Effects of natural iron fertilisation by baleen whales and Antarctic krill on the Southern Ocean carbon cycle
Creators: name: Ratnarajah, Lavenia
Publisher: name: University Of Tasmania
Year: 2023

description: Primary productivity in large areas of the Southern Ocean, known as High Nutrient Low Chlorophyll (HNLC) regions, is limited by the availability of a key micronutrient ‚Äö- the trace element iron (Fe). Iron is required for biochemical processes such as photosynthesis and respiration, as well as in the reduction of inorganic nitrogen species. There is growing evidence that marine animals could play an important role in recycling Fe through their diet and subsequent defecation, reviewed in Chapter 1. This dissertation adds significantly to our understanding on the influence of Antarctic krill (Euphausia superba) and baleen whales on the biogeochemical cycling of Fe, and other biologically important trace elements, in the Southern Ocean. The concentration of Fe, and other biologically important trace elements such as manganese (Mn), which is essential 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; copper (Cu) and Fe for nitrification, denitrification and organic N utilization; Zn for organic phosphorus (P) utilization; and Cu for methane oxidation, in whole Antarctic krill and baleen whale muscle and faecal samples were measured in Chapter 2 to explore the biogeochemical role of Antarctic krill and baleen whales in the Southern Ocean. Antarctic krill were found to be rich in Fe (174.5 ¬¨¬± 0.5 mg kg\\(^{-1}\\)), and these other biologically important trace elements. The elements stored in Antarctic krill are then transferred into the whales as they are consumed. Adult whales build blubber (fat) instead of muscle during their feeding season in Antarctic waters; consequently much of the nutrients get defecated. Iron concentrations in whale faecal material were found to be 145.9 ¬¨¬± 135.4 mg kg\\(^{-1}\\), over 10 million times higher than background seawater concentrations. Similarly, concentrations of other biologically important trace elements were elevated in whale faecal material compared to Southern Ocean seawater. The trace element to carbon ratio further suggests that whales are concentrating carbon and actively defecating trace elements. Based on these high Fe concentrations, a preliminary model was built in Chapter 3 to examine the potential Fe fertilisation by blue, fin and humpback whales, and Antarctic krill on the phytoplankton growth in the Southern Ocean. The model was used to examine the influence of historical whaling practices on the efficiency of Fe recycling. A local sensitivity analysis, which allowed for the use of the range of values (minimum, mean and maximum) for each parameter was used. The model suggested that historical populations of blue, fin and humpback whales might have enhanced primary productivity in the Southern Ocean. However there is a high degree of uncertainty around the magnitude of this enhancement due to uncertainty in model parameter estimates, which prioritised key areas for future research. Based on the model, the most influential parameters were: the Fe concentration in krill, the carbon-to-iron uptake ratio by phytoplankton, persistence and bioavailability of whale faecal material, and the consumption rate by whales. In order to constrain the key parameter Fe concentration in krill‚ÄövÑvp, the concentration of Fe in whole krill, distinct krill tissue material and krill faeces was analysed in Chapter 4. The results demonstrate that much of the Fe in krill is stored in the stomach (6 ‚Äö- 98 mg kg\\(^{-1}\\)) and digestive gland (14 ‚Äö- 82 mg kg\\(^{-1}\\)), and excreted as faecal material (683 ‚Äö- 1,039 mg kg\\(^{-1}\\)), instead of being stored in the muscle (4 ‚Äö- 7 mg kg\\(^{-1}\\)). This implies that Antarctic krill are ingesting more Fe than they require for physiological processes, and may be important recyclers of Fe in the Southern Ocean. Calculations suggest that the high Fe concentrations in the stomach and digestive gland can influence the overall Fe concentration in Antarctic krill. The large variability reported in the existing literature is very likely the result of a combination of seasonal and regional difference in quality and the quantity of their diet. The persistence and bioavailability of whale faecal material‚ÄövÑvp in surface seawater was subsequently investigated in Chapter 5 by size fractionating Fe particles in whale faecal samples into 4 different size fractions (&lt;0.2 ˜í¬¿m, 0.2 ‚Äö- 10 ˜í¬¿m, 10 ‚Äö- 60 ˜í¬¿m and &gt;60 ˜í¬¿m), and measuring the leaching Fe particles over time. The results suggest that the total particulate fraction (&gt;0.2 ˜í¬¿m, 5,026 ‚Äö- 22,526 nmol L\\(^{-1}\\)) dominated the total Fe pool (5,780 ‚Äö- 23,053 nmol L\\(^{-1}\\)). The concentrations of dissolved Fe (186 ‚Äö- 754 nmol L\\(^{-1}\\)) and particulate Fe in whale faecal samples, however, were significantly higher than published Southern Ocean surface seawater concentrations, and most other Fe sources in the region. A range of processes such as remineralisation, leaching, aggregation, precipitation, the recycling of biogenic particulate Fe in surface seawater, and leaching of particulate Fe will influence the bioavailable pool of Fe. Between 1 and 7% of the Fe leached from whale faeces in the first 5 minutes. Although the solubility of faecal particles seems low, the concentration of Fe being leached is high (51 - 143 nmol L\\(^{-1}\\)), and is greater than the solubility of Fe in seawater. In addition, calculations on the sinking rate of these particles (60 ˜í¬¿m, 10 ˜í¬¿m and 0.2 ˜í¬¿m would sink at a rate of 3 m day\\(^{-1}\\), 0.08 m day\\(^{-1}\\) and 3.3 x 10\\(^{-5}\\) m day\\(^{-1}\\) respectively) suggest that they may remain in the water column for an extended period, however many of these particles may aggregate and precipitate, or be transported laterally. In summary, this dissertation has demonstrated that Antarctic krill acts as an efficient reservoir of Fe, with much of the consumed Fe being stored in the digestive organs and not incorporated into the muscle. Baleen whales then recycle the Fe stored in Antarctic krill through their diet and subsequent defecation. Although whale faecal material consists mostly of particulate Fe, the concentration of dissolved Fe in whale faecal material is comparable to marine ice and continental ice, but considerably higher than other sources in the region. This suggests that baleen whales could play an important role in recycling Fe to HNLC regions of the Southern Ocean. Future research should focus on examining the importance of organic ligands in whale faecal material, the response of phytoplankton to faecal Fe, and the influence of historical whaling processes on the efficiency of Fe recycling in the Southern Ocean.