Photophysiological cycles in Arctic krill are entrained by weak midday twilight during the Polar Night.

Jonathan H Cohen,Laura Hobbs,Kim S Last,Corie L Charpentier,Jørgen Berge,Finlo Cottier,Geir Johnsen,Malin Daase

doi:10.1371/journal.pbio.3001413

Abstract

Light plays a fundamental role in the ecology of organisms in nearly all habitats on Earth and is central for processes such as vision and the entrainment of the circadian clock. The poles represent extreme light regimes with an annual light cycle including periods of Midnight Sun and Polar Night. The Arctic Ocean extends to the North Pole, and marine light extremes reach their maximum extent in this habitat. During the Polar Night, traditional definitions of day and night and seasonal photoperiod become irrelevant since there are only “twilight” periods defined by the sun’s elevation below the horizon at midday; we term this “midday twilight.” Here, we characterize light across a latitudinal gradient (76.5° N to 81° N) during Polar Night in January. Our light measurements demonstrate that the classical solar diel light cycle dominant at lower latitudes is modulated during Arctic Polar Night by lunar and auroral components. We therefore question whether this particular ambient light environment is relevant to behavioral and visual processes. We reveal from acoustic field observations that the zooplankton community is undergoing diel vertical migration (DVM) behavior. Furthermore, using electroretinogram (ERG) recording under constant darkness, we show that the main migratory species, Arctic krill (Thysanoessa inermis) show endogenous increases in visual sensitivity during the subjective night. This change in sensitivity is comparable to that under exogenous dim light acclimations, although differences in speed of vision suggest separate mechanisms. We conclude that the extremely weak midday twilight experienced by krill at high latitudes during the darkest parts of the year has physiological and ecological relevance.

Highlights

Using a spectroradiometer optimized for dim light detection [28], we quantified spectral irradiance of diffuse skylight at solar noon during the Polar Night from 76.5 ̊ N to 81 ̊ N under a range of weather conditions from a research vessel platform (Fig 1)
Integrated diffuse skylight irradiance measured at the water’s surface (400 to 700 nm, EPAR) ranged from 2.2 × 10−6 to 2.2 × 10−5 μmol photons m−2 s−1 (Fig 1C, inset). These measurements demonstrate that atmospheric light at midday during Polar Night is, as expected, characteristic of twilight spectral composition at lower latitudes (e.g., [30]) (Fig 1C, upper panel)
In order to capture the largest change in irradiance over the diel cycle, we measured a time series of diffuse skylight irradiance during Polar Night north of Rijpfjorden, Svalbard (80 ̊ 37.79N 22 ̊ 4.14E) over the midday period

Summary

Introduction

These measurements demonstrate that atmospheric light at midday during Polar Night is, as expected, characteristic of twilight spectral composition at lower latitudes (e.g., [30]) (Fig 1C, upper panel). In order to capture the largest change in irradiance over the diel cycle, we measured a time series of diffuse skylight irradiance during Polar Night north of Rijpfjorden, Svalbard (80 ̊ 37.79N 22 ̊ 4.14E) over the midday period.

Objectives

Results

Conclusion