Light pollution of freshwater ecosystems: principles, ecological impacts and remedies

Abstract

Light pollution caused by artificial light at night (ALAN) is increasingly recognized as a major driver of global environmental change. Since emissions are rapidly growing in an urbanizing world and half of the human population lives close to a freshwater shoreline, rivers and lakes are ever more exposed to light pollution worldwide. However, although light conditions are critical to aquatic species, and freshwaters are biodiversity hotspots and vital to human well-being, only a small fraction of studies conducted on ALAN focus on these ecosystems. The effects of light pollution on freshwaters are broad and concern all levels of biodiversity. Experiments have demonstrated diverse behavioural and physiological responses of species, even at low light levels. Prominent examples are skyglow effects on diel vertical migration of zooplankton and the suppression of melatonin production in fish. However, responses vary widely among taxa, suggesting consequences for species distribution patterns, potential to create novel communities across ecosystem boundaries, and cascading effects on ecosystem functioning. Understanding, predicting and alleviating the ecological impacts of light pollution on freshwaters requires a solid consideration of the physical properties of light propagating in water and a multitude of biological responses. This knowledge is urgently needed to develop innovative lighting concepts, mitigation strategies and specifically targeted measures. This article is part of the theme issue 'Light pollution in complex ecological systems'.

Keywords: aquatic community dynamics; circadian rhythms; conservation; ecosystem functioning; land–water interactions; light physics.

Figure 1.

(a) Global map of light pollution showing large geographical differences [4]. (b) The Nile river and its delta in Egypt as a global hotspot of light pollution (photo: NASA Earth Observatory, 2010). (c) Direct light pollution at Huangpu River in Shanghai, China (photo: A. Jechow). (d) Indirect light pollution by skyglow over the River Rhine, Germany (photo: A. Jechow). (e) Schematic summarizing diel changes in illuminance (note log scale) by major natural light sources during day, twilight and night as a function of elevation angle of Sun and Moon; dark yellow solid line—Sun illuminance on clear day, light yellow dashed line—moonlight full Moon masking natural darkness (adapted from [12]). (f) Examples of demonstrated effects [–17] of low-level light pollution (yellow field) resulting from direct light and skyglow and polluting natural light at night as visualized in (e). DVM = diel vertical migration. Fish icon made with Freepik (www.flaticon.com). (Online version in colour.)

Figure 2.

Empirical or modelled data on light spectra in relation to surface waters (A. Jechow 2023, unpublished data). (a) Selected spectra (log scale) of natural light sources, including daylight (25 000 lx, Sun 18° above horizon), twilight (25 lx, Sun 4° below horizon), light of a near full Moon (60 mlx, 88% illumination, 20° above horizon), night-time light with airglow (1 mlx, Sun more than 18° below horizon), and various artificial light sources at 25 lx, including high-pressure sodium lamps (HPS), mercury vapour lamps and LEDs at 3000 and 5000 K. (b) Spectral diffuse attenuation coefficients (log scale) for different Jerlov water types, covering clear ocean water (type I) to very turbid inland and coastal waters (type 7C) [24]. (c) Spectral irradiance of light from HPS lamps and 5000 K LEDs after passing through 50 m of clear ocean water (Jerlov water type I). (d) Spectral irradiance of light from HPS lamps and 5000 K LEDs after passing through 5 m of turbid inland and coastal waters (Jerlov water type 7C). (Online version in colour.)

Figure 3.

(a) Light refraction, reflection and polarization at the air–water interface: unpolarized light impinging perpendicularly on a water surface (incident angle α = 0°) remains unpolarized (yellow arrow); the proportion of reflected light incident at α = 53°, which is called the Brewster angle, will become fully polarized (orange arrow); and at α = 85° (dark red arrow) light will be partially polarized; light rays propagating upwards at an angle greater than 97° are subject to total internal reflection (bright red arrows) and refraction (grey arrows), with the resulting Snell's window (blue arrows). (b) Photograph of Snell's window (photo: David K. Lynch and Simon Higton, NASA's ‘Earth Science Picture of the Day’, https://epod.usra.edu/blog/2014/06/snells-window.html). (c–e) Illustration and quantification of ALAN polarized at a water surface, with (c) showing horizontal and (d) vertical polarization effects of LED street lights reflected at the water surface of a river, and (e) the degree of linear polarization (DOLP) [35] of the reflected polarized light indicated by colour coding (see electronic supplementary material, S1, for calculations). Fish icon made with Freepik (www.flaticon.com). (Online version in colour.)

Figure 4.

Examples of ecological consequences of artificial light at night (ALAN) along a river–lake continuum, showing interference (a) with zooplankton diel vertical migration [15], (b) longitudinal migration of fish [98] and (c) predator–prey interactions [68,69], including insect drift [70] and effects across the land–water interface [71,72]. The left side of the river illustrates the situation under naturally dark skies, and the right side highlights the impacts of ALAN. Arrows show the direction of river flow. (Online version in colour.)