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

Anna Belcher,Gabriele Stowasser,Louise Ireland,Peter Fretwell,Lauren Biermann,Geraint A Tarling,Andrew Gray,Sophie Fielding

doi:10.1017/s0954102021000262

Anna Belcher, Gabriele Stowasser + Show 6 more

Open Access

https://doi.org/10.1017/s0954102021000262

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Abstract

AbstractAntarctic 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.

Highlights

Antarctic krill (Euphausia superba) are central to Southern Ocean ecosystems, comprising the highest individual species biomass of any metazoan in the Southern Ocean and forming swarms that can extend over areas of ∼100 km2 (Tarling & Fielding 2016)
As these episodic pulse events of faecal egestion are difficult to capture with traditional 'snapshot'-style particulate organic carbon (POC) measurements made on oceanographic expeditions, estimates of the carbon exported by krill faecal pellets in the Southern Ocean have, far, been mainly addressed through models (Belcher et al 2019)
Krill were sampled from three main regions (Fig. 1): South Orkney Islands (SOI), South Georgia shelf (SG-shelf) and north-west South Georgia (NW-SG); SOI and SG-shelf in particular are regions known for high krill concentrations

Summary

Introduction

Antarctic krill (Euphausia superba) are central to Southern Ocean ecosystems, comprising the highest individual species biomass of any metazoan in the Southern Ocean and forming swarms that can extend over areas of ∼100 km (Tarling & Fielding 2016). Krill make a strong contribution to the Southern Ocean biological carbon pump, which is tightly coupled to levels of atmospheric carbon dioxide (Kwon et al 2009). As these episodic pulse events of faecal egestion are difficult to capture with traditional 'snapshot'-style particulate organic carbon (POC) measurements made on oceanographic expeditions, estimates of the carbon exported by krill faecal pellets in the Southern Ocean have, far, been mainly addressed through models (Belcher et al 2019). Present methodological limitations make it difficult to quantify Antarctic krill distribution and biomass and further progress is required in order to manage krill stocks and understand fully their biogeochemical contribution

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