bacterial origin and overly branched GDGTs (OB-GDGTs), sparsely branched GDGTs (SB-GDGTs), hybrid isoprenoid/branched GDGTs (IB-GDGTs) and a tentatively assigned H-shaped GDGT (H-1020) of unknown biological origin. The archaeal iso-GDGTs were the most abundant core lipids (89% of total), followed by iso-GDDs (4%), br-GDGTs (2%) and OH-GDGTs (1%). Archaeal intact polar GDGTs (IP GDGTs), including both mono- and diglycosidic iso-GDGTs, had depth profiles similar to OH-GDGTs and OH-GDDs, with a maximum concentration in the upper OMZ and secondary peaks in the mid and lower OMZ, suggesting similar but multiple planktonic sources. Core lipids of iso-GDGTs and other glycerol ethers showed deviating concentration profiles compared with IP GDGTs and OH-GDGTs and were most abundant in the mid OMZ. This is the first report of OH-GDDs, OB-GDGTs, SB-GDGTs, IB-GDGTs and H-1020 in the marine water column and the distribution patterns of these “orphan lipids” suggest that anaerobic planktonic microbes are their main source.

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 (0.2–0.7μm; “X-POM”) was sampled and analyzed separately from the size class typically collected as “suspended” POM (0.7–53μm; “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.

A biomarker perspective on coccolithophorid growth and export in a stratified sea

Summer cruises to the Gulf of California and adjacent Eastern Tropical North Pacific between 2004 and 2008 provided data on non-bloom coccolithophorid abundance and production in nitrate-depleted surface waters. Using lipid biomarkers specific to the dominant coccolithophorids in this region, it was found that these organisms routinely exhibited maximum production rates at depths associated with subsurface chlorophyll features and the nitracline, as opposed to the N-depleted surface. Estimates of integrated coccolithophorid production, relative to bulk primary production, showed no systematic relationship to availability of the limiting macronutrient (nitrate) in the euphotic zone. Our observations suggest that, in sufficiently well-stratified settings, the nutrient-depleted and nutrient-replete portions of the euphotic zone may become so sharply partitioned and temporally stable that a coccolithophorid-favorable ‘mid-to-low nutrient’ niche is absent. Consequently, the relative abundance and production rate of coccolithophorids are low, and the relative contribution of alkenones to carbon export is driven by variability in the significantly larger bulk carbon component. Study of this region provides important insights into the ecology of these calcifying organisms in a warm, quiescent ocean. Our findings suggest that, if water columns akin to those surveyed become more common, coccolithophorid-derived inorganic carbon export may serve as a weaker positive feedback on atmospheric CO2 than previously suggested.

Diversity and Spatial Distribution of Hydrazine Oxidoreductase (hzo) Gene in the Oxygen Minimum Zone Off Costa Rica

Anaerobic ammonia oxidation (anammox) as an important nitrogen loss pathway has been reported in marine oxygen minimum zones (OMZs), but the community composition and spatial distribution of anammox bacteria in the eastern tropical North Pacific (ETNP) OMZ are poorly determined. In this study, anammox bacterial communities in the OMZ off Costa Rica (CRD-OMZ) were analyzed based on both hydrazine oxidoreductase (hzo) genes and their transcripts assigned to cluster 1 and 2. The anammox communities revealed by hzo genes and proteins in CRD-OMZ showed a low diversity. Gene quantification results showed that hzo gene abundances peaked in the upper OMZs, associated with the peaks of nitrite concentration. Nitrite and oxygen concentrations may therefore colimit the distribution of anammox bacteria in this area. Furthermore, transcriptional activity of anammox bacteria was confirmed by obtaining abundant hzo mRNA transcripts through qRT-PCR. A novel hzo cluster 2x clade was identified by the phylogenetic analysis and these novel sequences were abundant and widely distributed in this environment. Our study demonstrated that both cluster 1 and 2 anammox bacteria play an active role in the CRD-OMZ, and the cluster 1 abundance and transcriptional activity were higher than cluster 2 in both free-living and particle-attached fractions at both gene and transcriptional levels.

Deoxygenation alters bacterial diversity and community composition in the ocean’s largest oxygen minimum zone

Oceanic oxygen minimum zones (OMZs) have a central role in biogeochemical cycles and are expanding as a consequence of climate change, yet how deoxygenation will affect the microbial communities that control these cycles is unclear. Here we sample across dissolved oxygen gradients in the oceans' largest OMZ and show that bacterial richness displays a unimodal pattern with decreasing dissolved oxygen, reaching maximum values on the edge of the OMZ and decreasing within it. Rare groups on the OMZ margin are abundant at lower dissolved oxygen concentrations, including sulphur-cycling Chromatiales, for which 16S rRNA was amplified from extracted RNA. Microbial species distribution models accurately replicate community patterns based on multivariate environmental data, demonstrate likely changes in distributions and diversity in the eastern tropical North Pacific Ocean, and highlight the sensitivity of key bacterial groups to deoxygenation. Through these mechanisms, OMZ expansion may alter microbial composition, competition, diversity and function, all of which have implications for biogeochemical cycling in OMZs.

Dynamics of satellite-derived interannual ocean bottom pressure variability in the western tropical North Pacific

[1] Ocean bottom pressure variability, derived from Release-05 Gravity Recovery and Climate Experiment time-variable gravity coefficients over the ocean, is investigated along the tropical North Pacific for the case of long time scales (>1 yr) and large space scales (>750 km). To interpret the observations, a linear model of the bottom pressure response to interior wind stress curl is derived on the basis of normal vertical modes; the adjustment comprises contributions from barotropic Sverdrup dynamics as well as first baroclinic mode Rossby waves. Model solutions are evaluated numerically using time-mean ocean stratification from the Ocean Comprehensible Atlas and time-varying surface wind stress from the European Centre for Medium-Range Weather Forecasts ERA-Interim reanalysis. In the western tropical North Pacific, model and data compare favorably; simulated and observed time series are significantly correlated, and the model generally explains more than half of the data variance; the good correspondence between model and data speaks to the good quality of the satellite-derived fields. In the central and eastern tropical North Pacific, findings are more ambiguous; model and data time series are mostly not significantly correlated, and simulations generally explain less than half of the data variance; discrepancies between model and data could point to physics absent from the model, for example, signals generated at the eastern boundary. Results provide observational demonstration that baroclinic contributions to bottom pressure changes can be important at low latitudes and low frequencies; findings hint at a basin-scale influence of tropical climate modes on the ocean bottom pressure field.

Composite Bacterial Hopanoids and Their Microbial Producers across Oxygen Gradients in the Water Column of the California Current

Hopanoids are pentacyclic triterpenoid lipids produced by many prokaryotes as cell membrane components. The structural variations of composite hopanoids, or bacteriohopanepolyols (BHPs), produced by various bacterial genera make them potentially useful molecular biomarkers of bacterial communities and metabolic processes in both modern and ancient environments. Building on previous work suggesting that organisms in low-oxygen environments are important contributors to BHP production in the marine water column and that there may be physiological roles for BHPs specific to these environments, this study investigated the relationship between trends in BHP structural diversity and abundance and the genetic diversity of BHP producers for the first time in a low-oxygen environment of the Eastern Tropical North Pacific. Amplification of the hopanoid biosynthesis gene for squalene hopene cyclase (sqhC) indicated far greater genetic diversity than would be predicted by examining BHP structural diversity alone and that greater sqhC genetic diversity exists in the marine environment than is represented by cultured representatives and most marine metagenomes. In addition, the genetic relationships in this data set suggest microaerophilic environments as potential "hot spots" of BHP production. Finally, structural analysis of BHPs showed that an isomer of the commonly observed BHP bacteriohopanetetrol may be linked to a producer that is more abundant in low-oxygen environments. Results of this study increase the known diversity of BHP producers and provide a detailed phylogeny with implications for the role of hopanoids in modern bacteria, as well as the evolutionary history of hopanoid biosynthesis, both of which are important considerations for future interpretations of the marine sedimentary record.

Nitrite oxidation in the upper water column and oxygen minimum zone of the eastern tropical North Pacific Ocean

Nitrogen (N) is an essential nutrient in the sea and its distribution is controlled by microorganisms. Within the N cycle, nitrite (NO2(-)) has a central role because its intermediate redox state allows both oxidation and reduction, and so it may be used by several coupled and/or competing microbial processes. In the upper water column and oxygen minimum zone (OMZ) of the eastern tropical North Pacific Ocean (ETNP), we investigated aerobic NO2(-) oxidation, and its relationship to ammonia (NH3) oxidation, using rate measurements, quantification of NO2(-)-oxidizing bacteria via quantitative PCR (QPCR), and pyrosequencing. (15)NO2(-) oxidation rates typically exhibited two subsurface maxima at six stations sampled: one located below the euphotic zone and beneath NH3 oxidation rate maxima, and another within the OMZ. (15)NO2(-) oxidation rates were highest where dissolved oxygen concentrations were <5 μM, where NO2(-) accumulated, and when nitrate (NO3(-)) reductase genes were expressed; they are likely sustained by NO3(-) reduction at these depths. QPCR and pyrosequencing data were strongly correlated (r(2)=0.79), and indicated that Nitrospina bacteria numbered up to 9.25% of bacterial communities. Different Nitrospina groups were distributed across different depth ranges, suggesting significant ecological diversity within Nitrospina as a whole. Across the data set, (15)NO2(-) oxidation rates were decoupled from (15)NH4(+) oxidation rates, but correlated with Nitrospina (r(2)=0.246, P<0.05) and NO2(-) concentrations (r(2)=0.276, P<0.05). Our findings suggest that Nitrospina have a quantitatively important role in NO2(-) oxidation and N cycling in the ETNP, and provide new insight into their ecology and interactions with other N-cycling processes in this biogeochemically important region of the ocean.

Zooplankton in the eastern tropical north Pacific: Boundary effects of oxygen minimum zone expansion

Oxygen minimum zones (OMZs) may be expanding in their worldwide spatial and vertical extent as a result of global climate change. Here, we highlight structural and functional features of pelagic OMZ zooplankton communities, especially the little known mesopelagic lower oxycline zooplankton assemblage in the eastern tropical north Pacific (ETNP). Day and night vertically-stratified zooplankton samples were collected with a MOCNESS plankton net system to depths of 1000m during two cruises in 2007 and 2008 as part of the Eastern Tropical Pacific Project. Size-fractionated biomass and selected species distributions were analyzed. A spatial comparison between two stations, Tehuantepec Bowl (TB) and Costa Rica Dome (CRD), with different OMZ thicknesses especially in 2008, was used as a proxy for future potential climate change to show how variability in oxycline depth at the upper and lower OMZ boundaries may have broad consequences for biological distributions and ecosystem function.The same zooplankton biomass features were present at both locations but responded differently to changes in OMZ thickness and boundary depths. The impact of habitat compression was quantified by the change in proportion of zooplankton biomass in the upper water column. The thermocline was the location of peak zooplankton biomass regardless of OMZ extent. At the lower oxycline, a unique zooplankton assemblage and secondary biomass peak (sharp order of magnitude jump from OMZ core biomass levels) occurred that was strongly associated with a specific oxygen concentration of 2μM. In 2008, the lower oxycline biomass peak deepened by over 200m at TB with its expanded OMZ compared to CRD, despite a temperature difference of ∼2°C, while remaining at the same oxygen level. In contrast, another secondary biomass peak corresponding to the daytime depth of diel vertical migration, and occurring within the upper oxycline or OMZ core, was present at the same depth and temperature at both locations, despite different oxygen concentrations. A dense monospecific layer of the copepod Eucalanus inermis was sometimes present just above the distinctive more persistent lower oxycline assemblage. While the general ontogenetic migration of this species was previously known, its precise positioning relative to oxygen concentration and the lower oxycline community is a new finding. The vertical re-positioning of biomass layers at the thermocline and lower OMZ boundaries, and the increased depth range of low oxygen water that diel vertical migrators and sinking particles must transit in an expanded OMZ, could have widespread effects on species distributions, the biological pump, and benthic–pelagic coupling.

Foraminiferal faunal evidence for Glacial–Interglacial variations in the ocean circulation and the upwelling of the Gulf of Tehuantepec (Mexico)

Abstract Three marine sediment cores from the Gulf of Tehuantepec (Eastern Tropical North Pacific) were used to determine variations in upwelling intensity and surface circulation patterns over the last 26 kyr BP. Changes in foraminiferal assemblages at glacial-interglacial timescale indicate a sequence of three major palaeoceanographic events in the study area: (1) The Late Pleistocene (including the Last Glacial Maximum, LGM), dominated by a Neogloboquadrina dutertrei–Globorotalia menardii assemblage, indicative of a stable thermocline, stratified water column and greater persistence of warm equatorial waters in the region. (2) The Termination 1, dominated by a Globigerinita glutinata assemblage, which indicates mixed surface water and transitional conditions between upwelling and stratified water column episodes. Changes in sedimentation rates and a clear reduction of the faunal abundances at this time, could be the result of eustatic sea level change. (3) The Holocene, dominated by a Globigerina bulloides–Globigerinoides ruber assemblage, indicates periods of elevated productivity, intense upwelling and the establishment of the modern ocean conditions. These findings are consistent with climate model results that suggest that regional surface ocean and upwelling conditions are mostly linked to changes in atmospheric circulation over the Laurentide Ice Sheet and shifts in the latitudinal position of the Intertropical Convergence Zone.

Reexamination of the species assignment of Diacavolinia pteropods using DNA barcoding.

Thecosome pteropods (Mollusca, Gastropoda) are an ecologically important, diverse, and ubiquitous group of holoplanktonic animals that are the focus of intense research interest due to their external aragonite shell and vulnerability to ocean acidification. Characterizing the response of these animals to low pH and other environmental stressors has been hampered by continued uncertainty in their taxonomic identification. An example of this confusion in species assignment is found in the genus Diacavolinia. All members of this genus were originally indentified as a single species, Cavolinia longirostris, but over the past fifty years the taxonomy has been revisited multiple times; currently the genus comprises 22 different species. This study examines five species of Diacavolinia, including four sampled in the Northeast Atlantic (78 individuals) and one from the Eastern tropical North Pacific (15 individuals). Diacavolina were identified to species based on morphological characteristics according to the current taxonomy, photographed, and then used to determine the sequence of the “DNA barcoding” region of the cytochrome c oxidase subunit I (COI). Specimens from the Atlantic, despite distinct differences in shell morphology, showed polyphyly and a genetic divergence of <3% (K2P distance) whereas the Pacific and Atlantic samples were more distant (∼19%). Comparisons of Diacavolinia spp. with other Cavolinia spp. reveal larger distances (∼24%). These results indicate that specimens from the Atlantic comprise a single monophyletic species and suggest possible species-level divergence between Atlantic and Pacific populations. The findings support the maintenance of Diacavolinia as a separate genus, yet emphasize the inadequacy of our current taxonomic understanding of pteropods. They highlight the need for accurate species identifications to support estimates of biodiversity, range extent and natural exposure of these planktonic calcifiers to environmental variability; furthermore, the apparent variation of the pteropods shell may have implications for our understanding of the species’ sensitivity to ocean acidification.

Nitrogen fixation in the Gulf of California and the Eastern Tropical North Pacific

Di-nitrogen (N2) fixation plays a well-recognized role in the enhancement of primary production and arguably particle export in oligotrophic regions of the subtropical and tropical oceans. However, recent evidence suggests that N2 fixation may also be significant in regions of the surface ocean proximate to or overlying zones of intense subsurface denitrification. In this study, we present results from a series of research cruises in the Gulf of California (GoCal) and adjacent waters of the Eastern Tropical North Pacific (ETNP). Measurements include microscopy, genomic analyses, incubations, stable isotopic measurements, and sediment traps coupled with 238U:234Th disequilibria. Combined, these results suggest that N2 fixing microorganisms are present and active throughout the region, with larger sized Richelia and Trichodesmium spp. recorded within the warmer waters at the entrance to and within the GoCal, and smaller, unicellular diazotrophs observed in the cooler waters of the northern ETNP. N2 fixation rates in the summer varied from 15–70μmolNm−2d−1, with episodic blooms contributing as much as 795μmolNm−2d−1. While the estimated contribution of N2 fixation to particle export was highly variable, blooms of diatom-Richelia symbioses accounted for as much as ∼44% of the measured summer carbon flux at 100m. Alternately, evaluation of the N isotopic composition of sinking material and the magnitude of measured N2 fixation rates indicate negligible to small enhancements of new production when blooms of either colonial Trichodesmium spp. or unicellular diazotrophs were encountered. Consistent with previous research, we also found that while fluxes of C to sediment traps are similar in winter and summer months, the efficiency of C export (export/surface productivity) in the GoCal region is elevated during summer relative to the more productive diatom-dominated winter phase of the seasonal cycle. The episodic and variable nature of N2 fixation recorded in this region make it unlikely that new production via diazotrophic activity is solely responsible for the observed patterns of C transport efficiency; rather, we hypothesize that eolian inputs and/or efficient transport of picocyanobacterial biomass via grazing or aggregation may further explain the enhanced export efficiency observed in the GoCal summer. In sum, diazotrophy typically supports <10%, but as much as 44% of export production. The high variability of direct measurements of N2 fixation implies that other mechanisms contribute to the seasonal invariance of C flux in this region. If this region is indicative of other oxygen minima zones with active diazotrophs, our results indicate that export-mediated feedback mechanisms between N2 fixation and denitrification are not as strong as previously hypothesized.

Latitudinal variations in intermediate depth ventilation and biological production over northeastern Pacific Oxygen Minimum Zones during the last 60 ka

Mechanisms affecting past variability in the Oxygen Minimum Zone (OMZ) in the Eastern Tropical North Pacific (ETNP) are poorly known. We analyzed core MD02-2524, obtained from the Nicaragua Margin in the present ETNP OMZ for major and minor elements (titanium (Ti), brome (Br), silicon (Si), potassium (K), and calcium (Ca)) using an X-ray Fluorescence (XRF) core scanner, and redox-sensitive trace elements (uranium (U), molybdenum (Mo), and nickel (Ni)) determined by ICP-MS. The U and Mo content was higher during the deglaciation than during the Holocene and the last glacial maximum, whereas enrichment was not observed for Ni, an element closely associated with organic matter. High-resolution XRF scanning indicated that the Ca-based carbonate content had millennial-scale variability inversely correlated with Br-based organic matter and Si/K-based opal content during the last glacial period. The available data suggest no clear regional trend in biological productivity during the last deglaciation, but significant local variability in the coastal eastern equatorial Pacific. The trace element enrichment and the lack of a concomitant increase in biogenic phases indicated that an enhanced ETNP OMZ, at least between 15°N and 12°N at a water depth of 500–900 m, was principally caused by a reduced oxygen supply driven by oceanic circulation to the Nicaragua Basin during the deglaciation. The observed patterns can be interpreted as the distinct changes in the oxygenation state of northern and southern water masses at intermediate depths.We also found evidence for a decoupling between local productivity and pore water oxygenation for several millennial-scale events during Marine Isotopic Stage 3, indicating that remote oxygen consumption and/or oceanic ventilation impacted OMZ intensity. Multi-millennial scale variations of the productivity at Papagayo upwelling cell displayed an opposite trend from productivity at the Costa Rica Dome, in relation with the latitudinal shift of the Inter Tropical Convergence Zone (ITCZ). Latitudinal variations of the OMZ intensity reflected the relative influence of the northern and southern intermediate water masses whose oxygenation was driven by high latitude climates and productivity over the intermediate water pathway.

Fixed nitrogen loss from the eastern tropical North Pacific and Arabian Sea oxygen deficient zones determined from measurements of N2:Ar

Previous work estimating the N2excess above background due to denitrification has suggested that nitrate deficit‐type methods may be an underestimate of fixed nitrogen (N) loss in the major oxygen deficient zones of the ocean. The N2excess approach has the advantage over nitrate deficit‐type methods in that it does not depend on stoichiometric assumptions of fixed N to phosphate or oxygen utilization and avoids any uncertainly regarding the pathway of N loss. Measurements of N2:Ar from two stations within the eastern tropical North Pacific and from one station within the Arabian Sea oxygen deficient zones were used to determine the N2 excess due to denitrification. In both of the regions, the N2 excess was comparable in shape and magnitude to the concurrent fixed nitrogen deficit. In the eastern tropical North Pacific oxygen deficient zone, the N2 excess was near zero at the surface and rose to maxima of 13.7 ± 1.8 and 10.8 ± 1.9 μM N, compared to maximum fixed N deficits of 13.5 ± 1.5 and 12.3 ± 1.5 μM N, respectively. In the Arabian Sea oxygen deficient zone, the maximum N2 excess was 11.1 ± 1.5 μM N, compared to a maximum deficit of 12.5 ± 1.0 μM N. These results suggests that previous estimates of fixed N loss based on fixed N deficit calculations in these regions are likely reasonable, given the same considerations of volume and residence time of the water of the oxygen deficient zone.

Metabolic suppression in thecosomatous pteropods as an effect of low temperature and hypoxia in the eastern tropical North Pacific

Many pteropod species in the eastern tropical North Pacific Ocean migrate vertically each day, transporting organic matter and respiratory carbon below the thermocline. These migrations take species into cold (15–10° C) hypoxic water (<20 μmol O2 kg−1) at depth. We measured the vertical distribution, oxygen consumption and ammonia excretion for seven species of pteropod, some of which migrate and some which remain in oxygenated surface waters throughout the day. Within the upper 200 m of the water column, changes in water temperature result in a ~60–75 % reduction in respiration for most species. All three species tested under hypoxic conditions responded to low O2 with an additional ~35–50 % reduction in respiratory rate. Combined, low temperature and hypoxia suppress the metabolic rate of pteropods by ~80–90 %. These results shed light on the ways in which expanding regions of hypoxia and surface ocean warming may impact pelagic ecology.

Microbial community structure and productivity in the oxygen minimum zone of the eastern tropical North Pacific

Microbial abundances and activities (heterotrophic and chemoautotrophic) were profiled in the fall of 2007 along a transect of four stations in the upwelling region between the Tehuantepec Bowl, off the coast of southern Mexico, and the Costa Rica Dome. Heterotrophic potential varied with depth exhibiting relatively low maximum values of 4–8nM leucine d−1, but varied among stations by a factor of 2. Generally, heterotrophic potential varied inversely with nitrate implying NO3− draw down by nitrate-reducing chemoorganotrophs. Dark dissolved inorganic carbon assimilation (≈chemoautotrophy) was also highly variable with depth and reached a maximum value of 6.4μMC d−1 at one station, but only attained maxima of 0.02 and 0.2μMCd−1 at other stations. Elevated values of chemoautotrophy corresponded with enrichments in nitrite concentrations suggesting nitrification as well as with depletions in nitrite and ammonium concentrations suggesting anammox processes. Additions of 30μM NH4+ at selected depths stimulated chemoautotrophic activity up to 33-fold. Fluorescence in situ hybridization (FISH) analyses revealed elevated abundances of Crenarchaeota and β-proteobacteria (up to 1.0×107 and 10.0×107cellsL−1, respectively) associated with NH4+ minima and NO3- maxima. Nitrifying β-proteobacteria were most abundant in the upper oxycline (1.1–1.8×107cellsL−1) at three stations and exhibited a secondary peak of 0.3×107cellsL−1 in the lower oxycline at one station. Anammox bacterial (Planctomycetes) abundances were as high as 2.1×106cellsL−1, but accounted for less than 1% of total DAPI counts. The concentration of diploptene (a hopanoid biomarker found in bacteria) varied from 2 to 15pgL−1 in the upper oxycline and from 51 to 160pgL−1 in the lower oxycline. Delta 13C values of particulate organic carbon (POC) varied from −30 to −21‰ in the water column (avg. −25‰). Distributions of isotopically light POC coincided with enrichments in diploptene, Planctomycetes cells, nitrite, and chemoautotrophy as well as with nitrate and ammonium depletions, suggesting that anammox and nitrification are important for secondary biological production in this system.

Quantification of ammonia oxidation rates and ammonia‐oxidizing archaea and bacteria at high resolution in the Gulf of California and eastern tropical North Pacific Ocean

Ammonia‐oxidizing microorganisms compete with phytoplankton for reduced nitrogen in the euphotic zone and provide oxidized nitrogen to other microbes present in the sea. We report 15NH4+ oxidation rate measurements made at 5–20‐m resolution using an in situ array and quantification of ammonia‐oxidizing archaea (AOA) and ammonia‐oxidizing bacteria (AOB) in corresponding samples from the upper water column and oxygen minimum zone (OMZ) of the Gulf of California (GOC) and eastern tropical North Pacific Ocean (ETNP). 15NH4+ oxidation rates varied substantially with depth and between stations: they were greatest at the base of the euphotic zone, and maximum rates were up to 28‐fold greater than rates measured within 5–10 m. Pyrosequencing and quantitative polymerase chain reactions (QPCR) indicated that AOA were present throughout the water column at all latitudes and always outnumbered AOB. AOB constituted only 39 of 432,240 16S ribosomal ribonucleic acid gene sequences produced via pyrosequencing but were more abundant at greater depths and higher latitudes. 15NH4+ oxidation rates were correlated with AOA abundance at some stations and were detectable in 96% of samples, including depths where oxygen concentrations were &lt; 5 µmol kg−1 and depths within the euphotic zone, where up to 42% of ammonia oxidation occurred. Ammonia is rapidly oxidized within discrete depth intervals in the GOC and ETNP; while pyrosequencing and QPCR demonstrate that AOB are confined to deeper portions of the water column, AOA appear to be active within the euphotic zone, where they may quickly respond to nitrogen inputs.

Biomarker evidence for anammox in the oxygen minimum zone of the Eastern Tropical North Pacific

Anaerobic oxidation of ammonium (anammox) is an important process in the marine N cycle. It has been estimated to contribute up to 50% of N loss from the ocean and is especially prevalent within the oxygen minimum zone (OMZ). We studied the presence and distribution of anammox in the extended OMZ of the Eastern Tropical North Pacific (ETNP) using ladderane fatty acids (FAs), specific biomarkers for anammox bacteria. The validity of ladderane FAs as a proxy for anammox bacteria was demonstrated by their excellent correspondence with anammox 16S rRNA functional gene abundances and their expression and intact polar ladderane lipid concentrations in suspended particulate matter (SPM) from the Arabian Sea. In the ETNP, SPM was collected from various water depths at four stations along a northwest to southeast cruise transect and ladderane FAs were analysed at each station. In all SPM samples where ladderanes were detected, C18 ladderane FAs were on average fivefold more abundant than C20 ladderane FAs. Maximum ladderane FA concentrations (1.1–2.3ngl−1) were recorded at 400–600m, often corresponding to the depth of the secondary nitrite maximum. At one of the four stations, a second maximum in concentration was noted at a shallower depth (85m), coinciding with higher nitrite availability at this depth. This suggests that nitrite, along with oxygen, may be a limiting factor for anammox activity in the ETNP. Anammox lipids were abundant within the OMZ at all stations and concentrations were comparable to those in other OMZs, suggesting that anammox may be responsible for a significant loss of N from the OMZ of the ETNP.

High-resolution opal records from the eastern tropical Pacific provide evidence for silicic acid leakage from HNLC regions during glacial periods

A shift from carbonate- to silica-dominated primary production could significantly affect the oceanic carbon cycle via changes in the particulate carbon rain-rate ratio ( C organic:C inorganic fluxes). An increase in C rain rate ratio has been invoked to explain lower glacial pCO 2; however, firm evidence of an ecological shift towards silica-dominated productivity during the last glacial period is lacking. Here, we present new high-resolution reconstructions of biogenic silica and total production over the past 40,000 yr BP in 3 cores from the eastern tropical North Pacific (ETNP) off Mexico and Nicaragua. These records reveal a clear regional pattern of higher siliceous productivity with higher opal accumulation during the last glacial period compared to interglacial times. Higher Si:C and Si:N ratios of glacial sediments in these records suggest a net increase in siliceous production over total production. We attribute this to the additional supply of silicic acid to the ETNP margins favouring diatoms over other non-siliceous algae. This suggestion for increased supply of Si during glacial periods is consistent with the proposed large-scale redistribution of excess silicic acid from High Nitrate Low Chlorophyll (HNLC) regions like the eastern equatorial Pacific (EEP) and the Southern Ocean by the Silicic Acid Leakage Hypothesis (SALH). In these HNLC regions, the Si-isotope composition of diatom frustules (δ 30Si) has provided evidence for the generation of surplus of silicic acid during diatom growth under conditions of higher Fe availability during glacial periods. We suggest that silicic acid leakage from the HNLC regions to the adjoining oceans may have increased the carbon rain rate ratio and ultimately, contributed to the decrease in glacial atmospheric pCO 2.

Coral Cd/Ca and Mn/Ca records of ENSO variability in the Gulf of California

Abstract. We analyzed the trace element ratios Cd/Ca and Mn/Ca in three coral colonies (Porites panamensis (1967–1989), Pavona clivosa (1967–1989) and Pavona gigantea (1979–1989)) from Cabo Pulmo reef, Southern Gulf of California, Mexico, to assess the oceanographic changes caused by El Niño – Southern Oscillation (ENSO) events in the Eastern Tropical North Pacific (ETNP). Interannual variations in the coral Cd/Ca and Mn/Ca ratios showed clear evidence that incorporation of Cd and Mn in the coral skeleton was influenced by ENSO conditions, but the response for each metal was controlled by different processes. The Mn/Ca ratios were significantly higher during ENSO years (p&lt;0.05) relative to non-ENSO years for the three species of coral. In contrast, the Cd/Ca was systematically lower during ENSO years, but the difference was significant (p&lt;0.05) only in Pavona gigantea. A decrease in the incorporation of Cd and a marked increase in Mn indicated strongly reduced vertical mixing in the Gulf of California during the mature phase of El Niño. The oceanic warming during El Niño events produces a relaxation of upwelling and a stabilization of the thermocline, which may act as a physical barrier limiting the transport of Cd from deeper waters into the surface layer. In turn, this oceanic condition can increase the residence time of particulate-Mn in surface waters, allowing an increase in the photo-reduction of particulate-Mn and the release of available Mn into the dissolved phase. These results support the use of Mn/Ca and Cd/Ca ratios in biogenic carbonates as tracers of increases in ocean stratification and trade wind weakening and/or collapse in the ETNP during ENSO episodes.