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Abstract An assay protocol, based on programmable Flow Injection (pFI), is optimized by tailoring flowrates appropriately to the individual steps of an assay, thus allowing sample and reagent metering, mixing, incubation, monitoring and washout to be carried out more efficiently and in different time frames. This novel approach to flow based methods is applied here to optimize the determination of orthophosphate at nanomolar levels. Programmable Flow Injection was also used to facilitate an investigation of the properties of the phosphomolybdenum blue (PMoB) formed during this assay, by using the stop flow technique – an approach that revealed for the first time the influence of surfactants on the kinetics of formation of PMoB and its spectral characteristics. It was discovered that the two most frequently used surfactants (SDS and Brij) have profound and different influences on the spectra and formation of PMoB and this finding was used to enhance the sensitivity of the phosphate assay at nanomolar levels. The method was applied to the assay of trace levels of phosphate in sea water. Graphical abstract fx1 Highlights • This work describes for the first time the use of Programmable Flow Injection (pFI), for an analysis based on two reagents. • The influence of surfactants on the spectral characteristics and kinetics of the formation of phosphomolybdenum blue is discovered and used to optimize reagent composition and sensitivity of the phosphate assay. • The stopped flow variant of the pFI technique for the determination of phosphate is insensitive to variations in the salinity of sea water. [ABSTRACT FROM AUTHOR]

The role that dissolved iron (dFe) rich Circumpolar Deep Water (CDW) might play in sustaining the consistently observed discrete patches of high chlorophyll biomass over Pennell Bank (PB) and Mawson Bank (MB) in the Ross Sea, was investigated during January/February 2011. Over a 26-day period, hydrographic and trace metal clean water sampling was carried out adjacent to both of these banks, in some cases repeatedly. Particulate sampling was also accomplished at selected stations by in situ pumping. The results indicate that the dFe content of the CDW is in fact reduced by on-shelf mixing with Antarctic Surface Water as it transitions into modified CDW (MCDW). Our stations above PB, where the maximum bloom is encountered, show no evidence of MCDW presence. In contrast, above MB, where there is a smaller persistent bloom, MCDW was observed. Although both of these stations displayed the imprint of sedimentary Fe input connected to the strong tidal cycles above the banks, the stronger near-bottom density gradient that MCDW produces appears to contribute to reduced vertical mixing of the sedimentary source. Thus, ironically, the presence of MCDW may be hindering the Fe supply to the surface waters, rather than being the source, as originally hypothesized. [ABSTRACT FROM AUTHOR]

The distribution of dissolved aluminium determined during GA03, the US GEOTRACES North Atlantic Transects (US GT NAZT) shows large inputs to the basin from three main sources, atmospheric deposition, outflow from the Mediterranean, and inputs from hydrothermal sources along the Mid Atlantic Ridge (MAR). The partial dissolution of atmospheric aerosols emanating from the Sahara yield high concentrations of dissolved Al in the surface waters of the basin and are used to estimate the geographical pattern of dust deposition. The Mediterranean outflow delivers a large source of dissolved Al to the intermediate waters of the eastern basin and its subsequent distribution within the basin can be explained by simple isopycnal mixing with surrounding water masses. Hydrothermal venting at the Trans-Atlantic Geotraverse (TAG) hydrothermal field in the MAR produces a neutrally buoyant plume that introduces copious quantities of dissolved Al (with concentrations of up to 40 nM) to the deeper waters of the North Atlantic that can be seen advecting to the west of the MAR. The concentration of dissolved Al in the deep waters of the eastern basin of the Atlantic can be accounted for by admixing the MAR Al enriched plume water and Antarctic Bottom Water (AABW) as they pass through the Vema Fracture Zone. The data sets show no evidence for biological remineralisation of dissolved Al from Si carrier phases in deep waters. [ABSTRACT FROM AUTHOR]

High-resolution dissolved Fe (dFe) and dissolved Mn (dMn) distributions were obtained using a trace metal clean rosette during the GEOTRACES GA03 zonal transect cruises (USGT10 and USGT11) across the North Atlantic Ocean. This manuscript provides a general overview of the dFe, as well as dMn and dissolved Al (dAl) distributions that reveal several Fe inputs at varying depths across the study region. Elevated dFe concentrations correlate with elevated dAl concentrations in the surface waters of the subtropical gyre, indicating a significant atmospheric source of Fe, in contrast there is no apparent significant dust source for Mn. In the subsurface waters, dFe maxima are a result of the remineralization process, as revealed by their correspondence with dissolved oxygen minima. Within the oxygen minimum, the ratio of dFe to apparent oxygen utilization (AOU) is lower than would be expected from the measured Fe content of surface water phytoplankton, suggesting that a significant amount of dFe that is remineralized at depth (~63–90%) is subsequently scavenged from the water column. The rate of remineralization, which is based on the slope of dFe:AOU plot, is similar across a wide area of the North Atlantic. In addition to the remineralization process, sedimentary inputs are apparent from elevated dMn signals in the eastern basin, particularly near the African coast. In the western basin, sedimentary input is also occurring along the advective flow path of the Upper Labrador Sea Water (ULSW), as ULSW transits along the North American continental shelf region. The largest dFe anomaly (~68 nM), which also corresponds to a dMn anomaly (up to ~33 nM) is seen in the neutrally buoyant hydrothermal plume sampled over the Mid-Atlantic Ridge, and that signal is visible for ~500 km to the west of the ridge. [ABSTRACT FROM AUTHOR]

The carbon to iron (C:Fe) ratio of planktonic biomass constrains net production in iron-limited regions of the ocean and is an important parameter for predicting biomass production from iron inputs. On a cruise to the southern Drake Passage in July–August 2006, we used two approaches to determine the C:Fe ratio of planktonic material: dual-radiotracer labeling and net biomass production in iron-limited grow-out experiments. There was variability in C:Fe ratios among experiments, but values from the two methods overlapped with average values of 1.4×105 (mol:mol) for the radiotracer method and 1.7×105 for the net biomass production method. This is notable since the net biomass production method is a new approach to determine C:Fe ratios. Although it has potential issues related to bottle effects and sensitivity to trace contamination, the method avoids some of the questions associated with iron speciation and bioavailability since ambient iron supports production. Because light intensity is known to affect C:Fe ratios in phytoplankton through photosynthetic iron demands, we tested the effect of light level on C:Fe in Antarctic assemblages. In contrast to what is seen in many phytoplankton cultures, C:Fe ratios increased at low-light, but we suspect that this is due to initial photoinhibition of the low-light adapted winter assemblages at higher light levels. [ABSTRACT FROM AUTHOR]

Abstract: Horizontal advection, vertical mixing, and mixed-layer entrainment all affect iron concentrations and biological productivity in the Ona Basin, near the Shackleton Transverse Ridge (STR) in southern Drake Passage. Trace metal sampling in the region indicates that dissolved iron concentrations are significantly higher on the continental shelf near the Antarctic Peninsula and the South Shetland Islands than they are in the deep waters away from the shelf. Comparisons between satellite-derived sea surface height (SSH) and Chlorophyll-a (Chl-a) levels in the Ona Basin show correlation between Chl-a concentrations and horizontal advection of these iron-rich shelf waters during the months of November and December for the years 1997–2010. However, no significant correlations are found for January–April, while high Chl-a concentrations in the Ona Basin persist through March. Enhanced vertical (diapycnal) mixing and mixed-layer entrainment are considered as alternative mechanisms for delivering iron into the Ona Basin mixed layer and sustaining the high Chl-a concentrations. Estimates of iron flux based on in situ measurements of dissolved iron concentrations suggest that diapycnal mixing alone can supply iron to the base of the mixed layer at a rate of 64±2nmolm−2 day−1 during the summer. In addition, the summer mixed layer in the Ona Basin deepens from January to April, allowing for iron-rich water to be steadily entrained from below. Estimates based on monthly mixed-layer climatologies produce average daily entrainment rates ranging from 5 to 25nmolm−2 day−1. While neither diapycnal mixing nor entrainment alone is always sufficient to meet the estimated iron demand for the Ona Basin bloom, numerical simulation suggests that the combined effect of the two processes can consistently supply sufficient iron to sustain the bloom. [Copyright &y& Elsevier]

Dissolved iron (Fe) and manganese (Mn) share common sources and sinks in the global ocean. However, Fe and Mn also have different redox reactivity and speciation that can cause their distributions to become decoupled. The Arctic Ocean provides a unique opportunity to compare Fe and Mn distributions because the wide Arctic continental shelves provide significant margin fluxes of both elements, yet in situ vertical regeneration inputs that can complicate scavenging calculations are negligible under the ice of the Arctic Ocean, making it easier to interpret the fate of lateral gradients. We present here a large-scale case study demonstrating a three-step mechanism for Fe and Mn decoupling in the upper 400 m of the Western Arctic Ocean. Both Fe and Mn are released during diagenesis in porewaters of the Chukchi Shelf, but they become immediately decoupled when Fe is much more rapidly oxidized and re-precipitated than Mn in the oxic Chukchi Shelf water column, leading to Fe hosted primarily in the particulate phase and Mn in the dissolved phase. However, as these shelf fluxes are transported toward the shelf break and subducted into the subsurface halocline water mass, the loss rates of all species change significantly, causing further Fe and Mn decoupling. In the second decoupling step in the shelf break region, the dominant shelf species are removed rapidly via particle scavenging, with smallest soluble Fe (sFe < 0.02 µm) being least subject to loss, while colloidal Fe (0.02 µm < cFe < 0.2 µm), dissolved Mn (dMn), and non-lithogenic particulate Fe (pFe xs) are all lost at similarly rapid rates. In the third decoupling step, once these species are swept >1000 km offshore with the prevailing current into the low-particle waters of the open Arctic, cFe and dMn appear conserved, while pFe, dFe, and sFe are very slowly removed with variable log-scale distances of transport: pFe ≪ dFe < sFe. To assess the role of physicochemical speciation on these trends, we observed that Fe(II) was a small (∼7%) fraction of total dFe in the upper 400 m of the Arctic, even over the shelf (∼2%). Also, colloidal contribution to dFe was very low (∼20%) in the open Arctic, in contrast to dFe in the North Atlantic, which is composed much more by colloids (≥50%). Throughout the Western Arctic Ocean, Fe and Mn are thus decoupled as a result of distinct oxidation kinetics and different scavenging rates within high- and low-particle regimes. As the "scavengers of the sea", the relative distribution of particulate Fe and Mn phases across the Arctic Ocean shelf and slope, respectively, will play an important role in determining the distribution and ultimate sediment burial site for other scavenging-prone trace elements. Additionally, we suggest that the future effects of climate change, including loss of sea ice that could impact the formation of the halocline, might change distributions of Fe and Mn species in the future Western Arctic. [ABSTRACT FROM AUTHOR]

Multi-year satellite records indicate an asymmetric spatial pattern in the summer bloom in the Northern Ross Sea, with the largest blooms over the shallows of Pennell Bank compared to Mawson Bank. In 2010–2011, high-resolution spatiotemporal in situ sampling focused on these two banks to better understand factors contributing to this pattern. Dissolved and particulate Fe profiles suggested similar surface water depletion of dissolved Fe on both banks. The surface sediments and velocity observations indicate a more energetic water column over Mawson Bank. Consequently, the surface mixed layer over Pennell Bank was more homogeneous and shallower. Over Mawson Bank we observed a thicker more homogeneous bottom boundary layer resulting from stronger tidal and sub-tidal currents. These stronger currents scour the seafloor resulting in sediments less likely to release additional sedimentary iron. Estimates of the quantum yield of photosynthesis and the initial slope of the photosynthesis-irradiance response were lower over Mawson Bank, indicating higher iron stress over Mawson Bank. Overall, the apparent additional sedimentary source of iron to, and longer surface residence time over Pennell Bank, as well as the reduced fluxes from the more isolated bottom mixed layer over Mawson Bank, sustain the observed asymmetric pattern across both banks. [ABSTRACT FROM AUTHOR]

The size partitioning of dissolved Fe (dFe<0.2 µm) into soluble (sFe<0.02 µm) and colloidal (0.02 µm

Abstract: The Drake Passage region near Elephant Island in the Southern Ocean displays patchy phytoplankton blooms. To test the hypothesis that natural Fe addition from localized sources promoted phytoplankton growth here, a grid of stations (59°S to 62°S, 59°W to 53°W, as well as four stations in the eastern Bransfield Strait) were occupied from 12 February–24 March 2004. Phytoplankton abundance was measured using shipboard flow cytometry (70 stations), with abundances conservatively converted to biomass, and compared with measurements of dissolved iron (dFe) at a subset of stations (30 stations). Based on T–S property plots, stations were divided into Antarctic Circumpolar Current (ACC), Water On Shelf (WOS), Bransfield Strait (BS), and Mixed water stations, the latter representing locations with T–S properties intermediate between ACC and WOS stations. The highest integrated phytoplankton biomass was found at Mixed water stations, however, the highest integrated abundance was found at WOS stations, demonstrating that abundance and biomass do not necessarily show the same patterns. The distributions of nano- and micro-phytoplankton (<20 and >20μm diameter cells, respectively) were also examined, with nano- and micro-plankton contributing equally to the total biomass at WOS and BS stations, but micro-plankton representing ∼2/3 of the biomass at Mixed and ACC stations. Increased inventories of dFe did not always correspond to increases in phytoplankton biomass – rather stations with lower mean light levels in the mixed layer (<110μEinsteinsm−2 s−1) had lower biomass despite higher ambient dFe concentrations. However, where the mean light levels in the mixed layer were >110μEinsteinsm−2 s−1, total biomass shows a positive trend with dFe, as does micro-phytoplankton biomass, but neither regression is significant at the 95% level. In contrast, if just nano-phytoplankton biomass is considered as a function of dFe, there is a significant correlation (r 2=0.62). These data suggest a dual mechanism for the patterns observed in biomass: an increasing reservoir of dFe allows increased phytoplankton biomass, but biomass can only accumulate where the light levels are relatively high, such that light is not limiting to growth. [Copyright &y& Elsevier]

Abstract: An austral winter cruise in July–August 2006 was conducted to study the winter circulation and iron delivery processes in the Southern Drake Passage and Bransfield Strait. Results from current and hydrographic measurements revealed a circulation pattern similar to that of the austral summer season observed in previous studies: The Shackleton Transverse Ridge (STR) in the southern Drake Passage blocks a part of the eastward Antarctic Circumpolar Current (ACC) which forces the ACC to detour southward, produces a Taylor Column over the STR, and forms an ACC jet within the Shackleton Gap, a deep channel between the STR and the shelf of Elephant Island. Observations show that to the west of the STR, the Upper Circumpolar Deep Water (UCDW) intruded onto the shelf around the South Shetland Islands while to the east of the STR, shelf waters were transported off the northern shelf of Elephant Island. Along a similar west–east transect approximately 50km off the shelf, the northward transport of shelf waters was approximately 2.4 and 1.2Sv in the austral winter and summer, respectively. The waters around Elephant Island primarily consist of the UCDW that has been modified by local cooling and freshening, unmodified UCDW that has recently intruded onto the shelf, and Bransfield Current water that is a mixture of shelf and Bransfield Strait waters. Weddell Sea outflows were observed which affect the hydrography and circulation in the Bransfield Strait and indirectly affect the circulation patterns in the southern Drake Passage and around Elephant Island. Two coupled Fe enrichment and transport mechanisms are proposed which operate together to firstly enrich water with iron and then transfer this water offshelf. These are an initial intrusion of UCDW onto the northern shelf region of the South Shetland Islands which occurs due to Ekman pumping driven by the prevailing westerly wind in the region, and subsequently, an offshelf transport of shelf waters which occurs east of Elephant Island due to acquisition of positive vorticity by shelf waters from horizontal mixing with ACC waters being intruded onto the shelf. [Copyright &y& Elsevier]

The Antarctic Peninsula (AP) shelf is an important source of dissolved iron (Fe) to the upper ocean in the southern Scotia Sea, one of the most productive regions of the Southern Ocean. Here we present results from a four-year (2003-2006) numerical simulation using a regional coupled physical-biogeochemical model to assess the Fe sources and transport on the AP shelf and toward the southern Scotia Sea. The model was validated with a suite of data derived from in situ surveys and remote sensing. Model results indicate that sediments in the AP shelf and the South Orkney Plateau (SOP) provide the dominant source of Fe to the upper 500 m in the southern Scotia Sea. Additional Fe inputs to the region are associated with the Antarctic Circumpolar Current (ACC) and the northern limb of the Weddell Gyre, deep-ocean sediment sources, dust deposition, and icebergs. Fe on the AP shelf originates primarily from sediments on the relatively shallow inner shelf and is directly injected into the water column and subsequently transported toward Elephant Island by the confluent shelf currents. Off-shelf Fe export is primarily through entrainment of shelf waters by the ACC's Southern Boundary frontal jet along the northern edge of the AP shelf, the Hesperides Trough, and the SOP shelf. About 70% of the off-shelf export takes place below the surface mixed layer, and is subsequently re-supplied to the euphotic zone through vertical mixing, mainly during austral fall and winter. The exported shelf-derived Fe is then advected downstream by the ACC and Weddell Gyre and spread over the southern and eastern Scotia Seas. Taken together, shelf Fe export witin top 500 m meets nearly all of the Fe demand of phytoplankton photosynthesis in the southern Scotia Sea. Waters with elevated Fe concentrations in the Scotia Sea are largely restricted to south of the Southern ACC Front. • Shelf Fe transport has complex pathways through shelf currents and ACC. • Shelf-derived Fe is the dominant Fe source to the southern Scotia Sea. • Shelf Fe originates mainly from inner shelf of the western Antarctic Peninsula. • Shelf-derived Fe meets phytoplankton Fe demand in the southern Scotia Sea. [ABSTRACT FROM AUTHOR]