Abstract
Antarctic krill (Euphausia superba) play a central role in the food web of the Southern Ocean, forming a link between primary production and large predators. Krill produce large, faecal pellets (FP) which can form a large component of mesopelagic particulate organic carbon (POC) fluxes. However, the patchy distribution of krill swarms, highly variable pellet composition, and variable sinking and attenuation rates means that these episodic, but potentially large, carbon fluxes are difficult to sample or model. We measured particle flux and type using Marine Snow Catchers (MSC) in the marginal ice zone near the South Orkneys, Antarctica. Krill FP were the dominant component of the POC flux in the upper 200 m (typically 60–85%). FP sinking velocities measured onboard were highly variable (15–507 m d−1) but overall high, with mean equivalent velocities of 172, 267, and 161 m d−1 at our three stations. The high numbers of krill FP sinking through the mesopelagic suggest that krill FP can be transferred efficiently and/or that rates of krill FP production are high. We compared our direct MSC-derived estimates of krill FP POC flux (33–154 mg C m−2 d−1) and attenuation to estimates of krill FP production based on previous measurements of krill density and literature FP egestion rates, and estimated net krill FP attenuation rates in the upper mesopelagic. Calculated attenuation rates are sensitive to krill densities in the overlying water column but suggest that krill FP could be transferred efficiently through the upper mesopelagic, and, in agreement with our MSC attenuation estimates, could make large contributions to bathypelagic POC fluxes. Our study contrasts with some others which suggest rapid FP attenuation, highlighting the need for further work to constrain attenuation rates and assess how important the contribution of Antarctic krill FP could be to the Southern Ocean biological carbon pump.
Highlights
Electronic supplementary material The online version of this article contains supplementary material, which is available to authorized users.Plymouth PL1 3DH, UK 5 Institute of Marine Research, PO Box 1870, Nordnes, 5817 Bergen, NorwayThe Southern Ocean is an important part of the global carbon cycle, with the region south of 40°S (~21% of the ocean area) estimated to account for 26 ± 6% of global export production (Primeau et al 2013)
Most of these faecal pellets (FP) belonged to Antarctic krill, with the exception of the mixed layer depth (MLD) +10 sample at ICE2 JR291, where krill FP accounted for 43.6% of the FP particulate organic carbon (POC)
Krill FP were the dominant component of the POC flux at ICE1 and ICE2 stations (Fig. 3) highlighting their importance for the transfer of POC through the upper mesopelagic layer in this marginal ice zone (MIZ) region
Summary
Polar Biol (2017) 40:2001–2013 factors including light and iron (Boyd et al 1999), leading to upwelled nutrients being subducted unused into the ocean interior This picture conceals the existence of high flux events in some regions of the Southern Ocean, associated with sinking diatoms (e.g., Smetacek 1985; Beaulieu 2002; Roca-Marti et al 2015), diatom resting spores (Rembauville et al 2015), enhanced iron supply (e.g., Bidigare et al 1999; Savoye et al 2008; Pollard et al 2009; Smetacek et al 2012; Jouandet et al 2014), and the influence of marginal ice zones (Smith and Nelson 1985; Fischer et al 1988; Buesseler et al 2001; Cavan et al 2015). In the Bransfield Strait, 97% of the annual POC flux to 1588 m occurred in the two most productive months, associated with Euphausia superba ( referred to as krill) faecal pellets, (Bodungen et al 1987; Wefer et al 1988), yet limited observations in these marginal ice zones mean that krill and their large FP fluxes are not well represented in global biogeochemical models
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