Eastern Tropical South Pacific Research Articles

Revisiting nitrification in the Eastern Tropical South Pacific: A focus on controls

AbstractNitrification, the oxidation of ammonium ( ) to nitrite ( ) and to nitrate ( ), is a component of the nitrogen (N) cycle internal to the fixed N pool. In oxygen minimum zones (OMZs), which are hotspots for oceanic fixed N loss, nitrification plays a key role because it directly supplies substrates for denitrification and anaerobic ammonia oxidation (anammox), and may compete for substrates with these same processes. However, the control of oxygen and substrate concentrations on nitrification are not well understood. We performed onboard incubations with 15N‐labeled substrates to measure rates of and oxidation in the eastern tropical South Pacific (ETSP). The spatial and depth distributions of and oxidation rates were primarily controlled by and availability, oxygen concentration, and light. In the euphotic zone, nitrification was partially photoinhibited. In the anoxic layer, oxidation was negligible or below detection, but high rates of oxidation were observed. oxidation displayed extremely high affinity for both and oxygen. The positive linear correlations between oxidation rates and in situ concentrations and ammonia monooxygenase subunit A (amoA) gene abundances in the upper oxycline indicate that the natural assemblage of ammonia oxidizers responds to in situ concentrations or supply by adjusting their population size, which determines the oxidation potential. The depth distribution of archaeal and bacterial amoA gene abundances and N2O concentration, along with independently reported simultaneous direct N2O production rate measurements, suggests that AOA were predominantly responsible for oxidation, which was a major source of N2O production at oxygen concentrations > 5 µM.

Influence of mesoscale eddies on the distribution of nitrous oxide in the eastern tropical South Pacific

Abstract. Recent observations in the eastern tropical South Pacific (ETSP) have shown the key role of meso- and submesoscale processes (e.g. eddies) in shaping its hydrographic and biogeochemical properties. Off Peru, elevated primary production from coastal upwelling in combination with sluggish ventilation of subsurface waters fuels a prominent oxygen minimum zone (OMZ). Given that nitrous oxide (N2O) production–consumption processes in the water column are sensitive to oxygen (O2) concentrations, the ETSP is a region of particular interest to investigate its source–sink dynamics. To date, no detailed surveys linking mesoscale processes and N2O distributions as well as their relevance to nitrogen (N) cycling are available. In this study, we present the first measurements of N2O across three mesoscale eddies (two mode water or anticyclonic and one cyclonic) which were identified, tracked, and sampled during two surveys carried out in the ETSP in November–December 2012. A two-peak structure was observed for N2O, wherein the two maxima coincide with the upper and lower boundaries of the OMZ, indicating active nitrification and partial denitrification. This was further supported by the abundances of the key gene for nitrification, ammonium monooxygenase (amoA), and the gene marker for N2O production during denitrification, nitrite reductase (nirS). Conversely, we found strong N2O depletion in the core of the OMZ (O2 < 5 µmol L−1) to be consistent with nitrite (NO2−) accumulation and low levels of nitrate (NO3−), thus suggesting active denitrification. N2O depletion within the OMZ's core was substantially higher in the centre of mode water eddies, supporting the view that eddy activity enhances N-loss processes off Peru, in particular near the shelf break where nutrient-rich, productive waters from upwelling are trapped before being transported offshore. Analysis of eddies during their propagation towards the open ocean showed that, in general, “ageing” of mesoscale eddies tends to decrease N2O concentrations through the water column in response to the reduced supply of material to fuel N loss, although hydrographic variability might also significantly impact the pace of the production–consumption pathways for N2O. Our results evidence the relevance of mode water eddies for N2O distribution, thereby improving our understanding of the N-cycling processes, which are of crucial importance in times of climate change and ocean deoxygenation.

Diazotroph community structure in the deep oxygen minimum zone of the Costa Rica Dome.

Oxygen minimum zones (OMZs), characterized by depleted dissolved oxygen concentration in the intermediate depth of the water column, are predicted to expand under the influence of global warming. Recent studies in the Eastern Tropical South Pacific Ocean and Arabian Sea have reported that heterotrophic nitrogen fixation is active in the OMZs. In this study, we investigated the community structure of diazotrophs in the OMZ of the Costa Rica Dome (CRD) upwelling region in the Eastern Tropical North Pacific Ocean, using 454-pyrosequencing of nifH gene amplicons. Comparing diazotroph assemblages in different depth strata of the OMZ (200-1000 m in depth), we found a unique diazotroph community in the OMZ core, which was mainly dominated by methanotroph-like diazotrophs, suggesting a potential coupling of nitrogen cycle and methane assimilation. In addition, some OTUs revealed in this study, especially those belonging to the large sub-cluster Vibrio diazotrophicus, were reported to be abundant and expressing the nifH gene in other OMZs. Our results suggest that the unique hydrographic conditions in OMZs may support similar assemblages of diazotrophs, and heterotrophic nitrogen fixation could also be occurring in our studied region. Our study provides the first insight into the composition and distribution of putative diazotrophs in the CRD OMZ.

The flow field of the upper hypoxic eastern tropical North Atlantic oxygen minimum zone

Abstract. A subsurface low oxygen zone is located in the eastern tropical North Atlantic Ocean (ETNA) in the upper ocean with the core of the hypoxic (O2 ≦60 µmol kg−1) oxygen minimum zone (OMZ) at 400 to 500 m depth. The subsurface circulation in the OMZ region is derived from observations and data assimilation results. Measurements in the ETNA of velocity, oxygen and of a tracer (CF3SF5) that was released in April 2008 at ∼ 8° N, 23° W (at ∼ 330 m depth) in November–December 2008, in November–December 2009 and October–November 2010 show the circulation in the upper part of the OMZ with spreading to the east in the North Equatorial Countercurrent (NECC) region and northwestward around the Guinea Dome. Three floats equipped with oxygen sensors deployed at ∼ 8° N, 23° W with parking depths at 330, 350 and 400 m depths were used to estimate velocity along the float trajectory at the surface and at the parking depth. At the 350 m park depth north of 9° N a cyclonic northwestward flow across the OMZ was observed. The northward drift of a float into the upper OMZ and a stronger cyclonic flow around the Guinea Dome seem to be connected to a strong Atlantic Meridional Mode (AMM) event in 2009. A near-surface cyclonic circulation cell east of the Cape Verde Islands reaches down into the OMZ layer. The circulation of the upper OMZ mirrors the near-surface circulation. Oxygen measurements from the cruises used here, as well as from other recent cruises up to the year 2014, confirm the continuous deoxygenation trend in the upper OMZ since the 1960s near the Guinea Dome. The three floats deployed with the tracer show spreading paths consistent with the overall observed tracer spreading. Oxygen sensors on the floats remained well calibrated for more than 20 months, and so the oxygen profiles can be used to investigate mesoscale eddy signatures. Mesoscale eddies may modify the oxygen distribution in OMZs. However, in general eddies are less energetic in the ETNA south of the Cape Verde Islands compared to similar latitudes in the eastern tropical South Pacific.

Centennial to millennial-scale changes in oxygenation and productivity in the Eastern Tropical South Pacific during the last 25,000 years

Oxygen minimum zones (OMZ) have expanded in all tropical oceans during the last 50 years resulting in habitat contraction and considerable changes in marine biogeochemistry. However, for a better understanding of the OMZ dynamics under the current climate change, two questions are relevant: 1) how do the magnitude and temporal changes in oceanic dissolved oxygen of the last few decades compare to the natural variability on longer timescales, and 2) what were the local and remote factors driving OMZ changes in the past. In the present study we use a stacked record covering the last 25 kyr from the Eastern Tropical South Pacific (ETSP) OMZ to reconstruct changes in oxygenation and productivity. We use a suite of proxies including the presence of laminations, redox sensitive metals (U, Mo, Re, Ni and Cu), total organic carbon and δ15N measurements. Water column denitrification and sediment redox conditions show pronounced centennial to millennial-scale variability during the last 25 kyr, with oxygenation levels as low as at present. Global cold periods at different timescales such as the Last Glacial Maximum (23–19 kyr BP) and the Little Ice Age (1500–1850 AD) were associated with a weak OMZ and low export production, while warm intervals such as the deglaciation, part of the Medieval Climate Anomaly and the last 100 years are associated with a stronger OMZ and high export production. Water column denitrification and sediment redox conditions were strongly coupled during the last 25 kyr BP apart from one remarkable exception: during the Antarctic Cold Reversal, sediments were less reducing but the water column denitrification was high resulting in a strong but shallow OMZ. This may have been produced by an enhanced Antarctic Intermediate Water flow. Contrary to our expectations and modeling predictions for the next few decades, we observe a weak ETSP-OMZ during the warm mid-Holocene, which may have been the result of a stronger Walker Circulation that brought oxygen-rich waters to intermediate depths off Peru via Equatorial undercurrents. In combination with other paleoceanographic reconstructions, our results show that oxygenation variability in the ETSP-OMZ was influenced by ocean circulation changes in the Tropical Pacific, high latitude oceanographic and climatic changes, and local productivity.

Meta-omic signatures of microbial metal and nitrogen cycling in marine oxygen minimum zones.

Iron (Fe) and copper (Cu) are essential cofactors for microbial metalloenzymes, but little is known about the metalloenyzme inventory of anaerobic marine microbial communities despite their importance to the nitrogen cycle. We compared dissolved O2, NO, NO, Fe and Cu concentrations with nucleic acid sequences encoding Fe and Cu-binding proteins in 21 metagenomes and 9 metatranscriptomes from Eastern Tropical North and South Pacific oxygen minimum zones and 7 metagenomes from the Bermuda Atlantic Time-series Station. Dissolved Fe concentrations increased sharply at upper oxic-anoxic transition zones, with the highest Fe:Cu molar ratio (1.8) occurring at the anoxic core of the Eastern Tropical North Pacific oxygen minimum zone and matching the predicted maximum ratio based on data from diverse ocean sites. The relative abundance of genes encoding Fe-binding proteins was negatively correlated with O2, driven by significant increases in genes encoding Fe-proteins involved in dissimilatory nitrogen metabolisms under anoxia. Transcripts encoding cytochrome c oxidase, the Fe- and Cu-containing terminal reductase in aerobic respiration, were positively correlated with O2 content. A comparison of the taxonomy of genes encoding Fe- and Cu-binding vs. bulk proteins in OMZs revealed that Planctomycetes represented a higher percentage of Fe genes while Thaumarchaeota represented a higher percentage of Cu genes, particularly at oxyclines. These results are broadly consistent with higher relative abundance of genes encoding Fe-proteins in the genome of a marine planctomycete vs. higher relative abundance of genes encoding Cu-proteins in the genome of a marine thaumarchaeote. These findings highlight the importance of metalloenzymes for microbial processes in oxygen minimum zones and suggest preferential Cu use in oxic habitats with Cu > Fe vs. preferential Fe use in anoxic niches with Fe > Cu.

The squat lobster Pleuroncodes monodon tolerates anoxic “dead zone” conditions off Peru

The squat lobster Pleuroncodes monodon is a key species of the highly productive, but oxygen-poor upwelling system of the Eastern Tropical South Pacific. Observations of P. monodon in the water column off Peru have led to the hypothesis that anoxic conditions force this otherwise primarily benthic species to adopt a pelagic lifestyle. Here we show that off Peru, P. monodon can be found in the oxygenated surface water, but also on the anoxic seafloor. Our physiological experiments demonstrate that juvenile and adult specimens have a very low critical respiratory pO2 of 0.5 kPa and that adults survive anoxia for 30.5–70.5 h. Anoxic conditions at the seafloor should therefore force P. monodon to regularly migrate to the oxic surface layer in order to restore energy reserves and recycle metabolic end products of anaerobic metabolism. It was recently estimated that the ammonium supply mediated by diel vertical migrations (DVMs) of zooplankton and nekton considerably fuels bacterial anaerobic ammonium oxidation—a major loss process for fixed nitrogen in the ocean. These estimates were based on the implicit assumption that anoxia does not result in a down-regulation of ammonium excretion. We here show that exposure to anoxia elicits a fourfold reduction in ammonium excretion from 2.1 ± 0.6 µmol h−1 g dry weight−1 under normoxic to 0.5 ± 0.6 µmol h−1 g DW−1 under anoxic conditions in P. monodon. Estimates of ammonium supply to the anoxic core of oxygen minimum zones via DVM therefore are likely too high.

Microbial community composition and nitrogen availability influence DOC remineralization in the South Pacific Gyre

Many environmental factors are thought to control the bioavailability of marine dissolved organic matter (DOM) for marine microbes including its composition, the microbial community structure, and nutrient availability, yet which factors dominate at the ocean basin scale remains uncertain. Understanding the controls on DOM lability is an important goal given the role of DOM in the marine carbon cycle. We performed DOM lability experiments at two contrasting stations, one oligotrophic and one mesotrophic, in the eastern tropical South Pacific (ETSP) to investigate the controls on microbial remineralization of surface ocean DOM. Surface layer dissolved organic carbon (DOC) and nitrogen (DON) were recalcitrant to remineralization over 9 to 14days when exposed to the microbial communities from the surface mixed layer, however exposure to microbial communities from the upper mesopelagic (twilight zone) allowed consumption of DOC but not DON. The DOC remineralization response differed between the mesotrophic site (~21μM consumed), likely experiencing allochthonous inputs of DOM from the adjacent eastern boundary upwelling system, versus the oligotrophic station (~3μM consumed) further offshore in the South Pacific gyre. DNA fingerprinting of the microbial communities across the ETSP with terminal restriction fragment length polymorphism (T-RFLP) analyses revealed greater differences between microbial communities in surface vs. subsurface (e.g., 100m) waters at the same station than between surface water microbial communities separated by 1000s of kilometers. The subsurface microbial community at the mesotrophic station responsible for the greatest observed DOC remineralization, with a concomitant consumption of nitrate, consumed DOC to concentrations below that observed in situ (at 100m), suggesting a potential role for co-metabolism of relatively labile with more recalcitrant DOC or relief from micronutrient limitation, in driving the additional DOC consumption. DOC remineralization by the mesopelagic (200m) microbial community was much less at the oligotrophic station and similar to previously published results from the Sargasso Sea. Both microbial community composition and nutrient availability contribute to DOM persistence over weekly timescales in the surface mixed layer with varying degrees of DOC lability in the subsurface waters of the ETSP.

What prevents nitrogen depletion in the oxygen minimum zone of the eastern tropical South Pacific?

Abstract. Local coupling between nitrogen fixation and denitrification in current biogeochemical models could result in runaway feedback in open-ocean oxygen minimum zones (OMZs), eventually stripping OMZ waters of all fixed nitrogen. This feedback does not seem to operate at full strength in the ocean, as nitrate does not generally become depleted in open-ocean OMZs. To explore in detail the possible mechanisms that prevent nitrogen depletion in the OMZ of the eastern tropical South Pacific (ETSP), we develop a box model with fully prognostic cycles of carbon, nutrients and oxygen in the upwelling region and its adjacent open ocean. Ocean circulation is calibrated with Δ14C data of the ETSP. The sensitivity of the simulated nitrogen cycle to nutrient and oxygen exchange and ventilation from outside the model domain and to remineralization scales inside an OMZ is analysed. For the entire range of model configurations explored, we find that the fixed-N inventory can be stabilized at non-zero levels in the ETSP OMZ only if the remineralization rate via denitrification is slower than that via aerobic respiration. In our optimum model configuration, lateral oxygen supply into the model domain is required at rates sufficient to oxidize at least about one fifth of the export production in the model domain to prevent anoxia in the deep ocean. Under these conditions, our model is in line with the view of phosphate as the ultimate limiting nutrient for phytoplankton, and implies that for the current notion of nitrogen fixation being favoured in N-deficit waters, the water column of the ETSP could even be a small net source of nitrate.

Upper-ocean gas dynamics from radon profiles in the Eastern Tropical South Pacific

Uncertainties in the dynamics of dissolved gases limit the accuracy of geochemical primary-productivity estimates. Mixed-layer ventilation, entrainment, and upwelling all affect the interpretation of geochemical data, yet they are rarely measured together. We report upper-ocean dissolved-gas dynamics in the Eastern Tropical South Pacific (10–20°S, 80–100°W) obtained from 222Rn distributions from a research cruise in February 2010. Radon-222-based surface ventilation measurements at seven stations were compared with gas transfer velocities calculated using empirical wind-speed parameterizations. Good agreement between the two methods was observed across different wind data products (ASCAT scatterometer and NCEP/NCAR meterological reanalysis) and grid scales (0.5°×0.5°, 1°×1°, and 2°×2°). Average wind speeds in the region were 5–8ms−1, corresponding to in situ gas transfer velocities of 1–3md−1. Averaged over the region, most recent gas exchange parameterizations performed similarly, and the results are robust with respect to both wind data products and grid scales. Non-steady-state conditions were observed at one station, possibly caused by a recent internal wave. These results suggest that 222Rn could be used to support measurements of biogeochemical O2 and CO2 fluxes in the upper ocean.

Biogenic particle flux and benthic remineralization in the Eastern Tropical South Pacific

We studied biogenic rain to the ocean interior and sea floor in the Eastern Tropical South Pacific (ETSP), a region at the intersection of three oceanic regimes; coastal upwelling, equatorial divergence and the South Pacific oligotrophic gyre. Sediment cores from ocean depths >4000m were collected, pore water was expressed using both whole core squeezing and rhizon techniques, and profiles of nitrate and dissolved Si were modeled to estimate remineralization fluxes. Nitrate modeling was interpreted as representative of Corg remineralization assuming the oxic transformation of ammonium to nitrate. A broad range of TCO2 fluxes were determined: 0.008–0.34mmol Cm−2d−1. The range in biogenic silica (bSi) remineralization flux was also large: 0.007–0.15mmol Sim−2d−1. The pattern of TCO2 flux showed higher particulate organic carbon (POC) inputs at sites closest to coastal and equatorial upwelling and lowest fluxes at the most oligotrophic site. Moored sediment traps, suspended at ~3700m at 10°S, 100°W and 20°S, 100°W captured the annual pattern of mass and biogenic rain. The annual average mass flux was over five times greater at a 10°S site (30.8mgm−2d−1) compared to a 20°S site (5.5mgm−2d−1). The relative wt% of POC, PIC and bSi at these two stations were 4.9, 8.0, 15.5 and 6.6, 8.3, 2.7, respectively. The deep trap POC and bSi annual rain rates were within 0.5–4 times the benthic fluxes estimated from pore water models. The annual averaged surface ocean chlorophyll concentration estimated from satellites is a good predictor of POC rain to the ocean interior at the ETSP sites studied, as is 14C primary production (PP). However, the POC rain into the deep ocean at ETSP sites per unit chlorophyll or per 14C PP is significantly less than values obtained from the equatorial Pacific at 140°W or subtropical gyre station HOT. It appears that the ETSP, although underlain by an intense oxygen minimum zone, is inefficient at transferring Corg production to deeply sinking POC rain.

Distribution of dissolved manganese in the Peruvian Upwelling and Oxygen Minimum Zone

The geochemistry of manganese (Mn) in seawater is dominated by its redox chemistry, as Mn(II) is soluble and Mn(IV) forms insoluble oxides, and redox transformations are mediated by a variety of processes in the oceans. Dissolved Mn (DMn) accumulates under reducing conditions and is depleted under oxidizing conditions. Thus the Peruvian upwelling region, characterized by highly reducing conditions over a broad continental shelf and a major oxygen minimum zone extending far offshore, is potentially a large source of Mn to the eastern Tropical South Pacific. In this study, DMn was determined on cruises in October 2005 and February 2010 in the Peruvian Upwelling and Oxygen Minimum Zone, to evaluate the relationship between Mn, oxygen and nitrogen cycle processes. DMn concentrations were determined using simple dilution and matrix-matched external standardization inductively coupled mass spectrometry. Surprisingly, DMn was depleted under the most reducing conditions along the Peruvian shelf. Concentrations of dissolved Mn in surface waters increased offshore, indicating that advection of Mn offshore from the Peruvian shelf is a minor source. Subsurface Mn maxima were observed within the oxycline rather than within the oxygen minimum zone (OMZ), indicating they arise from remineralization of organic matter rather than reduction of Mn oxides. The distribution of DMn appears to be dominated by non-redox processes and inputs from the atmosphere and from other regions associated with specific water masses. Lower than expected DMn concentrations on the shelf probably reflect limited fluvial inputs from the continent and efficient offshore transport. This behavior is in stark contrast to Fe, reported in a companion study which is very high on the shelf and undergoes dynamic redox cycling.

The microbial nitrogen cycle.

The microbial nitrogen cycle.

Evaluation of the eastern equatorial Pacific SST seasonal cycle in CMIP5 models

Abstract. The annual cycle of sea surface temperature (SST) in the eastern equatorial Pacific (EEP) has the largest amplitude in the tropical oceans, but it is poorly represented in the coupled general circulation models (CGCMs) of the Coupled Model Intercomparison Project Phase 3 (CMIP3). In this study, 18 models from CMIP5 are evaluated in terms of their capability of simulating the SST annual cycle in the EEP. Fourteen models are able to simulate the annual cycle fairly well, which suggests that the performances of CGCMs have been improved. The results of multi-model ensemble (MME) mean show that CMIP5 CGCMs can capture the annual cycle signal in the EEP with a correlation coefficient up to 0.9. Moreover, the CMIP5 models can simulate the westward propagation character of the EEP SST – in particular, EEP region 1 (EP1) near the eastern coast leading EEP region 2 (EP2) near the central equatorial Pacific by 1 to 2 months in spring. However, the models fail to reproduce the in-phase SST relationship between EP1 and EP2 in August and September. For amplitude simulations, the model SST in EP1 shows weaker seasonal variation than the observations due to the large warm SST biases from the southeastern tropical Pacific in the boreal autumn. In EP2, the simulated SST amplitudes are nearly the same as the observations while there is the presence of a quasi-constant cold bias associated with poor cold tongue simulation in the CGCMs. To improve CGCM simulation of a realistic SST seasonal cycle, local and remote SST biases that exist in both CMIP3 and CMIP5 CGCMs must be resolved at least for simulating the SSTs in the central equatorial Pacific and the southeastern tropical Pacific.

Upwelling velocity and eddy diffusivity from 7Be measurements used to compare vertical nutrient flux to export POC flux in the Eastern Tropical South Pacific

Five 7Be profiles, measured in an area bounded by 10°S–20°S and 80°W–100°W, were used to determine upwelling velocity (wH) and vertical diffusivity (Kz). A positive correlation between wH and 14C primary production rate and a negative correlation between the inventories of 7Be and phosphate were observed. We interpret this as the influence of deeper, nutrient-rich, 7Be-poor water brought up by upwelling. Excluding two stations that appear to be influenced by non-steady state dynamics or horizontal transport, upwelling velocities were estimated to be 0 to 1.0md−1 and Kz values ranged from 0.4 to 2.6cm2s−1. From these parameters, NO3− fluxes into the euphotic zone were assessed and ranged from 0.15 to 2.9mmolm−2d−1. Using these values, we estimate 1.0 to 19mmolCm−2d−1 of new production in the ETSP. New production based on 7Be-derived transport parameters agree with carbon export estimates using a 234Th balance, sediment traps and O2/Ar supersaturation for stations along 20°S, but are higher than export estimates at 10°S, 100°W.

The effect of organic carbon on fixed nitrogen loss in the eastern tropical South Pacific and Arabian Sea oxygen deficient zones

The three major oxygen deficient zones (ODZs) of the world oceans (eastern tropical North and South Pacific (ETNP and ETSP, respectively), and Arabian Sea (AS) host the vast majority of pelagic fixed nitrogen (N) loss and up to half of total marine N loss. The input of organic matter is an important control on the absolute and relative importance of the two main pathways of N removal (denitrification and anammox). We investigated the response of N loss in the ETSP and AS ODZs to additions of organic matter in the form of glucose and naturally derived dissolved and particulate organic matter (DOM and POM, respectively). In the ETSP ODZ, the addition of glucose stimulated denitrification (1.6‐fold increase after 5 d) but not anammox (14‐fold decrease after 5 d). In the AS ODZ, only POM, not DOM, significantly increased rates of denitrification at the base of the oxycline (5.4–6.4‐fold increase after 2 d), but not at the secondary nitrite maximum. These results suggest that denitrification was generally limited by organic matter supply at the time of this study in both the ETSP and AS ODZs, although the lability of the organic matter supplied was important. Interestingly, 15N2 produced in ETSP and AS incubations was not binomially distributed relative to the reactants after the influence of anammox was taken into account, suggesting an unknown production mechanism or pathway of N removal.

ENSO indices from sea surface salinity observed by Aquarius and Argo

Analysis of the first 26 months of data from the Aquarius satellite confirms the existence of a sharp sea surface salinity (SSS) front along the equator in the western equatorial Pacific. Following several earlier studies, we use the longitudinal location of the 34.8-psu isohaline as an index, termed Nino-S34.8, to measure the zonal displacement of the SSS front and consequently the eastern edge of the western Pacific warm pool. The on-going collection of the Array for Real-time Geostrophic Oceanography (ARGO) program data shows high correlations between Nino-S34.8 and the existing indices of El Nino, suggesting its potential important role in ENSO evolution. Further analysis of the ARGO data reveals that SSS variability in the southeastern tropical Pacific is crucial to identify the type of El Nino. A new SSS index, termed the southeastern Pacific SSS index (SEPSI), is defined based on the SSS variability in the region (0°–10°S, 150°–90°W). The SEPSI is highly correlated with the El Nino Modoki index, as well as the Trans-Nino index, introduced by previous studies. It has large positive anomalies during central Pacific El Nino or El Nino Modoki events, as a result of enhanced zonal sea surface temperature gradients between the central and eastern tropical Pacific, and can be used to characterize the type of El Nino. The processes that possibly control these SSS indices are also discussed.

The first report of a microdiverse anammox bacteria community in waters of Colombian Pacific, a transition area between prominent oxygen minimum zones of the eastern tropical Pacific.

Anaerobic ammonium oxidizers contribute to the removal of fixed nitrogen in oxygen-deficient marine ecosystems such as oxygen minimum zones (OMZ). Here we surveyed for the first time the occurrence and diversity of anammox bacteria in the Colombian Pacific, a transition area between the prominent South and North Pacific OMZs. Anammox bacteria were detected in the coastal and oceanic areas of the Colombian Pacific in low oxygen (< 22 μM), high nitrate (25–35 μM) and low nitrite (< 0.07 μM), and ammonium (< 1 μM) waters. In these waters, anammox bacteria were rich [∼ 7 operational taxonomic units (OTUs), 98% cut-off) and microdiverse (Shannon index H′ < 1.24), in comparison with the observed at the prominent OMZ of the Eastern Tropical South Pacific, Arabian Sea and Black Sea. Anammox bacteria-like sequences from the Colombian Pacific were grouped together with sequences retrieved from the distinct OMZ's marine subclusters (Peru, Northern Chile and Arabian Sea) within Candidatus ‘Scalindua spp’. Moreover, some anammox bacteria OTUs shared a low similarity with environmental phylotypes (86–94%). Our results indicated that a microdiverse anammox community inhabits the Colombian Pacific, generating new questions about the ecological and biogeochemical differences influencing its community structure.

The paradox of marine heterotrophic nitrogen fixation: abundances of heterotrophic diazotrophs do not account for nitrogen fixation rates in the Eastern Tropical South Pacific

Results of recent modelling efforts imply denitrification-influenced waters, such as those in the Eastern Tropical South Pacific (ETSP), may support high rates of biological nitrogen fixation (BNF), yet little is known about the N2 -fixing microbial community in this region. Our characterization of the ETSP diazotrophic community along a gradient from upwelling-influenced to oligotrophic waters did not detect cyanobacterial diazotrophs commonly found in other open ocean regions. Most of the nifH genes amplified by polymerase chain reaction (PCR) from DNA and RNA samples clustered with γ-proteobacterial nifH sequences, although a novel Trichodesmium phylotype was also recovered. Three quantitative PCR assays were developed to target γ-proteobacterial phylotypes, but all were found to be present at low abundances. An analysis of the expected BNF rates based on abundances and plausible cell-specific N2 fixation rates indicates that these γ-proteobacteria are unlikely to be responsible for previously reported BNF rates from corresponding samples. Therefore, the organisms responsible for the measured BNF rates remain poorly understood. Furthermore, there is little direct evidence, at this time, to support the hypothesis that heterotrophic N2 fixation contributes significantly to oceanic BNF rates based on our analysis of heterotrophic cell-specific N2 fixation rates required to explain BNF rates reported in previously published studies.

Aphotic N2 Fixation in the Eastern Tropical South Pacific Ocean

We examined rates of N2 fixation from the surface to 2000 m depth in the Eastern Tropical South Pacific (ETSP) during El Niño (2010) and La Niña (2011). Replicated vertical profiles performed under oxygen-free conditions show that N2 fixation takes place both in euphotic and aphotic waters, with rates reaching 155 to 509 µmol N m−2 d−1 in 2010 and 24±14 to 118±87 µmol N m−2 d−1 in 2011. In the aphotic layers, volumetric N2 fixation rates were relatively low (<1.00 nmol N L−1 d−1), but when integrated over the whole aphotic layer, they accounted for 87–90% of total rates (euphotic+aphotic) for the two cruises. Phylogenetic studies performed in microcosms experiments confirm the presence of diazotrophs in the deep waters of the Oxygen Minimum Zone (OMZ), which were comprised of non-cyanobacterial diazotrophs affiliated with nifH clusters 1K (predominantly comprised of α-proteobacteria), 1G (predominantly comprised of γ-proteobacteria), and 3 (sulfate reducing genera of the δ-proteobacteria and Clostridium spp., Vibrio spp.). Organic and inorganic nutrient addition bioassays revealed that amino acids significantly stimulated N2 fixation in the core of the OMZ at all stations tested and as did simple carbohydrates at stations located nearest the coast of Peru/Chile. The episodic supply of these substrates from upper layers are hypothesized to explain the observed variability of N2 fixation in the ETSP.