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Less well-known is their importance to the ocean’s carbon sink, where CO2 is removed from the atmosphere during photosynthesis by phytoplankton

Study shows krill's importance for the ocean’s carbon sink

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Friday, November 01, 2019, 18:00 (GMT + 9)

Antarctic krill (Euphausia superba) are well known for their role at the base of the Southern Ocean food web, where they’re food for marine predators such as seals, penguins and whales.  Less well known is their importance to the ocean’s carbon sink, where CO2 is removed from the atmosphere during photosynthesis by phytoplankton and sequestered to the seafloor through a range of processes.

Photo: oceanwide-expeditions.com

A new study published in the journal Nature Communications has highlighted the influence of krill in the carbon cycle and urged consideration of the impact of commercial krill fishing on ocean chemistry and the global climate.

Led by Dr Emma Cavan, a former IMAS researcher now at Imperial College London, the study reviewed current scientific knowledge of the role of krill in processes that each year remove up to 12 billion tonnes of carbon from Earth’s atmosphere.

“By eating phytoplankton and excreting carbon and nutrient-rich pellets that sink to the seafloor, Antarctic krill are an integral part of the carbon cycle and a key contributor of iron and other nutrients that fertilise the ocean,” Dr Cavan said.

“Krill faecal pellets constitute the majority of sinking carbon particles that scientists have identified in both shallow and deep waters in the Southern Ocean.

“Antarctic krill grow up to six centimetres long and weigh around one gram, but they swarm in such vast numbers that their combined contribution to the movement of ocean carbon and other nutrients can be huge.

“The Southern Ocean is one of the largest carbon sinks globally, so krill have an important influence on atmospheric carbon levels and therefore the global climate.”

Photo: Processes in the biological pump. Phytoplankton convert CO2, which has dissolved from the atmosphere into the surface oceans (90 Gt yr−1) into particulate organic carbon (POC) during primary production (~ 50 Gt C yr−1). Phytoplankton are then consumed by krill and small zooplankton grazers, which in turn are preyed upon by higher trophic levels. Any unconsumed phytoplankton form aggregates, and along with zooplankton faecal pellets, sink rapidly and are exported out of the mixed layer (< 12 Gt C yr−1 14). Krill, zooplankton and microbes intercept phytoplankton in the surface ocean and sinking detrital particles at depth, consuming and respiring this POC to CO2 (dissolved inorganic carbon, DIC), such that only a small proportion of surface-produced carbon sinks to the deep ocean (i.e., depths > 1000 m). As krill and smaller zooplankton feed, they also physically fragment particles into small, slower- or non-sinking pieces (via sloppy feeding, coprorhexy if fragmenting faeces), retarding POC export. This releases dissolved organic carbon (DOC) either directly from cells or indirectly via bacterial solubilisation (yellow circle around DOC). Bacteria can then remineralise the DOC to DIC (CO2, microbial gardening). Diel vertically migrating krill, smaller zooplankton and fish can actively transport carbon to depth by consuming POC in the surface layer at night, and metabolising it at their daytime, mesopelagic residence depths. Depending on species life history, active transport may occur on a seasonal basis as well. Numbers given are carbon fluxes (Gt C yr−1) in white boxes and carbon masses (Gt C) in dark boxes(click image to enlarge)


Dr Cavan said management of the krill fishery currently centres on sustainability and krill’s role in supporting megafauna such as whales, with little attention given to assessing the significance of krill to the carbon cycle and ocean chemistry.

“Today the fishery takes less than 0.5 per cent of the available krill and only adults are targeted.

“But there is no consensus on the effect that harvesting Antarctic krill could have on atmospheric carbon and ocean chemistry nor, for that matter, how growing whale populations might also affect krill numbers.

Photo: Cycling of nutrients by an individual krill. When krill moult they release dissolved calcium, fluoride and phosphorous from the exoskeleton (1). The chitin (organic material) that forms the exoskeleton contributes to organic particle flux sinking to the deep ocean. Krill respire a portion of the energy derived from consuming phytoplankton or other animals as carbon dioxide (2), when swimming from mid/deep waters to the surface in large swarms krill mix water, which potentially brings nutrients to nutrient-poor surface waters (3), ammonium and phosphate is released from the gills and when excreting, along with dissolved organic carbon, nitrogen (e.g., urea) and phosphorous (DOC, DON and DOP, 2 & 4). Krill release fast-sinking faecal pellets containing particulate organic carbon, nitrogen and phosphorous (POC, PON and POP) and iron, the latter of which is bioavailable when leached into surrounding waters along with DOC, DON and DOP (5)(click image to enlarge)

“Southern Ocean ecosystems and chemical processes are highly complex and poorly understood, and our lack of knowledge about the extent of krill’s ability to affect the carbon cycle is a concern, given that it is the region’s largest fishery.

“We don’t know, for example, whether a decline in krill might actually lead to an increase in the biomass of phytoplankton, which are also integral in transporting carbon to the seafloor.

”Conversely, a decline in krill would decrease the beneficial fertilisation effect that their faecal matter has on phytoplankton biomass, at the same time also jeopardising the important part krill play in circulating iron and other nutrients.

“Our study has shown there is a pressing need for further research to address these and other questions about the significance of krill, as well as for more accurate estimates of their biomass and distribution.

Photo: Role of E. superba in biogeochemical cycles. Krill (as swarms and individuals) feed on phytoplankton at the surface (1) leaving only a proportion to sink as phytodetrital aggregates (2), which are broken up easily and may not sink below the permanent thermocline. Krill also release faecal pellets (3) whilst they feed, which can sink to the deep sea but can be consumed (coprophagy) and degraded as they descend (4) by krill, bacteria and zooplankton. In the marginal ice zone, faecal pellet flux can reach greater depths (5). Krill also release moults, which sink and contribute to the carbon flux (6). Nutrients are released by krill during sloppy feeding, excretion and egestion, such as iron and ammonium (7, see Fig. 2 for other nutrients released), and if they are released near the surface can stimulate phytoplankton production and further atmospheric CO2 drawdown. Some adult krill permanently reside deeper in the water column, consuming organic material at depth (8). Any carbon (as organic matter or as CO2) that sinks below the permanent thermocline is removed from subjection to seasonal mixing and will remain stored in the deep ocean for at least a year (9). The swimming motions of migrating adult krill that migrate can mix nutrient-rich water from the deep (10), further stimulating primary production. Other adult krill forage on the seafloor, releasing respired CO2 at depth and may be consumed by demersal predators (11). Larval krill, which in the Southern Ocean reside under the sea ice, undergo extensive diurnal vertical migration (12), potentially transferring CO2 below the permanent thermocline. Krill are consumed by many predators including baleen whales (13), leading to storage of some of the krill carbon as biomass for decades before the whale dies, sinks to the seafloor and is consumed by deep sea organisms (click image to enlarge)

“This information would inform both our understanding of biogeochemical processes in the ocean and the management of the krill fishing industry.

“We also recommend that measures be put in place to ensure that as fishing technology advances, the fishery does not encroach on larval krill habitat near sea-ice, and steps should be taken to prevent potential larval by-catch when fishing for adults,” Dr Cavan said.

Funded by The Pew Charitable Trusts, the research also included scientists from the Australian Antarctic Division, the British Antarctic Survey and research institutes and universities from the UK, Germany, and the US.

Source: University of Tasmania


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