Oligotrophic Subtropical Gyres Research Articles

Coral photosymbiosis on Mid-Devonian reefs.

The ability of stony corals to thrive in the oligotrophic (low-nutrient, low-productivity) surface waters of the tropical ocean is commonly attributed to their symbiotic relationship with photosynthetic dinoflagellates1,2. The evolutionary history of this symbiosis might clarify its organismal and environmental roles3, but its prevalence through time, and across taxa, morphologies and oceanic settings, is currently unclear4-6. Here we report measurements of the nitrogen isotope (15N/14N) ratio of coral-bound organic matter (CB-δ15N) in samples from Mid-Devonian reefs (Givetian, around 385 million years ago), which represent a constraint on the evolution of coral photosymbiosis. Colonial tabulate and fasciculate (dendroid) rugose corals have low CB-δ15N values (2.51 ± 0.97‰) in comparison with co-occurring solitary and (pseudo)colonial (cerioidor phaceloid) rugose corals (5.52 ± 1.63‰). The average of the isotopic difference per deposit (3.01 ± 0.58‰) is statistically indistinguishable from that observed between modern symbiont-barren and symbiont-bearing corals (3.38 ± 1.05‰). On the basis of this evidence, we infer that Mid-Devonian tabulate and some fasciculate (dendroid) rugose corals hosted active photosymbionts, while solitary and some (pseudo)colonial (cerioidor phaceloid) rugose corals did not. The low CB-δ15N values of the Devonian tabulate and fasciculate rugose corals relative to the modern range suggest that Mid-Devonian reefs formed in biogeochemical regimes analogous to the modern oligotrophic subtropical gyres. Widespread oligotrophy during the Devonian may have promoted coral photosymbiosis, the occurrence of which may explain why Devonian reefs were the most productive reef ecosystems of the Phanerozoic.

Physical Mechanisms Driving Enhanced Carbon Sequestration by the Biological Pump Under Climate Warming

AbstractAs ocean Carbon Dioxide Removal techniques are being considered, it is critical that they be evaluated against our scientific understanding of the global biological carbon pump. In a recent paper Nowicki et al. (2022, https://doi.org/10.1029/2021GB007083) provide an innovative and comprehensive breakdown of the different mechanistic pathways of carbon sequestration through the present‐day biological pump but then speculate that “These results suggest that ocean carbon storage will weaken as the oceans stratify and the subtropical gyres expand due to anthropogenic climate change.” Essentially, the authors combine their steady state result that oligotrophic subtropical gyres have lower residence times than other areas with the climate change result of these areas increasing under climate warming and extrapolate—assuming “all else is equal”—that the overall ocean will suffer a reduction in carbon sequestration efficiency. Expressing global changes in carbon sequestered by the ocean's biological pump as the summation of local changes in the sequestered carbon, timescale of return to the surface, and biogeographical area, I discuss how all three terms are tightly coupled, and summarize decades of climate change modeling consistently indicating that the global scale physical sequestration response is an increase ‐ in opposition of what one would infer from changes in subtropical area alone.

Similarities and Contrasts in Time-Mean Striated Surface Tracers in Pacific Eastern Boundary Upwelling Systems: The Role of Ocean Currents in Their Generation

Eastern boundary upwelling systems feature strong zonal gradients of physical and biological properties between cool, productive coastal oceans and warm, oligotrophic subtropical gyres. Zonal currents and jets (striations) are therefore likely to contribute to the transport of water properties between coastal and open oceanic regions. For the first time, multi-sensor satellite data are used to characterize the time-mean signatures of striations in sea surface temperature (SST), salinity (SSS), and chlorophyll-a (Chl-a) in subtropical eastern North/South Pacific (ENP/ESP) upwelling systems. In the ENP, tracers exhibit striated patterns extending up to ~2500 km offshore. Striated signals in SST and SSS are highly correlated with quasi-zonal jets, suggesting that these jets contribute to SST/SSS mesoscale patterns via zonal advection. Striated Chl-a anomalies are collocated with sea surface height (SSH) bands, a possible result of mesoscale eddy trains trapping nutrients and forming striated signals. In the ESP, the signature of striations is only found in SST and coincides with the SSH bands, consistently with quasi-zonal jets located outside major zonal tracer gradients. An interplay between large-scale SST/SSS advection by the quasi-zonal jets, mesoscale SST/SSS advection by the large-scale meridional flow, and eddy advection may explain the persistent ENP hydrographic signature of striations. These results underline the importance of quasi-zonal jets for surface tracer structuring at the mesoscale.

Transcriptional responses of Trichodesmium to natural inverse gradients of Fe and P availability

The filamentous diazotrophic cyanobacterium Trichodesmium is responsible for a significant fraction of marine di-nitrogen (N2) fixation. Growth and distribution of Trichodesmium and other diazotrophs in the vast oligotrophic subtropical gyres is influenced by iron (Fe) and phosphorus (P) availability, while reciprocally influencing the biogeochemistry of these nutrients. Here we use observations across natural inverse gradients in Fe and P in the North Atlantic subtropical gyre (NASG) to demonstrate how Trichodesmium acclimates in situ to resource availability. Transcriptomic analysis identified progressive upregulation of known iron-stress biomarker genes with decreasing Fe availability, and progressive upregulation of genes involved in the acquisition of diverse P sources with decreasing P availability, while genes involved in N2 fixation were upregulated at the intersection under moderate Fe and P availability. Enhanced N2 fixation within the Fe and P co-stressed transition region was also associated with a distinct, consistent metabolic profile, including the expression of alternative photosynthetic pathways that potentially facilitate ATP generation required for N2 fixation with reduced net oxygen production. The observed response of Trichodesmium to availability of both Fe and P supports suggestions that these biogeochemically significant organisms employ unique molecular, and thus physiological responses as adaptations to specifically exploit the Fe and P co-limited niche they construct.

Spatial Distribution of Dissolved Methane Over Extreme Oceanographic Gradients in the Subtropical Eastern South Pacific (17° to 37°S)

AbstractMethane (CH4) is one of the most powerful greenhouse gases with the capacity to influence the Earth's radiative budget as well as contribute to atmospheric chemistry. Natural oceanic production makes up to ∼4% of the overall global CH4 emissions, however, there is uncertainty around the accuracy of this value due to a lack of accurate measurements. Such is the case in the Subtropical Eastern South Pacific Ocean (SESP), a region with pronounced chlorophyll‐a and oxygen gradients, which in turn affect the microbial CH4 cycling. This study was conducted during spring‐summer (2014–2016) in the SESP. The region (∼17°–37°S/71°–110°W) is separated into (i) eutrophic, (ii) mesotrophic, and (iii) oligotrophic areas, according to oceanographic and biogeochemical criteria. The SESP presents high CH4 zonal variability with levels ranging from 0.63 to 33.4 nmol L−1, corresponding to 29% and 1,423% saturation, respectively. High CH4 concentrations (>1,000% saturation) are observed in the narrow eutrophic area subjected to coastal upwelling. These conditions clearly differ to those observed in the extended oligotrophic subtropical gyre (∼100% saturation). Furthermore, CH4 also tends to accumulate in the mesotrophic area (with upto 1,423% saturation), where oceanographic conditions as stratification, mesoscale eddies and island mass effect could trigger the presence of a microbial biomass that may be able to induce CH4 regeneration. The CH4 efflux is estimated to be between 0.13 and 19.1 µmol m−2 d−1 (mean ± SD = 4.72 ± 4.67) and the SESP has an emission rate of ∼87.9 Gg CH4 yr−1.

Global Modeled Sinking Characteristics of Biofouled Microplastic.

Microplastic debris ending up at the sea surface has become a known major environmental issue. However, how microplastic particles move and when they sink in the ocean remains largely unknown. Here, we model microplastic subject to biofouling (algal growth on a substrate) to estimate sinking timescales and the time to reach the depth where particles stop sinking. We combine NEMO‐MEDUSA 2.0 output, that represents hydrodynamic and biological properties of seawater, with a particle‐tracking framework. Different sizes and densities of particles (for different types of plastic) are simulated, showing that the global distribution of sinking timescales is largely size‐dependent as opposed to density‐dependent. The smallest particles we simulate (0.1 μm) start sinking almost immediately around the globe and their trajectories take the longest time to reach their first sinking depth (relative to larger particles). In oligotrophic subtropical gyres with low algal concentrations, particles between 1 and 0.01 mm do not sink within the simulation time of 90 days. This suggests that in addition to the comparatively well‐known physical processes, biological processes might also contribute to the accumulation of floating plastic (of 1–0.01 mm) in subtropical gyres. Particles of 1 μm in the gyres start sinking largely due to vertical advection, whereas in the equatorial Pacific they are more dependent on biofouling. The qualitative impacts of seasonality on sinking timescales are small, however, localized sooner sinking due to spring algal blooms is seen. This study maps processes that affect the sinking of virtual microplastic globally, which could ultimately impact the ocean plastic budget.

A Lagrangian study of the contribution of the Canary coastal upwelling to the nitrogen budget of the open North Atlantic

Abstract. The Canary Current System (CanCS) is a major eastern boundary upwelling system (EBUS), known for its high nearshore productivity and for sustaining a large fishery. It is also an important but not well quantified source of nitrogen to the adjacent oligotrophic subtropical gyre of the North Atlantic. Here, we use a Lagrangian modeling approach to quantify this offshore transport and investigate its timescales, reach and contribution to the fueling of productivity in the offshore regions. In our Lagrangian model, we release nearly 10 million particles off the northwestern African coast and then track all those that enter the nearshore region and upwell along the coast between 14 and 35∘ N. We then follow them as they are transported offshore, also tracking the biogeochemical transformations, permitting us to construct biogeochemical budgets along the offshore moving particles. The three-dimensional velocity field as well as the biogeochemical tracers and fluxes are taken from an eddy-resolving configuration of the Regional Ocean Modeling System (ROMS). Our Lagrangian model analysis reveals a very intense offshore transport of nitrogen, with about 20 %–40 % in the form of organic nitrogen. The transport varies greatly along the coast. Even though the central CanCS (21–28∘ N) transports the largest amount of water offshore, its offshore transport of nitrogen is somewhat smaller than that in the southern CanCS (14–21∘ N), primarily because of the higher nitrogen content of the upwelling waters there. Around one-third of the total offshore transport of water occurs around major capes along the CanCS. The persistent filaments associated with these capes are responsible for an up to 4-fold enhancement of the offshore transport of water and nitrogen in the first 400 km. Much of this water and nitrogen stems from upwelling at quite some distance from the capes, confirming the capes' role in collecting water from along the coast. North of Cape Blanc and within the first 500 km from the coast, water recirculation is a dominant feature of offshore transport. This process, likely associated with mesoscale eddies, tends to reduce the efficiency of offshore transport. The Canary upwelling is modeled to supply around 44 and 7 mmol N m−2 yr−1 to the North Atlantic Tropical Gyral (NATR) and the North Atlantic Subtropical Gyral East (NASE) Longhurst provinces, respectively. In the NATR, this represents nearly half (53 ± 26 %) of the estimated total new production, while in the NASE, this fraction is small (4 ± 2 %). Our results highlight the importance of the CanCS upwelling as a key source of nitrogen to the open North Atlantic and stress the need for improving the representation of EBUS in global coarse-resolution models.

Sediment trap-derived particulate matter fluxes in the oligotrophic subtropical gyre of the South Indian Ocean

Oligotrophic areas cover about 75% of the ocean's surface, and these ocean regions are predicted to expand under global warming scenarios. To evaluate impacts on global marine biogeochemical cycles and changes in ocean-atmosphere carbon fluxes, it is essential to understand particulate matter fluxes and determine the amount of organic carbon that is exported to the ocean's interior. The oligotrophic Indian Ocean subtropical gyre (IOSG) is one of the least explored ocean regions in terms of particulate matter fluxes. Sediment trap-based particulate matter fluxes determined during a 4-year time series provide new information on the nature of export fluxes, their controlling factors, and on the spatial and temporal variability of oceanic processes in the IOSG. Trap-averaged total mass fluxes (~9.8 ± 3.7 mg m−2 day−1), as well as particulate organic carbon (POC) fluxes (0.50 ± 0.15 mg m−2 day−1) measured at 500–600 m above bottom (2600–3500 m water depth) are among the lowest fluxes recorded worldwide. These low flux values are a result of strongly stratified and nutrient-depleted upper waters in the gyre. Such oligotrophic conditions lead to low primary production rates in a relatively homogeneous and isolated ocean region. Consequently, we observe an almost constant rain of POC fluxes in space and time, although minor variations in the net primary production (NPP) and in the sea surface temperature (SST) are seen in satellite surveys and model estimations. Factors contributing to the lack of seasonality in the POC fluxes are intense organic matter degradation, variations in the ocean mixed layer depth (OMLD), and impacts of physical mixing (surface wind stress, cyclonic eddies). Preliminary estimates indicate that the average POC export efficiency (ε = 0.03 ± 0.01) is extremely low in the IOSG. Assuming that the IOSG, as well as comparable ocean regions, will expand under climate warming conditions, it is of major importance to investigate POC export fluxes to the deep ocean in order to predict changes in the global carbon cycle during the next decades.

Diel Measurements of Oxygen‐ and Carbon‐Based Ocean Metabolism Across a Trophic Gradient in the North Pacific

AbstractDaily in situ rates of gross production and community respiration estimated from high‐frequency diel cycles in oxygen (O2) and optically derived particulate carbon from several platforms (both ship based and via profiling floats) were made across an ecological gradient in the North Pacific spanning the high‐nutrient/low‐chlorophyll sub‐Arctic to the oligotrophic subtropical gyre. Both oxygen and carbon‐based gross primary production and respiration rates indicated a ~3× increase between subtropical and subpolar stations. We consistently found that gross production and community respiration rates were in approximate balance at all stations across the full ecological gradient, implying that community respiration is fueled by recently produced organic matter and that recycling efficiency (~90%) is similar along the gradient. We determined that phytoplankton turnover time doubles (from 2 to 4 days) between subtropical and subpolar regimes, whereas biomass increases by approximately tenfold. We found a consistent photosynthetic quotient (1.4 ± 0.2 mol O2 mol C−1), respiratory quotient (1.0 ± 0.2 mol O2 mol C−1), and gross to net production ratio (2.0 ± 0.3) at all stations which underscores the similarity of fundamental ecological characteristics despite the transition f rom nutrient deplete to replete conditions. That the float‐ and ship‐based estimates of in situ production and respiration generally agreed well suggests that float‐based diel O2 and particulate organic carbon measurements have the potential to greatly expand our knowledge of spatial and temporal variability of productivity and respiration in the ocean.

Homogeneous environmental selection dominates microbial community assembly in the oligotrophic South Pacific Gyre.

Oligotrophic subtropical gyres are the largest continuous biomes on Earth and play a key role in global biogeochemical cycles. Microbial communities govern primary production and carbon cycling in the oligotrophic South Pacific Gyre, yet the ecological processes which underpin microbial biogeography in the region remain understudied. We investigated microbial biogeography and community assembly processes at three depths over a ~2,000-km the transect was longitudinal, so ran from 32°S, 170°W to 32°S, 152°W). Thus the latitude (32°S) was constant. Microbial communities in the surface waters (15 and 50m) were remarkably similar across the transect, whilst communities at the deep chlorophyll maximum were distinct from the surface waters and displayed greater compositional heterogeneity. An ecological null model approach indicated that homogeneous selection was the dominant community assembly process in both the surface waters (100%) and at the deep chlorophyll maximum (91.81%), although variable selection (2.34%) and stochastic processes (5.85%) had a minor influence at the deep chlorophyll maximum. Homogeneous selection (76.69%77.90%), dispersal limitation (15.00%-20.05%) and variable selection (3.01%-7.11%) influenced community assembly between the surface waters and the deep chlorophyll maximum. Seawater density and temperature, which were correlated, were the most important environmental modulators of the balance between stochastic and deterministic assembly processes. Our findings demonstrate remarkable similarity in microbial community composition across longitudinal scales in the oligotrophic South Pacific Gyre, underpinned by strong environmental selection which overwhelms the influence of ecological drift. These data significantly advance our understanding of microbial community dynamics in the oligotrophic subtropical gyres which dominate the Earth's surface.

Carbon Export Buffering and CO2 Drawdown by Flexible Phytoplankton C:N:P Under Glacial Conditions

AbstractModern observations indicate that variations in marine phytoplankton stoichiometry correlate with the boundaries of major surface waters. For example, phytoplankton in the oligotrophic subtropical gyres typically have much higher C:N:P ratios (i.e., higher C:P and higher N:P ratios) than those in eutrophic upwelling regions and polar regions. Such a spatial pattern points to nutrient availability as a key environmental driver of stochiometric flexibility. Environmental dependence of phytoplankton C:N:P opens unexplored possibilities for modifying the strength of the biological pump under different climate conditions. Here we present a power law formulation of C:N:P flexibility that is driven by nutrients, temperature, and light. We embed the formulation in a global ocean carbon cycle model with multiple phytoplankton types and explore biogeochemical implications under glacial conditions. We find three key controls on export C:N:P ratio: phytoplankton physiology and community structure as well as the balance in regional production at the global level. Glacial inputs of iron and sea ice expansion are important modifiers of these three controls. We also find that global export C:N:P increases substantially under glacial conditions, and this strongly buffers global carbon export against decrease and draws down approximately 20 μatm of atmospheric CO2. These results point to the importance of including phytoplankton C:N:P flexibility in a mix of mechanisms that drive atmospheric CO2 over glacial‐interglacial time scale. Finally, our simulations indicate decoupling of nutrients, which may provide a resolution to the longstanding disagreement regarding nutrient utilization in the glacial Southern Ocean derived from different nutrient proxies.

Subtle bacterioplankton community responses to elevated CO2 and warming in the oligotrophic South Pacific gyre.

Bacterioplankton play a critical role in primary production, carbon cycling, and nutrient cycling in the oligotrophic ocean. To investigate the effect of elevated CO2 and warming on the composition and function of bacterioplankton communities in oligotrophic waters, we performed two trace-metal clean deck board incubation experiments during the New Zealand GEOTRACES transect of the South Pacific gyre (SPG). High-throughput amplicon sequencing of the 16S rRNA gene revealed that bacterioplankton community composition was distinct between the fringe and ultra-oligotrophic centre of the SPG and changed consistently in response to elevated CO2 at the ultra-oligotrophic centre but not at the mesotrophic fringe of the SPG. The combined effects of elevated CO2 and warming resulted in a high degree of heterogeneity between replicate communities. Community-level protein synthesis rates (3 H-Leucine incorporation) and bacterioplankton abundance were not affected by elevated CO2 alone or in combination with warming at the fringe or ultra-oligotrophic centre of the SPG. These data suggest bacterioplankton community responses to elevated CO2 may be modulated by nutrient regimes in open ocean ecosystems and highlight the need for further investigation in expanding oligotrophic subtropical gyres.

Active nitrogen fixation by Crocosphaera expands their niche despite the presence of ammonium \u2013 A case study

Unicellular nitrogen fixer Crocosphaera contributes substantially to nitrogen fixation in oligotrophic subtropical gyres. They fix nitrogen even when significant amounts of ammonium are available. This has been puzzling since fixing nitrogen is energetically inefficient compared with using available ammonium. Here we show that by fixing nitrogen, Crocosphaera can increase their population and expand their niche despite the presence of ammonium. We have developed a simple but mechanistic model of Crocosphaera based on their growth in steady state culture. The model shows that the growth of Crocosphaera can become nitrogen limited despite their capability to fix nitrogen. When they fix nitrogen, the population increases by up to 78% relative to the case without nitrogen fixation. When we simulate a simple ecological situation where Crocosphaera exists with non-nitrogen-fixing phytoplankton, the relative abundance of Crocosphaera increases with nitrogen fixation, while the population of non-nitrogen-fixing phytoplankton decreases since a larger fraction of fixed nitrogen is consumed by Crocosphaera. Our study quantitatively supports the benefit of nitrogen fixation despite the high electron/energy costs, even when an energetically efficient alternative is available. It demonstrates a competitive aspect of Crocosphaera, permitting them to be regionally significant nitrogen fixers.

Strategies among phytoplankton in response to alleviation of nutrient stress in a subtropical gyre.

Despite generally low primary productivity and diatom abundances in oligotrophic subtropical gyres, the North Atlantic Subtropical Gyre (NASG) exhibits significant diatom-driven carbon export on an annual basis. Subsurface pulses of nutrients likely fuel brief episodes of diatom growth, but the exact mechanisms utilized by diatoms in response to these nutrient injections remain understudied within near-natural settings. Here we simulated delivery of subsurface nutrients and compare the response among eukaryotic phytoplankton using a combination of physiological techniques and metatranscriptomics. We show that eukaryotic phytoplankton groups exhibit differing levels of transcriptional responsiveness and expression of orthologous genes in response to release from nutrient limitation. In particular, strategies for use of newly delivered nutrients are distinct among phytoplankton groups. Diatoms channel new nitrate to growth-related strategies while physiological measurements and gene expression patterns of other groups suggest alternative strategies. The gene expression patterns displayed here provide insights into the cellular mechanisms that underlie diatom subsistence during chronic nitrogen-depleted conditions and growth upon nutrient delivery that can enhance carbon export from the surface ocean.

Copepod carcasses in the subtropical convergence zone of the Sargasso Sea: implications for microbial community composition, system respiration and carbon flux

Abstract The oligotrophic subtropical gyre covers a vast area of the Atlantic Ocean. Decades of time-series monitoring have generated detailed temporal information about zooplankton species and abundances at fixed locations within the gyre, but their live/dead status is often omitted, especially in the dynamic subtropical convergence zone (STCZ) where the water column stratification pattern can change considerably across the front as warm and cold water masses converge. We conducted a detailed survey in the North Atlantic STCZ and showed that over 85% of the copepods were typically concentrated in the upper 200 m. Copepod carcasses were present in all samples and their proportional numerical abundances increased with depth, reaching up to 91% at 300–400 m. Overall, 14–19% of the copepods within the upper 200 m were carcasses. Shipboard experiments showed that during carcass decomposition, microbial respiration increased, and the bacterial community associated with the carcasses diverged from that in the ambient water. Combining field and experimental data, we estimated that decomposing copepod carcasses constitute a negligible oxygen sink in the STCZ, but sinking carcasses may represent an overlooked portion of the passive carbon sinking flux and should be incorporated in future studies of carbon flux in this area.

Isopycnal Processes Allow for Summertime Heterotrophy Despite Net Oxygen Accumulation in the Lower Euphotic Zone of the Western North Atlantic Subtropical Gyre

AbstractIn the oligotrophic subtropical gyre of the North Atlantic, the processes that allow for an imbalance between annual biological productivity and organic carbon export have been sought for decades. We use biogeochemical data from profiling floats and 26‐year bottle samples off Bermuda to provide the first evidence for a mechanism that allows for heterotrophy in the presence of oxygen accumulation in the lower euphotic zone (50–100 m) during the stratified season. After the spring bloom, surface waters that are enriched in oxygen and organic matter, but low in nitrate, are subducted and transported along the seasonal isopycnals that progressively displace downward. Due exclusively to this downward displacement, a positive 50‐ to 100‐m depth‐integrated O2 anomaly appears (1,688 ± 545 mmol O2/m2) from mid‐May to mid‐October. Neglecting this effect of isopycnal displacement would suggest an excess of biological productivity over remineralization at 50–100 m (344 ± 330 mmol O2/m2). Yet, when these changes are differenced, significant along‐isopycnal oxygen consumption (−1,344 ± 537 mmol O2/m2) is identified. After accounting for mixing, net biological‐driven oxygen consumption is still found (−827 ± 509 mmol O2/m2), which indicates heterotrophy. Remineralization of sinking and suspended organic matters at 50–100 m could support 90 ± 67% of the heterotrophic demand. Our analysis also shows that the spread in the biological‐driven oxygen sink is linked to the strength of isopycnal displacement that modulates the supply of nutrients and organic matters. This along‐isopycnal transport and heterotrophy in the lower euphotic zone reduces carbon export at 100 m and helps to resolve previously noted imbalances between surface biological productivity and total organic carbon export.

Distribution of 236U in the U.S. GEOTRACES Eastern Pacific Zonal Transect and its use as a water mass tracer

We report dissolved concentrations of the long-lived radioisotope 236U measured in the water column along the 2013 US GEOTRACES Eastern Pacific Zonal Transect (GP16). This transect followed a 10–15°S line from Manta, Ecuador, to Papeete, Haiti, French Polynesia, crossing the southern East Pacific Rise, intercepting one of the largest hydrothermal plumes as well as a productivity gradient, which includes the upwelling zone and associated low oxygen waters offshore from Peru and the oligotrophic sub-tropical gyre. Accelerator Mass Spectrometry was used to measure dissolved seawater 236U concentrations as low as 1 × 103 atom kg−1, which are among the lowest levels reported to date. Differences in upper water column 236U distributions from east to west are a result of variable contributions from different surface and intermediate waters encountered along the transect.The distribution of 236U, both in depth and geographically, provides complementary information to that obtained from Δ14C and helium isotopes, demonstrating that 236U concentrations are diagnostic in the identification of and contributions from the different deep and bottom water masses encountered along the EPZT (Jenkins et al., 2017). For example, we observe minimum 236U concentrations along the EPZT between 2000 and 3000 m that are consistent with contributions attributed to Pacific Deep Water. We also observe increases in 236U below 3000 m at the eastern and western termini of the EPZT. This is consistent with contributions associated with Antarctic Bottom Water and Lower Circumpolar Deep Water. Our results indicate that 236U may be used in conjunction with Δ14C and 3He isotopes as an additional tool with which to identify and resolve contributions from different water masses in the Pacific Ocean.This article is part of a special issue entitled: “Cycles of trace elements and isotopes in the ocean – GEOTRACES and beyond” - edited by Tim M. Conway, Tristan Horner, Yves Plancherel, and Aridane G. González.

Intra-host Symbiont Diversity and Extended Symbiont Maintenance in Photosymbiotic Acantharea (Clade F).

Photosymbiotic protists contribute to surface primary production in low-nutrient, open-ocean ecosystems and constitute model systems for studying plastid acquisition via endosymbiosis. Little is known, however, about host-symbiont dynamics in these important relationships, and whether these symbioses are mutualistic is debated. In this study, we applied single-cell sequencing methods and advanced fluorescent microscopy to investigate host-symbiont dynamics in clade F acantharians, a major group of photosymbiotic protists in oligotrophic subtropical gyres. We amplified the 18S rRNA gene from single acantharian hosts and environmental samples to assess intra-host symbiont diversity and to determine whether intra-host symbiont community composition directly reflects the available symbiont community in the surrounding environment. Our results demonstrate that clade F acantharians simultaneously host multiple species from the haptophyte genera Phaeocystis and Chrysochromulina. The intra-host symbiont community composition was distinct from the external free-living symbiont community, suggesting that these acantharians maintain symbionts for extended periods of time. After selectively staining digestive organelles, fluorescent confocal microscopy showed that symbionts were not being systematically digested, which is consistent with extended symbiont maintenance within hosts. Extended maintenance within hosts may benefit symbionts through protection from grazing or viral lysis, and therefore could enhance dispersal, provided that symbionts retain reproductive capacity. The evidence for extended symbiont maintenance therefore allows that Phaeocystis could glean some advantage from the symbiosis and leaves the possibility of mutualism.

The Impact of Variable Phytoplankton Stoichiometry on Projections of Primary Production, Food Quality, and Carbon Uptake in the Global Ocean

AbstractOcean biogeochemical models are integral components of Earth system models used to project the evolution of the ocean carbon sink, as well as potential changes in the physical and chemical environment of marine ecosystems. In such models the stoichiometry of phytoplankton C:N:P is typically fixed at the Redfield ratio. The observed stoichiometry of phytoplankton, however, has been shown to considerably vary from Redfield values due to plasticity in the expression of phytoplankton cell structures with different elemental compositions. The intrinsic structure of fixed C:N:P models therefore has the potential to bias projections of the marine response to climate change. We assess the importance of variable stoichiometry on 21st century projections of net primary production, food quality, and ocean carbon uptake using the recently developed Pelagic Interactions Scheme for Carbon and Ecosystem Studies Quota (PISCES‐QUOTA) ocean biogeochemistry model. The model simulates variable phytoplankton C:N:P stoichiometry and was run under historical and business‐as‐usual scenario forcing from 1850 to 2100. PISCES‐QUOTA projects similar 21st century global net primary production decline (7.7%) to current generation fixed stoichiometry models. Global phytoplankton N and P content or food quality is projected to decline by 1.2% and 6.4% over the 21st century, respectively. The largest reductions in food quality are in the oligotrophic subtropical gyres and Arctic Ocean where declines by the end of the century can exceed 20%. Using the change in the carbon export efficiency in PISCES‐QUOTA, we estimate that fixed stoichiometry models may be underestimating 21st century cumulative ocean carbon uptake by 0.5–3.5% (2.0–15.1 PgC).

Net community production in the South China Sea Basin estimated from in situ O2 measurements on an Argo profiling float

For the first time, the net community production (NCP) was estimated over a complete annual cycle in the basin of the South China Sea (SCS) using in situ oxygen measurements from an Argo profiling float and an oxygen mass balance model. The annual NCP from July 2014 to July 2015 was estimated to be 2.7molCm−2yr−1 (calculated to the deepest winter mixed layer depth of 56m), with the uncertainties ranging from 0.9 ~ 2.2molCm−2yr−1. NCP estimates followed a monsoonal pattern with higher values in the cold season (November to April) when northeast monsoon prevailed and low values in the warm season (June to September) when this area was dominated by the southwest monsoon. Most of the net heterotrophic events occurred in the warm season. The magnitude and seasonal pattern derived from our results agree with previous export production studies based on discrete measurements. Comparison with satellite-derived NCP revealed that the results derived with NPP from Carbon-based Production Model (CbPM) were closer to the Argo measurements than the results derived with NPP from Vertically Generalized Production Model (VGPM) in magnitude; while the VGPM-based approach did a better job in reproducing the seasonal cycle of NCP in this area. This novel approach provides the possibilities to study the carbon cycle in the SCS with a much higher temporal and spatial resolution, as well as more insights for metabolic state in the oligotrophic subtropical gyres.