High-nutrient Low-chlorophyll Region Research Articles

Drivers of ocean iron stress variability in high nutrient-low chlorophyll regions from ocean color

Drivers of ocean iron stress variability in high nutrient-low chlorophyll regions from ocean color

Estimates of the Isotope Effect for Nitrate Assimilation in the Indian Sector of the Southern Ocean

AbstractThe Southern Ocean is a high‐nutrient, low‐chlorophyll (HNLC) region characterized by incomplete nitrate (NO3−) consumption by phytoplankton in surface waters. During this incomplete consumption, phytoplankton preferentially assimilate the 14N‐ versus the 15N‐bearing form of NO3−, quantified as the NO3− assimilation isotope effect (15ε). Previous summertime estimates of the 15ε from HNLC regions range from 4 to 11‰. While culture work has shown that the 15ε varies among phytoplankton species, as well as with light and iron stress, we lack a systematic understanding of how and why the 15ε varies in the field. Here we estimate the 15ε from water‐column profile and surface‐water samples collected in the Indian sector of the Southern Ocean—the first leg of the Antarctic Circumnavigation Expedition (December 2016–January 2017) and the Crossroads transect (April 2016). Consistent with prior work in the mid‐to‐late summer Southern Ocean, we estimate a higher 15ε (8.9 ± 0.6‰) for the northern Subantarctic Zone and a lower 15ε (5.4 ± 0.9‰) at and south of the Subantarctic Front. We interpret our data in the context of coincident measurements of phytoplankton community composition and estimates of iron and light stress. Similar to prior work, we find a significant, negative relationship between the 15ε and the average mixed‐layer photosynthetically active radiation flux of 30–100 μmol m−2 s−1, while above 100 μmol m−2 s−1, 15ε increases again. In addition, while we observe no robust relationship of the 15ε to iron availability or phytoplankton community, mixed‐layer nitrification over the Kerguelen Plateau appears to strongly influence its magnitude.

Biogenic and lithogenic silicon along the GEOTRACES south West Indian Ocean section (SWINGS-GS02) and the islands mass effect on regional Si biogeochemical cycle

The distribution and cycling of biogenic silica (BSi) and lithogenic silicon (LSi) in the ocean play crucial roles in the global silicon cycle and marine ecosystem dynamics. This is especially the case in the Southern Ocean where diatoms constitute the predominant phytoplankton and participate in a major way to the biological carbon pump. This study presents an assessment of BSi and LSi concentrations along the GEOTRACES South West Indian Ocean Section (SWINGS, late austral summer 2021), where several and contrasting regions were encountered: oligotrophic Mozambique basin, HNLC (High Nutrient Low Chlorophyll) areas and regions fertilized by the Subantarctic islands. Suspended particles were sampled from Niskin bottles and in situ pumps, along with scanning electron microscope (SEM) observations and specific pigments measurements to support BSi and LSi analyses. With samples coming from a contrasting study area prone to diverse continental influences, our BSi and LSi results showed a reproducibility of 13 ± 7%, in the same range as the established protocol. BSi concentrations show a north-south gradient with maxima encountered in the Antarctic Zone, and contrasted results between HNLC open ocean areas and naturally fertilized regions in the vicinity of the Subantarctic islands. Some open ocean stations have unusually high BSi (e.g. > 5 μmol L−1) likely resulting from fertilization by aerosols, upwelling or island mass effect when they are downstream of the islands. Coupling of BSi with SEM observations and pigments measurements respectively showed diatoms were the most representative of the carrying phase of BSi and suggested silicification changes, induced either by heavily silicified diatoms or by micronutrient limitation in HNLC regions. BSi is often dominated by the smallest size fraction (0.45–5 μm) which represent 47 ± 23% of the total BSi based on 29 measurements on size fractionated samples. LSi results highlighted atmospheric inputs at the surface and nepheloid layers in the water column, which makes LSi overall a good indicator of the origin of lithogenic materials. SEM observations supported these results, enabling characterization of the diversity of lithogenic materials in the vicinity of the Subantarctic islands, more specifically volcanic ash around Heard Island, and within the nepheloid layers.

Assessing the Ecological Implications of Ocean Iron Fertilization: Insights from Post-Eruption Volcanic Processes

This study proposes the usage of post-eruption volcanic processes as a natural analog for assessing the potential impacts of man-made iron fertilization on ocean ecosystems. Data is compiled from the 14 most explosive volcanic eruptions since 1884 to establish correlations between climate temperature anomalies and phytoplankton growth induced by volcanic tephra fallout in addition to other post-eruption variables. Through a comparative analysis of the contrasting strengths of these relationships using a student t-test for statistical significance, multivariable regression analysis, and linear regression interpretation, the scale of ocean iron fertilization is quantified. Two well-documented modern volcanic eruptions in Hawaii and Iceland are additionally employed as case studies to provide a comprehensive understanding of micro phenomenons occurring post-eruption. Rapid algae growth is demonstrated in High Nutrient Low Chlorophyll regions (HNLC), where long-term growth can be sustained for up to six months. Statistical results isolate the effect of stratospheric injection from ocean iron fertilization, demonstrating the effectiveness of tephra ash fallout in inducing regional phytoplankton growth. In all, these findings contribute to a comprehension of the potential ecological consequences of iron fertilization efforts, an approach to assess environmental implications without direct experimentation.

Characterization of Bio‐Optical Anomalies in the Kerguelen Region, Southern Indian Ocean: A Study Based on Shipborne Sampling and BioGeoChemical‐Argo Profiling Floats

AbstractThe Southern Ocean (SO) is known for its atypical bio‐optical regime. This complicates the interpretation of proxies measured from satellite and in situ platforms equipped with optical sensors, which occupy an important niche for monitoring the vast and remote SO. A ship‐based field study in concert with time series observations from BioGeoChemical‐Argo (BGC‐Argo) profiling floats were used to investigate spatial and temporal variations in bio‐optical relationships in the open ocean waters surrounding the Kerguelen Plateau in the Indian sector of the SO. Compared to other regions with similar chlorophyll concentrations, chlorophyll‐specific phytoplankton absorption in the blue waveband presented a consistent negative anomaly. The anomaly was uniform over deep mixed layers and correlated with phytoplankton size, photoacclimation and atypically high concentrations of fucoxanthin. The BGC‐Argo observation‐based proxies revealed that the blue absorption anomaly increased with chlorophyll concentration both spatially and temporally and, while particularly pronounced in the naturally iron‐fertilized waters, was also found in the High Nutrient Low Chlorophyll region. While phytoplankton size was an important driver of the anomaly, photoacclimation associated with self‐shading of phytoplankton cells was also involved during intense booms. The backscattering coefficient exhibited negative and positive anomalies in the low and high biomass regimes, respectively. The large positive anomaly in high biomass regimes was attributed to the variable non‐algal particles characteristics associated with a relatively high production of bloom by‐products. With clear understanding of the bio‐optical anomalies, BGC‐Argo floats stand as unique tools for monitoring the bio‐optical spatio‐temporal complexity of the SO.

Synechococcus nitrogen gene loss in iron-limited ocean regions

Synechococcus are the most abundant cyanobacteria in high latitude regions and are responsible for an estimated 17% of annual marine net primary productivity. Despite their biogeochemical importance, Synechococcus populations have been unevenly sampled across the ocean, with most studies focused on low-latitude strains. In particular, the near absence of Synechococcus genomes from high-latitude, High Nutrient Low Chlorophyll (HNLC) regions leaves a gap in our knowledge of picocyanobacterial adaptations to iron limitation and their influence on carbon, nitrogen, and iron cycles. We examined Synechococcus populations from the subarctic North Pacific, a well-characterized HNLC region, with quantitative metagenomics. Assembly with short and long reads produced two near complete Synechococcus metagenome-assembled genomes (MAGs). Quantitative metagenome-derived abundances of these populations matched well with flow cytometry counts, and the Synechococcus MAGs were estimated to comprise >99% of the Synechococcus at Station P. Whereas the Station P Synechococcus MAGs contained multiple genes for adaptation to iron limitation, both genomes lacked genes for uptake and assimilation of nitrate and nitrite, suggesting a dependence on ammonium, urea, and other forms of recycled nitrogen leading to reduced iron requirements. A global analysis of Synechococcus nitrate reductase abundance in the TARA Oceans dataset found nitrate assimilation genes are also lower in other HNLC regions. We propose that nitrate and nitrite assimilation gene loss in Synechococcus may represent an adaptation to severe iron limitation in high-latitude regions where ammonium availability is higher. Our findings have implications for models that quantify the contribution of cyanobacteria to primary production and subsequent carbon export.

Global variability of high-nutrient low-chlorophyll regions using neural networks and wavelet coherence analysis

Abstract. We examine 20 years of monthly global ocean color data and modeling outputs of nutrients using self-organizing map (SOM) analysis to identify characteristic spatial and temporal patterns of high-nutrient low-chlorophyll (HNLC) regions and their association with different climate modes. The global nitrate-to-chlorophyll ratio threshold of NO3 : Chl > 17 (mmol NO3 mg Chl−1) is estimated to be a good indicator of the distribution limit of this unproductive biome that, on average, covers 92 × 106 km2 (∼ 25 % of the ocean). The trends in satellite-derived surface chlorophyll (0.6 ± 0.4 % yr−1 to 2 ± 0.4 % yr−1) suggest that HNLC regions in polar and subpolar areas have experienced an increase in phytoplankton biomass over the last decades, but much of this variation, particularly in the Southern Ocean, is produced by a climate-driven transition in 2009–2010. Indeed, since 2010, the extent of the HNLC zones has decreased at the poles (up to 8 %) and slightly increased at the Equator (< 0.5 %). Our study finds that chlorophyll variations in HNLC regions respond to major climate variability signals such as the El Niño–Southern Oscillation (ENSO) and Meridional Overturning Circulation (MOC) at both short (2–4 years) and long (decadal) timescales. These results suggest global coupling in the functioning of distant biogeochemical regions.

Iron and light limitation of phytoplankton growth off East Antarctica

Phytoplankton production controls the transfer of carbon dioxide from the atmosphere into the ocean through the biological carbon pump, facilitating the sequestration of carbon dioxide and thus, contributing to the regulation of global climate. In the Southern Ocean, a high-nutrient low-chlorophyll region, productivity is limited by micronutrients including iron. The limiting role of light is also important, especially in the context of Southern Ocean phytoplankton adaptation to the low-iron conditions. To investigate the relative dominance of iron and light limitation on phytoplankton growth, we conducted a series of incubation experiments during a 2019 summer research voyage off East Antarctica. Our results show that, while light was the primary limiting factor of phytoplankton growth, iron addition became favourable for growth under high light conditions. This supports similar findings from laboratory (Strzepek et al., 2019) and field experiments in the Ross Sea (Alderkamp et al., 2019) and provides a new insight from a different region of the Southern Ocean, at a later stage of the austral summer. Similar results have previously been documented in larger diatoms, and here we provide evidence that it is also true for smaller cells. Our findings add to the body of knowledge regarding iron and light limitation in the Southern Ocean, in a poorly sampled area, leading to a better understanding of how primary production will change in a future warming ocean.

The upper ocean silicon cycle of the subarctic Pacific during the EXPORTS field campaign

Diatoms are major contributors to marine primary productivity and carbon export due to their rapid growth in high-nutrient environments and their heavy silica ballast. Their contributions are highly modified in high-nutrient low-chlorophyll regions due to the decoupling of upper-ocean silicon and carbon cycling caused by low iron (Fe). The Si cycle and the role of diatoms in the biological carbon pump was examined at Ocean Station Papa (OSP) in the HNLC region of the northeastern subarctic Pacific during the NASA EXport Processes in the Ocean from RemoTe Sensing (EXPORTS) field study. Sampling occurred during the annual minimum in surface silicic acid (Si(OH)4) concentration. Biogenic silica (bSi) concentrations were low, being in the tens of nanomolar range, despite high Si(OH)4 concentrations of about 15 μM. On average, the >5.0-µm particle size fraction dominated Si dynamics, accounting for 65% of bSi stocks and 81% of Si uptake compared to the small fraction (0.6–5.0 μm). Limitation of Si uptake was detected in the small, but not the large, size fraction. Growth rate in small diatoms was limited by Fe, while their Si uptake was restricted by Si(OH)4 concentration, whereas larger diatoms were only growth-limited by Fe. About a third of bSi production was exported out of the upper 100 m. The contribution of diatoms to carbon export (9–13%) was about twice their contribution to primary productivity (3–7%). The combination of low bSi production, low diatom primary productivity and high bSi export efficiency at OSP was more similar to the dynamics in the subtropical gyres than to other high-nutrient low-chlorophyll regions.

Biogeochemical Equilibrium Responses to Maximal Productivity in High Nutrient Low Chlorophyll Regions

AbstractWhether ocean iron fertilization in high nutrient low chlorophyll (HNLC) regions is a practical geoengineering strategy to combat climate warming has been debated because the fate of absorbed carbon by the ocean remains unclear. We use an optimized oceanic biogeochemical inverse model to explore the equilibrium responses to increased productivity in major HNLC regions. We find that an overall downward shift of nutrients and carbon from the surface and intermediate to the deep waters with maximal productivity in the HNLC Southern Ocean (SO), North Pacific (NP) and eastern equatorial Pacific (EP). The efficiency of soft tissue pump is increased by 37.8%, 2.7% and 5.4% in the SO, EP and NP simulation while atmospheric CO2 concentration is drawn down by 45.1, 3.4 and 6.7 ppm, respectively. Meanwhile, global mean oxygen is decreased by 21.1%, 1.3% and 2.4% in the SO, EP and NP simulation, accompanied by an expansion of oxygen minimum zones by 400%, 26% and 63%, respectively. Our study indicates that iron fertilization has the potential to be an effective geoengineering approach to mitigate climate warming but also has a strong impact on the development of ocean hypoxia.

Macrozooplankton and micronekton diversity and associated carbon vertical patterns and fluxes under distinct productive conditions around the Kerguelen Islands

Mesopelagic communities are characterized by a large biomass of diverse macrozooplankton and micronekton (MM) performing diel vertical migration (DVM) connecting the surface to the deeper ocean and contributing to biogeochemical fluxes. In the Southern Ocean, a prominent High Nutrient Low Chlorophyll (HNLC) and low carbon export region, the contribution of MM to the vertical carbon flux of the biological pump remains largely unknown. Furthermore, few studies have investigated MM communities and vertical flux in naturally iron fertilized areas associated with shallow bathymetry. In this study, we assessed the MM community diversity, abundance and biomass in the Kerguelen Island region, including two stations in the HNLC region upstream of the islands, and two stations in naturally iron fertilized areas, one on the Plateau, and one downstream of the Plateau. The MM community was examined using a combination of trawl sampling and acoustic measurements at 18 and 38 kHz from the surface to 800 m. A conspicuous three-layer vertical system was observed in all areas - a shallow scattering layer, SSL, between 10 and 200 m; mid-depth scattering layer, MSL, between 200 and 500 m; deep scattering layer, DSL, between 500 and 800 m - but communities differing among stations. While salps ( Salpa thompsoni ) dominated the biomass at the productive Kerguelen Plateau and the downstream station, they were scarce in the HNLC upstream area. In addition, crustaceans (mainly Euphausia vallentini and Themisto gaudichaudii ) were particularly abundant over the Plateau, representing a large, although varying, carbon stock in the 0–500 m water layer. Mesopelagic fish were prominent below 400 m where they formed permanent or migrant layers accounting for the main source of carbon biomass. Through these spatial and temporal sources of variability, complex patterns of the MM vertical distribution and associated carbon content were identified. The total carbon flux mediated by migratory myctophids at the four stations was quantified. While this flux was likely underestimated, this study identified the main components and mechanisms of active carbon export in the region and how they are modulated by complex topography and land mass effects. • Macrozooplankton and micronekton communities are described in the Kerguelen area using trawl and acoustic • Despite different assemblages, a consistent three-layers system occurred in the first 800 m over the Kerguelen seascape • Salps dominated global biomass in productive areas but not in HNLC region while crustaceans were abundant on the Plateau • Myctophids dominated below 400 m as permanent or migrant layers accounting for the main source of carbon biomass • We estimate total carbon flux mediated by migratory myctophids in the contrasting productive areas

Calculating Feasibility of Ocean Iron Fertilization with Post-Volcanic Eruption Phenomena

This review proposes a novel method to accurately gauge the full ecological impact of manmade iron fertilization upon local environments not through direct experimentation, but by observing similar natural manifestations of fertilization in volcanic eruptions. Two case studies of specific volcanoes - Kilauea in Hawaii and Eyjafjallajökull in Iceland - show that firstly, ocean iron fertilization is capable of self-sustainment after ocean iron fertilization deployments in HNLC (High Nutrient Low Chlorophyll) regions, and secondly, ocean fertilization can be extended beyond the usage of iron dust fertilizer in ideal situations. For example, simple physical disturbances under the shores of one of Hawaii's islands propelled the large-scale development of phytoplankton colonies, in a process called "organic nitrate displacement". Kilauea's volcanic breakouts contributed to greenhouse gas emissions but were proportionally absorbed (7:1 ratio) by local phytoplankton growth. In another case of post-eruption phytoplankton development at Eyjafjallajökull in Iceland, while the volcanic eruptions dispersed a relatively small amount of iron-infused tephra through atmospheric injection and way of local winds, the airborne nutrients heavily stimulated phytoplankton colonies throughout surrounding waters and generated large biomasses of algae that were visible from space. After consecutive months of eruptions by Eyjafjallajökull in 2010, the airborne spread of iron-rich tephra particles lead to visible growths in phytoplankton colonies bordering other nearby countries. Through multiple comparative case studies of post volcanic eruption phenomena, ocean iron fertilization demonstrates its efficiency in initiating phytoplankton growth while revealing unexpected safety concerns in its deployment.

Pelagic food web structure in high nutrient low chlorophyll (HNLC) and naturally iron fertilized waters in the Kerguelen Islands region, Southern Ocean

The Kerguelen Plateau is a region of natural iron fertilization in the Southern Ocean, within the typically iron limited High Nutrient Low Chlorophyll (HNLC) waters of the eastward flowing Antarctic Circumpolar Current. Between 26 February and 19 March 2018, the MOBYDICK expedition investigated pelagic ecosystem dynamics in the Kerguelen Island region on the northern plateau during the post-phytoplankton bloom period. The survey specifically targeted sampling at two stations in the HNLC waters to the west of the Kerguelen Plateau, and in the iron enriched waters on and to the east of the plateau. A combination of WP2, WP3 and Mesopelagos midwater trawl were used to sample the mesozooplankton (125 μm to ≤ 10 mm), macrozooplankton (10–30 mm) and micronekton (> 30 to 200 mm) communities. Stable carbon (δ13C) and nitrogen (δ15N) isotopes were measured across representative samples of taxa from all stations. Trophic positions was estimated using a Bayesian isotope mixing model (tRophicPosition), that uses both the δ13C and δ15N values of baselines and consumers. Meso/macrozooplankton trophic positions (TPs) were ~ 2.4 on the plateau and > 0.6 TPs higher at the upstream HNLC stations. This provides empirical evidence for shorter food chains on the diatom dominated plateau and longer food chains in the HNLC region dominated by pico and nanophytopklankon. Meso/macrozooplankton TPs were also elevated downstream of the plateau, though to a lesser extent than in the upstream region, likely reflecting the effect of downstream iron transport, supporting elevated phytoplankton production in the downstream region. There was a consistent increase in trophic position between meso/macrozooplankton and micronekton of ~ 0.6 at all stations. Food chain length was therefore determined by the composition of and interactions between the trophic levels below micronekton. Similar trophic structure at the two upstream HNLC stations, on either side of the northern branch of the Polar Front, indicated that food chain length was robust to the differences in community composition, and supports the critical role of size structured trophic interactions in determining food web properties.

The Role of Zooplankton in Establishing Carbon Export Regimes in the Southern Ocean – A Comparison of Two Representative Case Studies in the Subantarctic Region

Marine ecosystems regulate atmospheric carbon dioxide levels by transporting and storing photosynthetically fixed carbon in the ocean’s interior. In particular, the subantarctic and polar frontal zone of the Southern Ocean is a significant region for physically driven carbon uptake due to mode water formation, although it is under-studied concerning biologically mediated uptake. Regional differences in iron concentrations lead to variable carbon export from the base of the euphotic zone. Contrary to our understanding of export globally, where high productivity results in high export, naturally iron-fertilized regions exhibit low carbon export relative to their surface productivity, while HNLC (High Nutrient, Low Chlorophyll) waters emerge as a significant area for carbon export. Zooplankton, an integral part of the oceanic food web, play an important role in establishing these main carbon export regimes. In this mini review, we explore this role further by focusing on the impact of grazing and the production of fecal pellets on the carbon flux. The data coverage in the subantarctic region will be assessed by comparing two case studies - the iron-replete Kerguelen Plateau and the HNLC region south of Australia. We then discuss challenges in evaluating the contributions of zooplankton to carbon flux, namely gaps in seasonal coverage of sampling campaigns, the use of non-standardized and biased methods and under-sampling of the mesopelagic zone, an important area of carbon remineralization. More integrated approaches are necessary to improve present estimates of zooplankton-mediated carbon export in the Southern Ocean.

Iron Fertilization with Enhanced Phytoplankton Productivity under Minimal Sulfur Compounds and Grazing Control Analysis in HNLC Region

The present study investigated quantitatively the significance of HNLC (high-nutrient low-chlorophyll) regions and its grazing control with the improved iron fertilization for climate change. The limitation of iron (Fe) for phytoplankton growth in HNLC regions was confirmed by sulfur compounds (S) such as volcanic ash and hydrogen sulfide (H2S) in batch cultures, whose chemical sediment of Fe3S4 showed 4.06 wt%. The technologies developed for iron fertilization since 1993 till now were not practical to provide sufficient amounts of bioavailable iron due to sedimentary iron sulfides induced by undersea volcanic sulfur compounds. The proposed technology for iron fertilization was improved to enhance the bioavailable iron to phytoplankton by keeping minimal sulfur compounds in HNLC regions. The low productivity of phytoplankton by grazing control in HNLC regions was 6% diatoms whose 52% was grazed by copepods and 42% by krill on the basis of data analysis in 2000 EisenEx Experiment at boundary of Antarctic and African tectonic plates. All of the previous iron fertilization experiments were conducted at volcanic sulfur compounds enriched HNLC regions. The present study revealed that the enhanced phytoplankton productivity in batch culture without sedimentary iron sulfides can be possible only if sulfur compounds are minimal, as is in Shag Rocks (53°S, 42°W) of South Georgia in Scotia Sea in the Southern Ocean.

Biological responses of the marine diatom Chaetoceros socialis to changing environmental conditions: A laboratory experiment.

Diatoms constitute a major group of phytoplankton, accounting for ~20% of the world’s primary production. It has been shown that iron (Fe) can be the limiting factor for phytoplankton growth, in particular, in the HNLC (High Nutrient Low Chlorophyll) regions. Iron plays thus an essential role in governing the marine primary productivity and the efficiency of biological carbon pump. Oceanic systems are undergoing continuous modifications at varying rates and magnitudes as a result of changing climate. The objective of our research is to evaluate how changing environmental conditions (dust deposition, ocean warming and acidification) can affect marine Fe biogeochemistry and diatom growth. Laboratory culture experiments using a marine diatom Chaetoceros socialis were conducted at two temperatures (13°C and 18°C) and under two pCO2 (carbon dioxide partial pressure) (400 μatm and 800 μatm) conditions. The present study clearly highlights the effect of ocean acidification on enhancing the release of Fe upon dust deposition. Our results also confirm that being a potential source of Fe, dust provides in addition a readily utilizable source of macronutrients such as dissolved phosphate (PO4) and silicate (DSi). However, elevated atmospheric CO2 concentrations may also have an adverse impact on diatom growth, causing a decrease in cell size and possible further changes in phytoplankton composition. Meanwhile, ocean warming may lead to the reduction of diatom production and cell size, inducing poleward shifts in the biogeographic distribution of diatoms. The changing climate has thus a significant implication for ocean phytoplankton growth, cell size and primary productivity, phytoplankton distribution and community composition, and carbon (C), nitrogen (N), phosphorus (P), silicon (Si) and Fe biogeochemical cycles in various ways.

Meltwater as a source of potentially bioavailable iron to Antarctica waters

AbstractRecent rapid retreat of glacial front lines and the loss of land ice along the Antarctic margins may play an important role in exporting suspended particulate matter (SPM) potentially rich in bioavailable (defined as ascorbate leachable) iron (FeA) to coastal areas of the Southern Ocean. Sediment ablation is an additional source of iron for this high-nutrient low-chlorophyll region. In Potter Cove, King George Island, meltwater streams discharge up to 18 000 mg l-1(average 283 mg l-1) of slightly weathered, finely ground bedrock particles into coastal waters during the summer. Approximately 15% of this SPM is exported within a low-salinity surface plume into Bransfield Strait. Based on our data, an estimated 12 mg m-2yr-1of FeAis exported from the South Shetland Island land surface (ice-free and subglacial areas) to the surrounding coastal waters. Extrapolated to an area of 2.5x104km2, this FeAinput is comparable to the contribution from icebergs andc.240-fold higher than aeolian input via dust. An observed rise in local sediment accumulation rates suggests that glacial erosion has been increasing over recent decades and that (sub-)glacially derived SPM is becoming more important as a source of iron to the Southern Ocean.

Role of zooplankton dynamics for Southern Ocean phytoplankton biomass and global biogeochemical cycles

Abstract. Global ocean biogeochemistry models currently employed in climate change projections use highly simplified representations of pelagic food webs. These food webs do not necessarily include critical pathways by which ecosystems interact with ocean biogeochemistry and climate. Here we present a global biogeochemical model which incorporates ecosystem dynamics based on the representation of ten plankton functional types (PFTs): six types of phytoplankton, three types of zooplankton, and heterotrophic procaryotes. We improved the representation of zooplankton dynamics in our model through (a) the explicit inclusion of large, slow-growing macrozooplankton (e.g. krill), and (b) the introduction of trophic cascades among the three zooplankton types. We use the model to quantitatively assess the relative roles of iron vs. grazing in determining phytoplankton biomass in the Southern Ocean high-nutrient low-chlorophyll (HNLC) region during summer. When model simulations do not include macrozooplankton grazing explicitly, they systematically overestimate Southern Ocean chlorophyll biomass during the summer, even when there is no iron deposition from dust. When model simulations include a slow-growing macrozooplankton and trophic cascades among three zooplankton types, the high-chlorophyll summer bias in the Southern Ocean HNLC region largely disappears. Our model results suggest that the observed low phytoplankton biomass in the Southern Ocean during summer is primarily explained by the dynamics of the Southern Ocean zooplankton community, despite iron limitation of phytoplankton community growth rates. This result has implications for the representation of global biogeochemical cycles in models as zooplankton faecal pellets sink rapidly and partly control the carbon export to the intermediate and deep ocean.

Microplankton trace element contents: implications for mineral limitation of mesozooplankton in an HNLC area.

Mesozooplankton production in high-nutrient low-chlorophyll regions of the ocean may be reduced if the trace element concentrations in their food are insufficient to meet growth and metabolic demands. We used elemental microanalysis (SXRF) of single-celled plankton to determine their trace metal contents during a series of semi-Lagrangian drift studies in an HNLC upwelling region, the Costa Rica Dome (CRD). Cells from the surface mixed layer had lower Fe:S but higher Zn:S and Ni:S than those from the subsurface chlorophyll maximum at 22-30 m. Diatom Fe:S values were typically 3-fold higher than those in flagellated cells. The ratios of Zn:C in flagellates and diatoms were generally similar to each other, and to co-occurring mesozooplankton. Estimated Fe:C ratios in flagellates were lower than those in co-occurring mesozooplankton, sometimes by more than 3-fold. In contrast, Fe:C in diatoms was typically similar to that in zooplankton. RNA:DNA ratios in the CRD were low compared with other regions, and were related to total autotrophic biomass and weakly to the discrepancy between Zn:C in flagellated cells and mesozooplankton tissues. Mesozooplankton may have been affected by the trace element content of their food, even though trace metal limitation of phytoplankton was modest at best.

Mercury and methylmercury incidence and bioaccumulation in plankton from the central Pacific Ocean

Open ocean samples of phytoplankton and zooplankton were collected from the central Pacific on board the R/V Kilo Moana in October of 2011. The cruise traveled from Hawaii to Samoa, progressing through a High Nutrient Low Chlorophyll (HNLC) zone, and an equatorial upwelling region. Phytoplankton samples were size fractioned into 0.2–5μm, 5–20μm, and >20μm samples. Methylmercury concentrations were 2.91±2.58pmolg−1 (wet weight) for the overall <200μm size fractions, and highest around the HNLC region. Phytoplankton bioconcentration factors (logBCFs) averaged to 5.69±0.98 and were higher than the values found for coastal regions. Both %MeHg ([MeHg]/[Hg]) and logBCF values indicated that the lowest size fraction had the largest fraction of the HgT as MeHg, signifying enhanced accumulation of MeHg into smaller organisms. Zooplankton vertical net tows were completed from depths of 200m up to ~10m. Zooplankton samples were analyzed for carbon, nitrogen and sulfur in addition to Hg and MeHg at size fractionations of 0.2–0.5mm, 0.5–1.0mm, 1.0–2.0mm and occasionally >2.0mm. Zooplankton abundance and MeHg concentrations both peaked at Stations 3 and 5 (upwelling region). The %MeHg in the organisms was highest in the >2.0mm size class, displaying MeHg bioaccumulation for increasing zooplankton sizes. Separate day and night net tows were collected at Stations 3 and 5 in order to investigate differences due to diurnal migration of zooplankton. There were higher concentrations of MeHg for all sizes in night collections of zooplankton at Station 5, but no discrepancy for Station 3. These results represent some of the few measurements of Hg and MeHg at the base of the open ocean food chain, and are significant as they represent the instigation of bioconcentration into the base of marine food webs.