Experimental determination of reflectance spectra of Antarctic krill (Euphausia superba) in the Scotia Sea

Abstract

Antarctic krill are the dominant metazoan in the Southern Ocean in terms of biomass; however, their wide and patchy distribution means that estimates of their biomass are still uncertain. Most currently employed methods do not sample the upper surface layers, yet historical records indicate that large surface swarms can change the water colour. Ocean colour satellites are able to measure the surface ocean synoptically and should theoretically provide a means for detecting and measuring surface krill swarms. Before we can assess the feasibility of remote detection, more must be known about the reflectance spectra of krill. Here, we measure the reflectance spectral signature of Antarctic krill collected in situ from the Scotia Sea and compare it to that of in situ water. Using a spectroradiometer, we measure a strong absorption feature between 500 and 550 nm, which corresponds to the pigment astaxanthin, and high reflectance in the 600–700 nm range due to the krill's red colouration. We find that the spectra of seawater containing krill is significantly different from seawater only. We conclude that it is tractable to detect high-density swarms of krill remotely using platforms such as optical satellites and unmanned aerial vehicles, and further steps to carry out ground-truthing campaigns are now warranted.

Observing change in pelagic animals as sampling methods shift: the case of Antarctic krill

“…Such data have already been used to detect aggregations of the pelagic copepod Calanus finmarchicus and global patterns of diurnal vertical migration (Basedow et al, 2019;Behrenfeld et al, 2019). A pilot study (Belcher et al, 2021) has shown that water containing krill indeed has a different reflectance spectrum, thus providing proof of concept for remote detection of surface swarms.…”

Commercial fishery disturbance of the global ocean biological carbon sink

“…In the Atlantic sector of the Southern Ocean, the carbon export can be up to 90 mg C m −2 day −1 (Cavan et al, 2015 ). There is some uncertainty in using satellite derived export estimates for this area due to a combination of high levels of cloud cover, ice cover and the importance of larger organisms such as Antarctic krill (Cavan et al, 2019 ), which are not detected by satellites, although this may change in the near future (Belcher et al, 2021 ).…”