Open Ocean Stations Research Articles

Biogenic and lithogenic silicon along the GEOTRACES south West Indian Ocean section (SWINGS-GS02) and the islands mass effect on regional Si biogeochemical cycle

The distribution and cycling of biogenic silica (BSi) and lithogenic silicon (LSi) in the ocean play crucial roles in the global silicon cycle and marine ecosystem dynamics. This is especially the case in the Southern Ocean where diatoms constitute the predominant phytoplankton and participate in a major way to the biological carbon pump. This study presents an assessment of BSi and LSi concentrations along the GEOTRACES South West Indian Ocean Section (SWINGS, late austral summer 2021), where several and contrasting regions were encountered: oligotrophic Mozambique basin, HNLC (High Nutrient Low Chlorophyll) areas and regions fertilized by the Subantarctic islands. Suspended particles were sampled from Niskin bottles and in situ pumps, along with scanning electron microscope (SEM) observations and specific pigments measurements to support BSi and LSi analyses. With samples coming from a contrasting study area prone to diverse continental influences, our BSi and LSi results showed a reproducibility of 13 ± 7%, in the same range as the established protocol. BSi concentrations show a north-south gradient with maxima encountered in the Antarctic Zone, and contrasted results between HNLC open ocean areas and naturally fertilized regions in the vicinity of the Subantarctic islands. Some open ocean stations have unusually high BSi (e.g. > 5 μmol L−1) likely resulting from fertilization by aerosols, upwelling or island mass effect when they are downstream of the islands. Coupling of BSi with SEM observations and pigments measurements respectively showed diatoms were the most representative of the carrying phase of BSi and suggested silicification changes, induced either by heavily silicified diatoms or by micronutrient limitation in HNLC regions. BSi is often dominated by the smallest size fraction (0.45–5 μm) which represent 47 ± 23% of the total BSi based on 29 measurements on size fractionated samples. LSi results highlighted atmospheric inputs at the surface and nepheloid layers in the water column, which makes LSi overall a good indicator of the origin of lithogenic materials. SEM observations supported these results, enabling characterization of the diversity of lithogenic materials in the vicinity of the Subantarctic islands, more specifically volcanic ash around Heard Island, and within the nepheloid layers.

Spatial and temporal variation of dissolved iodine in the eastern Arabian Sea

Here we report the seasonal and spatial variation of iodide (I−) and total dissolved iodine (TI) along a transect covering the oxygen minimum zone (OMZ) in the Arabian Sea (AS). This data is generated after a gap of more than two decades since the US-Joint Global Ocean Flux Study (JGOFS) reported observations of iodine speciation in the region. Also, this is the first study to report on iodine speciation from the coastal region of the west coast of India. Here the water samples were collected along the meridional transect (68°E) during spring inter-monsoon (SIM), southwest monsoon (SWM), and fall inter-monsoon (FIM), and from the coastal Goa transect during late SWM and FIM. In addition, sediment porewater was also collected from the coastal Goa transect during late SWM. Iodide was found to be dominant in the OMZ, whereas TI was majorly represented by iodate below the OMZ. The I− and TI concentrations were higher around the 15°N stations during all three seasons sampled. Seasonal variability was observed in the study, with high I− and TI during the SWM and FIM as compared to SIM. The coastal region was observed to have high I− values during seasonal anoxia in FIM as compared to the late SWM. This may be due to the release of I− from the sediments which are extremely rich in I−. The porewater showed extremely high I− (up to 4545 nM) at the shelf stations, which is 10 times higher than what is measured in the overlying waters. The US-JGOFS study reported iodide values as high as 953 nM in the AS OMZ with excess iodine (difference between the measured TI and the predicted TI) of 400 nM and attributed this to the input from the shelf sediments. In the present study TI values up to 811 nM were observed in the AS. To ascertain the source of excess iodine, porewater extracted from the sediments covering shelf and open ocean stations along the Goa transect was studied for iodine speciation. The diffusive flux of I− calculated from the surface sediments ranged between 14 and 43 μmol I− m−2 d−1 from the shelf stations off Goa, which is high enough to explain the excess iodine concentrations seen in the OMZ. Apart from the seasonality observed in iodine concentrations, our study infers that the shelf sediments along the west coast of India act as a source of iodine to the AS OMZ.

Fragility of marine photosynthesis

Ecosystem fragility is an often used term in oceanography yet to this day it lacks a precise and widely accepted definition. Defining and subsequently quantifying fragility would be of great value, for such measures could be used to objectively ascertain the level of risk marine ecosystems face. Risk assessments could further be used to define the level of protection a given ocean region requires from economic activity, such as fisheries. With this aim we introduce to the oceanographic literature the concepts of marginal production and fragility, which we define for marine photosynthesis, the base of the oceanic food web. We demonstrate that marine photosynthesis is always fragile with respect to light, implying variability in surface irradiance acts unfavourably on biomass. We also demonstrate that marine photosynthesis can be both fragile and antifragile with respect to the mixed-layer depth, implying variability in mixed-layer depth can act both favourably and unfavourably on biomass. Quantification of marginal production and fragility is presented on data from two open ocean stations: Hawaii Ocean Time Series and Bermuda Atlantic Time-Series Study. Seasonal cycle of biomass is modelled and the effects of primary production fragility are analysed. A new tipping point for marine phytoplankton is identified in the form of a depth horizon. Using the new definitions presented here a rich archive of data can be used straightforwardly to quantify primary production fragility. The definitions can also be used to predict when primary production enters the fragile state during the seasonal cycle.

Microbial community shifts along an estuarine to open ocean continuum

The water bodies along a continuum from estuaries to open ocean display significant changes in their physical, chemical and biological characteristics. In the current study, we report the influence of such changes on the abundance and community structure of bacteria and archaea in a continuum from Cochin Estuary, in the southwest coast of India, to the open ocean of Arabian Sea. The bacterial and archaeal abundances were in the range of 104 to 106 and 103 to 105 cells ml−1 respectively in all the stations and showed a clear dependence on nutrients. The bacterial abundance decreased with depth in the open ocean, whereas the archaea showed a similar reduction at both shelf and open ocean stations. The α- Proteobacteria (35 %) and Cyanobacteria (23 %) were dominant in the estuary. More than 70 % of the community of bacteria on the shelf and in the open ocean were similar. The α- and γ-Proteobacteria contributed 37 & 27 % of the OTUs in the clone library of shelf and 20 & 29 % in the open ocean. The archaeal community structure in the open ocean and shelf were similar (90 %) composed mostly of Nitrosopumilaceae (70%–75%), while they were highly diverse in the estuary. In short, our study shows that the archaea are more diverse in the estuary compared to shelf and open ocean, while bacteria are equally diverse in all ecosystems.

The Influence of pCO2-Driven Ocean Acidification on Open Ocean Bacterial Communities during A Short-Term Microcosm Experiment in the Eastern Tropical South Pacific (ETSP) off Northern Chile.

Due to the increasing anthropogenic CO2 emissions, Ocean Acidification (OA) is progressing rapidly around the world. Despite the major role that microorganisms play on the marine biogeochemical cycling and ecosystem functioning, the response of bacterial communities upon OA scenarios is still not well understood. Here, we have conducted a detailed characterization of the composition and relative abundance of bacterial communities in the water column of an open-ocean station in the Eastern Tropical South Pacific (ETSP) off northern Chile and their interactions with environmental factors. In addition, through a short-term microcosm experiment, we have assessed the effect of low pH/high pCO2 conditions over the abundance and genetic diversity of bacterial communities. Our results evidence a clear partitioning of community composition that could be attributed mostly to dissolved oxygen. However, our experimental approach demonstrated that low pH/high pCO2 conditions might modify the structure of the bacterial community, evidencing that small changes in pH may impact significantly the abundance and diversity of key microorganisms. This study constitutes a first step aiming to provide insight about the influence of changing carbonate chemistry conditions on natural bacterial communities and to shed light on the potential impact of OA in biogeochemical cycles on the ETSP region.

Surface cyclopoid copepod assemblages in relation to environmental conditions near undisturbed coral reefs of Minicoy, southeastern Arabian Sea, a part of the Indian Ocean

AbstractThe cruise conducted in the spring inter‐monsoon (pre‐monsoon) of April 2015 in the frame of a funded project epitomized an open ocean survey that allowed procuring a quasi‐synoptic picture of cyclopoid copepod community structure in 18 open ocean stations of the Minicoy Island, Lakshadweep archipelago, off the southwestern Indian coast, based on the analysis of standing stock and composition in the integrated 0–10 m water column. The main objective was to explore the community structure of cyclopoid copepods prevailing here and which environmental variable influences the same. It is witnessed that sea surface temperature (SST) (30.42 ± 0.19°C), sea surface salinity (SSS) (33.56 ± 1.15 psu), and dissolved oxygen (DO) (4.32 ± 0.06 ml/L) with SSS and DO are the best matching variables diversifying cyclopoid copepod species (av. abundance 700 ± 386 no. of individuals/100 m3). Average zooplankton biomass (0.03 ml/m3) and abundance (8,989 ± 3,866 no. of individuals/100 m3) were also observed. Fifty‐one cyclopoid copepod species belonging to four families and seven genera were identified, with the dominance of high saline species such as Sapphirina, Copilia, Farranula, and Oncaea. An abundance biomass curve (ABC) plot analysis indicated an undisturbed community with normal environmental conditions. TAXDTEST (taxonomic distinctness) depicted a diverse condition where all points clustered together within a 95% confidence level. Our results indicate that the cyclopoid copepod assemblage near Minicoy Island is a diverse, undisturbed community and hypothesize that the stable environmental conditions during pre‐monsoon (spring inter‐monsoon) preferred the diversification of cyclopoid copepods with the prevalence of high saline species. The data evolved could be used as a standard reference/benchmark to detect any deviation from an undisturbed/diverse community status of Minicoy Island in the looming scenario of climate change in and around the Indian Ocean.

Stratospheric Incursion as a Source of Enhancement of the Isotopic Ratios of Atmospheric N2O at Western Pacific

AbstractWe present analyses of nitrogen and oxygen isotopic ratios and site‐preference (SP) values of atmospheric nitrous oxide samples obtained from Taiwan in west Pacific. Air samples were collected during September 2013 to January 2016 from Taipei and during November 2011 to December 2014 from Keelung, located in the northeastern coast of Taiwan. The average values of δ15N, δ18O, and SP are 6.4 ± 0.3 ‰, 44.0 ± 0.4 ‰, and 17.1 ± 1.0 ‰, respectively, at Taipei and 6.4 ± 0.2 ‰, 44.6 ± 0.4 ‰, and 17.9 ± 1.3 ‰, for δ15N, δ18O, and SP respectively, at Keelung, comparable to the average values (6.6 ± 0.2, 44.2 ± 0.2, and 18.1 ± 0.5 ‰, scaled to year 2014) from Hateruma, an open ocean station in western Pacific. Strong temporal variations in the delta values, however, are seen. During a subtropical jet‐strengthening period in October to mid‐December 2014, the δ15N, δ18O and SP values increased at the rate of 0.006 ± 0.002 ‰/day, 0.010 ± 0.002 ‰/day and 0.008 ± 0.002 ‰/day, respectively at Taipei and 0.013 ± 0.005 ‰/day, 0.011 ± 0.005 ‰/day and 0.011 ± 0.027 ‰/day at Keelung. This suggests intrusion of air to the lower troposphere from the stratosphere where δ15N, δ18O and SP values are higher. This is consistent with earlier observations of oxygen isotope anomaly in atmospheric CO2 and elevated 35S in sulfate aerosols, indicating frequent stratospheric intrusions in this region. The observed δ15N and δ18O values, along with the oxygen isotope anomaly in CO2 and multiple sulfur isotopes in sulfate, can be used to constrain local emissions from the surface, improving our knowledge of anthropogenic contribution to the global budget of N2O.

Sea surface phytoplankton community response to nutrient and light changes

The sea surface microlayer (SML) is the boundary layer between the ocean and the atmosphere and plays a unique role in marine biogeochemistry. Phytoplankton communities in this uppermost surface layer are exposed to extreme ultraviolet (UV) radiation and potentially high nutrient supplies. In order to understand the response of SML communities to such contrasting conditions, we conducted experiments at three different sites, the North Sea (open ocean) and two sites, outer and middle fjord, in the Sognefjord, Norway, with differing physical and chemical parameters. We manipulated light, nitrogen (N) and phosphorus (P) supply to natural communities collected from the SML and compared their response to that of the underlying water (ULW) communities at 1-m depth. Phytoplankton communities in both SML and ULW responded significantly to N addition, suggesting the upper 1-m surface phytoplankton communities were N-limited. While phytoplankton growth rates were higher with high N and high light supply, biomass yield was higher under low light conditions and with a combined N and P supply. Furthermore, biomass yield was generally higher in the ULW communities compared to SML communities. Nutrient and light effects on phytoplankton growth rates, particulate organic carbon (POC) and stoichiometry varied with geographical location. Phytoplankton growth rates in both SML and ULW at the open ocean station, the site with highest salinity, did not respond to light changes, whereas the communities in the middle fjord, characterized by high turbidity and low salinity, did experience light limitation. This work on the upper surface phytoplankton communities provides new insights into possible effects of coastal darkening and increases understanding of oceanic biogeochemical cycling.

Close Encounters - Microplastic availability to pelagic amphipods in sub-Antarctic and Antarctic surface waters

This study investigated the distribution of plastic debris from the Atlantic portion of the Sub-Antarctic to the Antarctic Peninsula. This region is home to some of the highest concentrations of zooplankton biomass but is also threatened by increasing shipping traffic from fishing and the growing tourism market. Samples were collected using a surface-towed neuston net during the Austral summer 2018, aboard the RRS James Clark Ross. Using Fourier Transform Infrared Spectrometry it was found that 45.6% of the plastic particles isolated from seawater samples were sampling contamination, originating predominantly from the ship. Of the remaining particles, both low density (polyethylene, polypropylene) and high-density (phenoxy and epoxy resins) polymers were found in the surface water suggesting both long-range and local sources of origin. Whilst we found that micro and mesoplastic concentrations in seawater were significantly low (0.013 ± 0.005n/m3) compared to global averages, they were higher along the Antarctic Peninsula than the open ocean (Sub-Antarctic) stations. The potential availability of micro and mesoplastics (MP) to pelagic amphipods was explored, using an observed encounter rate (OER) and a possible encounter rate (PER). The total OER (0.8%) was higher than the PER (0.15%), suggesting that even at low concentrations, microplastics are encountered, and potentially consumed, by amphipods. This study highlights the need to prioritise regions of high zooplankton abundance and to investigate both water and biota to build up a picture of plastic pollution and its potential interaction with the Antarctic Ecosystem.

Metatranscriptomic identification of polyamine-transforming bacterioplankton in the Gulf of Mexico.

The importance of short-chained aliphatic polyamines (PAs) to bacterioplankton-mediated carbon and nitrogen cycles has been repeatedly proposed. However, bacterial taxa and genes involved in the transformations of different PA compounds and their potential spatial variations remain unclear. This study collected surface bacterioplankton from nearshore, offshore, and open ocean stations in the Gulf of Mexico and examined how metatranscriptomes responded to additions of three single PA model compounds (i.e. putrescine, spermidine, or spermine). Our data showed an overrepresentation of genes affiliated with γ-glutamylation and spermidine cleavage pathways in metatranscriptomes received PA amendments and the expression level of each pathway varied among different PA compounds and sampling locations. PA-transforming taxa were affiliated with Actinobacteria, Bacteroidetes, Cyanobacteria, Planctomycetes, and Proteobacteria and their relative importance was also compound and location specific. These findings suggest that PAs are transformed via multiple pathways and by a diversity of marine bacterioplankton in the Gulf of Mexico. The relative importance of different PA transforming pathways and composition of functional microbial communities may be regulated by nutrient status of local environments.

Coccolith mass and morphology of different Emiliania huxleyi morphotypes: A critical examination using Canary Islands material.

Different morphotypes of the abundant marine calcifying algal species Emiliania huxleyi are commonly linked to various degrees of E. huxleyi calcification, but few studies have been done to validate this assumption. This study investigated therefore whether E. huxleyi morphotypes can be related to coccolithophore calcification and coccolith mass. Samples from January (high productivity) and September (low productivity) 1997 at an open ocean and a coastal site near the Canary Islands were analysed using a combination of thickness measurements (Circular Polarizer Retardation estimates (CPR) method), Scanning Electron Microscope imaging, and Markov Chain Monte Carlo (MCMC) models. Mean E. huxleyi coccolith mass varied from a maximum of 2.9pg at the open ocean station in January to a minimum of 1.7pg in September at both stations. In contrast, overall calcite produced by E. huxleyi (assuming 23 coccoliths/cell) varied from a maximum of 2.6 μgL-1 at the coastal station in January to a minimum of 0.5 μgL-1 in September at the open ocean site. The relative abundance of "Overcalcified" Type A, Type A, Group B and malformed coccoliths was determined from SEM images. The mean coccolith mass of "Overcalcified" Type A was 2.0pg using the CPR-method, while mean mass of Type A and Group B coccoliths was determined using coccolith length measurements from SEM images and MCMC models relating thickness measurements to morphotype relative abundance. Type A cocccolith mass varied from a 1.6pg to 2.6pg and Group B coccolith mass varied from 1.5pg to 2.0pg. These results demonstrate that the coccolith mass of Type A, "Overcalcified" Type A, and Group B do not differ systematically and there is no systematic relationship between relative abundance of a morphotype and the overall calcite production of E. huxleyi. Therefore, morphotype appearance and relative abundance can not be uniformly used as reliable indicators of E. huxleyi calcification or calcite production.

The 210Po/210Pb method to calculate particle export: Lessons learned from the results of three GEOTRACES transects

The deviation from secular equilibrium between the natural radionuclide 210Po (half-life: 138.4 d) and its radioactive grandparent 210Pb (half-life: 22.3 y) has been used to examine particle export from the surface ocean. Here we combine 210Po and 210Pb activity results from three GEOTRACES transects: two transects of the North Atlantic Ocean (GA03: 15–40°N, and GA01: 40–60°N) and one transect of the South Pacific Ocean (GP16: 10–15°S), and estimate 210Po export fluxes at the base of the primary production zone (PPZ) by assuming steady state (SS) without advection or diffusion of the isotopes. The SS 210Po flux was sometimes lower at basin margins than at the open-ocean stations along the transects. High SS 210Po flux estimations derived in the North Atlantic subtropical gyre may be associated with the atmospheric deposition of 210Pb to the surface ocean. In this paper we also question the validity of the SS assumption and discuss the influence of vertical advection and diffusion on the overall 210Po activity balance. The SS model may have underestimated the export flux of 210Po at margin stations in the GA03 and GP16 transects and along the GA01 cruise track. We found that upwelling in the Peruvian coastal region and near the Greenland shelf had a dramatic impact on the estimated 210Po flux balance. Vertical diffusion had limited influence on the 210Po export fluxes along GA03 and GA01 in the North Atlantic whereas it added 210Po export fluxes by as much as 190% in GP16 in the Pacific, especially at the shelf stations 1 and 4. Further, analysis of the partitioning coefficient suggested the importance of small particles in the scavenging of radionuclides. This suggests it is wise to sample small particles along with large particles to determine the ratio of the concentration of particulate organic carbon (POC) to 210Po activity (POC/210Po) for the lower limit of POC export flux estimations. Finally, the observation of the deficit of 210Po relative to 210Pb activity (210Po/210Pb < 1) in seawater concurrent with a deficit of 210Po in particles contradicts our understanding of the conceptual 210Po flux model, which assumes that 210Po activity is more effectively removed from the surface ocean via particles than 210Pb activity. While this observation deserves more attention, we propose two possible solutions: (1) the deficit of total 210Po relative to 210Pb activity in the surface ocean may be due to an input of 210Pb activity instead of/concurrent with a relative removal of 210Po activity via particle export; or (2) the particles collected may not be identical to the ones that have originally created the observed deficit in total 210Po activity.

Single cell genomics yields a wide diversity of small planktonic protists across major ocean ecosystems

Marine planktonic protists are critical components of ocean ecosystems and are highly diverse. Molecular sequencing methods are being used to describe this diversity and reveal new associations and metabolisms that are important to how these ecosystems function. We describe here the use of the single cell genomics approach to sample and interrogate the diversity of the smaller (pico- and nano-sized) protists from a range of oceanic samples. We created over 900 single amplified genomes (SAGs) from 8 Tara Ocean samples across the Indian Ocean and the Mediterranean Sea. We show that flow cytometric sorting of single cells effectively distinguishes plastidic and aplastidic cell types that agree with our understanding of protist phylogeny. Yields of genomic DNA with PCR-identifiable 18S rRNA gene sequence from single cells was low (15% of aplastidic cell sorts, and 7% of plastidic sorts) and tests with alternate primers and comparisons to metabarcoding did not reveal phylogenetic bias in the major protist groups. There was little evidence of significant bias against or in favor of any phylogenetic group expected or known to be present. The four open ocean stations in the Indian Ocean had similar communities, despite ranging from 14°N to 20°S latitude, and they differed from the Mediterranean station. Single cell genomics of protists suggests that the taxonomic diversity of the dominant taxa found in only several hundreds of microliters of surface seawater is similar to that found in molecular surveys where liters of sample are filtered.

Ecological diversity of planktonic protists in spatial regimes of the Arabian Sea revealed through next-generation sequencing

Eukaryotic microbes inhabiting diverse ecological niches are capable of mediating biogeochemical shifts. Here, we studied the distribution patterns of protistan community in oxygen-deficient sites in the Arabian Sea and nearby estuarine waters. Protist diversity was quantified through Illumina Miseq sequencing of the V4 region of 18S rRNA gene amplicons. Overall, 12687 OTUs represented the diverse protist communities at various sampling sites such as the open ocean, outer shelf and inner shelf along the oxygen gradient. As per Alpha diversity estimation, estuarine communities were less diverse than the coastal, and open ocean sites. Multivariate analysis was applied to differentiate the community structure in estuarine, coastal and open ocean sites. The results indicated distinct community variation between oxic, hypoxic and suboxic water column at a comparatively deep sea station. However, the influence of dissolved oxygen was statistically insignificant for the protist distribution. The DistLM analysis suggests that the adaptation of protist communities across the spatial habitats could be significantly correlated with temperature, salinity, and nitrate. Moreover, chlorophyll a was the important environmental variable associated with the estuarine complex, whereas salinity, nitrate, and temperature influenced coastal and open ocean stations.

Water Mass Control on Phytoplankton Spatiotemporal Variations in the Northeastern East China Sea and the Western Tsushima Strait Revealed by Lipid Biomarkers

AbstractContinental margin ecosystems in the western North Pacific Ocean are subject to strong climate forcing and anthropogenic impacts. To evaluate mechanisms controlling phytoplankton biomass and community structure variations in marginal sea‐open ocean boundary regions, brassicasterol, dinosterol, and C37 alkenones were measured in suspended particles in summer and autumn from 2012 to 2013 in the northeastern East China Sea and the western Tsushima Strait. In summer, the concentrations of brassicasterol (40–1535 ng/L) and dinosterol (4.2–94 ng/L) were higher in the southwest of Cheju Island, while C37 alkenones (0–30 ng/L) were higher in the south of Cheju Island. In autumn, brassicasterol (12–106 ng/L), dinosterol (2.4–21 ng/L), and C37 alkenones (0.7–7.0 ng/L) were higher in the southwest of Cheju Island and the western Tsushima Strait, and higher C37 alkenones also occurred in the Okinawa Trough. Correlation analysis of biomarkers and environmental conditions (temperature, salinity, and inorganic nutrient concentrations) clearly demonstrated that phytoplankton biomass and community structure variations can be well elucidated by water masses as indexed by temperature and salinity. High nutrients from the Changjiang River were the main cause of high biomass in summer, while nutrients from subsurface water were likely the key factor regulating phytoplankton biomass in open ocean water stations in autumn. This study indicates that mechanisms controlling phytoplankton biomass in marginal sea‐open ocean boundary regions should be classified by various water masses with different nutrient concentrations, instead of by geography.

Dissolved copper (dCu) biogeochemical cycling in the subarctic Northeast Pacific and a call for improving methodologies

Abstract We investigated biogeochemical cycling of dissolved copper (dCu) along the Line P transect, spanning from the coastal waters of British Columbia, Canada, to the High Nutrient, Low Chlorophyll (HNLC) open Ocean Station Papa (OSP or P26), in the subarctic Northeast Pacific. DCu concentrations ranged from 1.4–3.7 nmol kg − 1 throughout the water column along the transect, and were elevated in the upper and bottom waters near the continental margin ( 1100 m respectively) as well as in the upper waters offshore (

Kinetics of thorium and particle cycling along the U.S. GEOTRACES North Atlantic Transect

The high particle reactivity of thorium has resulted in its widespread use in tracing processes impacting marine particles and their chemical constituents. The use of thorium isotopes as tracers of particle dynamics, however, largely relies on our understanding of how the element scavenges onto particles. Here, we estimate apparent rate constants of Th adsorption (k1), Th desorption (k−1), bulk particle degradation (β-1), and bulk particle sinking speed (w) along the water column at 11 open-ocean stations occupied during the GEOTRACES North Atlantic Section (GA03). First, we provide evidence that the budgets of Th isotopes and particles at these stations appear to be generally dominated by radioactive production and decay sorption reactions, particle degradation, and particle sinking. Rate parameters are then estimated by fitting a Th and particle cycling model to data of dissolved and particulate 228,230,234Th, 228Ra, particle concentrations, and 234,238U estimates based on salinity, using a nonlinear programming technique.We find that the adsorption rate constant (k1) generally decreases with depth across the section: broadly, the time scale 1/k1 averages 1.0 yr in the upper 1000m and (1.4–1.5) yr below. A positive relationship between k1 and particle concentration (P) is found, i.e., k1∝Pb, where b≥1, consistent with the notion that k1 increases with the number of surface sites available for adsorption. The rate constant ratio, K=k1/(k−1+β−1), which measures the collective influence of rate parameters on Th scavenging, averages 0.2 for most stations and most depths. We clarify the conditions under which K/P is equivalent to the distribution coefficient, KD, test that the conditions are met at the stations, and find that K/P decreases with P, in line with a particle concentration effect (dKD/dP<0). In contrast to the influence of colloids as envisioned by the Brownian pumping hypothesis, we provide evidence that the particle concentration effect arises from the joint effect of P on the rate constants for thorium attachment to, and detachment from, particles.

Characterization of the synoptic‐scale diversity, biogeography, and size distribution of diatoms in the North Pacific

AbstractThe diversity, biogeography, and size distribution of diatoms in the North Pacific and underlying mechanisms shaping those patterns have little been characterized despite their importance in marine ecosystems. Here, we examined the community structure of diatoms in the surface and subsurface chlorophyll a maximum (SCM) layers of the North Pacific using light and scanning electron microscopy. Diatom carbon biomass in both the subarctic and temperate coastal regions was higher than that in the open subtropical and tropical waters. Species density was high in the temperate coastal region and certain open ocean stations where coastal water could be intruded. Diversity was generally higher in the SCM layer than that in the surface layer without a clear latitudinal trend. All diatom species in the open subtropical waters were observed in the temperate coastal waters, suggesting that the coastal regions act as a species bank. Certain indigenous species in the subarctic waters were considered as cryophilic species. We found that the general patterns of the size spectrum were nearly identical among different geographical regions of the North Pacific, irrespective of the species composition, environmental conditions, and seasonality. Our results provide mechanistic insights indicating that the diatom biodiversity in the species bank around the coastal region and the following current systems from the coastal to oceanic regions could principally determine biodiversity patterns in the North Pacific. The species‐specific ecophysiological traits of diatoms and environmental conditions may further modify the biodiversity patterns and size distribution of diatoms.

Seasonal and long‐term changes in elemental concentrations and ratios of marine particulate organic matter

AbstractWhat is the temporal variability of the elemental stoichiometry of marine microbial communities across ocean regions? To answer this question, we present an analysis of environmental conditions, particulate organic carbon, nitrogen, and phosphorus concentrations and their ratios across 20 time series (3–25 years duration) representing estuarine, coastal, and open ocean environments. The majority of stations showed significant seasonal oscillations in particulate organic elemental concentrations and ratios. However, shorter‐term changes contributed most to overall variance in particulate organic matter concentrations and ratios. We found a correlation between the seasonal oscillations of environmental conditions and elemental ratios at many coastal but not open ocean and estuarine stations. C:N peaked near the seasonal temperature minimum and nutrient maximum, but some stations showed other seasonal links. C:N ratios declined with time over the respective observation periods at all open ocean and estuarine stations as well as at five coastal station but increased at the nine other coastal stations. C:P (but not N:P) declined slightly at Bermuda Atlantic Time‐series Study but showed large significant increases at Hawaii Ocean Time‐series and Arendal stations. The relationships between long‐term changes in environmental conditions and particulate organic matter concentrations or ratios were ambiguous, but interactions between changes in temperature and nutrient availability were important. Overall, our analysis demonstrates significant changes in elemental ratios at long‐term and seasonal time scales across regions, but the underlying mechanisms are currently unclear. Thus, we need to better understand the detailed mechanisms driving the elemental composition of marine microbial ecosystems in order to predict how oceans will respond to environmental changes.

Identification of polyamine-responsive bacterioplankton taxa in South Atlantic Bight.

Putrescine and spermidine are short-chained aliphatic polyamines (PAs) that are ubiquitously distributed in seawater. These compounds may be important sources of dissolved organic carbon and nitrogen for marine bacterioplankton. Here, we used pyrotag sequencing to quantify the response of bacterioplankton to putrescine and spermidine amendments in microcosms established using surface waters collected at various stations in the South Atlantic Bight in October 2011. Our analysis showed that PA-responsive bacterioplankton consisted of bacterial taxa that are typically dominant in marine systems. Rhodobacteraceae (Alphaproteobacteria) was the taxon most responsive to PA additions at the nearshore site. Gammaproteobacteria of the families Piscirickettsiaceae; Vibrionaceae; and Vibrionaceae and Pseudoalteromonadaceae, were the dominant PA-responsive taxa in samples from the river-influenced coastal station, offshore station and open ocean station, respectively. The spatial variability of PA-responsive taxa may be attributed to differences in composition of the initial bacterial community and variations of in situ physiochemical conditions among sites. Our results also provided the first empirical evidence that Gammaproteobacteria might play an important role in PA transformation in marine systems.