Oligotrophic North Pacific Subtropical Gyre Research Articles

Remote Impacts of Cyclonic Eddies on Productivity Around the Main Hawaiian Islands

AbstractThe Hawaiian Island chain lies on the southern limb of the oligotrophic North Pacific Subtropical Gyre, an area characterized by low productivity. In this region, productivity is controlled by several factors, including the island mass effect and mesoscale eddies generated by wind stress curl in the lee of the islands. This study identifies high‐chlorophyll events that occur periodically off the northern coasts of the Main Hawaiian Islands. These events are present in the satellite record and can be reproduced using a regional dynamical model. Model results indicate events are characterized by increases in total phytoplankton and total zooplankton, and a shift in phytoplankton size structure. We show these events are uniquely driven by the presence of cyclonic mesoscale eddies located downstream, on the opposite side of the island chain. While these eddies are known to impact productivity locally, we reveal that nutrients upwelled by these eddies can also be transported around the islands, counter to the mean background flow. These results demonstrate how leeward cyclonic eddies can have far‐reaching, remote impacts on productivity around the Hawaiian Islands.

Growth and mortality of aerobic anoxygenic phototrophs in the North Pacific Subtropical Gyre.

Aerobic anoxygenic phototrophic (AAP) bacteria harvest light energy using bacteriochlorophyll-containing reaction centers to supplement their mostly heterotrophic metabolism. While their abundance and growth have been intensively studied in coastal environments, much less is known about their activity in oligotrophic open ocean regions. Therefore, we combined in situ sampling in the North Pacific Subtropical Gyre, north of O'ahu island, Hawaii, with two manipulation experiments. Infra-red epifluorescence microscopy documented that AAP bacteria represented approximately 2% of total bacteria in the euphotic zone with the maximum abundance in the upper 50 m. They conducted active photosynthetic electron transport with maximum rates up to 50 electrons per reaction center per second. The in situ decline of bacteriochlorophyll concentration over the daylight period, an estimate of loss rates due to predation, indicated that the AAP bacteria in the upper 50 m of the water column turned over at rates of 0.75-0.90 d-1. This corresponded well with the specific growth rate determined in dilution experiments where AAP bacteria grew at a rate 1.05 ± 0.09 d-1. An amendment of inorganic nitrogen to obtain N:P = 32 resulted in a more than 10 times increase in AAP abundance over 6 days. The presented data document that AAP bacteria are an active part of the bacterioplankton community in the oligotrophic North Pacific Subtropical Gyre and that their growth was mostly controlled by nitrogen availability and grazing pressure.IMPORTANCEMarine bacteria represent a complex assembly of species with different physiology, metabolism, and substrate preferences. We focus on a specific functional group of marine bacteria called aerobic anoxygenic phototrophs. These photoheterotrophic organisms require organic carbon substrates for growth, but they can also supplement their metabolic needs with light energy captured by bacteriochlorophyll. These bacteria have been intensively studied in coastal regions, but rather less is known about their distribution, growth, and mortality in the oligotrophic open ocean. Therefore, we conducted a suite of measurements in the North Pacific Subtropical Gyre to determine the distribution of these organisms in the water column and their growth and mortality rates. A nutrient amendment experiment showed that aerobic anoxygenic phototrophs were limited by inorganic nitrogen. Despite this, they grew more rapidly than average heterotrophic bacteria, but their growth was balanced by intense grazing pressure.

Revealing the Marine Cycles of Volatile Sulfur Compounds and Their Biogeochemical Controls: A Case of the Western North Pacific.

Volatile sulfur compounds, such as dimethyl sulfide (DMS), carbonyl sulfide (OCS), and carbon disulfide (CS2), have significant implications for both atmospheric chemistry and climate change. Despite the crucial role of oceans in regulating their atmospheric budgets, our comprehension of their cycles in seawater remains insufficient. To address this gap, a field investigation was conducted in the western North Pacific to clarify the sources, sinks, and biogeochemical controls of these gases in two different marine environments, including relatively eutrophic Kuroshio-Oyashio extension (KOE) and oligotrophic North Pacific subtropical gyre. Our findings revealed higher concentrations of these gases in both seawater and the atmosphere in the KOE compared to the subtropical gyre. In the KOE, nutrient-rich upwelling stimulated rapid DMS biological production, while reduced seawater temperatures hindered the removal of OCS and CS2, leading to their accumulation. Furthermore, we have quantitatively evaluated the relative contribution of each pathway to the source and sink of DMS, OCS, and CS2 within the mixed layer and identified vertical exchange as a potential sink in most cases, transporting substantial amounts of these gases from the mixed layer to deeper waters. This research advances our understanding of sulfur gas source-sink dynamics in seawater, contributing to the assessment of their marine emissions and atmospheric budgets.

Amplicon sequencing with internal standards yields accurate picocyanobacteria cell abundances as validated with flow cytometry.

To understand ecosystem state and function, marine microbial ecologists seek measurements of organismal abundance and diversity at high taxonomic resolution. Conventional flow cytometry accurately estimates microbial cell abundance but only discerns broad groups with distinct optical properties. While amplicon sequencing resolves more comprehensive diversity within microbiomes, it typically only provides relative organismal abundances within samples, not absolute abundance changes. Internal genomic standards offer a solution for absolute amplicon-based measures. Here, we spiked genomic standards into plankton samples from surface seawater, gathered at 46-km intervals along a cruise transect spanning the southern California Current System and the oligotrophic North Pacific Subtropical Gyre. This enabled evaluation of the absolute volumetric gene copy abundances of 16S rRNA amplicon sequence variants (amplified with 515Y-926R universal primers, quantitatively validated with mock communities) and cell abundances of picocyanobacteria with known genomic 16S copy numbers. Comparison of amplicon-derived cell abundances of Prochlorococcus and Synechococcus with flow cytometry data from nearby locations yielded nearly identical results (slope = 1.01; Pearson's r = 0.9942). Our findings show that this amplicon sequencing protocol combined with genomic internal standards accurately measures absolute cell counts of marine picocyanobacteria in complex field samples. By extension, we expect this approach to reasonably estimate volumetric gene copies for other amplified taxa in these samples.

Geographical patterns of mesozooplankton functional diversity in the northwestern Pacific

To better understand how communities response to environmental change and how biodiversity affects ecosystem function, it is essential to quantify the functional diversity of communities. However, the current understanding of zooplankton functional traits and functional diversity in the northwestern Pacific (NWP) is inadequate. In this study, we analyzed the latitudinal changes in functional traits, diversities, and functional space of zooplankton in five biogeographic regions of the tropical and subtropical NWP. Zooplankton species richness varied unimodally over latitude, with the North Pacific Subtropical Gyre (NPSG) hosting the greatest number of species (367 taxa). The NWP was dominated by zooplankton of small and medium size (<2 mm; 72 %), current-feeding (47 %), omnivore–herbivore (71 %), and broadcast spawner (53 %). The functional trait composition of zooplankton varied significantly; for instance, omnivore–detritivores were prevalent in the Kuroshio Extension zone but few in the oligotrophic NPSG and North Equatorial Current regions. Protozoans with binary fission (primarily foraminifers) declined from the equator to high latitudes. The latitudinal variations of functional evenness, functional dispersion, and hypervolume of trait space of the zooplankton community were unimodal, with the maximum values occurring in the NPSG. This would suggest that the zooplankton community in the NPSG was more efficient in utilizing resources than in other regions, and the competition across functional groups was mild. Stochastic processes were crucial in zooplankton community assembly in tropical and subtropical NWP. This study enhanced our understanding of zooplankton community assembly and offered new insights into the functional traits of zooplankton communities in the NWP.

Genome Sequence of a Heterocystous Diazotroph Isolated from a Trichodesmium Consortium from the North Pacific Subtropical Gyre.

Diazotrophic cyanobacteria play a vital role in the nitrogen influx of the global marine ecosystem. In July 2010, colonies of Trichodesmium spp. were picked near Station ALOHA in the oligotrophic North Pacific Subtropical Gyre, and a novel heterocystous diazotroph (strain HetDA_MAG_MS3) belonging to the genus Rivularia was found living in close association; it was cultured and sequenced.

Microbial functional diversity across biogeochemical provinces in the central Pacific Ocean

Enzymes catalyze key reactions within Earth's life-sustaining biogeochemical cycles. Here, we use metaproteomics to examine the enzymatic capabilities of the microbial community (0.2 to 3 µm) along a 5,000-km-long, 1-km-deep transect in the central Pacific Ocean. Eighty-five percent of total protein abundance was of bacterial origin, with Archaea contributing 1.6%. Over 2,000 functional KEGG Ontology (KO) groups were identified, yet only 25 KO groups contributed over half of the protein abundance, simultaneously indicating abundant key functions and a long tail of diverse functions. Vertical attenuation of individual proteins displayed stratification of nutrient transport, carbon utilization, and environmental stress. The microbial community also varied along horizontal scales, shaped by environmental features specific to the oligotrophic North Pacific Subtropical Gyre, the oxygen-depleted Eastern Tropical North Pacific, and nutrient-rich equatorial upwelling. Some of the most abundant proteins were associated with nitrification and C1 metabolisms, with observed interactions between these pathways. The oxidoreductases nitrite oxidoreductase (NxrAB), nitrite reductase (NirK), ammonia monooxygenase (AmoABC), manganese oxidase (MnxG), formate dehydrogenase (FdoGH and FDH), and carbon monoxide dehydrogenase (CoxLM) displayed distributions indicative of biogeochemical status such as oxidative or nutritional stress, with the potential to be more sensitive than chemical sensors. Enzymes that mediate transformations of atmospheric gases like CO, CO2, NO, methanethiol, and methylamines were most abundant in the upwelling region. We identified hot spots of biochemical transformation in the central Pacific Ocean, highlighted previously understudied metabolic pathways in the environment, and provided rich empirical data for biogeochemical models critical for forecasting ecosystem response to climate change.

The dynamic trophic architecture of open-ocean protist communities revealed through machine-guided metatranscriptomics

Intricate networks of single-celled eukaryotes (protists) dominate carbon flow in the ocean. Their growth, demise, and interactions with other microorganisms drive the fluxes of biogeochemical elements through marine ecosystems. Mixotrophic protists are capable of both photosynthesis and ingestion of prey and are dominant components of open-ocean planktonic communities. Yet the role of mixotrophs in elemental cycling is obscured by their capacity to act as primary producers or heterotrophic consumers depending on factors that remain largely uncharacterized. Here, we develop and apply a machine learning model that predicts the in situ trophic mode of aquatic protists based on their patterns of gene expression. This approach leverages a public collection of protist transcriptomes as a training set to identify a subset of gene families whose transcriptional profiles predict trophic mode. We applied our model to nearly 100 metatranscriptomes obtained during two oceanographic cruises in the North Pacific and found community-level and population-specific evidence that abundant open-ocean mixotrophic populations shift their predominant mode of nutrient and carbon acquisition in response to natural gradients in nutrient supply and sea surface temperature. Metatranscriptomic data from ship-board incubation experiments revealed that abundant mixotrophic prymnesiophytes from the oligotrophic North Pacific subtropical gyre rapidly remodeled their transcriptome to enhance photosynthesis when supplied with limiting nutrients. Coupling this approach with experiments designed to reveal the mechanisms driving mixotroph physiology provides an avenue toward understanding the ecology of mixotrophy in the natural environment.

Optical determinations of photophysiology along an ecological gradient in the North Pacific Ocean

AbstractPhotosynthesis acts as a fundamental control in the cycling of biologically reactive elements in the ocean. Modeling photosynthesis requires an understanding of its response to light, specifically the maximum rate of photosynthesis per photon absorbed and the irradiance level at which it becomes light‐saturated (Ek), though field measurements of these parameters are both time and labor intensive. As absorbed light either drives photosynthesis, is re‐emitted as fluorescence, or is converted to heat, fluorescence can be related to the photosynthetic response to light in that, as light increases, there exists an inflection point where the probability that excess absorbed energy is dissipated as heat increases and fluorescence yield is decreased. Accordingly, we use a combination of in vivo chlorophyll fluorescence, particulate matter absorption spectra, and photosynthetically active radiation measurements to approximate this inflection irradiance (termed EFT) and relate it to modeled Ek along a transect from the oligotrophic North Pacific Subtropical Gyre to the edge of the more eutrophic subpolar gyre (~ 45°N). We find that EFT declines by a factor of 4× from values of 200–300 μmol photons m−2 s−1 in the oligotrophic gyre to 50–100 μmol photons m−2 s−1 north of the transition zone and correlates well with Ek from traditional data and models. This latitudinal pattern is associated with changes in biomass concentrations and the phytoplankton carbon to chlorophyll ratio, as well as with changes in particulate carbon to nitrogen ratios. Collectively, these results demonstrate a promising framework to capture high‐resolution variability in a key photosynthetic parameter.

Micro-size plankton abundance and assemblages in the western North Pacific Subtropical Gyre under microscopic observation.

While primary productivity in the oligotrophic North Pacific Subtropical Gyre (NPSG) is changing, the micro-size plankton community has not been evaluated in the last 4 decades, prompting a re-evaluation. We collected samples over three years (2016-2018) from depths of 10 to 200 m (n = 127), and the micro-size plankton were identified and counted to understand the heterogeneity of micro-size plankton community structure. The assemblages were consistent to the those of 4 decades ago. Dinophyceae (dinoflagellates) were the most numerically abundant, followed by Cryptophyceae and Bacillariophyceae (diatoms). The other micro-size plankton classes (Cyanophyceae, Haptophyceae, Dictyochophyceae, Euglenophyceae, and Prasinophyceae) were not always detected, whereas only Trichodesmium spp. was counted in the Cyanophyceae. Other unidentified autotrophic and heterotrophic flagellates were also significantly present, and their numeric abundance was higher than or at the same level as was that of the Dinophyceae. In the Dinophyceae, Gymnodiniaceae and Peridiniales were abundant. The chlorophyll a concentration and these class-level assemblages suggested micro-size plankton is not a major primary producer in this area. We applied generalized additive models (GAMs) and principal coordination analyses (PCoAs) to evaluate the habitats of every plankton group and the heterogeneity of the assemblages. The GAMs suggested that every classified plankton abundance showed a similar response to salinity, and we observed differences in habitats in terms of temperature and nitrate concentrations. Based on the PCoAs, we observed unique communities at the 200 m depth layer compared with those at the other sampling layers. The site scores of PCoAs indicated that the micro-size plankton assemblages are most heterogeneous at the 10 m depth layer. At such depth, diazotrophic Cyanophyceae (Trichodesmium spp.) are abundant, particularly in less-saline water. Therefore, nitrogen fixation may contribute to the heterogeneity in the abundance and assemblages in the western NPSG.

Come rain or shine: Depth not season shapes the active protistan community at station ALOHA in the North Pacific Subtropical Gyre

Protists are extremely diverse morphologically and physiologically, and they play important ecological roles at multiple trophic levels as primary producers and consumers in nearly all microbial communities. In spite of their fundamental importance in marine ecosystems, protistan diversity and distribution has yet to be comprehensively characterized in much of the world ocean, particularly in the vast expanses below the euphotic zone. We examined protistan community structure and species diversity in the oligotrophic North Pacific Subtropical Gyre. Our primary goal was to better characterize the breadth of metabolically active protistan species throughout the water column spanning 12 depths from 5 m to 770 m (~600 m below the euphotic zone) across three seasons using 18S rRNA gene V4 amplicon sequencing of RNA (cDNA).Protistan community structure changed markedly across relatively narrow ranges of increasing depths between 75 m-100 m, and again between 175 m-300 m. Changes were driven by depth-specific distributions among major protistan taxa associated with the upper mixed layer, deep chlorophyll maximum and aphotic zone, respectively, in this permanently stratified water column. Diatoms and some heterotrophic protists (MAST, choanoflagellates) were important contributors in the upper mixed layer, while haptophytes and pelagophytes increased in relative abundances in the lower euphotic zone. Radiolaria, ciliates and syndinians (putative parasitic protists within the order Syndiniales) increased in relative abundances below the euphotic zone. Overall, the highest taxonomic richness of amplicon sequence variants (ASVs) was observed in aphotic samples. Additionally, the dominant ASVs within some taxonomic groups that were observed in deep water were different than those observed in the upper water column, implying adaptations to specific depth strata rather than passive transport of surface-dwelling species. In contrast to depth-related changes, seasonal changes in protistan community structure were not significant.

Ecosystem implications of fine-scale frontal disturbances in the oligotrophic ocean – An idealized modeling approach

In the subtropical ocean, physical and biogeochemical patchiness exists across a range of spatial and temporal scales. As ocean surface temperatures rise, climate models suggest that the strength of the basin-scale biological carbon pump may diminish due to increased stratification and depleted surface nutrients. However, global model predictions often cannot account for climate-physical-ecosystem interactions occurring at finer spatial scales (e.g., less than a few tens of kilometers). In order to study such fine-scale interactions, we developed a computationally tractable, idealized modeling framework that allows for a systematic assessment of the impact of fine-scale frontal disturbances (i.e., short-lived upwelling and downwelling events) on biogeochemical and ecosystem dynamics. The model successfully captured both the mean dynamics and the range of ecosystem variability observed at the Hawaii Ocean Time-series (HOT) site in the oligotrophic North Pacific Subtropical Gyre. Fine-scale frontal disturbances were shown to impact both phytoplankton assemblages and carbon cycling. Specifically, disturbances of shorter duration but higher intensity favored the growth of large phytoplankton and contributed to elevated carbon export efficiency. However, for disturbances of longer duration and higher intensity, a decoupling between physical changes and biological responses led to a reduced utilization efficiency of upwelled nutrients and thus a reduction in new production. Our results emphasize that future changes in both large and fine-scale physical dynamics may play an important role in shaping marine ecosystems, and could be accounted for in the next-generation global climate models using this type of parameterization.

Cell sorting reveals few novel prokaryote and photosynthetic picoeukaryote associations in the oligotrophic ocean.

SummaryClose associations between single‐celled marine organisms can have a central role in biogeochemical processes and are of great interest for understanding the evolution of organisms. The global significance of such associations raises the question of whether unidentified associations are yet to be discovered. In this study, fluorescence‐activated cell sorted photosynthetic picoeukayote (PPE) populations and single cells were analysed by sequencing of 16S rRNA genes in the oligotrophic North Pacific Subtropical Gyre. Samples were collected during two cruises, spanning depths near the deep chlorophyll maximum, where the abundance of PPEs was highest. The association between the widespread and significant nitrogen (N2)‐fixing cyanobacterium, UCYN‐A and its prymnesiophyte host was prevalent in both population and single‐cell sorts. Several bacterial sequences, affiliating with previously described symbiotic taxa were detected but their detection was rare and not well replicated, precluding identification of novel tightly linked species‐specific associations. Similarly, no enrichment of dominant seawater taxa such as Prochlorococcus, SAR11 or Synechococcus was observed suggesting that these were not systematically ingested by the PPE in this study. The results indicate that apart from the UCYN‐A symbiosis, similar tight species‐specific associations with PPEs are unusual in the oligotrophic ocean.

Plasticity in the grazing ecophysiology of Florenciella (Dichtyochophyceae), a mixotrophic nanoflagellate that consumes Prochlorococcus and other bacteria

AbstractMixotrophic nanoflagellates can account for more than half of the bacterivory in the sunlit ocean, yet very little is known about their ecophysiology. Here, we characterize the grazing ecology of an open‐ocean mixotroph in the genus Florenciella (class Dictyochophyceae). Members of this class were indirectly implicated as major consumers of Prochlorococcus and Synechococcus in the oligotrophic North Pacific Subtropical Gyre, but their phagotrophic capabilities have never been investigated. Our studies showed that Florenciella readily consumed Prochlorococcus, Synechococcus, and heterotrophic bacteria, and that the ingested prey relieved nutrient limitations on growth. Florenciella grew faster (3 d−1) in nitrogen‐deplete medium given sufficient live Synechococcus, than in nitrogen‐replete K medium (2 d−1), but it did not grow in continuous darkness. Grazing rates were substantially higher under nutrient limitation and showed a hint of diel variability, with rates tending to be highest near the end of the light period. An apparent trade‐off between the maximum clearance rate (5 nL Florenciella−1 h−1) and the maximum ingestion rate (up to ∼ 10 prey cells Florenciella−1 h−1) across experiments suggests that grazing behavior may also vary in response to prey concentration. If the observed grazing rates are representative of other open‐ocean mixotrophs, their collective activity could account for a significant fraction of the daily cyanobacterial mortality. This study provides essential parameters for understanding the grazing ecology of a common marine mixotroph and the first characterization of mixotrophic nanoflagellate functional responses when feeding on unicellular cyanobacteria, the dominant marine primary producers in the oligotrophic ocean.

Sinking Versus Suspended Particle Size Distributions in the North Pacific Subtropical Gyre

AbstractThe particle size distribution (PSD) is a fundamental property that influences all aspects of phytoplankton ecology. In particular, the size (e.g., diameter d [μm]) and sinking speed w (m/day) of individual particles are inextricable, but much remains unknown about how d and w are related quantitatively for bulk particulate matter. There is significant interest in inferring sinking mass fluxes from PSDs, but doing so requires knowing how both mass and w scale with d. To this end, using both laser diffraction and imaging, we characterized for the first time both sinking and suspended PSDs in the oligotrophic North Pacific subtropical gyre. Comparing these PSDs via a power law parameterization indicates an approximately linear w‐to‐d scaling, suggesting particles are more fractal‐like than sphere‐like in this respect. This result is robust across multiple instruments, depths, and sediment trap deployments and is made comparatively precise by a high degree of replication.

Anomalous wind triggered the largest phytoplankton bloom in the oligotrophic North Pacific Subtropical Gyre

In summer 2010, a massive bloom appeared in the middle (16–25°N, 160–200°E) of the North Pacific Subtropical Gyre (NPSG) creating a spectacular oasis in the middle of the largest oceanic desert on Earth. Peaked in June 2010 covering over two million km2 in space, this phytoplankton bloom is the largest ever recorded by ocean color satellites in the NPSG over the period from 1997 to 2013. The initiation and mechanisms sustaining the massive bloom were due to atmospheric and oceanic anomalies. Over the north (25–30°N) of the bloom, strong anticyclonic winds warmed sea surface temperature (SST) via Ekman convergence. Subsequently, anomalous westward ocean currents were generated by SST meridional gradients between 19°N and 25°N, producing strong velocity shear that caused large number of mesoscale (100-km in order) cyclonic eddies in the bloom region. The ratio of cyclonic to anticyclonic eddies of 2.7 in summer 2010 is the highest over the 16-year study period. As a result of the large eddy-number differences, eddy-eddy interactions were strong and induced submesoscale (smaller than 100 km) vertical pumping as observed in the in-situ ocean profiles. The signature of vertical pumping was also presented in the in-situ measurements of chlorophyll and nutrients, which show higher concentrations in 2010 than other years.

Effects of nutrient enrichment on surface microbial community gene expression in the oligotrophic North Pacific Subtropical Gyre.

Marine microbial communities are critical for biogeochemical cycles and the productivity of ocean ecosystems. Primary productivity in the surface ocean is constrained by nutrients which are supplied, in part, by mixing with deeper water. Little is known about the time scales, frequency, or impact of mixing on microbial communities. We combined in situ sampling using the Environmental Sample Processor and a small-scale mixing experiment with lower euphotic zone water to determine how individual populations respond to mixing. Transcriptional responses were measured using the MicroTOOLs (Microbiological Targets for Ocean Observing Laboratories) microarray, which targets all three domains of life and viruses. The experiment showed that mixing substantially affects photosynthetic taxa as expected, but surprisingly also showed that populations respond differently to unfiltered deep water which contains particles (organisms and detritus) compared to filtered deep water that only contains nutrients and viruses, pointing to the impact of biological interactions associated with these events. Comparison between experimental and in situ population transcription patterns indicated that manipulated populations can serve as analogs for natural populations, and that natural populations may be frequently or continuously responding to nutrients from deeper waters. Finally, this study also shows that the microarray approach, which is complementary to metatranscriptomic sequencing, is useful for determining the physiological status of in situ microbial communities.

Nitrogen fixation rates diagnosed from diurnal changes in elemental stoichiometry

AbstractThe carbon, nitrogen, and phosphorus (C, N, and P, respectively) composition and elemental ratios were measured in the 20–200 μm size fraction during July 2015 in the surface waters of an anticyclonic eddy encountered north of Hawaii in the oligotrophic North Pacific Subtropical Gyre. The observed particulate N : P ratio fluctuated by approximately a factor of two over the diel cycle. We present a simple mathematical argument connecting this change to a rate of biological nitrogen fixation, and calculate the nitrogen fixation rate to be ≥ 13 nmol L−1 d−1 for this size class. This value is higher than simultaneous bottle‐incubation based rates measured with isotopic tracers, yet is consistent with historic rate measurements from the region. As confirmation of our methods, diurnal changes in C : N : P of laboratory cultures of the diazotrophic genus Trichodesmium were measured. In the laboratory, we show that estimates of nitrogen fixation from stoichiometric time series are equivalent to those derived directly from mass balance. The disparity between nitrogen fixation rates derived from tracer measurements and particulate stoichiometry in the field suggests that large diazotrophs may be underestimated in small volume (∼ 4 L) bottle incubations as a result of either spatial heterogeneity or vertical migration of large cells. Otherwise, processes other than diazotrophy must cause the observed changes in stoichiometry. This approach represents a novel and scalable means of quantifying in situ nitrogen fixation rates from diurnal changes in size‐fractionated stoichiometry. We also infer carbon fixation, growth rates, and phosphorus uptake in the 20–200 μm size class.

Decoupling between bacterial production and primary production over multiple time scales in the North Pacific Subtropical Gyre

We measured rates of 3H-leucine (3H-Leu) incorporation, as a proxy for bacterial production, at Station ALOHA (22°45′N, 158°W) in the oligotrophic North Pacific Subtropical Gyre (NPSG). We report measurements conducted between January 2011 and April 2013, examining variability in 3H-Leu incorporation over diel, daily, and monthly time scales. Rates of 3H-Leu were evaluated in the context of contemporaneous 14C-based primary productivity (14C-PP) to identify potential temporal coupling between these measures of productivity. Throughout the upper ocean (0–125m), rates of 3H-Leu incorporation measured in the light (3H-LeuLight) were stimulated (1.5-fold, on average) relative to measurements in the dark (3H-LeuDark). At monthly scales, rates of 3H-LeuLight and 3H-LeuDark varied 4.9-fold and 3.8-fold, respectively, while rates of 14C-PP varied 1.7-fold. Rates of 14C-PP were often elevated during summer months (May through August) when incident light flux was greatest, while rates of both 3H-LeuLight and 3H-LeuDark often peaked in early fall (August through October) when seawater temperatures were maximal. Near-daily measurements of 3H-Leu incorporation and 14C-PP conducted over a 62-day period in the summer of 2012 revealed that rates of 3H-LeuLight and 3H-LeuDark varied ~2.5 and 2.0-fold, respectively, similar to ~1.8-fold daily variability observed in rates of 14C-PP. Over diel time scales, rates of 3H-LeuLight and 3H-LeuDark demonstrated different patterns, with rates of 3H-LeuLight elevated at mid-day and rates of 3H-LeuDark greatest in the early evening. Together, these results suggest that in this oligotrophic ecosystem, photosynthetic production of organic matter and bacterial production can be temporally uncoupled across daily to seasonal scales.

Application of membrane inlet mass spectrometry to measure aquatic gross primary production by the18O in vitro method

The 18O technique is considered the most direct in vitro method for measuring gross primary production (GPP) in aquatic ecosystems. This method measures the 18O enrichment of the dissolved O2 pool through photosynthesis after spiking a water sample with a tracer amount of 18O-labeled water (18O-H2O) and incubating it under natural light conditions. Despite its advantages, the 18O technique has only scarcely been used to measure GPP in the ocean. The lack of 18O-based primary productivity measurements is most likely due to the technical difficulty associated with sample collection, handling, and processing, and to the need of an isotope ratio mass spectrometer (IRMS) for sample analysis, which is not available for the majority of research groups. The current procedure also precludes at sea measurements. In this manuscript, we demonstrate that the biological 18O enrichment of dissolved O2, after incubation of seawater enriched with 18O-H2O, can be precisely measured by shipboard or laboratory-based membrane inlet mass spectrometry (MIMS). The method was validated in the low-productivity oligotrophic North Pacific Subtropical Gyre, where the measured GPP ranged from 0.2 to 1.1 μmol O2 L−1 d−1, with an approximate precision for surface waters of ± 0.02 μmol O2 L−1 d−1. This new approach has the advantages of simple water sample handling and analysis, accurate dissolved gas measurements, capability of analysis on board of a ship, and use of relatively inexpensive instrumentation, and therefore has the potential to improve our understanding of primary production in the ocean and other aquatic environments.