Influence of atmospheric inputs on the iron distribution in the subtropical North-East Atlantic Ocean

Aerosol (soluble and total) iron and water-column dissolved (DFe, < 0.2 μm) and total dissolvable (TDFe, unfiltered) iron concentrations were determined in the Canary Basin and along a transect towards the Strait of Gibraltar, in order to sample across the Saharan dust plume. Cumulative dust deposition fluxes estimated from direct aerosol sampling during our one-month cruise are representative of the estimated deposition fluxes based on near surface water dissolved aluminium concentrations measured on board. Iron inventories in near surface waters combined with flux estimates confirmed the relatively short residence time of DFe in waters influenced by the Saharan dust plume (6–14 months). Enhanced near surface water concentrations of DFe (5.90–6.99 nM) were observed at the Strait of Gibraltar mainly due to inputs from metal-rich rivers. In the Canary Basin and the transect towards Gibraltar, DFe concentrations (0.07–0.76 nM) were typical of concentrations observed in the surface North Atlantic Waters, with the highest concentrations associated with higher atmospheric inputs in the Canary Basin. Depth profiles showed that DFe and TDFe were influenced by atmospheric inputs in this area with an accumulation of aeolian Fe in the surface waters. The sub-surface minimum of both DFe and TDFe suggests that a simple partitioning between dissolved and particulate Fe is not obvious there and that export may occur for both phases. At depths of around 1000–1300 m, both regeneration and Meddies may explain the observed maximum. Our data suggest that, in deep waters, higher particle concentrations likely due to dust storms may increase the scavenging flux and thus decrease DFe concentrations in deep waters.     

The chemical speciation of iron in the north-east Atlantic Ocean

The distribution of dissolved iron and its chemical speciation (organic complexation and redox speciation) were studied in the northeastern Atlantic Ocean along 23°W between 37 and 42°N at depths between 0 and 2000 m, and in the upper-water column (upper 200 m) at two stations further east at 45°N10°W and 40°N17°W in the early spring of 1998. The iron speciation data are here combined with phytoplankton data to suggest cyanobacteria as a possible source for the iron binding ligands. The organic Fe-binding ligand concentrations were greater than that of dissolved iron by a factor of 1.5–5, thus maintaining iron in solution at levels well above it solubility. The water column distribution of the organic ligand indicates in-situ production of organic ligands by the plankton (consisting mainly of the cyanobacteria Synechococcus sp.) in the euphotic layer and a remineralisation from sinking biogenic particles in deeper waters. Fe(II) concentrations varied from below the detection limit (<0.1 nM) up to 0.55 nM but represented only a minor fraction of 0% to occasionally 35% of the dissolved iron throughout the water column. The water column distribution of the Fe(II) suggests biologically mediated production in the deep waters and photochemical production in the euphotic layer. Although there was no evidence of iron limitation in these waters, the aeolian iron input probably contributed to a shift in the phytoplankton assemblage towards increased Synechococcus growth.     

Size-fractionated biological iron and carbon uptake along a coastal to offshore transect in the NE Pacific

Iron has been shown to limit phytoplankton growth in high-nutrient low-chlorophyll (HNLC) regions such as the NE subarctic Pacific. We report size-fractionated Fe-uptake rates by the entire plankton community in short (6–8 h) light and dark incubations along an E–W transect from P04 (a coastal ocean station) to OSP (an open-ocean HNLC station) during August–September 1997. Size-fractionated primary productivity and chl a were measured to monitor algal Fe : C uptake ratios and Fe-uptake relative to phytoplankton biomass. The >5.0 μm size-class, which consisted mostly of large diatoms, had the highest Fe-uptake rate at nearshore stations (P04 and P8), but Fe-uptake rates for this size class decreased despite increases in biomass and primary productivity when transecting westwards to HNLC waters. Fe-uptake rates of the small size class (0.2–1.0 μm, including heterotrophic bacteria and autotrophs) were inversely related to the >5.0 μm size-class uptake rates, in that stations with high dissolved Fe (DFe) concentrations had relatively low uptake rates compared to those in the low-Fe offshore region. The 1.0–5.0 μm size-class Fe-uptake rates were low, relatively invariant along the transect, and differed little between light and dark incubations. Dark Fe-uptake rates averaged 10–20% less than those in the light for the >5.0 μm size class. Dark uptake rates however, were higher than light uptake rates for the 0.2–1.0 μm size class at all stations. Fe : C uptake ratios were high for all size classes at P04, but decreased as DFe concentrations decreased offshore. The prokaryote-dominated 0.2–1.0 μm size class had the highest Fe : C uptake ratios at all stations. These data suggest that prokaryotic organisms make an important contribution to biological Fe uptake in this region. Our experiments support the results of previous culture work, suggesting higher Fe : C ratios in coastal phytoplankton compared to open-ocean species, and demonstrate that light can have a large effect on Fe partitioning between size classes in subarctic Pacific HNLC waters.     

Organic complexation of iron in the Southern Ocean

The chemical speciation of iron was determined in the Southern Ocean along a transect from 48 to 70°S at 20°E. Dissolved iron concentrations were low at 0.1–0.6 nM, with average concentrations of 0.25±0.13 nM. Organic iron complexing ligands were found to occur in excess of the dissolved iron concentration at 0.72±0.23 nM (equivalent to an excess of 0.5 nM), with a complex stability of log K_FeL′=22.1±0.5 (on the basis of Fe³⁺ and L′). Ligand concentrations were higher in the upper water column (top 200 m) suggesting in situ production by microorganisms, and less at the surface consistent with photochemical breakdown. Our data are consistent with the presence of stable organic iron-complexing ligands in deep global ocean waters at a background level of ∼0.7 nM. It has been suggested that this might help stabilise iron at levels of ∼0.7 nM in deep ocean waters. However, much lower iron concentrations in the waters of the Southern Ocean suggest that these ligands do not prevent the removal of iron (by scavenging or biological uptake) to well below the concentration of these ligands. Scavenging reactions are probably inhibited by such ligand competition, so it is likely that biological uptake is the chief cause for the further removal of iron to these low levels in waters that suffer from very low iron inputs.     

Particulate trace element fluxes in the deep northeast Atlantic Ocean

Particulate fluxes of trace elements (Al, Cd, Co, Cu, Fe, Mn, Ni, P, Ti, V and Zn) in the northeast Atlantic Ocean (three positions at latitudes from 33°N to 54°N along ∼20°W) were measured using time-series sediment traps between March 1992 and September 1994. Significant variabilities of fluxes with season and depth (1000 m to maximum of 4000 m) were observed only for ‘biogenic elements’, such as Cd, Ni, Zn or P. On the other hand, we found a distinct large-scale increase of fluxes into the deep-sea traps to the south for Al, Co, Fe, Mn and V. We attribute this latitudinal gradient to the increasing influence of the Saharan dust plume. The biogenic flux decreased towards the south. This trend was clearly visible for Cd and P only. The fluxes of other ‘nutrient-like’ elements, such as Ni or Zn, exhibited a general decrease between 53°N and 33°N. We compared our sedimentation flux data with published data from the western North Atlantic basins. For this purpose we corrected the deep-sea fluxes of Cu, Mn, Ni and Zn for their lithogenic fractions on the basis of Al, with average crustal material and granitic rocks as references. The comparison indicates that these ‘excess’ fluxes are a factor of at least 2 higher in the western basins for the selected elements. Estimated fluxes are in good agreement with reported atmospheric deposition in the two areas. The noted imbalance between the non-lithogenic atmospheric input of Mn and the determined ‘excess flux’ in the deep northeast Atlantic indicates an additional input in the form of a lateral flux of dissolved Mn(II) species and scavenging onto sinking particles. With respect to the mechanism of sedimentation, a unique behaviour is noticed for the refractory elements Co, Fe, Mn, Ti and V, which were found to correlate with the vertical transport of Al (clay). The ‘excess’ fluxes of Cu, Ni and Zn are linearly related to C_org, whereas the overall relation of Cd to P fluxes exhibits a molar Cd/P ratio of 2.0×10^-4, which is close to the ratio in the dissolved fractions in the northeast Atlantic.     

Seasonality of particle-associated trace element fluxes in the deep northeast Atlantic Ocean

In the framework of the German contribution to the Joint Global Ocean Flux Study (JGOFS), deep-water fluxes of particle-associated trace elements were measured in the northeast Atlantic Ocean. The sinking particles were collected almost continuously from 1992 to 1996 at three time-series stations, L1 (33°N/22°W), L2 (47°N/20°W), and L3 (54°N/21°W), using sediment traps. The focus of the present study is the temporal variability of the particle-associated elemental fluxes of Al, Ca, Cd, Co, Cu, Fe, Mn, Ni, P, Pb, Ti, V, and Zn at a depth of 2000 m.A clear seasonality of the fluxes that persisted for several years was documented for the southernmost station (L1) at stable oligotrophic conditions in the area of the North Atlantic Subtropical Gyre East (NASTE). At L2 and L3, an episodic nature of the elemental fluxes was determined. Mesoscale eddies are known to frequently cause temporal and spatial variability in the flux of biogenic components in that area. These events modified the simple seasonal pattern controlled by the annual cycle at L2, in the North Atlantic Drift Region (NADR), and at L3, which was influenced by the Atlantic Arctic province (ARCT). All stations were characterized by an additional episodic lithogenic atmospheric supply reaching the deep sea.The integrated annual fluxes during the multi-year study revealed similar flux magnitudes for lithogenic elements (Al, Co, Fe, Ti, and V) at L2 and L3 and roughly twofold fluxes at L1. Biogenic elements (Cd, P, and Zn) showed the opposite trend, i.e., two to fourfold higher values at L2 and L3 than at L1. For Mn, Ni, and Cu, the spatial differences were smaller, perhaps because of the intermediate behavior, between lithogenic and biogenic, of these elements. Similarly, among the three study sites, there were no noticeable differences in the total annual flux of Pb.The respective lithogenic fractions of the deep-sea fluxes of Cd, Co, Cu, Mn, Ni, V, and Zn were subtracted based on the amount of Al, with the average composition of the continental crust as reference. This procedure allowed estimation of the labile trace element fraction (TE_exc) of the particles, i.e., TE taken up or scavenged during particle production and sedimentation. The ratios of TE_exc/P clearly demonstrated an enrichment of TE over labile P from biogenic surface material to the deep sea for Zn (factor 4–6), Mn (12–27), Ni (3–5), and Cu (9–25); an intermediate status for Co (0.5–2.2); and depletion for Cd vs. P (0.2–0.4). Surprisingly, the recycling behavior of excess Co was found to be similar to that of P. Hence, Co_exc behaved like a biogenic element; this is in contrast to total Co, which is dominated by the refractory lithogenic fraction.Moreover, it is argued that these excess elemental fluxes caused a loss of the dissolved elements in upper waters, since their transport reaches the deep-sea waters at 2000 m, a depth far below of deep-winter mixing and upwelling. The annual amount of excess TE exported from surface waters was estimated to be 1.3×10⁹ mol Zn y^?1, 4.4×10⁹ mol Mn y^?1, 4.9×10⁸ mol Ni y^?1, 2.2×10⁷ mol Cd y^?1, 7.4×10⁸ mol Cu y^?1, and 2.7×10⁷ mol Co y^?1 for the whole North Atlantic Ocean. Important primary sources that could replenish these losses are the aeolian and fluvial supply processes.     

Nutrient gradients in the western North Atlantic Ocean: Relationship to microbial community structure and comparison to patterns in the Pacific Ocean

Studies of nitrogen and phosphorus dynamics in the oligotrophic surface waters of the western North Atlantic Ocean have been constrained because ambient concentrations are typically at or below the detection limits of standard colorometric methods, except during periods of deep vertical mixing. Here we report the application of high-sensitivity analytical methods—determinations of nitrate plus nitrite (N+N) by chemiluminescence and soluble reactive phosphorus (SRP) by the magnesium induced co-precipitation (MAGIC) protocol—to surface waters along a transect from the Sargasso Sea at 26°N through the Gulf Stream at 37°N, including sampling at the JGOFS Bermuda Atlantic Time-series Study (BATS) station. The results were compared with data from the BATS program, and the HOT station in the Pacific Ocean, permitting cross-ecosystem comparisons. Microbial populations were analyzed along the transect, and an attempt was made to interpret their distributions in the context of the measured nutrient concentrations.Surface concentrations of N+N and SRP during the March 1998 transect separated into 3 distinct regions, with the boundaries corresponding roughly to the locations of the BATS station (∼31°N) and the Gulf Stream (∼37°N). Although N+N and SRP co-varied, the [N+N] : [SRP] molar ratios increased systematically from ∼1 to 10 in the southern segment, remained relatively constant at ∼40–50 between 31°N and 37°N, then decreased again systematically to ratios <10 north of the Gulf Stream. Dissolved organic N (DON) and P (DOP) dominated (⩾90%) the total dissolved N (TDN) and P (TDP) pools except in the northern portion of the transect. The [DON] : [DOP] molar ratios were relatively invariant (∼30–60) across the entire transect.Heterotrophic prokaryotes (operationally defined as “bacteria”), Prochlorococcus, Synechococcus, ultra- and nanophytoplankton, cryptophytes, and coccolithophores were enumerated by flow cytometry. The abundance of bacteria was well correlated with the concentration of SRP, and that of the ultra- and nanophytoplankton was well correlated with the concentration of N+N. The only group whose concentration was correlated with temperature was Prochlorococcus, and its abundance was unrelated to the concentrations of nutrients measured at the surface.We combined our transect results with time-series measurements from the BATS site and data from select depth profiles, and contrasted these North Atlantic data sets with time-series of N and P nutrient measurements from a station in the North Pacific subtropical gyre near Hawaii [Hawaii Ocean Time-series (HOT) site]. Two prominent differences are readily observed from this comparison. The [N+N] : [SRP] molar ratios are much less than 16 : 1 during stratified periods in surface waters at the BATS site, as is the case at the HOT site year round. However, following deep winter mixing, this ratio is much higher than 16 : 1 at BATS. Also, SRP concentrations in the upper 100 m at BATS fall in the range 1–10 nM during stratified periods, which is at least one order of magnitude lower than at the HOT site. That two ecosystems with comparable rates of primary and export production would differ so dramatically in their nutrient dynamics is intriguing, and highlights the need for detailed cross ecosystem comparisons.     

Nitrous oxide and methane in the Atlantic Ocean between 50°N and 52°S: Latitudinal distribution and sea-to-air flux

We discuss nitrous oxide (N₂O) and methane (CH₄) distributions in 49 vertical profiles covering the upper ∼300 m of the water column along two ∼13,500 km transects between ∼50°N and ∼52°S during the Atlantic Meridional Transect (AMT) programme (AMT cruises 12 and 13). Vertical N₂O profiles were amenable to analysis on the basis of common features coincident with Longhurst provinces. In contrast, CH₄ showed no such pattern. The most striking feature of the latitudinal depth distributions was a well-defined “plume” of exceptionally high N₂O concentrations coincident with very low levels of CH₄, located between ∼23.5°N and ∼23.5°S; this feature reflects the upwelling of deep waters containing N₂O derived from nitrification, as identified by an analysis of N₂O, apparent oxygen utilization (AOU) and NO₃⁻, and presumably depleted in CH₄ by bacterial oxidation. Sea-to-air emissions fluxes for a region equivalent to ∼42% of the Atlantic Ocean surface area were in the range 0.40–0.68 Tg N₂O yr⁻¹ and 0.81–1.43 Tg CH₄ yr⁻¹. Based on contemporary estimates of the global ocean source strengths of atmospheric N₂O and CH₄, the Atlantic Ocean could account for ∼6–15% and 4–13%, respectively, of these source totals. Given that the Atlantic Ocean accounts for around 20% of the global ocean surface, on unit area basis it appears that the Atlantic may be a slightly weaker source of atmospheric N₂O than other ocean regions but it could make a somewhat larger contribution to marine-derived atmospheric CH₄ than previously thought.     

Dry atmospheric deposition and diazotrophy as sources of new nitrogen to northwestern Mediterranean oligotrophic surface waters

Atmospheric dry deposition of nitrogen (N) and dinitrogen (N₂) fixation rates were assessed in 2004 at the time-series DYFAMED station (northwestern Mediterranean, 43°25′N, 7°52′E). The atmospheric input was monitored over the whole year. Dinitrogen fixation was measured during different seasonal trophic states (from mesotrophy to oligotrophy) sampled during nine cruises. The bioavailability of atmospherically deposited nutrients was estimated by apparent solubility after 96 h. The solubility of dry atmospheric N deposition was highly variable (from ∼18% to more than 96% of total N). New N supplied to surface waters by the dry atmospheric deposition was mainly nitrate (NO₃⁻) (∼57% of total N, compared to ∼6% released as ammonium (NH₄⁺)). The mean bioavailable dry flux of total N was estimated to be ∼112 μmol m⁻² d⁻¹ over the whole year. The NO₃⁻ contribution (70 μmol NO₃⁻ m⁻² d⁻¹) was much higher than the NH₄⁺ contribution (1.2 μmol NH₄⁺ m⁻² d⁻¹). The N:P ratios in the bioavailable fraction of atmospheric inputs (122.5–1340) were always much higher than the Redfield N:P ratio (16). Insoluble N in atmospheric dry deposition (referred to as “organic” and believed to be strongly related to anthropogenic emissions) was ∼40 μmol m⁻² d⁻¹. N₂ fixation rates ranged from 2 to 7.5 nmol L⁻¹ d⁻¹. The highest values were found in August, during the oligotrophic period (7.5 nmol L⁻¹ at 10 m depth), and in April, during the productive period (4 nmol L⁻¹ d⁻¹ at 10 m depth). Daily integrated values of N₂ fixation ranged from 22 to 100 μmol N m⁻² d⁻¹, with a maximum of 245 μmol N m⁻² d⁻¹ in August. No relationship was found between the availability of phosphorus or iron and the observed temporal variability of N₂ fixation rates. The atmospheric dry deposition and N₂ fixation represented 0.5–6% and 1–20% of the total biological nitrogen demand, respectively. Their contribution to new production was more significant: 1–28% and 2–55% for atmospheric dry deposition and N₂ fixation, respectively. The dry atmospheric input was particularly significant in conditions of water column stratification (16–28% of new production), while N₂ fixation reached its highest values in June (46% of new production) and in August (55%).     

Dissolved iron(II) in the Pacific Ocean: Measurements from the PO2 and P16N CLIVAR/CO2 repeat hydrography expeditions

The redox speciation of dissolved iron in seawater was evaluated at 121 locations in the Pacific Ocean at depths of 15-1000 m, using the method of luminol chemiluminescence. The results indicate that reduced iron, Fe(II), is ubiquitous in surface seawater with a relatively consistent pattern of occurrence. Surface maxima were present in most profiles, with median concentrations of 25-30 pM representing 12-14% of the total dissolved iron. Concentrations decreased monotonically with depth to<12 pM within the upper euphotic zone. This pattern was observed during both day and nighttime sampling events, which suggests that non-photochemical production mechanisms can produce photochemical-like signatures. Further, if theoretical rates of Fe(II) oxidation are applicable to the open ocean, then the employed sampling methods precluded assessment of photochemically-produced Fe(II), regardless of ambient light conditions. For this and other reasons, the concentrations reported here for the upper water column likely represent lower limits of labile iron concentration, and suggest that dissolved iron may be more available for uptake than previously believed. Deeper in the water column, Fe(II) was also frequently detected, though it constituted a small fraction of the total dissolved iron. Possible source mechanisms at these depths include thermal (dark) reduction of Fe(III) organic complexes or remineralization of sinking biogenic particles containing Fe(II). In the northern Philippine Sea between the Japanese coast and the Izu-Bonin volcanic arc system, Fe(II) concentrations were found to be atypically high, possibly because of high atmospheric dust deposition near the surface and transport of sediment-derived iron at depth.     

Silver in the far North Atlantic Ocean

Total (unfiltered) silver concentrations in higher latitudes of the North Atlantic (52–68°N) are reported for the second Intergovernmental Oceanographic Commission (IOC) Global Investigation of Pollutants in the Marine Environment (GIPME) baseline survey of 1993. These silver concentrations (0.69–7.2 pM) are oceanographically consistent with those (0.24–9.6 pM) previously reported for lower latitudes in the eastern North and South Atlantic (Flegal et al., 1995). However, surface (⩽200 m) water concentrations of silver (0.69–4.6 pM) in the northern North Atlantic waters are, on average, ten-fold larger than those (0.25 pM) considered natural background concentrations in surface waters of the central Atlantic. In contrast, variations in deep far North Atlantic silver concentrations are associated with discrete water masses. Consequently, the cycling of silver in the far North Atlantic appears to be predominantly controlled by external inputs and the advection of distinct water masses, in contrast to the nutrient-like biogeochemical cycling of silver observed in the central Atlantic and Pacific oceans.     

Vertical distributions of dissolved organic carbon and nitrogen in the Southern Ocean

Detailed vertical profiles of dissolved organic carbon (DOC), nitrogen (DON) and DOC/DON ratios in the Australian sector of the Southern Ocean (56°–65°S around 140°W transect) were obtained by a high-temperature catalytic oxidation method with a modified Shimadzu TOC-5000 unit. The simultaneous analyses of DOC and total dissolved nitrogen (TDN) gave high precision results (±0.7 μMC and ±0.3 μMN). In surface layers, DOC concentrations were lower (45–55 μM) than those generally obtained from other oceanic environments (60–90 μM). The surface concentrations of DON varied from 4 to 9 μM, but within the range generally reported for other ocean regions. Surface excesses of DOC and DON were calculated against nearly constant subsurface concentrations. A consistent contribution of low C/N ratio (2.7–5.0) was found in the mid-surface layer (30–75 m), suggesting more extensive degradation of carbon-enriched materials and/or enhanced supply of nitrogen-enriched ones.     

Automated high resolution determination of the trace elements iron and aluminium in the surface ocean using a towed Fish coupled to flow injection analysis

A towed surface sampling device coupled to two automated flow injection analysis (FIA) systems is described. The towed system permits uncontaminated sampling of seawater from research vessels while underway at full speed. Coupling the sampler to the FIA systems permits automatic determination of Al and Fe in surface waters at natural levels at 5 min intervals, equivalent to ∼1.5 km spacing at a ship speed of 10 knots (5 m s⁻¹). Results from the tropical Atlantic indicate significant (50%) variation in concentrations of both Al and Fe on space scales of less than 90 km. The combined system facilitates surface mapping of large regions of the ocean for dissolved Al and Fe, thus identifying the sites and magnitude of eolian deposition to the surface ocean. In combination with the determination of nutrients and other biological parameters this permits the investigation of the role that eolian deposition plays in modifying surface water biogeochemical cycles.     

Atmospheric nitrogen deposition to the Mullica River-Great Bay Estuary

Measurements of nitrate and ammonium in precipitation and associated with aerosols were conducted at Rutgers University Marine Field Station in Tuckerton, New Jersey from March 2004 to March 2005 to characterize atmospheric nitrogen deposition to the Mullica River-Great Bay Estuary. The arithmetic means of nitrate and ammonium concentrations for precipitation samples were 2.3 mg L⁻¹ and 0.42 mg L⁻¹, respectively. Nitrate and ammonium concentrations in aerosol samples averaged 3.7 μg m⁻³ and 1.6 μg m⁻³, respectively. Wet deposition rates appeared to vary with season; the highest rate of inorganic nitrogen deposition (nitrate + ammonium) occurred in the spring with an average value of 1.33 kg-N ha⁻² month⁻¹. On an annual basis, the total (wet and dry) direct atmospheric deposition fluxes into the Mullica River-Great Bay Estuary were 7.08 kg-N ha⁻² year⁻¹ for nitrate and 4.44 kg-N ha⁻² year⁻¹ for ammonium. The total atmospheric inorganic nitrogen directly deposited to the Mullica River-Great Bay Estuary was estimated to be 4.79 × 10⁴ kg-N year⁻¹, and the total atmospheric inorganic nitrogen deposited to the Mullica River watershed was estimated to be 1.69 × 10⁶ kg-N year⁻¹. Only a fraction of the nitrogen deposited on the watershed will actually reach the estuary; most of the nitrogen will be retained in the watershed due to utilization and denitrification during transport. The amount of N reaching the Mullica River-Great Bay Estuary indirectly is estimated to be 5.07 × 10⁴ kg-N year⁻¹, approximately 97% is retained within the watershed. This atmospheric nitrogen deposition may stimulate phytoplankton productivity in the Mullica River-Great Bay ecosystem.     

Speciation of Fe in the Eastern North Atlantic Ocean

In the Eastern North Atlantic Ocean iron (Fe) speciation was investigated in three size fractions: the dissolvable from unfiltered samples, the dissolved fraction (<0.2 μm) and the fraction smaller than 1000 kDa (<1000 kDa). Fe concentrations were measured by flow injection analysis and the organic Fe complexation by voltammetry. In the research area the water column consisted of North Atlantic Central Water (NACW), below which Mediterranean Overflow Water (MOW) was found with the core between 800 and 1000 m depth. Below 2000 m depth the North Atlantic Deep Water (NADW) proper was recognised. Dissolved Fe and Fe in the <1000 kDa fraction showed a nutrient like profile, depleted at the surface, increasing until 500–1000 m depth below which the concentration remained constant. Fe in unfiltered samples clearly showed the MOW with high concentrations (4 nM) compared to the overlying NACW and the underlying NADW, with 0.9 nM and 2 nM Fe, respectively. By using excess ligand (Excess L) concentrations as parameter we show a potential to bind Fe. The surface mixed layer had the highest excess ligand concentrations in all size fractions due to phytoplankton uptake and possible ligand production. The ratio of Excess L over Fe proved to be a complementary tool in revealing the relative saturation state of the ligands with Fe. In the whole water column, the organic ligands in the larger colloidal fraction (between 0.2 μm and 1000 kDa) were saturated with Fe, whereas those in the smallest fraction (<1000 kDa) were not saturated with Fe, confirming that this fraction was the most reactive one and regulates dissolution and colloid aggregation and scavenging processes. This regulation was remarkably stable with depth since the alpha factor (product of Excess L and K′), expressing the reactivity of the ligands, did not vary and was 10¹³. Whereas, in the NACW and the MOW, the ligands in the particulate (>0.2 μm) fraction were unsaturated with Fe with respect to the dissolved fraction, thus these waters had a scavenging potential.     

Carbon and biogenic silica export influenced by the Amazon River Plume: Patterns of remineralization in deep-sea sediments

The Amazon River Plume delivers freshwater and nutrients to an otherwise oligotrophic western tropical North Atlantic (WTNA) Ocean. Plume waters create conditions favorable for carbon and nitrogen fixation, and blooms of diatoms and their diazotrophic cyanobacterial symbionts have been credited with significant CO₂ uptake from the atmosphere. The fate of the carbon, however, has been measured previously by just a few moored or drifting sediment traps, allowing only speculation about the full extent of the plume's impact on carbon flux to the deep sea. Here, we used surface (0.5 m) sediment cores collected throughout the Demerara Slope and Abyssal Plain, at depths ranging from 1800 to 5000 m, to document benthic diagenetic processes indicative of carbon flux. Pore waters were extracted from sediments using both mm- and cm-scale extraction techniques. Profiles of nitrate (NO₃) and silicate (Si(OH)₄) were modeled with a diffusion-reaction equation to determine particulate organic carbon (POC) degradation and biogenic silica (bSi) remineralization rates. Model output was used to determine the spatial patterns of POC and bSi arrival at the sea floor. Our estimates of POC and Si remineralization fluxes ranged from 0.16 to 1.92 and 0.14 to 1.35 mmol m⁻² d⁻¹, respectively. A distinct axis of POC and bSi deposition on the deep sea floor aligned with the NW axis of the plume during peak springtime flood. POC flux showed a gradient along this axis with highest fluxes closest to the river mouth. bSi had a more diffuse zone of deposition and remineralization. The impact of the Amazon plume on benthic fluxes can be detected northward to 10°N and eastward to 47°W, indicating a footprint of nearly 1 million km². We estimate that 0.15 Tmol C y⁻¹ is remineralized in abyssal sediments underlying waters influenced by the Amazon River. This constitutes a relatively high fraction (~7%) of the estimated C export from the region.; the plume thus has a demonstrable impact on C_org export in the western Atlantic. Benthic fluxes under the plume were comparable to and in some cases greater than those observed in the eastern equatorial Atlantic, the southeastern Atlantic, and the Southern Ocean.     

Latitudinal distribution of microbial plankton abundance,production, and respiration in the Equatorial Atlantic in autumn 2000

Phytoplankton and bacterial abundance, size-fractionated phytoplankton chlorophyll-a (Chl-a) and production together with bacterial production, microbial oxygen production and respiration rates were measured along a transect that crossed the Equatorial Atlantic Ocean (10°N–10°S) in September 2000, as part of the Atlantic Meridional Transect 11 (AMT 11) cruise. From 2°N to 5°S, the equatorial divergence resulted in a shallowing of the pycnocline and the presence of relatively high nitrate (>1 μM) concentrations in surface waters. In contrast, a typical tropical structure (TTS) was found near the ends of the transect. Photic zone integrated ¹⁴C primary production ranged from ∼200 mg C m⁻² d⁻¹ in the TTS region to ∼1300 mg C m⁻² d⁻¹ in the equatorial divergence area. In spite of the relatively high primary production rates measured in the equatorial upwelling region, only a moderate rise in phytoplankton biomass was observed as compared to nearby nutrient-depleted areas (22 vs. 18 mg Chl-a m⁻², respectively). Picophytoplankton were the main contributors (>60%) to both Chl-a biomass and primary production throughout the region. The equatorial upwelling did not alter the phytoplankton size structure typically found in the tropical open ocean, which suggests a strong top-down control of primary producers by zooplankton. However, the impact of nutrient supply on net microbial community metabolism, integrated over the euphotic layer, was evidenced by an average net microbial community production within the equatorial divergence (1130 mg C m⁻² d⁻¹) three-fold larger than net production measured in the TTS region (370 mg C m⁻² d⁻¹). The entire region under study showed net autotrophic community metabolism, since respiration accounted on average for 51% of gross primary production integrated over the euphotic layer.     

Seasonal and interannual variability in particle fluxes of carbon,nitrogen and silicon from time series of sediment traps at Ocean Station P, 1982–1993: relationship to changes in subarctic primary productivity

An extended time series of particle fluxes at 3800 m was recorded using automated sediment traps moored at Ocean Station Papa (OSP, 50°N, 145°W) in the northeast Pacific Ocean for more than a decade (1982–1993). Time-series observations at 200 and 1000 m, and short-term measurements using surface-tethered free-drifting sediment traps also were made intermittently. We present data for fluxes of total mass (dry weight), particulate organic carbon (POC), particulate organic nitrogen (PON), biogenic Si (BSi), and particulate inorganic carbon (PIC) in calcium carbonate. Mean monthly fluxes at 3800 m showed distinct seasonality with an annual minimum during winter months (December–March), and maximum during summer and fall (April–November). Fluxes of total mass, POC, PIC and BSi showed 4-, 10-, 7- and 5-fold increases between extreme months, respectively. Mean monthly fluxes of PIC often showed two plateaus, one in May–August dominated by <63 μm particles and one in October–November, which was mainly >63 μm particles. Dominant components of the mass flux throughout the year were CaCO₃ and opal in equal amounts. The mean annual fluxes at 3800 m were 32±9 g dry weight g m⁻² yr⁻¹, 1.1±0.5 g POC m⁻² yr⁻¹, 0.15±0.07 g PON m⁻² yr⁻¹, 5.9±2.0 g BSi m⁻² yr⁻¹ and 1.7±0.6 g PIC m⁻² yr⁻¹. These biogenic fluxes clearly decreased with depth, and increased during “warm” years (1983 and 1987) of the El Niño, Southern Oscillation cycle (ENSO). Enhancement of annual mass flux rates to 3800 m was 49% in 1983 and 36% in 1987 above the decadal average, and was especially rich in biogenic Si. Biological events allowed estimates of sinking rates of detritus that range from 175 to 300 m d⁻¹, and demonstrate that, during periods of high productivity, particles sink quickly to deep ocean with less loss of organic components. Average POC flux into the deep ocean approximated the “canonical” 1% of the surface primary production.     

Export fluxes of particulate organic carbon in the Chukchi Sea: A comparative study using 234Th/238U disequilibria and drifting sediment traps

Large-volume sampling of ²³⁴Th and drifting sediment trap deployments were conducted as part of the 2004 Western Arctic Shelf–Basin Interactions (SBI) spring (May 15–June 23) and summer (July 17–August 26) process cruises in the Chukchi Sea. Measurements of ²³⁴Th and particulate organic carbon (POC) export fluxes were obtained at five stations during the spring cruise and four stations during the summer cruise along Barrow Canyon (BC) and along a parallel shelf-to-basin transect from East Hanna Shoal (EHS) to the Canada Basin. ²³⁴Th and POC fluxes obtained with in situ pumps and drifting sediment traps agreed to within a factor of 2 for 70% of the measurements. POC export fluxes measured with in situ pumps at 50 m along BC were similar in spring and summer (average = 14.0 ± 8.0 mmol C m^− 2 day^− 1 and 16.5 ± 6.5 mmol C m^− 2 day^− 1, respectively), but increased from spring to summer at the EHS transect (average = 1.9 ± 1.1 mmol C m^− 2 day^− 1 and 19.5 ± 3.3 mmol C m^− 2 day^− 1, respectively). POC fluxes measured with sediment traps at 50 m along BC were also similar in both seasons (31.3 ± 9.3 mmol C m^− 2 day^− 1 and 29.1 ± 14.2 mmol C m^− 2 day^− 1, respectively), but were approximately twice as high as POC fluxes measured with in situ pumps. Sediment trap POC fluxes measured along the EHS transect also increased from spring to summer (3.0 ± 1.9 mmol C m^− 2 day^− 1 and 13.0 ± 6.4 mmol C m^− 2 day^− 1, respectively), and these fluxes were similar to the POC fluxes obtained with in situ pumps. Discrepancies in POC export fluxes measured using in situ pumps and sediment traps may be reasonably explained by differences in the estimated POC/²³⁴Th ratios that arise from differences between the techniques, such as time-scale of measurement and size and composition of the collected particles. Despite this variability, in situ pump and sediment trap-derived POC fluxes were only significantly different at a highly productive station in BC during the spring.     

Seasonal influences on size-fractionated chlorophyll a concentrations and primary production in the north-west Indian Ocean

Chlorophyll a (chl a) concentrations and primary production by the 0.2–2, 2–18 and >18 μm phytoplankton size-fractions were estimated along a transect in the NW Indian Ocean extending from the coast of Oman to 8°N 68°E during the late SW monsoon and autumn intermonsoonal seasons in 1994. Primary production was estimated using the ¹⁴C technique with either in situ or simulated in situ incubations. During the late monsoon season, maximal chl a and production values were recorded in the coastal upwelling zone with values of 69 mg m^-2 and 3800 mg C m^-2 d^-1, respectively. The maxima, which were distributed patchily in this region, were dominated by the >18 μm size-fraction. Over the remainder of the transect chl a concentrations and production averaged 30 mg m^-2 and 1500 mg C m^-2 d^-1, respectively, with approximately equal contributions by the three size-fractions in the case of chl a at the majority of stations, but in general, with a maximum in production in the 0.2–2 μm fraction. Immediately following cessation of the SW monsoon wind, chl a and production values over the northern part of the transect decreased to values similar to those over the southern part of the transect at the time of the SW monsoon, with the contributions by the three size-fractions being approximately equal. During the following intermonsoonal season, both chl a concentrations and production across the section were dominated by the 0.2–2 μm size-fraction, with average chl a and production values of the order of 20 mg m^-2 and 750 mg C m^-2 d^-1, respectively. Considerable variation in production values, however, was exhibited across the transect. A clearly defined subsurface chl a maximum was only recorded at the southernmost stations of the transect in oligotrophic waters: the feature did not develop universally across the transect during the intermonsoon.