Bermuda Atlantic Time-series Study Research Articles

Deep water trends and variability at the BATS site in the subtropical North Atlantic and consequences on local sea level budget

There is no robust evidence for warming of the global upper ocean and an associated sea-level rise based on a nearly global and continuous data coverage of the surface and upper ocean. The amount of sea level change contributed by the deep ocean is unclear, however, because of limited data availability below 2000m. The Bermuda Atlantic Time-series Study (BATS) started monthly cruises in 1988 in the subtropical North Atlantic, and during each cruise at least one vertical profile is conducted to a depth >4000m. The BATS dataset provides a unique estimation of the seasonal to inter-annual hydrographic variability in the deep ocean as well as long-term trends at a fixed location. In this paper, we focus on the 2000–4000m deep layer, and find (1) an isopycnal cooling and freshening trend in the deep subtropical North Atlantic, (2) an isobaric warming since ∼ year 2000, despite the continued isopycnal cooling, driven by a gradual deepening of neutral surfaces with unknown spatial extent, and (3) a previously undocumented high frequency variability (>1yr−1) of the deep ocean characterized by large (100m) episodic vertical displacements of neutral surfaces. The observed water mass changes below 2000m contribute ∼1mm/yr to the 0–4000m steric height budget, and is driven mainly by the halosteric contribution.

Oxygen dynamics in the North Atlantic subtropical gyre

Dissolved oxygen (DO) in the ocean is a tracer for most ocean biogeochemical processes including net community production and remineralization of organic matter which in turn constrains the biological carbon pump. Knowledge of oxygen dynamics in the North Atlantic Ocean is mainly derived from observations at the Bermuda Atlantic Time-series Study (BATS) site located in the western subtropical gyre which may skew our view of the biogeochemistry of the subtropical North Atlantic. This study presents and compares a 15yr record of DO observations from ESTOC (European Station for Time-Series in the Ocean, Canary Islands) in the eastern subtropical North Atlantic with the 20yr record at BATS. Our estimate for net community production of oxygen was 2.3±0.4mol O2m−2yr−1 and of oxygen consumption was −2.3±0.5mol O2m−2yr−1 at ESTOC, and 4mol O2m−2yr−1 and −4.4±1molm−2yr−1 at BATS, respectively. These values were determined by analyzing the time-series using the Discrete Wavelet Transform (DWT) method. These flux values agree with similar estimates from in-situ observational studies but are higher than those from modeling studies. The difference in net oxygen production rates supports previous observations of a lower carbon export in the eastern compared to the western subtropical Atlantic. The inter-annual analysis showed clear annual cycles at BATS whereas longer cycles of nearly 4 years were apparent at ESTOC. The DWT analysis showed trends in DO anomalies dominated by long-term perturbations at a basin scale for the consumption zones at both sites, whereas yearly cycles dominated the production zone at BATS. The long-term perturbations found are likely associated with ventilation of the main thermocline, affecting the consumption and production zones at ESTOC.

A new time series of particle export from neutrally buoyant sediments traps at the Bermuda Atlantic Time-series Study site

Neutrally buoyant sediment traps (NBSTs) were deployed concurrently with particle interceptor traps (PITs) on monthly cruises of the Bermuda Atlantic Time-series Study (BATS) from June 2007 to July 2010. In most months, particulate organic carbon flux measured by the two sediment trap systems agreed within the range of variability for two separate NBSTs. In three periods of this study however, the PITs collected more than three times the amount of particulate organic carbon (POC) than the NBSTs. One of these periods coincided with two instances when the mixed layer was deeper than the trap deployment depth, which may lead to biased collection. One of the anomalous periods occurred during summer months, for which we were unable to find existing evidence of biogeochemical changes that would lead to such increases in POC flux. The third anomalous period occurred when the POC flux measured by the NBSTs was near zero. We hypothesize that the PITs may have a positive bias at low fluxes relative to the NBSTs. Zooplankton swimmers accounted for 49±20% of the total measured carbon flux over the three year period and swimmer removal by screening of samples through a 350μm mesh was established as an effective method for this site. Process blank corrections based on sample-identical blanks were important for determining mass and carbon flux from both the NBSTs and the PITs. This new record of particle export from NBSTs suggests that the observed carbon imbalance in this region of the ocean (Michaels et al., 1994) is not due to undersampling by the PITs and that export ratios may be even lower than previously thought.

Multi-decadal uptake of carbon dioxide into subtropical mode water of the North Atlantic Ocean

Abstract. Natural climate variability impacts the multi-decadal uptake of anthropogenic carbon dioxide (Cant) into the North Atlantic Ocean subpolar and subtropical gyres. Previous studies have shown that there is significant uptake of CO2 into subtropical mode water (STMW) of the North Atlantic. STMW forms south of the Gulf Stream in winter and constitutes the dominant upper-ocean water mass in the subtropical gyre of the North Atlantic Ocean. Observations at the Bermuda Atlantic Time-series Study (BATS) site near Bermuda show an increase in dissolved inorganic carbon (DIC) of +1.51 ± 0.08 μmol kg−1 yr−1 between 1988 and 2011, but also an increase in ocean acidification indicators such as pH at rates (−0.0022 ± 0.0002 yr−1) higher than the surface ocean (Bates et al., 2012). It is estimated that the sink of CO2 into STMW was 0.985 ± 0.018 Pg C (Pg = 1015 g C) between 1988 and 2011 (70 ± 1.8% of which is due to uptake of Cant). The sink of CO2 into the STMW is 20% of the CO2 uptake in the North Atlantic Ocean between 14°–50° N (Takahashi et al., 2009). However, the STMW sink of CO2 was strongly coupled to the North Atlantic Oscillation (NAO), with large uptake of CO2 into STMW during the 1990s during a predominantly NAO positive phase. In contrast, uptake of CO2 into STMW was much reduced in the 2000s during the NAO neutral/negative phase. Thus, NAO induced variability of the STMW CO2 sink is important when evaluating multi-decadal changes in North Atlantic Ocean CO2 sinks.

Apparent oxygen utilization rates calculated from tritium and helium-3 profiles at the Bermuda Atlantic Time-series Study site

Abstract. We present three years of Apparent Oxygen Utilization Rates (AOUR) estimated from oxygen and tracer data collected over the ocean thermocline at monthly resolution between 2003 and 2006 at the Bermuda Atlantic Time-series Study (BATS) site. We estimate water ages by calculating a transit time distribution from tritium and helium-3 data. The vertically integrated AOUR over the upper 500 m, which is a regional estimate of export, during the three years is 3.1 ± 0.5 mol O2 m−2 yr−1. This is comparable to previous AOUR-based estimates of export production at the BATS site but is several times larger than export estimates derived from sediment traps or 234Th fluxes. We compare AOUR determined in this study to AOUR measured in the 1980s and show AOUR is significantly greater today than decades earlier because of changes in AOU, rather than changes in ventilation rates. The changes in AOU are likely a methodological artefact associated with problems with early oxygen measurements.

Evaluating triple oxygen isotope estimates of gross primary production at the Hawaii Ocean Time‐series and Bermuda Atlantic Time‐series Study sites

The triple oxygen isotopic composition of dissolved oxygen (17Δ) is a promising tracer of gross oxygen productivity (P) in the ocean. Recent studies have inferred a high and variable ratio of P to 14C net primary productivity (12–24 h incubations) (e.g., P:NPP(14C) of 5–10) using the 17Δ tracer method, which implies a very low efficiency of phytoplankton growth rates relative to gross photosynthetic rates. We added oxygen isotopes to a one‐dimensional mixed layer model to assess the role of physical dynamics in potentially biasing estimates ofP using the 17Δ tracer method at the Bermuda Atlantic Time‐series Study (BATS) and Hawaii Ocean Time‐series (HOT). Model results were compared to multiyear observations at each site. Entrainment of high17Δ thermocline water into the mixed layer was the largest source of error in estimating P from mixed layer 17Δ. At both BATS and HOT, entrainment bias was significant throughout the year and resulted in an annually averaged overestimate of mixed layer P of 60 to 80%. When the entrainment bias is corrected for, P calculated from observed 17Δ and 14C productivity incubations results in a gross:net productivity ratio of 2.6 (+0.9 −0.8) at BATS. At HOT a gross:net ratio decreasing linearly from 3.0 (+1.0 −0.8) at the surface to 1.4 (+0.6 −0.6) at depth best reproduced observations. In the seasonal thermocline at BATS, however, a significantly higher gross:net ratio or large lateral fluxes of 17Δ must be invoked to explain 17Δ field observations.

Interannual variability of chlorophyll and the influence of low‐frequency climate modes in the North Atlantic subtropical gyre

The impact of low‐frequency climate modes on the large‐scale variability of chlorophyll‐a (a proxy for phytoplankton biomass) concentration in the subtropics is little known. Here we examined the concurrent monthly chlorophyll‐a and hydrographical records obtained at two subtropical time series stations (Bermuda Atlantic Time series Study, BATS and European Station for Time series in the Ocean Canary Islands, ESTOC) from the beginning of the time series (1989 at BATS and 1994 at ESTOC) to 2003, in order to analyze its seasonal and interannual variability and investigate the potential correlation with large‐scale atmospheric oscillations. At BATS, Chl‐a variations are mainly caused by differences in the convective mixing and mesoscale phenomena. Variability in winter mixing is a significant factor at ESTOC as exemplified by years with anomalously deep mixed layer depths. An additional nutrient source causing Chl‐a variability at this station likely occurs due to nutrient advection driven by the baroclinic flow caused by the Trade Winds during summertime. We found that interannual variability in mean integrated total Chl‐a (TChl‐a) is significantly correlated with temperature and salinity anomalies at BATS. Chl‐a also covaried with changes in temperature although the correlation was not significant at ESTOC. We could not find any direct correlation between TChl‐a and NAO at BATS; the correlation improved between TChl‐a and NAO +1 year but was still insignificant. However, significant correlations were found for ESTOC between TChl‐a anomalies and ENSO (El Niño Southern Oscillation) +3 years and NAO +1 year time lag.

Environmental, biochemical and genetic drivers of DMSP degradation and DMS production in the Sargasso Sea

Dimethylsulfide (DMS) is a climatically relevant trace gas produced and cycled by the surface ocean food web. Mechanisms driving intraannual variability in DMS production and dimethylsulfoniopropionate (DMSP) degradation in open-ocean, oligotrophic regions were investigated during a 10-month time-series at the Bermuda Atlantic Time-series Study site in the Sargasso Sea. Abundance and transcription of bacterial DMSP degradation genes, DMSP lyase enzyme activity, and DMS and DMSP concentrations, consumption rates and production rates were quantified over time and depth. This interdisciplinary data set was used to test current hypotheses of the role of light and carbon supply in regulating upper-ocean sulfur cycling. Findings supported UV-A-dependent phytoplankton DMS production. Bacterial DMSP degraders may also contribute significantly to DMS production when temperatures are elevated and UV-A dose is moderate, but may favour DMSP demethylation under low UV-A doses. Three groups of bacterial DMSP degraders with distinct intraannual variability were identified and niche differentiation was indicated. The combination of genetic and biochemical data suggest a modified 'bacterial switch' hypothesis where the prevalence of different bacterial DMSP degradation pathways is regulated by a complex set of factors including carbon supply, temperature and UV-A dose.

Long‐term increase in mesozooplankton biomass in the Sargasso Sea: Linkage to climate and implications for food web dynamics and biogeochemical cycling

Changes in zooplankton biomass and species composition over long time scales can have significant effects on biogeochemical cycling and transfer of energy to higher trophic levels. We analyzed size‐fractionated mesozooplankton biomass (>200μm) from biweekly to monthly day and night tows taken from 1994 to 2010 in the epipelagic zone at the Bermuda Atlantic Time series Study (BATS) site in the oligotrophic North Atlantic subtropical gyre. During this 17‐year period total mesozooplankton biomass increased 61% overall, although a few short‐term downturns occurred over the course of the time series. The overall increase was higher in the nighttime compared to daytime, resulting in an increase in calculated diel vertical migrator biomass. The largest seasonal increase in total biomass was in the late‐winter to spring (February–April). Associated with the larger increase in late‐winter/spring biomass was a shift in the timing of annual peak biomass during the latter half of the time series (from March/April to a distinct March peak for all size fractions combined, and April to March for the 2–5 mm size fractions). Zooplankton biomass was positively correlated with sea‐surface temperature, water column stratification, and primary production, and negatively correlated with mean temperature between 300 and 600 m. Significant correlations exist between multidecadal climate indices–the North Atlantic Oscillation plus three different Pacific Ocean climate indices, and BATS zooplankton biomass, indicating connections between patterns in climate forcing and ecosystem response. Resultant changes in biogeochemical cycling include an increase in the magnitude of both active carbon flux by diel vertical migration and passive carbon flux of fecal pellets as components of the export flux. The most likely mechanism driving the zooplankton biomass increase is bottom‐up control by smaller phytoplankton, which has also increased in biomass and production at BATS, translating up the microbial food web into mesozooplankton. Decreases in top‐down control or expansion of the range of tropical species northward as a result of warming may also play a role.

High-precision and accurate determinations of neodymium isotopic compositions at nanogram levels in natural materials by MC-ICP-MS

A comparatively simple, rapid, low-blank, high-precision and reproducible technique for Nd isotope analysis (143Nd/144Nd) of trace amounts (1–5 ng) of Nd by multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) was developed and is reported here. The technique utilizes a combination of a high-sensitivity desolvator (Aridus II or APEX-IR) and high-efficiency Jet-sample/X-skimmer cones, together with high-yield pre-concentration and column chemistry, to achieve optimal conditions for Nd isotope determination in natural materials with a variety of Nd-depleted matrices. This new method was validated by analyzing a series of international reference materials (RMs), which span a wide range of the Nd isotopic composition and cover different sample matrices in order to provide a useful benchmark for future studies. The long-term precision of 143Nd/144Nd (±0.000016, 2SD) for a sample size of ca. 1.25 ng Nd is a factor of 5 better than the existing high-sensitivity MC-ICP-MS methods using <2 ng of Nd and is comparable to the best TIMS methods. Replicate analyses of the RMs, including JNdi-1 (0.512116 ± 0.000016, 2SD), AGV-1 (0.512794 ± 0.000013, 2SD), BHVO-1 (0.512992 ± 0.000015, 2SD), Nod-P-1 (0.512430 ± 0.000015, 2SD) and two sets of synthetic solutions for seawater and calcium carbonate matrices, are in excellent agreement with the published values, demonstrating the robustness of the present protocol for natural samples with different matrices. A depth profile of the seawater Nd isotopes near the Bermuda Atlantic Time-series Study station was also analyzed for comparison, and is identical to the reported seawater profile in this region. Applying this improved technique, we are able to generate high-precision and reproducible Nd isotope data for Nd-depleted natural materials, such as seawater, continental runoff, benthic fluxes at continental margins, and marine biogenic carbonate, which can provide pertinent and complementary information about the marine Nd cycle and past ocean circulation.

Phytoplankton distribution patterns in the northwestern Sargasso Sea revealed by small subunit rRNA genes from plastids

Phytoplankton species vary in their physiological properties, and are expected to respond differently to seasonal changes in water column conditions. To assess these varying distribution patterns, we used 412 samples collected monthly over 12 years (1991-2004) at the Bermuda Atlantic Time-Series Study site, located in the northwestern Sargasso Sea. We measured plastid 16S ribosomal RNA gene abundances with a terminal restriction fragment length polymorphism approach and identified distribution patterns for members of the Prymnesiophyceae, Pelagophyceae, Chrysophyceae, Cryptophyceae, Bacillariophyceae and Prasinophyceae. The analysis revealed dynamic bloom patterns by these phytoplankton taxa that begin early in the year, when the mixed layer is deep. Previously, unreported open-ocean prasinophyte blooms dominated the plastid gene signal during convective mixing events. Quantitative PCR confirmed the blooms and transitions of Bathycoccus, Micromonas and Ostreococcus populations. In contrast, taxa belonging to the pelagophytes and chrysophytes, as well as cryptophytes, reached annual peaks during mixed layer shoaling, while Bacillariophyceae (diatoms) were observed only episodically in the 12-year record. Prymnesiophytes dominated the integrated plastid gene signal. They were abundant throughout the water column before mixing events, but persisted in the deep chlorophyll maximum during stratified conditions. Various models have been used to describe mechanisms that drive vernal phytoplankton blooms in temperate seas. The range of taxon-specific bloom patterns observed here indicates that different 'spring bloom' models can aptly describe the behavior of different phytoplankton taxa at a single geographical location. These findings provide insight into the subdivision of niche space by phytoplankton and may lead to improved predictions of phytoplankton responses to changes in ocean conditions.

The annual silica cycle of the North Pacific subtropical gyre

Silica cycling in the upper 175 m of the North Pacific Subtropical Gyre was examined over a two year period (January 2008–December 2009) at the Hawaii Ocean Time-series (HOT) station ALOHA. Silicic acid concentrations in surface waters ranged from 0.6 to 1.6 μM, exhibiting no clear seasonal trends. Biogenic silica concentrations and silica production rates increased by an order of magnitude each summer following stratification of the upper 50 m reaching values of 157 nmol Si L −1 and 81 nmol Si L −1 d −1, in 2008 and 2009, respectively. Sea surface height anomalies together with analyses of variability in isothermal surfaces at 150–175 m indicated that the summer periods of elevated biogenic silica were associated with anticyclonic mesoscale features during both years. Lithogenic silica concentrations increased in the spring during the known period of maximum atmospheric dust concentrations with maximum values of 36 nmol Si L −1 in the upper 10 m. Dust deposition would enhance levels of dissolved iron in surface waters, but there was no response of diatom biomass or silica production to increases in near-surface ocean lithogenic silica concentrations suggesting iron sufficiency of diatom silica production rates. Low ambient silicic acid concentrations restricted silica production rates to an average of 43% of maximum potential rates. Si sufficiency only occurred during the summer period when diatom biomass was elevated suggesting that bloom diatoms are adapted to exploit low silicic acid concentrations. Annual silica production at HOT is estimated to be 63 mmol Si m −2 a −1 with summer blooms contributing 29% of the annual total. Diatoms are estimated to account for 3–7% of total phytoplankton primary productivity, but 9–20% of organic carbon export confirming past suggestions that diatoms are relatively minor contributors to primary productivity and autotrophic biomass, but important contributors to new and export production in oligotrophic open-ocean ecosystems. Annual silica production at HOT is nearly 4-fold lower than estimates at the Bermuda Atlantic Time-series Study (BATS) site in the Sargasso Sea from the 1990s, but annual silica export at the base of the euphotic zone is similar between the two gyres indicating very different balances between silica production and its loss in surface waters. On a relative basis, BATS is a more productive system with respect to silica, where biogenic silica is recycled with high efficiency in surface waters; in contrast the NPSG is a lower productivity system with respect to silica, but where lower recycling efficiency leads to a much higher fraction of new silica production. The two gyres show contrasting long-term trends in diatom biomass as biogenic silica concentrations at HOT have been increasing since 1997, but they have been decreasing at BATS suggesting very different forcing of decadal trends in the contribution of diatoms in carbon cycling between these gyres. Combining the data from both gyres indicates that globally subtropical gyres produce 13 Tmol Si a −1, which is only 51% of previous estimates reducing the contribution of subtropical gyres to 5–7% of global annual marine silica production.

Similarity in microbial amino acid uptake in surface waters of the North and South Atlantic (sub-)tropical gyres

The Earth’s most extensive biomes – the oceanic subtropical gyres – are considered to be expanding with current surface ocean warming. Although it is well established that microbial communities control gyre biogeochemistry, comparisons of their metabolic activities between gyres are limited. In order to estimate metabolic activities including production of microbial communities, the uptake rates of amino acids leucine, methionine and tyrosine at ambient concentrations were estimated in surface waters of the Atlantic Ocean using radioisotopically labelled tracers. Data were acquired during six research cruises covering main oceanic provinces herein termed: North and South Atlantic Gyres, Bermuda Atlantic Time-series Study site (BATS), Equatorial region, and Mauritanian Upwelling (off Cape Blanc). Data were divided between provinces, the extents of which were identified by ocean colour data, in order to achieve provincial mean uptake rates. Leucine and methionine uptake rates did not differ between sampling periods, and were comparable between the North and South subtropical gyres. Furthermore, variation in uptake rates measured throughout the two oligotrophic gyres, where sampling covered ∼4 × 10 6 km 2, was considerably lower than that measured within the Mauritanian Upwelling and Equatorial regions, and even at the BATS site. Tyrosine was generally the slowest of the amino acids to be taken up, however, it was assimilated faster than methionine within the Mauritanian Upwelling region. Thus, we propose that one value for leucine (12.6 ± 3.2 pmol L −1 h −1) and methionine (10.0 ± 3.3 pmol L −1 h −1) uptake could be applied to the oligotrophic subtropical gyres of the Atlantic Ocean. However, with the significantly lower uptake rates observed at the BATS site, we would not advise extrapolation to the Sargasso Sea.

Silicate:nitrate ratios of upwelled waters control the phytoplankton community sustained by mesoscale eddies in sub-tropical North Atlantic and Pacific

Abstract. Mesoscale eddies in sub-tropical gyres physically perturb the water column and can introduce macronutrients to the euphotic zone, stimulating a biological response in which phytoplankton communities can become dominated by large phytoplankton. Mesoscale eddies may therefore be important in driving export in oligotrophic regions of the modern ocean. However, the character and magnitude of the biological response sustained by eddies is variable. Here we present data from mesoscale eddies in the Sargasso Sea (Atlantic) and the waters off Hawai'i (Pacific), alongside mesoscale events that affected the Bermuda Atlantic Time-Series Study (BATS) over the past decade. From this analysis, we suggest that the phytoplankton community structure sustained by mesoscale eddies is predetermined by the relative abundance of silicate over nitrate (Si*) in the upwelled waters. We present data that demonstrate that mode-water eddies (MWE) in the Sargasso Sea upwell locally formed waters with relatively high Si* to the euphotic zone, and that cyclonic eddies in the Sargasso Sea introduce waters with relatively low Si*, a signature that originated in the iron-limited Southern Ocean. We propose that this phenomenon can explain the observed dominance of the phytoplankton community by large-diatom species in MWE and by small prokaryotic phytoplankton in cyclonic features. In contrast to the Atlantic, North Pacific Intermediate Water (NPIW) with high Si* may influence the cyclonic eddies in waters off Hawai'i, which also appear capable of sustaining diatom populations. These observations suggest that the structure of phytoplankton communities sustained by eddies may be related to the chemical composition of the upwelled waters in addition to the physical nature of the eddy.

Analysis of trace metals (Cu, Cd, Pb, and Fe) in seawater using single batch nitrilotriacetate resin extraction and isotope dilution inductively coupled plasma mass spectrometry

A simple and accurate low-blank method has been developed for the analysis of total dissolved copper, cadmium, lead, and iron in a small volume (1.3–1.5 mL per element) of seawater. Pre-concentration and salt-separation of a stable isotope spiked sample are achieved by single batch extraction onto nitrilotriacetate (NTA)-type Superflow ® chelating resin beads (100–2400 beads depending on the element). Metals are released into 0.1–0.5 M HNO 3, and trace metal isotope ratios are determined by ICPMS. The benefit of this method compared to our previous Mg(OH) 2 coprecipitation method is that the final matrix is very dilute so cone-clogging and matrix sensitivity suppression are minimal, while still retaining the high accuracy of the isotope dilution technique. Recovery efficiencies are sensitive to sample pH, number of resin beads added, and the length of time allowed for sample–resin binding and elution; these factors are optimized for each element to yield the highest recovery. The method has a low procedural blank and high sensitivity sufficient for the analysis of pM–nM open-ocean trace metal concentrations. Application of this method to samples from the Bermuda Atlantic Time-Series Study station provides oceanographically consistent Cu, Cd, Pb, and Fe profiles that are in good agreement with other reliable data for this site. In addition, the method can potentially be modified for the simultaneous analysis of multiple elements, which will be beneficial for the analysis of large number of samples.

Diversity and distribution of single-stranded DNA phages in the North Atlantic Ocean

Knowledge of marine phages is highly biased toward double-stranded DNA (dsDNA) phages; however, recent metagenomic surveys have also identified single-stranded DNA (ssDNA) phages in the oceans. Here, we describe two complete ssDNA phage genomes that were reconstructed from a viral metagenome from 80 m depth at the Bermuda Atlantic Time-series Study (BATS) site in the northwestern Sargasso Sea and examine their spatial and temporal distributions. Both genomes (SARssφ1 and SARssφ2) exhibited similarity to known phages of the Microviridae family in terms of size, GC content, genome organization and protein sequence. PCR amplification of the replication initiation protein (Rep) gene revealed narrow and distinct depth distributions for the newly described ssDNA phages within the upper 200 m of the water column at the BATS site. Comparison of Rep gene sequences obtained from the BATS site over time revealed changes in the diversity of ssDNA phages over monthly time scales, although some nearly identical sequences were recovered from samples collected 4 years apart. Examination of ssDNA phage diversity along transects through the North Atlantic Ocean revealed a positive correlation between genetic distance and geographic distance between sampling sites. Together, the data suggest fundamental differences between the distribution of these ssDNA phages and the distribution of known marine dsDNA phages, possibly because of differences in host range, host distribution, virion stability, or viral evolution mechanisms and rates. Future work needs to elucidate the host ranges for oceanic ssDNA phages and determine their ecological roles in the marine ecosystem.

Direct comparison of 210Po, 234Th and POC particle-size distributions and export fluxes at the Bermuda Atlantic Time-series Study (BATS) site

Particle-reactive, naturally occurring radionuclides are useful tracers of the sinking flux of organic matter from the surface to the deep ocean. Since the Joint Global Ocean Flux Study (JGOFS) began in 1987, the disequilibrium between 234Th and its parent 238U has become widely used as a technique to measure particle export fluxes from surface ocean waters. Another radionuclide pair, 210Po and 210Pb, can be used for the same purpose but has not been as widely adopted due to difficulty with accurately constraining the 210Po/ 210Pb radiochemical balance in the ocean and because of the more time-consuming radiochemical procedures. Direct comparison of particle flux estimated in different ocean regions using these short-lived radionuclides is important in evaluating their utility and accuracy as tracers of particle flux. In this paper, we present paired 234Th/ 238U and 210Po/ 210Pb data from oligotrophic surface waters of the subtropical Northwest Atlantic and discuss their advantages and limitations. Vertical profiles of total and particle size-fractionated 210Po and 234Th activities, together with particulate organic carbon (POC) concentrations, were measured during three seasons at the Bermuda Atlantic Time-series Study (BATS) site. Both 210Po and 234Th reasonably predict sinking POC flux caught in sediment traps, and each tracer provides unique information about the magnitude and efficiency of the ocean's biological pump.

Challenges of modeling depth‐integrated marine primary productivity over multiple decades: A case study at BATS and HOT

The performance of 36 models (22 ocean color models and 14 biogeochemical ocean circulation models (BOGCMs)) that estimate depth‐integrated marine net primary productivity (NPP) was assessed by comparing their output to in situ 14C data at the Bermuda Atlantic Time series Study (BATS) and the Hawaii Ocean Time series (HOT) over nearly two decades. Specifically, skill was assessed based on the models' ability to estimate the observed mean, variability, and trends of NPP. At both sites, more than 90% of the models underestimated mean NPP, with the average bias of the BOGCMs being nearly twice that of the ocean color models. However, the difference in overall skill between the best BOGCM and the best ocean color model at each site was not significant. Between 1989 and 2007, in situ NPP at BATS and HOT increased by an average of nearly 2% per year and was positively correlated to the North Pacific Gyre Oscillation index. The majority of ocean color models produced in situ NPP trends that were closer to the observed trends when chlorophyll‐a was derived from high‐performance liquid chromatography (HPLC), rather than fluorometric or SeaWiFS data. However, this was a function of time such that average trend magnitude was more accurately estimated over longer time periods. Among BOGCMs, only two individual models successfully produced an increasing NPP trend (one model at each site). We caution against the use of models to assess multiannual changes in NPP over short time periods. Ocean color model estimates of NPP trends could improve if more high quality HPLC chlorophyll‐a time series were available.

Transcriptomic analysis of a marine bacterial community enriched with dimethylsulfoniopropionate

Dimethylsulfoniopropionate (DMSP) is an important source of reduced sulfur and carbon for marine microbial communities, as well as the precursor of the climate-active gas dimethylsulfide (DMS). In this study, we used metatranscriptomic sequencing to analyze gene expression profiles of a bacterial assemblage from surface waters at the Bermuda Atlantic Time-series Study (BATS) station with and without a short-term enrichment of DMSP (25 nM for 30 min). An average of 303 143 reads were obtained per treatment using 454 pyrosequencing technology, of which 51% were potential protein-encoding sequences. Transcripts from Gammaproteobacteria and Bacteroidetes increased in relative abundance on DMSP addition, yet there was little change in the contribution of two bacterioplankton groups whose cultured members harbor known DMSP degradation genes, Roseobacter and SAR11. The DMSP addition led to an enrichment of transcripts supporting heterotrophic activity, and a depletion of those encoding light-related energy generation. Genes for the degradation of C3 compounds were significantly overrepresented after DMSP addition, likely reflecting the metabolism of the C3 component of DMSP. Mapping these transcripts to known biochemical pathways indicated that both acetyl-CoA and succinyl-CoA may be common entry points of this moiety into the tricarboxylic acid cycle. In a short time frame (30 min) in the extremely oligotrophic Sargasso Sea, different gene expression patterns suggest the use of DMSP by a diversity of marine bacterioplankton as both carbon and sulfur sources.

A comparison of the variability of biological nutrients against depth and potential density

Abstract. The main biogeochemical nutrient distributions, along with ambient ocean temperature and the light field, control ocean biological productivity. Observations of nutrients are much sparser than physical observations of temperature and salinity, yet it is critical to validate biogeochemical models against these sparse observations if we are to successfully model biological variability and trends. Here we use data from the Bermuda Atlantic Time-series Study and the World Ocean Database 2005 to demonstrate quantitatively that over the entire globe a significant fraction of the temporal variability of phosphate, silicate and nitrate within the oceans is correlated with water density. The temporal variability of these nutrients as a function of depth is almost always greater than as a function of potential density, with he largest reductions in variability found within the main pycnocline. The greater nutrient variability as a function of depth occurs when dynamical processes vertically displace nutrient and density fields together on shorter timescales than biological adjustments. These results show that dynamical processes can have a significant impact on the instantaneous nutrient distributions. These processes must therefore be considered when modeling biogeochemical systems, when comparing such models with observations, or when assimilating data into such models.