Michael J. Ellwood

Zinc speciation in the Northeastern Atlantic Ocean

Measurements of zinc and zinc complexation by natural organic ligands in the northeastern part of the Atlantic Ocean were made using cathodic stripping voltammetry with ligand competition. Total zinc concentrations ranged from 0.3 nM in surface waters to 2 nM at 2000 m for open-ocean waters, whilst nearer the English coast, zinc concentrations reached 1.5 nM in the upper water column. In open-ocean waters zinc speciation was dominated by complexation to a natural organic ligand with conditional stability constant (log KZnL′) ranging between 10.0 and 10.5 and with ligand concentrations ranging between 0.4 and 2.5 nM. The ligand was found to be uniformly distributed throughout the water column even though zinc concentrations increased with depth. Organic ligand concentrations measured in this study are similar to those published for the North Pacific. However the log KZnL′ values for the North Atlantic are almost and order of magnitude lower than those reported by Bruland [Bruland, K.W., 1989. Complexation of zinc by natural organic-ligands in the central North Pacific. Limnol. Oceanogr., 34, 269–285.] using anodic stripping voltammetry for the North Pacific. Free zinc ion concentrations were low in open-ocean waters (6–20 pM) but are not low enough to limit growth of a typical oceanic species of phytoplankton.

Determination of organic complexation of cobalt in seawater by cathodic stripping voltammetry

A method was developed to determine the extent of cobalt complexation with organic ligands in seawater, and applied to samples from the North Eastern Atlantic Ocean. The cobalt speciation was determined using catalytic cathodic stripping voltammetry with ligand competition against the adsorptive ligand (nioxime). Optimised conditions include a nioxime concentration of 200 nM, 0.07 M ammonia/ammonium chloride pH buffer (pH 9.1), and 0.5 M nitrite. The stability of the mixed complex of cobalt, nioxime and ammonia was calibrated against EDTA. The cobalt speciation in the open-ocean surface waters was found to be dominated by complexation to natural organic ligands with conditional stability constants (log KCoL′) ranging between 15.6 and 16.1 and with ligand concentrations between 22 and 60 pM. The cobalt concentrations varied between 25 pM in the open-ocean waters and 103 pM in the English Channel, and were less than the ligand concentrations in many of the surface oceanic waters. The ligand concentration at depths between 30 and 115 m (average 22 pM) was lower than that of cobalt, whereas at other depths (shallower as well as deeper), the ligand concentrations were between 26 and 31 pM, sometimes greater than the cobalt concentrations. The cobalt and ligand concentrations in these waters are finely balanced, strongly binding all cobalt when the ligands are in excess. Free Co2+ concentrations calculated for the open-ocean surface waters are extremely low (<5 fM) which could cause cobalt limitation to certain phytoplankton such as coccolithophores and cyanobacteria if the organic complexes are unavailable. If this organic complexation of cobalt occurs also in other regions characterised by low cobalt concentrations, iron-limited waters in high-nitrate low-chlorophyll regions may be cobalt-limited too.

Microbial control of diatom bloom dynamics in the open ocean

[1] Diatom blooms play a central role in supporting foodwebs and sequestering biogenic carbon to depth. Oceanic conditions set bloom initiation, whereas both environmental and ecological factors determine bloom magnitude and longevity. Our study reveals another fundamental determinant of bloom dynamics. A diatom spring bloom in offshore New Zealand waters was likely terminated by iron limitation, even though diatoms consumed <1/3 of the mixed-layer dissolved iron inventory. Thus, bloom duration and magnitude were primarily set by competition for dissolved iron between microbes and small phytoplankton versus diatoms. Significantly, such a microbial mode of control probably relies both upon out-competing diatoms for iron (i.e., K-strategy), and having high iron requirements (i.e., r-strategy). Such resource competition for iron has implications for carbon biogeochemistry, as, blooming diatoms fixed three-fold more carbon per unit iron than resident non-blooming microbes. Microbial sequestration of iron has major ramifications for determining the biogeochemical imprint of oceanic diatom blooms. Citation: Boyd, P. W., et al. (2012), Microbial control of diatom bloom dynamics in the open ocean, Geophys. Res. Lett., 39, L18601, doi:10.1029/2012GL053448.

Measurement of arsenic species in marine macroalgae by microwave-assisted extraction and high performance liquid chromatography-inductively coupled plasma mass spectrometry

The measurement of arsenic species (arsenoribosides, arsenate, dimethyl arsenic and monomethyl arsenic) in marine macroalgae by microwave-assisted extraction and HPLC–ICP-MS is described. The extraction of arsenic from three different macroalgae classes was optimised using a chemometric approach, with solvent composition and sample mass being the two significant factors influencing the extraction of arsenic. Extraction temperature and extraction time did not significantly influence the extraction of arsenic from macroalgae. The optimised conditions for arsenic extraction (methanol (%)) were: 56% for phaeophyta, 66% for rhodophyta and 78% for chlorophyta, (sample mass in 10 ml of solvent) 0.05 g for phaeophyta, 0.07 g for rhodophyta and 0.08 g for chlorophyta. When two extractions were used, the percentage of arsenic extracted from macroalgae was greater than 88%. Unambiguous separation and identification of three arsenoribosides (phosphate-, sulfonate- and sulfate-arsenoriboside) was achieved by chromatographing extracts on a Hamilton PRP X-100 anion exchange column with ammonium phosphate buffer as the mobile phase at a pH of 9.2. The unambiguous separation and identification of the glycerol-arsenoriboside was achieved by chromatographing extracts on a Supelcosil SCX cation exchange column with a pyridine–formic acid buffer as the mobile phase at a pH of 2.6.

Bioaccumulation of antimony and arsenic in a highly contaminated stream adjacent to the Hillgrove Mine, NSW, Australia

Environmental context. Concern over the presence of antimony (Sb) in the environment because of chemical similarities with arsenic (As) has prompted a need to better understand its environmental behaviour and risks. The present study investigates the bioaccumulation and uptake of antimony in a highly contaminated stream near the Hillgrove antimony–gold mine in NSW, Australia, and reports high Sb (and As) concentrations in many components of the ecosystem consisting of three trophic levels, but limited uptake into aboveground parts of riparian vegetation. The data suggest that Sb can transfer into upper trophic levels of a creek ecosystem, but that direct exposure of creek fauna to creek sediment and soil, water and aquatic autotrophs are more important metalloid uptake routes than exposure via riparian vegetation. Abstract. Bioaccumulation and uptake of antimony (Sb) were investigated in a highly contaminated stream, Bakers Creek, running adjacent to mining and processing of Sb–As ores at Hillgrove Mine, NSW, Australia. Comparisons with arsenic (As) were included owing to its co-occurrence at high concentrations. Mean metalloid creek rhizome sediment concentrations were 777 ± 115 μg g–1 Sb and 60 ± 6 μg g–1 As, with water concentrations at 381 ± 23 μg L–1 Sb and 46 ± 2 μg L–1 As. Antimony and As were significantly elevated in aquatic autotrophs (96–212 μg g–1 Sb and 32–245 μg g–1 As) but Sb had a lower uptake efficiency. Both metalloids were elevated in all macroinvertebrates sampled (94–316 μg g–1 Sb and 1.8–62 μg g–1 As) except Sb in gastropods. Metalloids were detected in upper trophic levels although biomagnification was not evident. Metalloid transfer to riparian vegetation leaves from roots and rhizome soil was low but rhizome soil to leaf As concentration ratios were up to 2–3 times greater than Sb concentration ratios. Direct exposure to the rhizosphere sediments and soils, water ingestion and consumption of aquatic autotrophs appear to be the major routes of Sb and As uptake for the fauna of Bakers Creek.

Determination of the Zn/Si ratio in diatom opal: a method for the separation, cleaning and dissolution of diatoms

A procedure is presented for the separation and the purification of diatoms from marine sediments prior to chemical analysis of the opal. The method has been optimised for the measurement of the Zn/Si ratio of the frustule. The separation procedure eliminates artifacts due to the presence of clay contaminants in the sample and the adsorption of clays onto the frustule surface. The concentration of trace elements (Fe, Al, Ba and Ge) in the sample digest were low indicating that the samples were almost pure biogenic opal. For a number of samples, Zn and Si were measured as a function of time as the opal is dissolved. Both Zn and Si were released at similar rates, thereby indicating a frustule source. On the other hand, Fe and Al were released to solution at a constant rate, even after all of the opal had dissolved, indicating a non-opal source. However, the amount of Zn from clay contaminants was small and likely negligible. The Zn/Si ratios from four core top samples were found to be good agreement (3.4±0.1 μmol/mol) with each other. These results were also close to Zn/Si ratios reported for biogenic opal collected in plankton tows from the South Atlantic sector of the Southern Ocean and the Pacific Ocean.

Occurrence and chemical form of arsenic in marine macroalgae from the east coast of Australia

Arsenic concentrations were measured in thirteen macroalgal species from Sydney, Australia. Brown macroalgae contained, on average, more arsenic (range, mean ± s.e.: 5-173 µg g -1 , 39 ± 4 µg g -1 ) than either green (0.12-30.2 µg g -1 , 10.7 ± 0.7 µg g -1 ) or red macroalgae (0.11-16.9 µg g -1 , 4.3 ± 0.3 µg g -1 ). Despite the overlap in arsenic concentrations between different macroalgal species, inter-species arsenic variation was apparent with arsenic concentrations following the order brown > green > red macroalgal species. It was concluded that the main contribution to the variation in arsenic concentration was from natural variability expected to occur between individuals of any species as a result of physiological differences. Most of the arsenic compounds in macroalgae (70-108%) could be extracted using methanol/water mixtures, with 38-95% of the arsenic compounds present in characterizable forms. All macroalgal species contained arsenoribosides (9-99%). The distribution of arsenoribosides followed a general pattern; glycerol-arsenoriboside and phosphate-arsenoriboside were common to all macroalgal species. Sulfonate-arsenoriboside and sulfate-arsenoriboside were found in brown macroalgal species and one red macroalgal species. Six macroalgal species contained high concentrations of inorganic arsenic (14.2-62.9%) and four species contained high concentrations of dimethylarsinic acid (13.3-41.1%). The variation in the distribution of arsenic compounds in marine macroalgal species appears to be related to taxonomic differences in storage and structural polysaccharides.

Arsenic Species Determination in Biological Tissues by HPLC–ICP–MS and HPLC–HG–ICP–MS

The use of high-pressure liquid chromatography coupled directly or by a hydride generation system to an inductively coupled plasma mass spectrometer for the unambiguous measurement of 13 arsenic species in marine biological extracts is described. The use of two chromatography systems; a Supelcosil LC-SCX cation-exchange column eluted with a 20 mM pyridine mobile phase adjusted to pH 2.2 and 2.6 with formic acid, with a flow rate of 1.5 mL min−1 at 40°C, and a Hamilton PRP-X100 anion-exchange column eluted with 20 mM NH4H2PO4 buffer at pH 5.6, with a flow rate of 1.5 mL min−1 at 40°C, was required to separate and quantify cation and anion arsenic species. Under these conditions, arsenous acid could not be separated from other arsenic species and required the use of an additional hydride generation step. Arsenic species concentrations in a locally available Tasmanian kelp (Durvillea potatorum), a certified reference material (DORM-2), and a range of commercially available macroalgae supplements and sushi seaweeds have been measured and are provided for use as in-house quality control samples to assess the effectiveness of sample preparation, extraction, and measurement techniques.

Physical mixing effects on iron biogeochemical cycling: FeCycle experiment

The effects of physical processes on the distribution, speciation, and sources/sinks for Fe in a high-nutrient low-chlorophyll (HNLC) region were assessed during FeCycle, a mesoscale SF6 tracer release during February 2003 (austral summer) to the SE of New Zealand. Physical mixing processes were prevalent during FeCycle with rapid patch growth (strain rate γ = 0.17–0.20 d−1) from a circular shape (50 km2) into a long filament of ∼400 km2 by day 10. Slippage between layers saw the patch-head overlying noninfused waters while the tail was capped by adjacent surface waters resulting in a SF6 maximum at depth. As the patch developed it entrained adjacent waters containing higher chlorophyll concentrations, but similar dissolved iron (DFe) levels, than the initial infused patch. DFe was low ∼60 pmol L−1 in surface waters during FeCycle and was dominated by organic complexation. Nighttime measurements of Fe(II) ∼20 pmol L−1 suggest the presence of Fe(II) organic complexes in the absence of an identifiable fast Fe(III) reduction process. Combining residence times and phytoplankton uptake fluxes for DFe it is cycled through the biota 140–280 times before leaving the winter mixed layer (WML). This strong Fe demand throughout the euphotic zone coupled with the low Fe:NO3 − (11.9 μmol:mol) below the ferricline suggests that vertical diffusion of Fe is insufficient to relieve chronic iron limitation, indicating the importance of atmospheric inputs of Fe to this region.

Glacial Silicic Acid Concentrations in the Southern Ocean

Silicon Leakage Silicon is a major structural component of many marine organisms, whose chemistry is affected by oceanic nutrient distributions. To constrain nutrient changes since the last glacial period, Ellwood et al. (p. 1088, published online 21 October) measured the isotopic compositions of silicon obtained from the skeletons of deep-sea sponges found in deep cores from the Atlantic and Pacific sectors of the Southern Ocean and compared them to the silicon signatures in the skeletons of modern sponges. The results indicate that nutrient redistribution, related to iron fertilization from dust deposition, boosted the growth of organisms that transferred silicon to mid-latitudes during the last glacial period. Silicon isotope distributions in sponges contain the signature of ocean nutrient distributions during the last glacial period. Reconstruction of nutrient concentrations in the deep Southern Ocean has produced conflicting results. The cadmium/calcium (Cd/Ca) data set suggests little change in nutrient concentrations during the last glacial period, whereas the carbon isotope data set suggests that nutrient concentrations were higher. We determined the silicon isotope composition of sponge spicules from the Atlantic and Pacific sectors of the Southern Ocean and found higher silicic acid concentrations in the Pacific sector during the last glacial period. We propose that this increase results from changes in the stoichiometric uptake of silicic acid relative to nitrate and phosphate by diatoms, thus facilitating a redistribution of nutrients across the Pacific and Southern Oceans. Our results are consistent with the global Cd/Ca data set and support the silicic acid leakage hypothesis.