17 Apr 2015: Ratnarajah L, Bowie AR, Lannuzel D, Meiners KM, Nicol S (2015) Correction: The Biogeochemical Role of Baleen Whales and Krill in Southern Ocean Nutrient Cycling. PLOS ONE 10(4): e0125134. doi: 10.1371/journal.pone.0125134 View correction
The availability of micronutrients is a key factor that affects primary productivity in High Nutrient Low Chlorophyll (HNLC) regions of the Southern Ocean. Nutrient supply is governed by a range of physical, chemical and biological processes, and there are significant feedbacks within the ecosystem. It has been suggested that baleen whales form a crucial part of biogeochemical cycling processes through the consumption of nutrient-rich krill and subsequent defecation, but data on their contribution are scarce. We analysed the concentration of iron, cadmium, manganese, cobalt, copper, zinc, phosphorus and carbon in baleen whale faeces and muscle, and krill tissue using inductively coupled plasma mass spectrometry. Metal concentrations in krill tissue were between 20 thousand and 4.8 million times higher than typical Southern Ocean HNLC seawater concentrations, while whale faecal matter was between 276 thousand and 10 million times higher. These findings suggest that krill act as a mechanism for concentrating and retaining elements in the surface layer, which are subsequently released back into the ocean, once eaten by whales, through defecation. Trace metal to carbon ratios were also higher in whale faeces compared to whale muscle indicating that whales are concentrating carbon and actively defecating trace elements. Consequently, recovery of the great whales may facilitate the recycling of nutrients via defecation, which may affect productivity in HNLC areas.
Citation: Ratnarajah L, Bowie AR, Lannuzel D, Meiners KM, Nicol S (2014) The Biogeochemical Role of Baleen Whales and Krill in Southern Ocean Nutrient Cycling. PLoS ONE 9(12): e114067. doi:10.1371/journal.pone.0114067
Editor: Syuhei Ban, University of Shiga Prefecture, Japan
Received: July 16, 2014; Accepted: October 29, 2014; Published: December 3, 2014
Copyright: © 2014 Ratnarajah et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Data Availability: The authors confirm that all data underlying the findings are fully available without restriction. The data is publicly available through the Australian Oceans Data Network (AODN). The data can be downloaded from (http://mest.imas.utas.edu.au/geonetwork/srv/en/metadata.show?uuid=c744388c-b914-4771-99e2-870bd8114e30).
Funding: This work was supported by the Australian Government's Cooperative Research Centres Programme through the Antarctic Climate and Ecosystem Cooperative Research Centre, by the Australian Antarctic Division and by the Australian Marine Mammal Centre. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
Competing interests: The authors have declared that no competing interests exist.
Large regions of the Southern Ocean are characterized by low phytoplankton biomass despite high concentrations of major nutrients (e.g. nitrate, phosphate and silicate), and have been characterised as High Nutrient Low Chlorophyll (HNLC) waters . Phytoplankton forms the base of the marine food chain, supporting everything from microscopic animals to large marine mammals –. It also plays an important role in carbon sequestration by converting carbon dioxide (CO2) to biomass through photosysnthesis, and through sinking, transferring the carbon to the deep ocean and sea floor sediments , . Marine ecosystems can either act as a source or sink of atmospheric CO2 depending on the relative rates of photosynthesis and overall total respiration. One factor responsible for limiting the accumulation of phytoplankton in HNLC waters has been the availability of essential trace elements, particularly iron (Fe), that are required for biochemical processes such as photosynthesis and respiration, as well as in the reduction of inorganic nitrogen species .
The major sources of trace elements in marine ecosystems are from atmospheric deposition, continental run-off, shelf sediments, hydrothermal vents and ocean crust . However the Southern Ocean is remote from most of these sources; consequently the concentration of trace elements in surface waters is low. Some of the important trace elements underpinning biogeochemical processes are: Fe and manganese (Mn) for carbon fixation; zinc (Zn), cadmium (Cd), and cobalt (Co) for CO2 acquisition; Zn and Cd for silica uptake by large diatoms; Co and Zn as calcifiers; Fe for nitrogen (N2) fixation; copper (Cu) and Fe for nitrification, denitrification and organic N utilization; Zn for organic phosphorus (P) utilization; Fe for synthesis of photopigments; and Cu for methane oxidation , . As Fe, Mn, and Cu have a short residence time, while Cd, Zn and P have an intermediate residence time in oxygenated waters –, any mechanism that can increase the persistence of trace elements in surface waters should enhance overall marine primary productivity.
Until recently, the primary biogeochemical role of marine animals was considered to be as consumers of carbon, converting it into fast-sinking faecal material or returning it to the atmosphere through respiration . However, a number of recent studies instead suggest that marine animals and seabirds are part of a positive feedback loop that retains nutrients in the surface waters, thus enhancing primary productivity and stimulating carbon export –.
All animals require a range of nutrients that they mostly obtain from their diet. Different marine animal groups have requirements for particular nutrients: e.g. crustaceans require Cu for their respiratory pigment , whereas marine mammals require Fe for the oxygen (O2) storage protein in muscles; myoglobin . Thus animals tend to concentrate the range of nutrients that are important for their metabolic processes. Marine mammals, being air-breathing, spend most of their lives in the surface layer and are thought to defecate exclusively in the euphotic zone . In addition, some animals inhabit or migrate to water deeper than the euphotic zone, where they feed and then return the scavenged nutrients to the surface layer when they defecate , . Animals such as seabirds and whales are capable of converting the concentrated elements found in solid form in their prey into a liquid form in their faecal material that is released into the euphotic zone , , . This plume of liquid, rich in trace elements, could act as a fertiliser for phytoplankton production , . Dense aggregations of large animals may also have a significant local effect on mixing of water and nutrients across the thermocline by generating turbulence .
The objective of our study was to determine the degree to which a variety of trace elements are concentrated in krill tissue, and subsequently taken up into whale muscle or defecated, to evaluate their potential role in recycling nutrients in the Southern Ocean. In addition to Fe, we report the concentrations of carbon and six other biologically important elements (Cd, Mn, Co, Cu, P and Zn) measured in five species of baleen whales and four species of krill, including Antarctic krill (Euphausia superba). Iron concentrations and diet analysis on these samples have been presented and discussed in Nicol et al.  and Jarman et al. , respectively.
Whale muscle samples were collected from stranded and dead blue (Baleoptera musculus) and fin (Baleoptera physalus) whales in South-western Australia. Blue, fin, sperm (Physeter macrocephalus), humpback (Megaptera novaeangliae) and pygmy blue (Baleoptera musculus brevicauda) whale faecal samples were collected opportunistically from a range of locations by trawling 0.5 mm mesh nets over the surface waters following defecation. Four species of krill (Nyctiphanes australis, Meganyctiphanes norvegica, Euphausia pacifica and Euphausia superba) were collected from various locations worldwide. All sample tissue and faecal matter were stored in individual 50 ml polycarbonate screw cap bottles, preserved in>70% ethanol and frozen at −20°C until analyses.
Analysis of the trace element concentration
Samples were dried at 60°C until constant weight was attained. Subsequently they were crushed using an acid-cleaned pipette tip and shaken vigorously to homogenise the samples. Digestion of 2–100 mg subsamples were performed in acid-cleaned 15 ml Teflon perfluoroalkoxy (PFA) vials (Savillex, Minnetonka, MN, USA) by adding 1 ml of concentrated nitric acid and 0.125 ml of hydrogen peroxide (all Ultrapure, Seastar Baseline, Choice Analytical). The samples were then heated at 125°C for 8 hours on Teflon coated digestion hotplate, housed in a bench-top fume hood coupled with HEPA filters to ensure clean input air (Digiprep, France). Identical procedures were applied to blanks (n = 6) and to two certified referenced materials (n = 5) (DORM-3 fish protein; National Research Council, Ottawa, Canada; and NIST 1566a oyster tissue; National Institute of Standards and Technology, Gaithersburg, Maryland, USA). Certified materials, blanks and samples were resuspended in 10–100 mL of 10% v:v nitric acid (Ultrapure, Seastar Baseline) and analysed by sector field inductively coupled plasma mass spectrometry (SF-ICP-MS) (Finnigan MAT ELEMENT 1 Bremen Germany), following methods described in Cullen and Sherrell  and Townsend .
Analysis of carbon
All glass- and metal-ware in contact with the carbon samples were pre-combusted at 450°C for 12 hours. Subsamples (2–100 mg) of dried faecal matter were placed in 13 mm diameter silver capsules (Sercon, Australia) and carbon content was then determined at the Central Science Laboratory, University of Tasmania, using a Thermo Finnigan EA 1112 Series Flash Elemental Analyser (estimated precision ∼1%).
Results for certified reference materials are presented in Table 1 and were found fit for purpose. Mean and standard deviation of C, Fe, Cd, P, Co, Mn, Cu and Zn for five species of whale faeces, two species of whale muscle and four species of krill are summarised in Table 2; with published comparative values of dissolved and particulate trace elements in Southern Ocean surface waters in Table 3, marine phytoplankton in Table 4, and Antarctic krill and marine mammals in Table 5. Concentrations of metals varied between the specimens. In krill tissue, the highest concentration was observed for Zn followed by Fe and Cu. In whale muscle, the highest concentration was observed for Fe followed by Zn and Cu. Lastly, in whale faeces, the highest concentration was observed for Zn, followed by Cu and Fe. Consistently, the three elements with the lowest mean concentrations in krill tissue and whale muscle and faeces were Mn followed by Cd and Co. There are some differences in concentration of the various elements between our results and published data (Table 5). These differences may be a result of seasonal or regional effects and variability in trace element concentrations in krill and baleen whales, which is a topic for future studies.
Mean concentrations of trace elements were higher in whale faecal matter compared to whale muscle and krill tissues. When compared to published Southern Ocean seawater concentrations in HNLC waters , the metal content of krill tissue was between 22 thousand (for Co) and 4.8 million (for Fe) times higher than surface water concentrations, while whale faecal matter was between 276 thousand (for Co) and 9.2 million times (for Fe) times higher.
Metal: Carbon and carbon to phosphorus ratio
When normalised to C, the concentration of Cd, Cu, Co, Mn and Zn were higher in krill tissue compared to whale muscle, whereas Fe was higher in whale muscle compared to krill tissue (Table 6 and Figure 1). All metal to C ratios were higher in whale faeces compared to whale muscle. When normalised to P, the C content was highest in whale muscle followed by krill tissue and lastly whale faeces (Table 6 and Figure 2). Redfield C:P molar ratio of 106∶1 mol:mol is typical of phytoplankton . Here, whale faeces and krill tissue are below the C:P Redfield ratio and whale muscle are higher.
Data points above the third quartile for whale faeces are 3 or more times higher than the interquartile range.
Data point above the third quartile for whale faeces is 3 or more times higher than the interquartile range.
Comparison to published analyses
The concentrations of trace elements in krill from this study were within the reported ranges for the Antarctic krill (Table 5) , . For whale muscle, the concentration of Cd, Cu and Zn were similar to published values from other Southern Ocean marine mammals: Crabbeater seal (Lobodon carcinophagus), Leopard seal (Hydrurga leptonyx), and Weddell seal (Leptonychotes weddellii) (Table 5) .
Most studies investigating trace element concentration in marine vertebrates have used liver or kidney tissue as a means of quantifying the bioaccumulation of metal contaminants. However, as liver plays an important role in accumulation and detoxification of elements, it is expected that the concentration of elements in liver and kidney would not be comparable with trace element concentrations in muscle samples analysed in this study . Unfortunately we did not have any samples from other whale tissue to compare with the literature values. The concentration of Fe, Mn, Zn, Cd and Cu in whale muscle from this study was much higher than published muscle concentrations of the Southern minke whale (Baleoptera acutorostrata) (Table 5) . In whale faeces, the concentration of Cd, Cu and Zn were higher than published values for faeces from Antarctic chinstrap penguins (Pygoscelis antarctica) (Table 5) . To the best of our knowledge, there are no other studies that have reported trace element concentrations in faecal matter from Antarctic vertebrates.
Antarctic krill and baleen whales as sources of trace elements to ocean surface waters
Iron has been demonstrated to be the primary factor controlling marine primary productivity in one third of the world's oceans, including the climatically important Southern Ocean. Iron-containing proteins are essential for photosynthetic and respiratory electron transport , and iron been demonstrated to limit the growth rates of the diatom Thalassiosira weissflogii and the dinoflagellate Prorocentrum minimum when the unchelated Fe concentrations in seawater fall below 0.1 nmol L−1 . This is further supported by the 100-fold increase in diatom concentrations following natural and artificial Fe-fertilization experiments in HNLC surface waters (see Boyd  and de Baar  for a synthesis).
Dissolved and particulate Fe concentration in surface seawater of HNLC regions is typically less than 1 nmol L−1 , , . This micronutrient can be passively scavenged onto particles or actively taken up by organisms. Nicol  indicated that the Southern Ocean krill population could contain approximately 24% of the total Fe in the surface waters within its range, and whale faecal Fe content (145±133.7 mg kg−1) was approximately ten million times that of Southern Ocean surface seawater concentrations. Here we confirm that krill concentrate the Fe derived from phytoplankton into its tissue, with the Fe:C ratio in krill 3 times higher than the averaged published value for Southern Ocean phytoplankton in low Fe conditions (Table 6). In whale muscle, the Fe:C ratio was almost double that of krill and in whale faecal matter it was over 5 times higher than krill tissue. This indicates that whales are concentrating the carbon and actively defecating the Fe.
Manganese is also a crucial trace element in seawater, and it is required by the water oxidizing complex of photosystem II in phytoplankton . The concentration of Mn in Southern Ocean surface waters is typically low (dissolved and particulate 0.02 – 6.77 nmol L−1, but 19.33 – 199.2 nmol L−1 in the Ross Sea , and 8.81 – 39.4 nmol L−1, particulate only, in the Amundsen Sea ). However published average cellular concentrations of Mn in diatoms from low Fe waters in Southern Ocean were between 200 and 2 million times higher than surface water concentrations suggesting that phytoplankton is enriched in Mn. Manganese is also an essential element for metabolism in crustaceans . Accordingly, krill tissue showed even higher concentrations of Mn (8.4±6.1 mg kg−1), which is over 300,000 times higher than typical HNLC seawater concentrations of 0.52 nmol L−1 (dissolved and particulate) . Whale muscle had low concentrations of Mn (2.4±2.3 mg kg−1), and lower Mn:C ratio compared to whale faeces. This is because Mn is not assimilated and consequently is often used as a measure of assimilation efficiency in marine mammals . As a result, and similar to Fe, whales defecate most of their dietary Mn as demonstrated by high Mn content in their faeces (27.3±16.3 mg kg −1) compared to their muscle (2.4±2.3 mg kg −1).
The Zn, Co and Cd concentrations in Southern Ocean surface waters are low (0.24 – 9.4 nmol L−1,0.00006 pmol L−1 and 0.04 – 0.905 nmol L−1, respectively – Table 2), however, these elements are essential cofactors in metalloenzymes in marine phytoplankton. All marine phytoplankton have adapted to limitations of CO2 diffusion in water by evolving carbon concentrating mechanisms (CCMs) to support photosynthetic carbon fixation . The CCM catalyses the equilibrium between bicarbonate (HCO3−) and CO2 using the Zn metalloenzyme carbonic anyhydrase . Under Zn limitation, the carbonic anhydrase can function with Co or Cd instead of Zn . Therefore the ability of marine phytoplankton to acquire CO2 also depends on the availability of Zn, Co and Cu in surface waters.
The mean cellular concentrations of Zn in diatoms vary by 2 orders of magnitude (3.43 – 982 µmol L−1 – Table 3); however diatoms show cellular accumulation of Zn, with concentrations between 1000 to 100,000 times higher than seawater (Table 4). Zinc is then further concentrated in krill tissue (275.4±137.2 mg kg−1). Whale muscle was relatively low in Zn (74.9±40.9 mg kg−1) compared to krill tissue, and Zn:C ratios were lower in whale muscle compared to whale faeces suggesting the low requirement of whales on this element. As such, most of the Zn is released through whale faecal matter (621.5±432.9 mg kg−1).
Cobalt and Cd were present in very low concentrations in krill tissue (0.08±0.03 mg kg−1 and 2.8±0.7 mg kg−1, respectively) suggesting that relative to other trace elements measured in this study, krill may have little use for Co and Cd. When normalised to C, Co and Cd were higher in phytoplankton compared to the average among krill (Table 6). Similarly Co and Cd were scarce in whale muscle (0.04±0.04 mg kg−1 and 0.1±0.2 mg kg−1, respectively). When normalised to C, Co and Cd were lower in whale muscle compared to whale faeces, indicating that these elements are expelled through their faecal matter (0.94±0.87 mg kg−1 and 34.7±88.9 mg kg−1, respectively). Interestingly, the concentration of Cd in sperm whale faeces was much higher compared to other species of whales in this study (575 mg kg−1), which may reflect the different diet of this species. Sperm whales in the Southern Ocean predominantly consume squid which may predate on Antarctic krill .
Copper is one element that shows clear differential uptake and utilization across the food web compared to other elements in this study. Copper concentration in seawater is low (dissolved and particulate 0.48 – 12.96 nmol L−1 - Table 2) and is little concentrated by phytoplankton (3.48 – 79.3 µmol L−1) , which appear to have little physiological use for it. Studies have demonstrated that Cu is toxic to the dinoflagellate Gonyaulax tamarensis and the diatom T. pseudonana, and is able to decrease their growth at only a few pmol L−1 , . Krill, like most crustaceans however, require Cu, as it is an essential element in their respiratory pigment; hemocyanin . Accordingly, krill tissues show a marked bio-concentration of Cu (49.1±30.5 mg kg−1 – Table 5, and Cu:C 20.5±10.4 µmol mol−1 - Table 6), 100,000 times higher than Southern Ocean surface waters and over 1.5 million times higher than that measured for Southern Ocean diatoms. Whale muscle was relatively low in Cu (5.3±4.5 mg kg−1) compared to their prey, which reflects the lower physiological dependency of mammals on this element. Consequently, whale faeces contained high levels of Cu (1635 5.3 mg kg−1 in sperm whales, 253.5±100.4 mg kg−1, all other species), and higher Cu:C ratio compared to whale muscle, suggesting that whales take up relatively little Cu from their diet.
Phosphorus is an essential nutrient required for structural and functional components of all organisms. Despite a high range, the mean C:P ratio in whale muscle from our study was 30 times higher than mean whale faeces ratio and 3 times higher than the Redfield ratio (Figure 2), indicating that whales are actively storing the P in their muscle. When nutrients are not limiting, the C:P ratio in most phytoplankton is 106∶1 . When P is scarce, phytoplankton have been demonstrated to reduce their cellular P requirements by substituting phospholipids for non-P membrane lipids . In the Southern Ocean, surface water phosphate concentrations (16.6 – 44.5 nmol L−1)  are much higher than the other elements we report here. Despite this, the concentration of P in krill was over 30 million times higher than median surface water concentrations (28,304.1±23,286.7 mg kg−1). Whales concentrate the P from krill for biochemical processes.
Our results suggest that Antarctic krill and whales may be a key part of marine biogeochemical cycling and act as a source of essential and limiting trace elements to phytoplankton in surface waters of the Southern Ocean. Krill and whales are long-lived, actively swimming animals that do not undergo any form of dormancy. As such, the large stock of krill can act as a mechanism of retaining trace elements in the surface waters whereas whales concentrate certain elements required for physiological processes from the krill, but actively defecate other elements that can be used for phytoplankton production. In addition, krill are capable of absorbing elements such as fluorine directly from seawater suggesting that they can concentrate some elements despite their scarcity in surface waters .
Ecological importance of whales – past, present and future
The loss of large predators from marine ecosystems has the potential to affect marine biogeochemistry, and consequently marine primary productivity and carbon sequestration , , . Because of their vast size and huge consumption of krill, blue and fin whales would have been the dominant krill consumers in the Southern Ocean before the era of commercial whaling and thus would have been the significant contributors to ocean nutrient recycling. Although their large size acts as a carbon store, their major role is in how they affect the recycling of critical elements, and it is the availability of these elements that affects the ocean's ability to sequester carbon. Consequently it has been suggested that the efficiency of recycling and supply of essential nutrients to surface waters has diminished in the Southern Ocean due to massive reductions in whale numbers through commercial whaling , , .
The pre-exploitation population of Antarctic blue whales was estimated to be between 202,000 to 311,000 individuals and was expected to have exported approximately 72,172 tons C yr−1 , . Current estimates of Antarctic blue whales are approximately 4,727 individuals, less than 2% of mean pre-exploitation levels , , with a predicted recovery rate of 8.2% per year . There is no reliable data on pygmy blue whale abundances. Fin whales are thought to be more abundant and their numbers may be increasing; however, current estimates of population sizes are not available. Many humpback whale populations are recovering quickly but their current numbers are still considerably below pre-exploitation population sizes. The recovery of the great whales could increase the spatial extent of productive regions in the Southern Ocean through the recycling of essential nutrients to surface layers from their faecal matter , .
There is accumulating evidence of the role of whales in the ocean nutrient cycling and their importance relative to their abundance (see Nicol , Lavery , Pershing , Wing , Lavery , Roman  for synthesis). Our results show that krill can act as a reservoir of essential trace elements in surface waters, and whales can release these stored elements through feeding and defecation. This study further extends the role of larger animals as important components of ocean biogeochemical cycling for a range of elements. To fully understand the role of large marine mammals in ocean biogeochemical cycling future studies will have to determine the bioavailability of the elements contained in whale faeces, and to quantify the combined effects of, nutrient recycling in the surface layer, the effects of nutrient scavenging from deep water and biogenic turbulence caused by vertically migrating whales.
We would like to thank Dr Pier van der Merwe and Dr Ashley Townsend for assistance in the laboratory and Dr Marion Fourquez and Dr Simon Jarman for their comments and suggestions during the development of this project and manuscript. Samples were collected under the Australian Antarctic Animals Ethics permit for project 2926. No whales were killed in the pursuit of this research.
Conceived and designed the experiments: ARB DL KM SN. Performed the experiments: DL KM. Analyzed the data: LR DL KM. Contributed reagents/materials/analysis tools: ARB DL KM. Wrote the paper: LR ARB DL KM SN.
- 1. Moore JK, Abbott MR (2000) Phytoplankton chlorophyll distributions and primary production in the Southern Ocean. Journal of Geophysical Research: Oceans 105:28709–28722.
- 2. Frederiksen M, Edwards M, Richardson AJ, Halliday NC, Wanless S (2006) From plankton to top predators: bottom-up control of a marine food web across four trophic levels. Journal of Animal Ecology 75:1259–1268.
- 3. Perissinotto R (1997) In situ grazing rates and daily ration of Antarctic krill Euphausia superba feeding on phytoplankton at the Antarctic Polar Front and the Marginal Ice Zone. Marine ecology. Progress series (Halstenbek) 160:77.
- 4. Sheldon RW, Sutcliffe Jr WH, Paranjape MA (1977) Structure of Pelagic Food Chain and Relationship Between Plankton and Fish Production. Journal of the Fisheries Research Board of Canada 34:2344–2353.
- 5. Blain S, Queguiner B, Armand L, Belviso S, Bombled B, et al. (2007) Effect of natural iron fertilization on carbon sequestration in the Southern Ocean. Nature 446:1070–4.
- 6. Boyd PW, Watson AJ, Law CS, Abraham ER, Trull T, et al. (2000) A mesoscale phytoplankton bloom in the polar Southern Ocean stimulated by iron fertilization. Nature 407:695–702.
- 7. Morel FMM, Rueter JG, Price NM (1991) Iron nutrition of phytoplankton and its possible importance in the ecology of ocean regions with high nutrient and low biomass. Oceanography (Washington, D.C.) 4:56.
- 8. SCOR Working Group (2007) GEOTRACES - an international study of the global marine biogeochemical cycles of trace elements and their isotopes. Chemie der Erde - Geochemistry 67:85–131.
- 9. Morel FMM, Milligan AJ, Saito MA (2003) Marine bioinorganic chemistry: the role of trace metals in the oceanic cycles of major nutrients. The Oceans and Marine Geochemistry. Treatise on Geochemistry, ed. H Elderfield., Oxford: Elsevier.
- 10. Morel FMM, Price NM (2003) The biogeochemical cycles of trace metals in the oceans. Science 300:944–7.
- 11. Landing WM, Bruland KW (1987) The contrasting biogeochemistry of iron and manganese in the Pacific Ocean. Geochimica et Cosmochimica Acta 51:29–43.
- 12. Orians KJ, Bruland KW (1986) The biogeochemistry of aluminum in the Pacific Ocean. Earth and Planetary Science Letters 78:397–410.
- 13. Coale KH, Johnson KS, Fitzwater SE (1996) A massive phytoplankton bloom induced by an ecosystem-scale iron fertilization experiment in the equatorial pacific ocean. Nature 383:495–501.
- 14. Chester R (1990) Trace elements in the oceans, in Marine Geochemistry. Springer Netherlands. p. 346–421.
- 15. Huntley ME, Lopez MDG, Karl DM (1991) Top Predators in the Southern Ocean: A Major Leak in the Biological Carbon Pump. Science 253:64–66.
- 16. Nicol S, Bowie A, Jarman SN, Lannuzel D, Meiners KM, et al. (2010) Southern Ocean iron fertilization by baleen whales and Antarctic krill. Fish and Fisheries 11:203–209.
- 17. Jennings S, Wilson RW (2009) Fishing impacts on the marine inorganic carbon cycle. Journal of Applied Ecology 46:976–982.
- 18. Lavery TJ, Roudnew B, Gill P, Seymour J, Seuront L, et al. (2010) Iron defecation by sperm whales stimulates carbon export in the Southern Ocean. Proceedings of the Royal Society. B, Biological sciences 277:3527–31.
- 19. Pershing AJ, Christensen LB, Record NR, Sherwood GD, Stetson PB (2010) The impact of whaling on the ocean carbon cycle: why bigger was better. PLoS One 5:e12444.
- 20. Wing SR, Jack L, Shatova O, Leichter JJ, Barr D, et al. (2014) Seabirds and marine mammals redistribute bioavailable iron in the Southern Ocean. Marine Ecology Progress Series 510:1–13.
- 21. Spicer JI, Saborowski R (2010) Physiology and Metabolism of Northern Krill (Meganyctiphanes norvegica Sars). Advances in Marine Biology, ed. M Lesser. Vol. 57.: Elsevier.
- 22. Ordway GA, Garry DJ (2004) Myoglobin: an essential hemoprotein in striated muscle. The Journal of Experimental Biology 207:3441–3446.
- 23. Kooyman GL, Castellini MA, Davis RW (1981) Physiology of diving in marine mammals. Annual Review of Physiology 43:343–356.
- 24. Roman J, McCarthy JJ (2010) The Whale Pump: Marine Mammals Enhance Primary Productivity in a Coastal Basin. PloS one 5:e13255.
- 25. Clarke A, Tyler PA (2008) Adult Antarctic Krill Feeding at Abyssal Depths. Current Biology 18:282–285.
- 26. Smetacek V, Nicol S (2005) Polar ocean ecosystems in a changing world. Nature 437:362–8.
- 27. Smith LV, McMinn A, Martin A, Nicol S, Bowie AR, et al. (2013) Preliminary investigation into the stimulation of phytoplankton photophysiology and growth by whale faeces. Journal of Experimental Marine Biology and Ecology 446:1–9.
- 28. Katija K (2012) Biogenic inputs to ocean mixing. The Journal of Experimental Biology 215:1040–1049.
- 29. Jarman SN, Gales NJ, Tierney M, Gill PC, Elliott NG (2002) A DNA-based method for identification of krill species and its application to analysing the diet of marine vertebrate predators Molecular ecology. 11:2679–2690.
- 30. Cullen JT, Sherrell RM (1999) Techniques for determination of trace metals in small samples of size fractionated particulate matter: phytoplankton metals of central California Marine Chemistry. 67:233–247.
- 31. Townsend AT (2000) The accurate determination of the first row transition metals in water, urine, plant, tissue and rock samples by sector field ICP-MS. Journal of Analytical Spectometry 15:307–314.
- 32. Cullen JT, Chase Z, Coale KH, Fitzwater SE, Sherrell RM (2003) Effect of iron limitation on the cadmium to phosphorus ratio of natural phytoplankton assemblages from the Southern Ocean. Limnology Oceanography 48:1079–1087.
- 33. Redfield AC (1958) THE BIOLOGICAL CONTROL OF CHEMICAL FACTORS IN THE ENVIRONMENT. American Scientist 46:230A–221.
- 34. Barbante C, Cozzi G, Capodaglio G, Cescon P (2000) Trace element determination in a candidate reference material (Antarctic Krill) by ICP-sector field MS. Journal of Analytical Atomic Spectrometry 15:377–382.
- 35. Yamamoto Y, Honda K, Tatsukawa R (1987) Heavy metal accumulation in Antarctic krill, Euphausia superba. Polar Biology 1:198–204.
- 36. Szefer P, Szefer K, Pempkowiak J, Skwarzec B, Bojanowski R, et al. (1994) Distribution and coassociations of selected metals in seals of the Antarctic. Environmental Pollution 83:341–349.
- 37. Honda K, Yamamoto Y, Tatsukawa R (1987) Distribution of heavy metals in Antarctic marine ecosystems. Polar Biol 1:184–197.
- 38. Espejo W, Celis JE, Gonzalez-Acuna D, Jara S, Barra R (2014) Concentration of trace metals in excrements of two species of penguins from different locations of the Antarctic Peninsula. Polar biology 37:675–683.
- 39. Sandmann G (1985) Consequences of iron deficiency on photosynthetic and respiratory electron transport in blue-green algae. Photosynthesis Research 6:261–271.
- 40. Sunda WG, Huntsman SA (1997) Interrelated influence of iron, light and cell size on marine phytoplankton growth. Nature 390:389–392.
- 41. Boyd PW, Jickells T, Law CS, Blain S, Boyle EA, et al. (2007) Mesoscale Iron Enrichment Experiments 1993–2005: Synthesis and Future Directions. Science 315:612–617.
- 42. de Baar HJW, Boyd PW, Coale KH, Landry MR, Tsuda A, et al. (2005) Synthesis of iron fertilization experiments: From the Iron Age in the Age of Enlightenment. Journal of Geophysical Research: Oceans 110:C09S16.
- 43. de Baar HJW, de Jong JTM, Bakker DCE, Loscher BM, Veth C, et al. (1995) Importance of iron for plankton blooms and carbon dioxide drawdown in the Southern Ocean. Nature 373:412–415.
- 44. Bowie AR, Maldonado MT, Frew R, Croot PL, Achterberg EP, et al. (2001) The fate of added iron during a mesoscale fertilisation experiment in the Southern Ocean. Deep Sea Research II 48:2703–2743.
- 45. Corami F, Capodaglio G, Turetta C, Soggia F, Magi E, et al. (2005) Summer distribution of trace metals in the western sector of the Ross Sea, Antarctica. Journal of Environmental Monitoring 7:1256–1264.
- 46. Planquette H, Sherrell RM, Stammerjohn S, Field MP (2013) Particulate iron delivery to the water column of the Amundsen Sea, Antarctica. Marine Chemistry 153:15–30.
- 47. Twining BS, Baines SB (2004) Element stoichiometries of individual plankton cells collected during the Southern Ocean Iron Experiment(SOFeX). Limnology and oceanography 49:2115.
- 48. Baden SP, Eriksson SP (2006) Role, Routes and Effects of Manganese in Crustaceans. Oceanography and Marine Biology 44:61–83.
- 49. Fadely BS, Worthy GAJ, Costa DP (1990) Assimilation Efficiency of Northern Fur Seals Determined Using Dietary Manganese. The Journal of Wildlife Management 54:246–251.
- 50. Reinfelder JR (2010) Carbon Concentrating Mechanisms in Eukaryotic Marine Phytoplankton. Annual Review of Marine Science 3:291–315.
- 51. Lane TW, Morel FMM (2000) A biological function for cadmium in marine diatoms. Proceedings of the National Academy of Sciences 97:4627–4631.
- 52. Nemoto T, Okiyama M, Iwasaki N, Kikuchi T (1988) Squid as Predators on Krill (Euphausia superba) and Prey for Sperm Whales in the Southern Ocean, in Antarctic Ocean and Resources Variability, D Sahrhage, Editor, Springer Berlin Heidelberg. p. 292–296.
- 53. Annett AL, Lapi S, Ruth TJ, Maldonado MT (2008) The Effects of Cu and Fe Availability on the Growth and Cu:C Ratios of Marine Diatoms. Limnology and Oceanography 53:2451–2461.
- 54. Sunda WG, Guillard RRL (1976) The relationship between cupric ion activity and the toxicity of copper to phytoplankton. Journal of Marine Research 37:761–777.
- 55. Anderson DM, Morel FMM (1978) Copper sensitivity of Gonyaulax-tamarensis. Limnology and Oceanography 23:283–295.
- 56. Van Mooy BAS, Fredricks HF, Pedler BE, Dyhrman ST, Karl DM, et al. (2009) Phytoplankton in the ocean use non-phosphorus lipids in response to phosphorus scarcity. Nature 458:69–72.
- 57. Nicol S, Stolp M (1991) Molting, Feeding, and Fluoride Concentration of the Antarctic Krill Euphausia superba Dana. Journal of Crustacean Biology 11:10–16.
- 58. Lavery TJ, Roudnew B, Seymour J, Mitchell JG, Smetacek V, et al. (2014) Whales sustain fisheries: Blue whales stimulate primary production in the Southern Ocean. Marine Mammal Science 30:888–904.
- 59. Smetacek V (2008) Are declining Antarctic krill stocks a result of global warming or of the decimation of the whales. Impacts of global warming on polar ecosystems.: Fundación BBVA.
- 60. Branch TA, Matsuoka K, Miyashita T (2004) Evidence for increases in Antarctic blue whales based on Bayesian modelling. Marine Mammal Science 20:726–754.
- 61. International Whaling Commission (2000) Report of the Sub-Committee on the Comprehensive Assessment of Other Whale Stocks - Annex G. Journal of Cetacean Research and Management 2 (Suppl.): 167–208
- 62. Roman J, Estes JA, Morissette L, Smith C, Costa D, et al.. (2014) Whales as marine ecosystem engineers. Frontiers in Ecology and the Environment:
- 63. Hendry KR, Meredith MP, Measures CI, Carson DS, Rickaby REM (2010) The role of sea ice formation in cycling of aluminium in northern Marguerite Bay, Antarctica. Estuarine, Coastal and Shelf Science 87:103–112.
- 64. Fitzwater SE, Johnson KS, Gordon RM, Coale KH, Smith Jr WO (2000) Trace metal concentrations in the Ross Sea and their relationship with nutrients and phytoplankton growth. Deep-sea research. Part II, Topical studies in oceanography 47:3159–3179.
- 65. Grotti M, Soggia F, Abelmoschi ML, Rivaro P, Magi E, et al. (2001) Temporal distribution of trace metals in Antarctic coastal waters. Marine Chemistry 76:189–209.
- 66. Westerlund S, Öhman P (1991) Iron in the water column of the Weddell Sea. Marine Chemistry 35:199–217.
- 67. Löscher BM, de Jong JTM, de Baar HJW (1998) The distribution and preferential biological uptake of cadmium at 6°W in the Southern Ocean. Marine Chemistry 62:259–286.
- 68. Löscher BM (1999) Relationships among Ni, Cu, Zn, and major nutrients in the Southern Ocean. Marine Chemistry 67:67–102.
- 69. Frew R, Bowie A, Croot P, Pickmere S (2001) Macronutrient and trace-metal geochemistry of an in situ iron-induced Southern Ocean bloom. Deep-sea research. Part II, Topical studies in oceanography 48:2467–2481.
- 70. Lannuzel D, Bowie AR, van der Merwe PC, Townsend AT, Schoemann V (2011) Distribution of dissolved and particulate metals in Antarctic sea ice. Marine Chemistry 124:134–146.