Conservation ecologist Thomas Davies has long known that natural light plays a pivotal role in the lives of many marine organisms.

“They use it as a clock,” Davies, a lecturer in marine conservation at the University of Plymouth, U.K., told Mongabay in a video interview. “They use it to regulate the timing of particular events like broadcast spawning in corals, for example. Marine species can use it as a compass to navigate around the environment. And they can use it to guide things like their migrations up and down the water column.”

But until recently, many researchers hadn’t considered the potential impacts of artificial light at night on the marine environment, Davies says. According to him, some experts have even suggested that light pollution isn’t a serious issue for the underwater world since only small amounts of light reach the depths of the water column. Yet Davies argues that artificial light can have far-reaching impacts on a range of marine organisms — even deep-dwelling ones — as they’ve evolved over millions of years to be extremely sensitive to natural light such as moonlight.

In December 2021, Davies and colleagues published a paper in Elementa: Science of the Anthropocene that introduced the first global atlas that quantifies artificial light at night on underwater habitats. The researchers generated the atlas by drawing on a range of data sources, including the highly cited atlas of artificial night sky brightness developed by Fabio Falchi and colleagues in 2016, as well as measurements of artificial light in the northern Gulf of Aqaba in the Red Sea, a marine region rich with coral reefs.

The research team figured out how much, and how deeply, light spectrums were entering the ocean, and also how things like phytoplankton and sediment could change the optimal properties of the water. On top of this, they set out to determine when artificial light became biologically important enough to have a substantial impact on the marine environment. To do this, they turned to copepods in the Calanus genus, zooplankton that play an important role in the marine food web and are particularly sensitive to light. According to another study in Polar Biology, Calanus copepods can respond to moonlight at depths of 170 meters (560 feet) during dark polar nights, and are known to make vertical migrations when there is very little light in the sky.“In the paper, we talked about a ‘critical depth,’” lead author Tim Smyth, a scientist at the Plymouth Marine Laboratory, told Mongabay in a video interview. “And that critical depth is basically the light sensitivity of a zooplankton — so a copepod organism in the water.”

One of the key findings of the paper was that at a depth of 1 m (3 ft), 1.9 million square kilometers (734,000 square miles) of the world’s coastal oceans were exposed to biologically important artificial light at night. This accounts for more than 3% of the world’s exclusive economic zones, parts of the ocean that extend 200 nautical miles (370 kilometers) from nations’ shorelines. The amount of impacted coastal waters decreased to 1.6 million km2 (618,000 mi2) at 10 m (33 ft) and 840,000 km2 (324,000 mi2) at 20 m (66 ft), according to the study.

Smyth says these calculations are likely conservative since the researchers made a number of assumptions, including the premise that all cities would have a similar light spectrum as Plymouth, a city with a population of about 260,000 residents. The researchers expect that as they refine their model, the impact of artificial light at night on the underwater world will significantly increase.

The researchers also found that LED light, which uses far less energy than traditional incandescent bulbs and is considered by many to be environmentally friendly, can actually penetrate deeper into the water column, potentially causing more issues.

“[Cities] are shifting towards this much more LED-lit spectrum,” Smyth said. “Now the consequences of that are that the LEDs are very much more peaked in the blue end of the spectrum. And the problem with blue light is that it is much more energetic, and it can penetrate much deeper in the water column than in the orangey-red area of the spectrum.”

Another study found that the blue tones of LED lights were impacting birds, insects, fish and sea turtles, but that filtered yellow‐green and amber LEDs would have a much lower impact.

Both Davies and Smyth say they hope the atlas can increase awareness about the impacts of light pollution on the underwater world, and that artificial light will come to be seen as an additional threat to the marine environment, alongside other stressors like ocean acidification and plastic pollution.

“There’s a lot of interest now in ecology and conservation on being able to understand what the combined effects of multiple human stressors are on the environment,” Davies said. “And part of that is about building up multiple layers of maps of different impacts, and this is yet another impact that needs to be considered in building that picture.”

Banner image: Bright artificial lights at Hamburg coastline. Image by KarstenBergmann via Pixabay.

Citations:

Båtnes, A. S., Miljeteig, C., Berge, J., Greenacre, M., & Johnsen, G. (2013). Quantifying the light sensitivity of Calanus spp. during the polar night: Potential for orchestrated migrations conducted by ambient light from the sun, moon, or aurora borealis? Polar Biology, 38(1), 51-65. doi:10.1007/s00300-013-1415-4

Longcore, T., Rodríguez, A., Witherington, B., Penniman, J. F., Herf, L., & Herf, M. (2018). Rapid assessment of lamp spectrum to quantify ecological effects of light at night. Journal of Experimental Zoology Part A: Ecological and Integrative Physiology, 329(8-9), 511-521. doi:10.1002/jez.2184

Smyth, T. J., Wright, A. E., McKee, D., Tidau, S., Tamir, R., Dubinsky, Z., … Davies, T. W. (2021). A global atlas of artificial light at night under the sea. Elementa: Science of the Anthropocene, 9(1). doi:10.1525/elementa.2021.00049

Tamir, R., Lerner, A., Haspel, C., Dubinsky, Z., & Iluz, D. (2017). The spectral and spatial distribution of light pollution in the waters of the northern Gulf of Aqaba (Eilat). Scientific Reports, 7(1). doi:10.1038/srep42329

Elizabeth Claire Alberts is a staff writer for Mongabay. Follow her on Twitter @ECAlberts.

FEEDBACK: Use this form to send a message to the author of this post. If you want to post a public comment, you can do that at the bottom of the page.