We report measurements of the transient species iron(II) in filtered water column samples collected during the U.S. GEOTRACES North Atlantic transect cruise (GEOTRACES GA03), which was comprised of ...legs from Lisbon to Cape Verde in October–November 2010, and from Woods Hole to Cape Verde in November–December 2011. Dissolved iron(II) (dFe(II)) was determined at sea in 0.2 µm filtered samples as soon as possible after collection and filtration, using flow injection analysis, with mean detection limits of 0.06nM and 0.01nM during the 2010 and 2011 cruise legs, respectively. Water column concentrations along the cruise transects were generally low (<0.2nM), with the exception of deep water samples collected over the Trans Atlantic Geotraverse hydrothermal field, in which dFe(II) was as high as 70nM and accounted for more than 80% of the dissolved iron pool in the near-field hydrothermal plume. Smaller local concentration maxima were observed near 1000m depth in the low-oxygen, iron-rich waters to the east of Cape Verde, where dFe(II) is correlated with apparent oxygen utilization, and in the upper water column at several stations in the Subtropical Gyre, where dFe(II) can account for greater than 50% of the dissolved iron pool in the lower euphotic zone. Elevated dFe(II) concentrations were also observed over much of the water column on the Bermuda platform, although the source of this enrichment remains uncertain, in the absence of data between Woods Hole and Bermuda.
During two cruises to the Ross Sea, Antarctica in austral spring and summer, fast repetition rate fluorometry was used to investigate the relationship between phytoplankton photophysiology and water ...mass characteristics, micronutrient availability, and composition. Particulate organic matter proxies for phytoplankton biomass (chlorophyll a, particulate organic carbon and nitrogen, and biogenic silica) were all elevated in the photic zone during spring and summer. Biogenic silica concentrations were an order of magnitude higher in summer relative to spring, reflecting a shift in composition from Phaoecystis antarctica to diatoms. Quantum yields of PS II (Fv/Fm) were generally higher in spring relative to summer, coincident with weaker vertical and horizontal gradients in hydrographic properties. Reduced Fv/Fm values (<0.4) were observed in the upper 30m in both seasons, with maximum values (ca. 0.55) observed near base and below the euphotic zone. No significant relationship between Fv/Fm values and dissolved Fe could be identified in the merged spring/summer data set. Functional absorption cross sections were significantly higher in spring than summer, presumably reflecting adaptations to lower irradiance in spring; little variation with depth was observed. Phytoplankton composition did not appear to be a major determinant of bulk quantum yield, although diatom-dominated waters exhibited significantly higher functional absorption cross sections when compared to waters dominated by P. antarctica. Dominance of P. antarctica appears to be related to greater photophysiological resilience and faster photoacclimation to changing light conditions, whereas diatoms were prevalent in shallow summer mixed layers, which likely reflects their enhanced photosynthetic capacity at high irradiance levels.
•Quantum yields of PS II were generally higher in spring than summer.•Low Fv/Fm values (<0.4) occurred in the upper 30m, with greater values near the base of the euphotic zone.•No relationship between Fv/Fm values and dissolved Fe was identified in the spring/summer data set.•Phytoplankton composition did not appear to be a major determinant of quantum yield.•Irradiance was the major factor influcencing photochemical capacity in the Ross Sea.
A shipboard-deployable, flow-injection (FI) based instrument for monitoring iron(II) in surface marine waters is described. It incorporates a miniature, low-power photon-counting head for measuring ...the light emitted from the iron(II)-catalyzed chemiluminescence (CL) luminol reaction. System control, signal acquisition, and data processing are performed in a graphical programming environment. The limit of detection for iron(II) is in the range 8−12 pmol L-1 (based on 3s of the blank), and the precision over the range 8−1000 pmol L-1 varies between 0.9 and 7.6% (n = 4). Results from a day−night deployment during a north-to-south transect of the Atlantic Ocean and a daytime transect in the Sub-Antarctic Front are presented together with ancillary temperature, salinity, and irradiance data. The generic nature of the components used to assemble the instrument make the technology readily transferable to other laboratories and the modular construction makes it easy to adapt the system for use with other CL chemistries.
We report water column dissolved iron (dFe) and particulate iron (pFe) concentrations from 50 stations sampled across the Ross Sea during austral summer (January–February) of 2012. Concentrations of ...dFe and pFe were measured in each of the major Ross Sea water masses, including the Ice Shelf Water and off‐shelf Circumpolar Deep Water. Despite significant lateral variations in hydrography, macronutrient depletion, and primary productivity across several different regions on the continental shelf, dFe concentrations were consistently low (<0.1 nM) in surface waters, with only a handful of stations showing elevated concentrations (0.20–0.45 nM) in areas of melting sea ice and near the Franklin Island platform. Across the study region, pFe associated with suspended biogenic material approximately doubled the inventory of bioavailable iron in surface waters. Our data reveal that the majority of the summertime iron inventory in the Ross Sea resides in dense shelf waters, with highest concentrations within 50 m of the seafloor. Higher dFe concentrations near the seafloor are accompanied by an increased contribution to pFe from authigenic and/or scavenged iron. Particulate manganese is also influenced by sediment resuspension near the seafloor but, unlike pFe, is increasingly associated with authigenic material higher in the water column. Together, these results suggest that following depletion of the dFe derived from wintertime convective mixing and sea ice melt, recycling of pFe in the upper water column plays an important role in sustaining the summertime phytoplankton bloom in the Ross Sea polynya.
Key Points
Summertime dissolved and particulate iron concentrations are reported for all Ross Sea water masses and off‐shelf Circumpolar Deep Water
Summertime inventories of dissolved iron and particulate trace elements are strongly influenced by high concentrations near the seafloor
Surface water concentrations of bioavailable iron are approximately doubled if biogenic pFe is considered along with dFe concentrations
The GEOTRACES Intermediate Data Product 2017 (IDP2017) is the second publicly available data product of the international GEOTRACES programme, and contains data measured and quality controlled before ...the end of 2016. The IDP2017 includes data from the Atlantic, Pacific, Arctic, Southern and Indian oceans, with about twice the data volume of the previous IDP2014. For the first time, the IDP2017 contains data for a large suite of biogeochemical parameters as well as aerosol and rain data characterising atmospheric trace element and isotope (TEI) sources. The TEI data in the IDP2017 are quality controlled by careful assessment of intercalibration results and multi-laboratory data comparisons at crossover stations. The IDP2017 consists of two parts: (1) a compilation of digital data for more than 450 TEIs as well as standard hydrographic parameters, and (2) the eGEOTRACES Electronic Atlas providing an on-line atlas that includes more than 590 section plots and 130 animated 3D scenes. The digital data are provided in several formats, including ASCII, Excel spreadsheet, netCDF, and Ocean Data View collection. Users can download the full data packages or make their own custom selections with a new on-line data extraction service. In addition to the actual data values, the IDP2017 also contains data quality flags and 1-σ data error values where available. Quality flags and error values are useful for data filtering and for statistical analysis. Metadata about data originators, analytical methods and original publications related to the data are linked in an easily accessible way. The eGEOTRACES Electronic Atlas is the visual representation of the IDP2017 as section plots and rotating 3D scenes. The basin-wide 3D scenes combine data from many cruises and provide quick overviews of large-scale tracer distributions. These 3D scenes provide geographical and bathymetric context that is crucial for the interpretation and assessment of tracer plumes near ocean margins or along ridges. The IDP2017 is the result of a truly international effort involving 326 researchers from 25 countries. This publication provides the critical reference for unpublished data, as well as for studies that make use of a large cross-section of data from the IDP2017.
This article is part of a special issue entitled: "Cycles of trace elements and isotopes in the ocean – GEOTRACES and beyond" - edited by Tim M. Conway, Tristan Horner, Yves Plancherel, and Aridane G. González.
The Ross Sea is home to some of the largest phytoplankton blooms in the Southern Ocean. Primary production in this system has previously been shown to be iron limited in the summer and periodically ...iron and vitamin B
12
colimited. In this study, we examined trace metal limitation of biological activity in the Ross Sea in the austral spring and considered possible implications for vitamin B
12
nutrition. Bottle incubation experiments demonstrated that iron limited phytoplankton growth in the austral spring while B
12
, cobalt, and zinc did not. This is the first demonstration of iron limitation in a
Phaeocystis antarctica
-dominated, early season Ross Sea phytoplankton community. The lack of B
12
limitation in this location is consistent with previous Ross Sea studies in the austral summer, wherein vitamin additions did not stimulate
P. antarctica
growth and B
12
was limiting only when bacterial abundance was low. Bottle incubation experiments and a bacterial regrowth experiment also revealed that iron addition directly enhanced bacterial growth. B
12
uptake measurements in natural water samples and in an iron fertilized bottle incubation demonstrated that bacteria serve not only as a source for vitamin B
12
, but also as a significant sink, and that iron additions enhanced B
12
uptake rates in phytoplankton but not bacteria. Additionally, vitamin uptake rates did not become saturated upon the addition of up to 95 pM B
12
. A rapid B
12
uptake rate was observed after 13 min, which then decreased to a slower constant uptake rate over the next 52 h. Results from this study highlight the importance of iron availability in limiting early season Ross Sea phytoplankton growth and suggest that rates of vitamin B
12
production and consumption may be impacted by iron availability.
Laboratory culture experiments were used to investigate the growth rate of colonial Phaeocystis antarctica as a function of irradiance and dissolved iron concentration. The experiments were conducted ...with a P. antarctica strain isolated from the southern Ross Sea, Antarctica, and made use of natural, low-iron (<0.2 nM dissolved Fe) filtered seawater as a growth medium, thereby avoiding the addition of synthetic organic ligands to regulate dissolved iron concentrations. Under iron- and nutrient-replete conditions, colonial P. antarctica attained an average maximum cell-specific growth rate of 0.37 d super(-1) at an irradiance of 68 kE m super(-2) s super(-1), above which growth rates decreased to 0.27 d super(-1) at an irradiance of 314 kE m super(-2) s super(-1). The dependence of growth rate on ambient dissolved iron concentration was examined in dose-response type bioassay experiments using realistic sub-nanomolar additions of dissolved iron. The experimental results indicate significant changes in the iron requirements for growth of colonial P. antarctica as a function of irradiance, with our estimates of the half-saturation constant for growth with respect to dissolved iron (K sub(k)) ranging from 0.26 nM at ~20 kE m super(-2) s super(-1), to 0.045 nM at ~40 kE m super(-2) s super(-1) and to 0.19 nM at ~90 kE m super(-2) s super(-1). We interpret these variations in K sub(k) as reflecting an increase in the cellular iron requirements of colonial P. antarctica at sub-optimal and supraoptimal irradiance, such that the cells require higher ambient dissolved iron concentrations to attain maximum growth rates under such irradiance conditions. The experiments also provide evidence of a relationship between iron availability and the relative proportion of colonial versus solitary P. antarctica cells, whereby the colonial form appears to be favored by higher dissolved iron concentrations. Our experimental results suggest that the initiation and termination of colonial P. antarctica blooms in the Ross Sea are determined by the combined effects of irradiance-driven changes in cellular iron requirements and a seasonal decrease in dissolved iron availability.
A sensitive flow‐injection method with chemiluminescence detection (FI‐CL) for the determination of dissolved cobalt in open ocean samples, suitable for shipboard use has been developed. To date, FI ...methods for dissolved cobalt have been used only in coastal and estuarine waters. Therefore, significant modifications to existing methods were required, including (1) the use of a commercially available iminodiacetate (IDA) resin (Toyopearl AF‐chelate 650M) in place of resin immobilized 8‐hydroxyquinoline for online preconcentration and matrix removal, (2) the introduction of acidified ammonium acetate (pH 4) as a column‐conditioning step before sample loading and rinse steps, and most importantly, (3) UV irradiation of acidified seawater samples to determine total dissolved cobalt, rather than an operationally defined fraction. This method had a detection limit of 4.5 pM (3σ of the blank). The accuracy of the method was evaluated by determining total dissolved cobalt in acidified North Pacific deep seawater (1000 m) samples from the Sampling and Analysis of Iron (SAFe) program and NASS‐5. The method yields a mean (± SD) value of 40.9 ± 2.6 pM (n = 9), which is in excellent agreement with the SAFe consensus value of 43 ± 4 pM, and 208 ± 30 pM for NASS‐5 (certified value 187 ± 51 pM). This study demonstrates that UV irradiation is an essential step for the determination of total dissolved cobalt in seawater by FI‐CL. The method was applied to vertical profiles from the Sargasso Sea, indicating that total dissolved cobalt is influenced by both biological and physical processes.
We report the distribution of cobalt (Co) in the Ross Sea polynya during austral summer 2005–2006 and the following austral spring 2006. The vertical distribution of total dissolved Co (dCo) was ...similar to soluble reactive phosphate (PO43−), with dCo and PO43− showing a significant correlation throughout the water column (r2 = 0.87, 164 samples). A strong seasonal signal for dCo was observed, with most spring samples having concentrations ranging from ~45–85 pM, whereas summer dCo values were depleted below these levels by biological activity. Surface transect data from the summer cruise revealed concentrations at the low range of this seasonal variability (~30 pM dCo), with concentrations as low as 20 pM observed in some regions where PO43− was depleted to ~0.1 μM. Both complexed Co, defined as the fraction of dCo bound by strong organic ligands, and labile Co, defined as the fraction of dCo not bound by these ligands, were typically observed in significant concentrations throughout the water column. This contrasts the depletion of labile Co observed in the euphotic zone of other ocean regions, suggesting a much higher bioavailability for Co in the Ross Sea. An ecological stoichiometry of 37.6 μmol Co:mol−1 PO43− calculated from dissolved concentrations was similar to values observed in the subarctic Pacific, but approximately tenfold lower than values in the Eastern Tropical Pacific and Equatorial Atlantic. The ecological stoichiometries for dissolved Co and Zn suggest a greater overall use of Zn relative to Co in the shallow waters of the Ross Sea, with a Co:PO43−/Zn:PO43− ratio of 1:17. Comparison of these observed stoichiometries with values estimated in culture studies suggests that Zn is a key micronutrient that likely influences phytoplankton diversity in the Ross Sea. In contrast, the observed ecological stoichiometries for Co were below values necessary for the growth of eukaryotic phytoplankton in laboratory culture experiments conducted in the absence of added zinc, implying the need for significant Zn nutrition in the Zn-Co cambialistic enzymes. The lack of an obvious kink in the dissolved Co:PO43− relationship was in contrast to Zn:PO43− and Cd:PO43− kinks previously observed in the Ross Sea. An excess uptake mechanism for kink formation is proposed as a major driver of Cd:PO43− kinks, where Zn and Cd uptake in excess of that needed for optimal growth occurs at the base of the euphotic zone, and no clear Co kink occurs because its abundances are too low for excess uptake. An unusual characteristic of Co geochemistry in the Ross Sea is an apparent lack of Co scavenging processes, as inferred from the absence of dCo removal below the euphotic zone. We hypothesize that this vertical distribution reflects a low rate of Co scavenging by Mn oxidizing bacteria, perhaps due to Mn scarcity, relative to the timescale of the annual deep winter mixing in the Ross Sea. Thus Co exhibits nutrient-like behavior in the Ross Sea, in contrast to its hybrid-type behavior in other ocean regions, with implications for the possibility of increased marine Co inventories and utility as a paleooceanographic proxy.
Over the Ross Sea shelf, annual primary production is limited by dissolved iron (DFe) supply. Here, a major source of DFe to surface waters is thought to be vertical resupply from the benthos, which ...is assumed most prevalent during winter months when katabatic winds drive sea ice formation and convective overturn in coastal polynyas, although the impact of these processes on water‐column DFe distributions has not been previously documented. We collected hydrographic data and water‐column samples for trace metals analysis in the Terra Nova Bay and Ross Ice Shelf polynyas during April–May 2017 (late austral fall). In the Terra Nova Bay polynya, we observed intense katabatic wind events, and surface mixed layer depths varied from ∼250 to ∼600 m over lateral distances <10 km; there vertical mixing was just starting to excavate the dense, iron‐rich Shelf Waters, and there was also evidence of DFe inputs at shallower depths in the water column. In the Ross Ice Shelf polynya, wind speeds were lower, mixed layers were <300 m deep, and DFe distributions were similar to previous, late‐summer observations, with concentrations elevated near the seafloor. Corresponding measurements of dissolved manganese and zinc, and particulate iron, manganese, and aluminum, suggest that deep DFe maxima and some mid‐depth DFe maxima primarily reflect sedimentary inputs, rather than remineralization. Our data and model simulations imply that vertical resupply of DFe in the Ross Sea occurs mainly during mid‐late winter, and may be particularly sensitive to changes in the timing and extent of sea ice production.
Plain Language Summary
The Ross Sea is among the most productive areas on the Antarctic continental shelf. Here, primary production during the austral summer season is limited by the availability of dissolved iron (DFe), an essential micronutrient. A major source of DFe to Ross Sea surface waters is thought to be deep convective mixing, driven by strong, cold katabatic (offshore) winds and associated sea ice formation during austral winter (∼May–September). This mixing is assumed to deliver DFe‐rich bottom waters to the surface, particularly in coastal polynyas, where sea ice is formed and exported northwards. However, the impact of winter mixing on the distribution of DFe has not been previously documented in the Ross Sea. We report such observations from the Terra Nova Bay polynya (TNBP) and Ross Ice Shelf polynya (RISP) at the onset of winter (late April to late May) 2017. In the TNBP, intense katabatic winds and vigorous sea ice formation drove convective mixing that was penetrating into dense, DFe‐rich bottom waters. In the RISP, with milder local winds and less sea ice formation, vertical mixing had not reached DFe‐rich deep waters. Our results indicate that deep mixing does indeed supply substantial DFe to Ross Sea surface waters, although primarily in mid‐late winter (∼June–September).
Key Points
Late‐fall observations from two Ross Sea polynyas reveal variability in the extent of vertical mixing and distributions of dissolved iron
In Terra Nova Bay, katabatic winds and sea ice formation drove deep convective mixing that had extended into dense, iron‐rich Shelf Waters
Vertical mixing was less advanced in the Ross Ice Shelf polynya, suggesting that vertical resupply of iron mainly occurs later in winter
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