Evaluating the summer night sky brightness at a research field site on Lake Stechlin in northeastern Germany

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Abstract

We report luminance measurements of the summer night sky at a field site on a freshwater lake in northeastern Germany (Lake Stechlin) to evaluate the amount of artificial skyglow from nearby and distant towns in the context of a planned study on light pollution. The site is located about 70 km north of Berlin in a rural area possibly belonging to one of the darkest regions in Germany. Continuous monitoring of the zenith sky luminance between June and September 2015 was conducted utilizing a Sky Quality Meter. With this device, typical values for clear nights in the range of 21.5–21.7 magSQM/arcsec2 were measured, which is on the order of the natural sky brightness during starry nights. On overcast nights, values down to 22.84 magSQM/arcsec2 were obtained, which is about one third as bright as on clear nights. The luminance measured on clear nights as well as the darkening with the presence of clouds indicates that there is very little influence of artificial skyglow on the zenith sky brightness at this location. Furthermore, fish-eye lens sky imaging luminance photometry was performed with a digital single-lens reflex camera on a clear night in the absence of moonlight. The photographs unravel several distant towns as possible sources of light pollution on the horizon. However, the low level of artificial skyglow makes the field site at Lake Stechlin an excellent location to study the effects of skyglow on a lake ecosystem in a controlled fashion.

Introduction

Since the invention of electric lighting in the 19th century, artificial light at night (ALAN) has had a major impact on both human society and the environment. ALAN propagates into nocturnal landscapes and has hence been recognized as one type of environmental pollution, light pollution (LP). The fact that ALAN can be seen from space should be well known since its observation during the first crewed US American orbital flight more than 50 years ago [1]. However, the public is much less aware of LP than of other types of environmental pollution. Although ALAN has globally increased by 3–6% per year [2], [3] over the last decades, the effects on the environment and on human well-being are still poorly understood [2].

Early work on LP concentrated mostly on the decrease of contrast of the night sky and the resulting lack of the ability to observe stars during the night (see [3] for an overview). Later it was found that LP has strong effects on flora, fauna [4] and human well-being [5]. Recently, concerns were raised that LP might have an impact on whole ecosystems and biodiversity, and several investigations in that context have been performed [6], [7]. While most work has focused on terrestrial animals, some recent work investigated the influence of ALAN on aquatic life [8]. This includes studies on the diel vertical migration of zooplankton in lakes [9], the alteration of microbial communities [10] and photophysiology of freshwater cyanobacteria [11]. Since ALAN is increasingly recognized as an environmental stressor, several studies to quantify the amount of ALAN leaving the Earth׳s surface have been performed. A combination of satellite images and theoretical modeling resulted in the World Atlas of night sky brightness [12].

Artificial skyglow is one type of LP that is mainly observed in densely populated areas where ALAN is scattered by atmospheric molecules or aerosols and returned to Earth [13]. Skyglow can be drastically amplified by several orders of magnitude due to the presence of clouds as shown by empirical [14], [15] and theoretical [16] work. This amplification depends not only on the amount of upwelling radiation caused by ALAN but also on its spectral distribution, the topography of the landscape, the vegetation cover and especially the weather condition [17].

Despite its possible impact on ecosystems [18], it is not straightforward to estimate the amount of skyglow at a specific location from satellite images alone, as these are only usable in clear conditions. Furthermore, the seminal World Atlas of night sky brightness [12] is based on almost 20 year old satellite data; and unfortunately no such updated global data base is available, yet. ALAN, however, is rapidly changing especially very recently due to the availability of cheap and efficient solid state lighting. As a result, prediction of the current exposure to LP at a field site, particularly for overcast conditions, is difficult.

This lack of reliable data, together with the development of cheap handheld sky brightness meters [19] and novel measurement schemes based on imaging technology [20], [21], has triggered several local [1], [22] and global [23] ground-based studies of the night sky brightness (NSB). In this work, we measured the NSB at Lake Stechlin over a period of four months in summer and early autumn to evaluate the amount of artificial skyglow at an experimental field site. We employed continuous four-month measurements with a sky quality meter (SQM) as well as imaging with a digital single lens reflex (DSLR) camera. The motivation of our work is to characterize skyglow at a field site where an experiment is planned to assess the impact of LP on a lake ecosystem. For this experiment it has to be verified that the field site is located in one of the darkest regions of Germany [12], which makes it possible to use unmanipulated experimental units as controls.

Section snippets

The field site

Measurements were made at the LakeLab (Fig. 1; www.lake-lab.de), which is an experimental mesocosm facility installed in Lake Stechlin, a clearwater lake with a surface area of 4.25 km and a maximum depth of 69.5 m. The lake is situated about 70 km north of the city of Berlin in a forested area. The nearest village is Neuglobsow (53°848N,13°249E) with less than 300 inhabitants. The lake is part of a nature reserve belonging to one of the darkest areas of Germany (according to [12]).

The

The sky quality meter (SQM)

Continuous night sky luminance measurements were performed between June and September 2015 with an SQM manufactured by Unihedron (Grimsby, Ontario, Canada). The SQM version with an integrated lens (SQM-Lxx) measures luminance for a patch of the sky with an opening angle of 20° [24]. The combination of a silicon photo diode (TSL237S) and a band-pass filter (HOYA CM-500) results in a spectral response roughly similar to that of the scotopic human vision [19]. The SQM provides the luminance in

Continuous zenith sky brightness measurements

Fig. 3 shows the variation in sky luminance in NSU on a logarithmic scale as a function of time from the beginning of June until the end of September. NSU values were recorded automatically during twilight and the night, while the SQM saturates in bright conditions and therefore no data was recorded during daytime.

The diel change in sky brightness from nighttime to daytime is clearly visible in the plot. No filtering or averaging was used in the plot and the full data is shown. Each spike

Summary and conclusion

We investigated the NSB at a research field site situated on a lake 70 north of the city of Berlin. Continuous monitoring between the beginning of June and the end of September 2015 was performed using a small photometer (an SQM) collecting data every 10 min. In addition, fish-eye lens imaging was performed on a moonless clear night to spatially resolve and quantify distant skyglow caused by LP.

With the SQM, zenith NSBs as low as 0.32 NSU (22.84 magSQM/arcsec2) were found under overcast

Acknowledgments

This work was supported by the ILES project funded by the Leibniz Association, Germany (SAW-2015-IGB-1), the “Verlust der Nacht” project funded by the Federal Ministry of Education and Research, Germany (BMBF-033L038A) and the EU COST Action ES1204 (Loss of the Night Network). The DSLR camera was provided by the GFZ German Research Centre for Geoscience. The authors would like to thank the ILES team for fruitful discussions and Stefan Heller, Armin Penske and Michael Sachtleben for help with

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