Light Pollution From Coastal Development Can Affect Seafloor Organisms.

Review of Davies, T. W., McKee, D., Fishwick, J., Tidau, S., & Smyth, T. (2020). Biologically important artificial light at night on the seafloor. Scientific Reports, 10(1), 12545. https://doi.org/10.1038/s41598-020-69461-6: Available online

Roll on, thou deep and dark blue Ocean – roll! -- Lord Byron

For generation, the seas have been synonymous with darkness.  As Lord Byron will attest, deep and dark are among its most common descriptors.  We all know that light gets absorbed quickly by water.  We see it when we’re swimming at the beach.  At night, we know that below the surface the sea is dark, black, devoid of light.  To poets and philosophers, it is the inky depths, the great void. 

Well, then again, maybe not. 

To light sensitive organisms living in the oceans near coastal development, the ocean is not as dark as it used to be.  For many marine organisms, the lights from our cities and towns are infiltrating their world all the way to the seafloor.  A recent paper by Thomas W. Davies and colleagues concludes that light pollution from coastal cities is likely affecting seafloor ecosystems in negative ways. 

Zooplankton

Credit: Matt Wilson/Jay Clark, NOAA NMFS AFSC via Wikimedia Commons.

Scientists, and fishermen, have long known that marine organisms respond to changing light levels in the water column.  The fishing community often uses lights to attract gamefish, and tackle shops are happy to offer a wealth of products to meet the demand.  In support for this conventional wisdom, biologists have demonstrated that lights affect marine organisms in numerous ways, often using bright lights near the surface of the water to illicit a response.  What has been less clear is the extent to which more diffuse light and sky glow from coastal population centers affect marine organisms, and how much of that light actually reaches ecologically important benthic or bottom dwelling communities. These are the questions that Davis, et al. explore. 

It is true that most light is scattered and absorbed as it travels through water and very little reaches the seafloor. In typical ocean environments, 99% of light is absorbed and scattered within the first 100 meters. However, most marine organisms have evolved to be extremely sensitive to even the smallest fluctuations in illumination.

In terms of sheer numbers, the greatest migration on earth happens every morning as microscopic animals, or zooplankton, near the surface dive into deeper waters.  When the sun begins to rise, copepods and other zooplankton, who have been happily munching on phytoplankton (microscopic plants) all night, head for deeper waters in an effort to avoid becoming snacks themselves. The trigger for this migration is a change in illumination levels.  Fortunately, the levels that trigger this behavior are known for one specific type of organism involved in the migration: Calanus copepods.  These levels were used by researchers in this study as a threshold for “biologically significance.”

Plymouth Sound

Credit: Robert Brimacombe, CC BY 2.0, via Wikimedia Commons

The authors measured anthropogenic light levels at the surface of Plymouth Sound and Tamar Estuary, two connected coastal waterbodies in southwest England.  The area is bordered by a large naval base and a city of more than 250,000 people.  Accounting for cloud cover and water depth (tide), the authors then calculated the amount of light that would reach the sea floor and the extend of the seafloor that would be affected. The ability of light to travel through water depends on its wavelength.  Longer wavelengths, such as red are absorbed the fastest and shorter wavelengths such as green and blue penetrate the furthest. In this study, Davies and his colleagues predicted light levels for three different light frequencies or colors.  The results showed that green light reached biologically significant levels over 76% of the seafloor under cloudy conditions at low tide, while blue light covered 70% of the seafloor under the same conditions. Red light, which is more readily absorbed by seawater, affected only 0.4% of the seafloor.  The effected areas were reduced by about 30% under clear skies and high tide.

These results could have profound implications for coastal marine environments. The study demonstrates that large areas of seafloor can be affected by light and skyglow from coastal cities and towns with biologically significant effects on benthic communities.  These effects will most likely ripple through entire coastal ecosystems -  some of the most productive and ecologically important areas on earth.  More research on these effects is desperately needed.  The authors note that several trends will likely increase the importance of these results in the near future.  First, coastal development is growing at dramatic rates and human coastal population is projected to double by 2060. Second, cities and towns across the globe are transitioning from sodium vapor and other technologies that generate a lot of short wavelength red and amber light to LEDs that generate more light in the blue portion of the spectrum.  Uncontrolled, this shift will result in higher intensity light reaching larger areas of seafloor.   Fortunately, well designed LED lighting can reduce overall levels of light pollution and prevent some of these impacts. For example, lighting manufacturers are developing luminaires that produce lower levels of blue light. Using lower light levels, shielding and carefully aiming lights, and using timers, motion sensors and other types of controls can also minimize skyglow.  These efforts must continue to protect our coastal marine environments.