E. Malcolm S. Woodward

Large-scale distribution of Atlantic nitrogen fixation controlled by iron availability

Oceanic fixed-nitrogen concentrations are controlled by the balance between nitrogen fixation and denitrification1, 2, 3, 4. A number of factors, including iron limitation5, 6, 7, can restrict nitrogen fixation, introducing the potential for decoupling of nitrogen inputs and losses2, 5, 8. Such decoupling could significantly affect the oceanic fixed-nitrogen inventory and consequently the biological component of ocean carbon storage and hence air–sea partitioning of carbon dioxide2, 5, 8, 9. However, the extent to which nutrients limit nitrogen fixation in the global ocean is uncertain. Here, we examined rates of nitrogen fixation and nutrient concentrations in the surface waters of the Atlantic Ocean along a north–south 10,000 km transect during October and November 2005. We show that rates of nitrogen fixation were markedly higher in the North Atlantic compared with the South Atlantic Ocean. Across the two basins, nitrogen fixation was positively correlated with dissolved iron and negatively correlated with dissolved phosphorus concentrations. We conclude that inter-basin differences in nitrogen fixation are controlled by iron supply rather than phosphorus availability. Analysis of the nutrient content of deep waters suggests that the fixed nitrogen enters North Atlantic Deep Water. Our study thus supports the suggestion that iron significantly influences nitrogen fixation5, and that subsequent interactions with ocean circulation patterns contribute to the decoupling of nitrogen fixation and loss2, 4, 8.

Plankton respiration in the Eastern Atlantic Ocean

Concurrent measurements of dark community respiration (DCR), gross production (GP), size fractionated primary production (14C PP), nitrogen uptake, nutrients, chlorophyll a concentration, and heterotrophic and autotrophic bacterial abundance were collected from the upper 200 m of a latitudinal (32°S–48°N) transect in the Eastern Atlantic Ocean during May/June 1998. The mean mixed layer respiration rate was 2.5±2.1 mmol O2 m−3 d−1 (n=119) for the whole transect, 2.2±1.1 mmol O2 m−3 d−1 (n=32) in areas where chlorophyll a was <0.5 mg m−3 and 1.5±0.7 mmol O2 m−3 d−1 (n=10) where chlorophyll a was <0.2 mg m−3. These values lie within the range of published data collected in comparable waters, they co-vary with indicators of heterotrophic and autotrophic biomass (heterotrophic bacterial abundance, chlorophyll a concentration, beam attenuation and particulate organic carbon concentration) and they can be reconciled with accepted estimates of total respiratory activity. The mean and median respiratory quotient (RQ), calculated as the ratio of dissolved inorganic carbon production to dissolved oxygen consumption, was 0.8 (n=11). At the time of the study, plankton community respiration exceeded GP in the picoautotroph dominated oligotrophic regions (Eastern Tropical Atlantic [15.5°S–14.2°N] and North Atlantic Subtropical Gyre [21.5–42.5°N]), which amounted to 50% of the stations sampled along the 12,100 km transect. These regions also exhibited high heterotrophic: autotrophic biomass ratios, higher turnover rates of phytoplankton than of bacteria and low f ratios. However, the carbon supply mechanisms required to sustain the rates of respiration higher than GP could not be fully quantified. Future research should aim to determine the temporal balance of respiration and GP together with substrate supply mechanisms in these ocean regions.

Nitrogen biogeochemical cycling in the northwestern Indian Ocean

Abstract The vertical distribution and fine scale structure of nitrate (NO 3 ), nitrite (NO 2 ), nitrous Oxide (N 2 O), phosphate (PO 4 ), oxygen (O 2 ) and chlorophyll α (chl α) were determined in the North Western Indian Ocean (NWIO) along a meridional section (67°E) from the Equator to the Gulf of Oman using an Autoanalyser for micromolar levels of nutrients, and chemiluminescence and gas chromatographic methods for nanomolar levels of NO 3 and NO 2 and N 2 O respectively. Three biogeochimically contrasting regimes were investigated: (1) the highly oligotrophic nutrient-depleted subtropical gyre; (2) the nonsoonal upwelling of nutrient-rich intermediate waters of the southeastern Arabian Coast; and (3) the denitrifying O 2 -depleted zone (ODZ; ca 150–1200 m depth) in the Arabian Sea. Concentrations of NO 3 and NO 2 were severely depleted in surface oligotrophic waters from the equator (average 43 and 3.6 nM respectively) to the subtropical gyre (12–15°N; average 13.3 and 2.0 nM respectively) with similar levels in the more stratified Gulf of Oman. Upwelling waters off Southern Arabia had three orders of magnitude higher NO 3 levels, and throughout the NWIO, the calculated NO 3 -fuelled primary production appeared to be regulated by NO 3 concentration. Existing Redfield ΔO 2 /ΔNO 3 regeneration ratios (=9.1) previously derived for the deep Indian Ocean were confirmed (= 9.35) within the oxic upper layers of the NWIO. The “NO”-potential temperature relationship ( Broecker , 1974 Earth and Planetary Science Letters , 23 , 100–107) needed for the derivation of expected NO 3 and NO 3 -deficits within the denitrifying ODZ were refined using an isopycnal, binary mixing model along the σ θ = 26.6%, density layer to take into account the inflowing contribution of NO 3 -depleted Persian Gulf Water. Vertically integrated NO 3 -deficits increased northwards from 0.8 mol NO 3 -N m −2 at Sta. 2 (04°N), up to 6.49 mol NO 3 -N m −2 at Sta. 9, at the mouth of the Gulf of Oman, then decreased to 4.10 moles NO 3 -N m −2 toward Sta. 11, near the Straits of Hormuz. When averaged for the denitrification area of the Arabian Sea, this corresponds to a deficit of 118 Tg NO 3 -N. Adopting a recent Freon-11 based estimate of water residence time of 10 years ( Olson et al. , 1993, Deep-Sea Research II , 40 , 673–685) for the O 2 -depleted layer, we calculate an annual net denitrification flux of 11.9 Tg N to the atmosphere or approximately 10% of the global water column denitrification rates. Supersaturated N 2 O concentrations were found in both surface oxic and upwelling waters (up to 246%) and peaked at the base of the ODZ (up to 1264%) in the northern Arabian Sea. Both nitrification in oxic waters and denitrification in hypoxic layers can be invoked as sources of N 2 O. The inventory of excess N 2 O amounted to 2.55 ± 1.3 Tg N 2 O-N, corresponding to annual production of 0.26 ± 0.13 Tg from denitrification. This is comparable to earlier ( Law and Owens , 1990, Nature , 346 , 826–828) estimates of the ventilation flux of N 2 O (0.22–0.39 Tg yr −1 ) from the upwelling region of the Arabian Sea. The decadal response times for circulation, deoxygenation, denitrification and ventilation of the ODZ-derived N 2 O and CO 2 greenhouse gases and their monsoonal coupling implies the Arabian Sea is a sensitive oceanic recorder of global climate change.

Nitrogen fixation and nitrogenase (nifH) expression in tropical waters of the eastern North Atlantic

Expression of nifH in 28 surface water samples collected during fall 2007 from six stations in the vicinity of the Cape Verde Islands (north-east Atlantic) was examined using reverse transcription-polymerase chain reaction (RT-PCR)-based clone libraries and quantitative RT-PCR (RT-qPCR) analysis of seven diazotrophic phylotypes. Biological nitrogen fixation (BNF) rates and nutrient concentrations were determined for these stations, which were selected based on a range in surface chlorophyll concentrations to target a gradient of primary productivity. BNF rates greater than 6 nmolN l−1 h−1 were measured at two of the near-shore stations where high concentrations of Fe and PO43− were also measured. Six hundred and five nifH transcripts were amplified by RT-PCR, of which 76% are described by six operational taxonomic units, including Trichodesmium and the uncultivated UCYN-A, and four non-cyanobacterial diazotrophs that clustered with uncultivated Proteobacteria. Although all five cyanobacterial phylotypes quantified in RT-qPCR assays were detected at different stations in this study, UCYN-A contributed most significantly to the pool of nifH transcripts in both coastal and oligotrophic waters. A comparison of results from RT-PCR clone libraries and RT-qPCR indicated that a γ-proteobacterial phylotype was preferentially amplified in clone libraries, which underscores the need to use caution interpreting clone-library-based nifH studies, especially when considering the importance of uncultivated proteobacterial diazotrophs.

Primary production and nutrient assimilation in the Iberian upwelling in August 1998

Primary production was measured during two Lagrangian experiments in the Iberian upwelling. The first experiment, in a body of upwelled water, measured day-to-day changes in phytoplankton activity as the water mass moved south along the shelf break. Nutrient concentrations decreased over a five day period, with concomitant increases in phytoplankton biomass. Initially the maximum phytoplankton biomass was in the upper 10m but after four days, a sub-surface chlorophyll maximum was present at 30m. Depth-integrated primary production at the beginning of the experiment was 70mmolC.m−2.d−1 (838mgC.m−2.d−1) and reached a maximum of 88mmolC.m−2.d−1 (1053mgC.m−2.d−1) on day 3. On day 1, the picoplankton fraction ( 5μm) phytoplankton, but the increase in overall production during the drift experiment was by these larger cells. Nitrate was the dominant nitrogen source. As nutrient concentrations declined, ammonium became increasingly more important as a nitrogen source and the f-ratio decreased from 0.7 to 0.5. Picoplankton cells (<2μm) were responsible for most (65–80%) of the ammonium uptake. The C:N:P uptake ratios were very close to the Redfield ratio for the first four days but as nutrients became depleted high C:N uptake ratios (11 to 43) were measured. Over the period of the experiment, nitrate concentration within the upper 40m decreased by 47.91mmolN.m−2. In vitro estimates, based on 15N nitrate uptake, accounted for 56% of the decrease in nitrate concentration observed in the drifting water mass. Ammonium uptake over the same four day period was 16.28mmolN.m−2, giving a total nitrogen uptake of 43.18mmolN.m−2. In the second experiment, an offshore filament was the focus and a water mass was sampled as it moved offshore. Nutrient concentrations were very low (nitrate was <10nmol l−1 and ammonium was 20–40nmol l−1). Primary production rate varied between 36mmolC.m−2.d−1 (436mgC.m−2.d−1) and 21mmolC.m−2.d−1 (249mgC.m−2.d−1). Picophytoplankton was the most productive fraction and was responsible for a constant proportion (ca 0.65) of the total carbon fixation. Uptake rates of both nitrate and ammonium were between 10 and 20% of those measured in the upwelling region. Urea could be a very significant nitrogen source in these waters with much higher uptake rates than nitrate or ammonium; urea turnover times were ca. one day but the source of the urea remains unknown. Urea uptake had a profound effect on calculated f ratios. If only nitrate and ammonium uptake was considered, f ratios were calculated to be 0.42–0.46 but inclusion of urea uptake reduced the f ratio to <0.1. The primary production of this oligotrophic off-shore filament was driven by regenerated nitrogen.

Two Lagrangian experiments in the Iberian upwelling system: tracking an upwelling event and an off-shore filament

Abstract Two Lagrangian drift experiments were carried out at the NW Iberian margin. The first tracked a body of nutrient-rich, upwelled water as it moved south along the shelf break over a 5 day period. The second experiment, of similar duration, followed a water mass as it moved into the deep ocean in an off-shelf filament. This paper describes the background to and aims of each experiment. The overall objective was to quantify chemical and biological processes relating to the additional potential for the ocean at the shelf margins to sequester atmospheric CO2 in upwelling regions. The first experiment began at a time of intense wind-driven upwelling; within 2 days, the wind speed had moderated and the system entered a relaxation period with greatly reduced upwelling. The patch of upwelled water was marked by a single buoy array and it moved south along the shelf break. Transport was initially rapid but slowed with reducing wind speed. The temperature–salinity characteristics were consistent with sampling only a single water mass throughout the experiment. A model of particle trajectories showed slight deviation from the actual movement of the marked water mass, but overall the data support the assumption that the experiment was Lagrangian. During a 5 day experimental period, nutrients were utilised with a N:P ratio of 18.3 and N:Si of 4. Nutrient concentrations first reduced in the near-surface but depletion deepened in the water column during the experiment. At the beginning of the experiment, the highest chlorophyll concentrations were in the surface 15m but this was replaced by a subsurface chlorophyll maximum at 30m. There was a shift from a small flagellate and dinoflagellate dominated photosynthetic phytoplankton assemblage to a diatom dominated assemblage. A high biomass of heterotrophic dinoflagellates and ciliates was also present. Canonical correlation analysis between environmental variables and microplankton assemblages, as defined by principal component analysis, suggested that a considerable part of DON production resulted from trophic relationships rather than direct release from phytoplankton. The second experiment followed a water mass marked with 5 Argos drifting buoys for 5 days as the water drifted off shelf in an off-shore filament. This water mass was extremely oligotrophic; nitrate concentrations were typically The data presented in this paper are a general description of the experiments and form the background to the more detailed descriptions given in the individual papers that make up this Special Issue of Progress in Oceanography.

How widespread and important is N2 fixation in the North Atlantic Ocean

The spatial extent of N2 fixation in the Atlantic Ocean is examined by determining the isotopic composition of N in suspended particulate organic nitrogen (δ15N PONsusp). The samples were collected from zonal and meridional transects of the Atlantic Ocean during a 3-year period. There is a consistent depleted δ15N PONsusp signal extending over the center of the northern subtropical gyre, which partly coincides with a region where the tracer N* increases westward following the gyre circulation. This nonconservative behavior of N* implies that N2 fixation is responsible for the depleted δ15N PONsusp. A mixing model suggests that N2 fixation over parts of the northern gyre provides up to 74% of the N utilized by phytoplankton. However, since the PONsusp represents only a small fraction of the total N pool, N2 fixation probably only plays a minor role in supplying new N to the euphotic zone in the surface waters of the northern subtropical gyre.

Fine-scale nutrient and carbonate system dynamics around cold-water coral reefs in the northeast Atlantic

Ocean acidification has been suggested as a serious threat to the future existence of cold-water corals (CWC). However, there are few fine-scale temporal and spatial datasets of carbonate and nutrients conditions available for these reefs, which can provide a baseline definition of extant conditions. Here we provide observational data from four different sites in the northeast Atlantic that are known habitats for CWC. These habitats differ by depth and by the nature of the coral habitat. At depths where CWC are known to occur across these sites the dissolved inorganic carbon ranged from 2088 to 2186 μmol kg−1, alkalinity ranged from 2299 to 2346 μmol kg−1, and aragonite Ω ranged from 1.35 to 2.44. At two sites fine-scale hydrodynamics caused increased variability in the carbonate and nutrient conditions over daily time-scales. The observed high level of variability must be taken into account when assessing CWC sensitivities to future environmental change.

Nitrate uptake at photic zone depths is not important for export in the subtropical ocean

Observations of nitrate (NO3?) uptake across the photic zone of the tropical and subtropical Atlantic Ocean are described. High NO3? uptake rates were commonly found at depth (>75 m), coincident with the deep chlorophyll maximum and the nitracline, whereas highest rates of carbon fixation were found in near-surface waters (0?50 m). Thus NO3? based estimates of in situ new production (NO3? uptake × 6.6) at depth are in excess of in situ measurements of carbon fixation implying NO3? uptake exceeds production requirements. Such surplus NO3? uptake may ultimately result in the production of dissolved organic nitrogen with its measurement within the particulate phase representing a transient enrichment. When only upper euphotic zone (to 14% irradiance depth) rates of NO3? uptake and carbon fixation are considered, new production is less than total carbon fixation, suggesting, as expected, that additional nitrogen sources are required to support the observed production levels. Comparison of the deep NO3? uptake rates with literature estimates of seasonal NO3? drawdown within the nitracline suggests that the drawdown observed represents only 1% of the actual NO3? uptake occurring in this region of the water column, and hence most NO3? uptake must be recycled locally rather than exported.

Mercury presence and speciation in the South Atlantic Ocean along the 40°S transect

Mercury (Hg) natural biogeochemical cycle is complex and a significant portion of biological and chemical transformation occurs in the marine environment. To better understand the presence and abundance of Hg species in the remote ocean regions, waters of South Atlantic Ocean along 40°S parallel were investigated during UK-GEOTRACES cruise GA10. Total mercury (THg), methylated mercury (MeHg), and dissolved gaseous mercury (DGM) concentrations were determined. The concentrations were very low in the range of pg/L (femtomolar). All Hg species had higher concentration in western than in eastern basin. THg did not appear to be a useful geotracer. Elevated methylated Hg species were commonly associated with low-oxygen water masses and occasionally with peaks of chlorophyll a, both involved with carbon (re)cycling. The overall highest MeHg concentrations were observed in themixed layer (500m) and in the vicinity of the Gough Island. Conversely, DGM concentrations showed distinct layering and differed between the water masses in a nutrient-like manner. DGM was lowest at surface, indicating degassing to the atmosphere, and was highest in the Upper Circumpolar Deep Water, where the oxygen concentration was lowest. DGM increased also in Antarctic Bottom Water. At one station, dimethylmercury was determined and showed increase in region with lowest oxygen saturation. Altogether, our data indicate that the South Atlantic Ocean could be a source of Hg to the atmosphere and that its biogeochemical transformations depend primarily upon carbon cycling and are thereby additionally prone to global ocean change.