Light pollution offshore: Zenithal sky glow measurements in the mediterranean coastal waters

Xavier Ges,Salvador Bará,Manuel García-Gil,Jaime Zamorano,Salvador J Ribas,Eduard Masana

doi:10.1016/j.jqsrt.2018.02.014

Abstract

Light pollution is a worldwide phenomenon whose consequences for the natural environment and the human health are being intensively studied nowadays. Most published studies address issues related to light pollution inland. Coastal waters, however, are spaces of high environmental interest, due to their biodiversity richness and their economical significance. The elevated population density in coastal regions is accompanied by correspondingly large emissions of artificial light at night, whose role as an environmental stressor is increasingly being recognized. Characterizing the light pollution levels in coastal waters is a necessary step for protecting these areas. At the same time, the marine surface environment provides a stage free from obstacles for measuring the dependence of the skyglow on the distance to the light polluting sources, and validating (or rejecting) atmospheric light propagation models. In this work we present a proof-of-concept of a gimbal measurement system that can be used for zenithal skyglow measurements on board both small boats and large vessels under actual navigation conditions. We report the results obtained in the summer of 2016 along two measurement routes in the Mediterranean waters offshore Barcelona, travelling 9 and 31.7 km away from the coast. The atmospheric conditions in both routes were different from the ones assumed for the calculation of recently published models of the anthropogenic sky brightness. They were closer in the first route, whose results approach better the theoretical predictions. The results obtained in the second route, conducted under a clearer atmosphere, showed systematic differences that can be traced back to two expected phenomena, which are a consequence of the smaller aerosol content: the reduction of the anthropogenic sky glow at short distances from the sources, and the slower decay rate of brightness with distance, which gives rise to a relative excess of brightness at large distances from the coastline.

Highlights

Light pollution is a widespread phenomenon whose consequences can be detected at places located hundreds of kilometers away from the light polluting sources [1,2,3,4]
Light pollution has been recognized as a relevant environmental stressor, and a large variety of ecological effects of artificial light at night have been reported in recent works [see, e.g. 11-17]
The SQMshp measured brightness in the SQM spectral band was fairly coincident with the V-band estimates of the NWA in the first route, whose atmospheric conditions were relatively close to the ones used for the calculations of the NWA

Summary

Introduction

Light pollution is a widespread phenomenon whose consequences can be detected at places located hundreds of kilometers away from the light polluting sources [1,2,3,4]. Most research in this field has traditionally addressed the unwanted effects of artificial light inland, there is a growing interest in the study of light pollution in coastal regions, due to both the relevance of the marine areas for the preservation of biodiversity, and the opportunity they present for validating or otherwise contradicting physical models of atmospheric propagation with relatively simple source distributions. We are still far from having a complete picture of the complex effects induced by artificial light in the marine environment, several relevant impacts have been reported and are well documented They include changes in the zooplankton diel vertical migration, bird collisions with ships, intensified visual predation and foraging in illuminated shorelines, sessile larvae settlement disruption, desynchronization of reproductive behaviour, nesting site displacement, and streetlight-induced disorientation in seaward or long-range migration of several species [18, 21]. Such low levels can be detected tens of kilometers away from big cities [27]

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