Oligotrophic North Atlantic Research Articles

Organic nitrogen in aerosols and precipitation at Barbados and Miami: Implications regarding sources, transport and deposition to the western subtropical North Atlantic

The deposition of anthropogenic nitrogen (N) species is believed to have a significant impact on the oligotrophic North Atlantic, but the magnitude of ecological effects remains uncertain because the deposition of water soluble organic N (WSON) is poorly quantified. Here we present measurements of water soluble inorganic N (WSIN) and WSON in aerosol and rain at two subtropical North Atlantic time series sites: Barbados and Miami. WSON total deposition rates ranged from 17.9 mmol m−2 yr−1 to 49.6 mmol m−2 yr−1, contributing on average only 6–14% of total N deposition, less than half the poorly constrained global average which is typically cited as 30%. On an event basis, biomass burning and dust events yielded the largest concentrations of WSON. However, biomass burning was relatively infrequent and highly variable in composition, and much of the organic N associated with dust appeared to be externally adsorbed from pollution sources. Conversely, in Miami pollution made relatively small contributions of WSON on an event basis, but impacts were relatively frequent, making pollution one of the largest sources of WSON during the year. The largest contributor to WSON was volatile basic organic N (VBON) species, which were present at concentrations 1–2 times higher than particulate WSON. Despite VBON inputs, samples associated with pollution-source trajectories yielded much more inorganic N than WSON. Consequently, we would expect that in the future as anthropogenic N emissions increase, inorganic nitrogen will remain the dominant form of N that is deposited to the western North Atlantic.

Interbasin isotopic correspondence between upper-ocean bulk DON and subsurface nitrate and its implications for marine nitrogen cycling

[1] Measurements to date have shown that both bulk and high molecular weight marine dissolved organic nitrogen (DON) have a 15N/14N that is substantially higher than the 15N/14N of suspended particulate organic nitrogen (PNsusp) found in the same surface waters (with δ15N of ∼4 to 5‰ and ∼−1 to 1‰, respectively). Moreover, the concentration and 15N/14N of DON are much less dynamic than those of PNsusp. These observations raise questions regarding the role of DON in the upper ocean nitrogen (N) cycle. In this study, the concentration and 15N/14N of nitrate and DON was measured in the upper 300 m of the oligotrophic North Atlantic and North Pacific Oceans. Comparing these two regions, the average DON concentration in the upper 100 m is similar, between 4.5 and 5.0 μM, but the average δ15N of DON is significantly different, 3.9‰ versus air in the North Atlantic and 4.7‰ in the North Pacific. This difference parallels a similar isotopic difference between shallow nitrate in these two regions; at 200 m in the North Atlantic, the δ15N of nitrate is 2.6‰, while it is 4.0‰ in the North Pacific. This isotopic correlation between surface DON and subsurface nitrate indicates that DON is actively participating in the upper ocean N cycle of each region. We describe a conceptual model that explains the elevation of the 15N/14N of DON relative to surface ocean PNsusp as well as the interbasin difference in the 15N/14N of DON. In this model, DON is produced from PNsusp without isotopic fractionation but DON is removed by fractionating processes. The ammonium and simple organic N compounds released by DON decomposition reactions are reassimilated by algae into the PNsusp pool, as an integral part of the ammonium-centered cycle that lowers the 15N/14N of PNsusp relative to the nitrate supply from below. This interpretation is consistent with the understanding of the chemical controls on isotope fractionation and is analogous to the previously posed explanation for the 15N/14N elevation of herbivorous zooplankton. In addition, it explains a lack of correlation between in situ N2 fixation rates and DON concentration and 15N/14N on short time scales.

Rapid biological oxidation of methanol in the tropical Atlantic: significance as a microbial carbon source

Abstract. Methanol is the second most abundant organic gas in the atmosphere after methane, and is ubiquitous in the troposphere. It plays a significant role in atmospheric oxidant chemistry and is biogeochemically active. Large uncertainties exist about whether the oceans are a source or sink of methanol to the atmosphere. Even less is understood about what reactions in seawater determine its concentration, and hence flux across the sea surface interface. We report here concentrations of methanol between 151–296 nM in parts of the oligotrophic North Atlantic, with corresponding microbial uptake rates between 2–146 nM d−1, suggesting turnover times as low as 1 day (1–25 days) in surface waters of the oligotrophic tropical North East Atlantic. Methanol is mainly (≥97%) used by microbes for obtaining energy in oligotrophic regions, which contrasts with shelf and coastal areas where between 20–50% can be used for cell growth. Comparisons of microbial methanol oxidation rates with parallel determinations of bacterial leucine uptake suggest that methanol contributes on average 13% to bacterial carbon demand in the central northern Atlantic gyre (maximum of 54%). In addition, the contribution that methanol makes to bacterial carbon demand varies as a power function of chlorophyll a concentrations; suggesting for concentrations <0.2 μg l−1 that methanol can make a significant contribution to bacterial carbon demand. However, our low air to sea methanol flux estimates of 7.2–13 μmol m−2 d−1 suggest that the atmosphere is not a major methanol source. We conclude that there must be a major, as yet unidentified, in situ oceanic methanol source in these latitudes which we suggest is sunlight driven decomposition of organic matter.

Linking microbial heterotrophic activity and sediment lithology in oxic, oligotrophic sub-seafloor sediments of the North Atlantic Ocean

Microbial heterotrophic activity was investigated in oxic sub-seafloor sediments at North Pond, a sediment pond situated at 23°N on the western flank of the Mid-Atlantic Ridge. The North Pond sediments underlie the oligotrophic North Atlantic Gyre at 4580-m water depth and cover a 7–8 million-year-old basaltic crust aquifer through which seawater flows. Discrete samples for experimentation were obtained from up to ~9 m-long gravity cores taken at 14 stations in the North Pond area. Potential respiration rates were determined in sediment slurries incubated under aerobic conditions with 14C-acetate. Microbial heterotrophic activity, as defined by oxidation of acetate to CO2 (with O2 as electron acceptor), was detected in all 14 stations and all depths sampled. Potential respiration rates were generally low (<0.2 nmol of respired acetate cm−3 d−1) in the sediment, but indicate that microbial heterotrophic activity occurs in deep-sea, oxic, sub-seafloor sediments. Furthermore, discernable differences in activity existed between sites and within given depth profiles. At seven stations, activity was increased by several orders of magnitude at depth (up to ~12 nmol of acetate respired cm−3 d−1). We attempted to correlate the measures of activity with high-resolution color and element stratigraphy. Increased activities at certain depths may be correlated to variations in the sediment geology, i.e., to the presence of dark clay-rich layers, of sandy layers, or within clay-rich horizons presumably overlying basalts. This would suggest that the distribution of microbial heterotrophic activity in deeply buried sediments may be linked to specific lithologies. Nevertheless, high-resolution microbial examination at the level currently enjoyed by sedimentologists will be required to fully explore this link.

Nutrient distributions in baroclinic eddies of the oligotrophic North Atlantic and inferred impacts on biology

Abstract High-sensitivity (nanomolar) techniques for nitrate and phosphate were applied to study nutrient patterns in the euphotic zone of mesoscale eddies in the Sargasso Sea during the EDDIES project. Surface concentrations of nitrate plus nitrite (DNN) and phosphate (DIP) were found in the range of 1–20 nM with substantial spatial variability in the eddies, with resulting mean N:P molar ratios of ⩽2.1. Chlorophyll biomass was well correlated with DNN but not DIP in the upper euphotic zone, suggesting N-limitation of marine phytoplankton at this time of year. Within the upper 140 m, the water column experienced a transition from a P-enriched (relative to Redfield ratio) shallow layer to a N-enriched deep layer, which may suggest downward transport and subsequent remineralization of high N/P biogenic products presumably originating from N2 fixation. Chlorophyll biomass in the deep chlorophyll maximum of eddies was found to be tightly related to eddy–induced variability in major nutrients (N, P, Si) and nutrient stoichiometry, suggesting that the impact of eddies on biology is through control of nutrient availability. Because the eddies were likely to be in various phases of development (different degrees of both biological and physical maturity), full interpretation of eddy data and dynamics will require better coverage of a full eddy life cycle.

Nitrogen and phosphorus co‐limitation of bacterial productivity and growth in the oligotrophic subtropical North Atlantic

Bacterial productivity and biomass are thought to be limited by dissolved organic carbon (DOC) in much of the world's oceans. However, the mixed layer of oligotrophic oceans is often depleted in dissolved inorganic nitrogen and phosphate, raising the possibility that macronutrients may also limit heterotrophic bacterial growth. We used nutrient bioassay experiments to determine whether inorganic nutrients (N, P, Fe) and/or DOC could limit bacterial productivity and biomass in the central North Atlantic during the spring of 2004 (Mar‐Apr). We observed that both heterotrophic bacterial productivity and biomass were co‐limited by N and P in the oligotrophic North Atlantic, and additions of labile DOC (glucose) provided no stimulation unless N and P were also added. Flow cytometry results indicated that only a small subset of large cells high in nucleic acid content were responsible for the increased productivity in the combined NP amendments. In contrast, nutrient additions elicited no net change on the dominant component of the bacterial population, composed of small cells with relatively low nucleic acid content. In the combined NP treatments the relative increase in bacterial production was greater than that measured when phytoplankton productivity was relieved of nitrogen limitation. These results suggest that N and P co‐limitation in the bacterial community results in increased competition between the heterotrophic and autotrophic components of the surface communities in the Central North Atlantic Ocean, and potentially impacts the cycling of organic matter by the bacterioplankton.

Bacterial assemblage structure and carbon metabolism along a productivity gradient in the NE Atlantic Ocean

AME Aquatic Microbial Ecology Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsSpecials AME 46:43-53 (2007) - doi:10.3354/ame046043 Bacterial assemblage structure and carbon metabolism along a productivity gradient in the NE Atlantic Ocean Laura Alonso-Sáez1,*, Javier Arístegui2, Jarone Pinhassi3, Laura Gómez-Consarnau3, José M. González4, Dolors Vaqué1, Susana Agustí5, Josep M. Gasol1 1Departament de Biologia Marina i Oceanografia, Institut de Ciències del Mar—CSIC, Pg. Marítim de la Barceloneta 37–49, 08003 Barcelona, Catalunya, Spain 2Departamento de Biología, Universidad de las Palmas de Gran Canaria, Campus Universitario de Tafira, PO Box 550, 35017 Las Palmas de Gran Canaria, Spain 3Marine Microbiology, Department of Biology and Environmental Sciences, University of Kalmar, Barlastgatan 11, 39182 Kalmar, Sweden 4Departamento de Microbiología y Biología Celular, Facultad de Farmacia, Universidad de La Laguna, Avda. Astrofísico Francisco Sánchez s/n 38071 La Laguna, Tenerife, Spain 5Institut Mediterrani d’Estudis Avançats (IMEDEA), CSIC-Universitat de les Illes Balears, Miquel Marquès, 21, 07190 Esporles, Mallorca, Spain *Email: lalonso@icm.csic.es ABSTRACT: Bacterioplankton have the potential to significantly affect the cycling of organic matter in the ocean; however, little is known about the linkage between bacterial assemblage structure and carbon metabolism. In this study, we investigated whether changes in the phylogenetic composition of bacterioplankton were associated with changes in bacterial carbon processing (bacterial production, respiration and biomass) in the subtropical NE Atlantic Ocean. We found consistent differences in the composition of the bacterial assemblage, as revealed by denaturing gradient gel electrophoresis (DGGE) and catalyzed reporter deposition-fluorescence in situ hybridization (CARD-FISH), along a gradient from the NW African upwelling to the oligotrophic North Atlantic Subtropical Gyre. The percent contribution of Bacteroidetes, Roseobacter and Gammaproteobacteria significantly increased towards more productive waters, whereas the SAR11 clade of the Alphaproteobacteria remained relatively constant (average 28% of DAPI-stained cells) throughout the area. Changes in the composition of the bacterial assemblage detected by DGGE were weakly but significantly correlated with changes in carbon processing variables. The abundances of Roseobacter and Gammaproteobacteria were highly correlated with the concentration of particulate organic carbon and chlorophyll a, reflecting the affinity of these groups to nutrient-enriched conditions. The abundance of Roseobacter was also positively correlated with heterotrophic bacterial production, suggesting their active participation in carbon processing. KEY WORDS: Bacteria · Atlantic Ocean · Diversity · Metabolism · CARD-FISH · DGGE Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in AME Vol. 46, No. 1. Online publication date: January 19, 2007 Print ISSN: 0948-3055; Online ISSN: 1616-1564 Copyright © 2007 Inter-Research.

Modeling abiotic production of apparent oxygen utilisation in the oligotrophic subtropical North Atlantic

Apparent oxygen utilisation is potentially biased by abiotic, physical processes. Using a coupled 3-D circulation-oxygen model, this potential is quantitatively estimated for a region in the eastern subtropical North Atlantic, called the Beta Triangle, where an inconsistency exists between observational estimates of high carbon export from the euphotic zone, based on oxygen utilisation rates in the thermocline (Jenkins 1982), and those of low nutrient supply to the euphotic zone (Lewis et al. 1986, 2004). Our results indicate that in the upper ocean, the Jenkins (1982) estimate is indeed biased high by approximately 10% due to abiotic processes feigning respiration, thus contributing to the apparent inconsistency. Vertical integration, however, yields an abiotic fraction of less than 3%, so the apparent observational discrepancy can not be resolved.

Nitrogen fixation and nitrogen isotope abundances in zooplankton of the oligotrophic North Atlantic

Deep‐water nitrate is a major reservoir of oceanic combined nitrogen and has long been considered to be the major source of new nitrogen supporting primary production in the oligotrophic ocean. 15N:14N ratios in plankton provide an integrative record of the nitrogen cycle processes at work in the ocean, and near‐surface organic matter in oligotrophic waters like the Sargasso Sea is characterized by an unusually low 15N content relative to average deep‐water nitrate. Herein we show that the low dΔ15N of suspended particles and zooplankton from the tropical North Atlantic cannot arise through isotopic fractionation associated with nutrient uptake and food web processes but are instead consistent with a significant input of new nitrogen to the upper water column by N2 fixation. These results provide direct, integrative evidence that N2 fixation makes a major contribution to the nitrogen budget of the oligotrophic North Atlantic Ocean.

Decoupling of primary production and community respiration in the ocean: implications for regional carbon studies

Primary production (P) and community respiration (R) were measured in vitro along 2 transects of a trans-oceanic section crossing the oligotrophic North Atlantic waters, and during a drifter experiment in the eutrophic waters of the Northwest Africa upwelling system. Our results indi- cate that the scales of variability of P and R are different, R being less variable than P in the 2 studies. In the first transect of the oceanic study, the mean P:R ratio was 1 (1.1), the difference being statistically significant. The cause was a significant increase in P in the second transect, which was decoupled from changes in chlorophyll a (chl a) or R. In upwelling waters, however, R was always significantly lower than P (P:R > 1), and enhancements in P were paralleled by increases in chl a but not in R. The close correlation between the P:R ratio and P (and the lack of correlation between P:R and R) supports the view that changes in P but not in R control the transition of the system from net heterotrophy to net autotrophy (in our study at P > 56 mmol C m -2 d -1 ). The observed decoupling between P and R at temporal scales of days to weeks, has strong implications for addressing metabolic balances from in vitro experiments carried out during limited field samplings. We conclude that unless the scales of variability are established for the 2 metabolic rates, and particularly for P, it will not be feasible to accurately establish regional metabolic balances at seasonal to annual scales.

Dissolved organic carbon production by microbial populations in the Atlantic Ocean

Dissolved organic carbon (DOC) production by microbial populations was measured at 19 stations in the Atlantic Ocean to quantify the fraction of photoassimilated carbon that flows through the dissolved organic pool at basin scale and to assess the relationship between the percentage of DOC production, phytoplankton size structure, and rates of net community production. Experiments were conducted during four cruises carried out between May 1998 and October 1999, covering three upwelling regions: Benguela (SW Africa), Mauritania (NW Africa) and NW Spain, and the oligotrophic North Atlantic subtropical gyre between 30°N and 36°N. Photic zone integrated particulate organic carbon (POC) production rates ranged from 10 to 1,178 mg C m−2 h−1, thus covering a wide productivity spectrum. The percentage of DOC production with respect to total integrated primary production ranged from 4 to 42%, being larger in oligotrophic, picoplanktonߚdominated waters, where a balanced metabolism of the microbial community was observed, than in productive, net autotrophic waters, where largeߚsized cells formed the bulk of the phytoplankton biomass. A highly significant relationship was calculated between DOC and POC production rates in upwelling conditions. By contrast, the relationship between these variables in oligotrophic environments was weak, which suggests that different processes could be controlling the release of dissolved organic matter in productive and unproductive waters.

Soluble and Colloidal Iron in the Oligotrophic North Atlantic and North Pacific

In the oligotrophic North Atlantic and North Pacific, ultrafiltration studies show that concentrations of soluble iron and soluble iron-binding organic ligands are much lower than previously presumed "dissolved" concentrations, which were operationally defined as that passing through a 0.4-micrometer pore filter. Our studies indicate that substantial portions of the previously presumed "dissolved" iron (and probably also iron-binding ligands) are present in colloidal size range. The soluble iron and iron-binding organic ligands are depleted at the surface and enriched at depth, similar to distributions of major nutrients. By contrast, colloidal iron shows a maximum at the surface and a minimum in the upper nutricline. Our results suggest that "dissolved" iron may be less bioavailable to phytoplankton than previously thought and that iron removal through colloid aggregation and settling should be considered in models of the oceanic iron cycle.

Late Quaternary high‐resolution biomarker and other sedimentary climate proxies in a Northeast Atlantic Core

We studied variations in terrigenous input and sea surface temperature over the last 208 ka using a multiproxy approach, involving direct comparison of terrigenous and marine molecular biomarker, foraminiferal abundance, and other sedimentological data for the same horizons in a single northeast Atlantic core (core T88‐9P; 48°23′N, 25°05′W, 3193 m water depth, 790 cm core length) with a well‐resolved δ18O stratigraphy. The abundances of three land‐derived, long‐chain compound classes, the n‐alkanes (C27, C29, and C31), the n‐alkanoic acids (C24, C26, and C28), and the n‐alkanols (C24, C26, and C28) show glacial/interglacial related changes in the past 208 ka which generally parallel the weight percent noncarbonate. The abundances of these three compound classes are higher in glacial than in interglacial sediments. Such changes are consistent with the expected higher dust inputs during glaciations as well as enhanced advection of particulate matter in current systems. Ice‐rafted debris (Heinrich events) may also contribute to the terrigenous biomarker record. The sea surface temperature (SST) signals derived from the alkenone unsaturation index (UK′37) give an average SST during interglacial periods of 13°C, which is about 3–4°C higher than the average glacial SST estimate. The UK′37− derived SST values dropped markedly in some Heinrich layers, while sometimes leading the onset of the layer; the percent Neogloboquadrina pachyderma(s), which is another frequently used qualitative temperature proxy, generally shows maxima coinciding with the layers. Our results show that most Heinrich layers were produced during times of intensely cold waters in the eastern North Atlantic. They demonstrate that sufficient alkenones are present in the sediments for alkenone stratigraphy to be used as a tool for the assessment of short‐term variations in sea surface temperatures even in these oligotrophic North Atlantic waters.

Seasonal and interannual variability of oceanic carbon dioxide species at the U.S. JGOFS Bermuda Atlantic Time-series Study (BATS) site

The seasonal and interannual dynamics of the oceanic carbon cycle and the strength of air-sea exchange of carbon dioxide are poorly known in the North Atlantic subtropical gyre. Between October 1988 and December 1993, a time series of oceanic measurements of total carbon dioxide (TCO 2), alkalinity (TA) and calculated pCO 2 was obtained at the Bermuda Atlantic Time-series Study (BATS) site (31°50′N, 64°10′W) in the Sargasso Sea. These measurements constitute the most extensive set of CO 2 species data collected in the oligotrophic North Atlantic. Seasonal changes in surface and water-column CO 2 species were ∼40–50 μmol kg −1 in TCO 2, ∼20 μmol kg −1 in TA, and ∼90–100 μatm in calculated pCO 2. These large changes were driven principally by deep convective winter mixing, temperature forcing and biological activity. TA was well correlated with salinity (with the exception of a 15–25 μmol kg −1 drawdown of TA on one cruise resulting from open-ocean calcification). TCO 2 and pCO 2 were well correlated with seasonal temperature changes (8–9°C). Other underlying processes, such as biological production, advection, gas exchange of CO 2 and vertical entrainment, were important modulators of the carbon cycle, and their importance varied seasonally. Each spring-to-summer, despite the absence of measurable nutrients in the euphotic zone, a 35–40 μmol kg −1 decrease in TCO 2 was attributed primarily to the biological uptake of TCO 2 (evaporation/precipitation balance, gas exchange, and advection were also important). An increase in TCO 2 during the fall months was associated primarily with entrainment of higher TCO 2 subsurface waters. These seasonal patterns require a reassessment of the modelling of the carbon cycle using nutrient tracers and Redfield stoichiometries. Overall, the region is a weak sink (0.22–0.83 mol C m −2 year −1) for atmospheric CO 2. Upper ocean TCO 2 levels increased between 1988 and 1993, at a rate of ∼ 1.7 μmol kg −1 year −1. This increase appears to be in response to the uptake of atmospheric CO 2 through gas exchange or natural variability of the subtropical gyre.