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Human impacts

Shine on…..or should we?

Article: Davies, Thomas W., James P. Duffy, Jon Bennie, and Kevin J. Gaston. “The nature, extent, and ecological implications of marine light pollution.” Frontiers in Ecology and the Environment 12, no. 6 (2014): 347-355. DOI: 10.1890/130281

 Light Pollution

Figure 1: Satellite image of the Earth at night. Credit: NASA Earth Observatory/NOAA NGDC

Figure 1: Satellite image of the Earth at night. Credit: NASA Earth Observatory/NOAA NGDC

Pollution is a very broad term for such a great issue facing our planet. If you Google “pollution,” you immediately become flooded with images of plastics, fossil fuel emissions, and sewage. While all of these pollution types are important and absolutely harmful to both the land and aquatic environments, there are other types of pollution that often get overlooked.

A prime example of an under-studied pollution type is artificial light pollution. As humans, we need light. It is a wonderful advancement for human civilization! However, artificial lights used to light up harbors, oil platforms, boats, etc. disrupt the natural order. Think about the stars at night. In a large city, you can barely see anything celestial while in an unlit field the night sky is dark, clear, and aglow with stars. We can even “see” the continents at night by satellite (Figure 1).

Figure 2: An example of light pollution in the Tamar, as viewed from Cremyll, UK.

Figure 2: An example of light pollution in the Tamar, as viewed from Cremyll, UK.

As of 2010, initial estimates have predicted that 22.2% of all coastlines have some exposure to artificial light at night. This estimate is likely to grow, similar to the ~6% annual light use increase on land due to population and socioeconomic growth.

There are many sources of artificial marine light, from the dim glowing of neighboring commercial and residential harbors (Figure 2) to direct light input from oil and fishing platforms. Anyone who has been night fishing can tell you that shining a light onto the surface reveals all sorts of interesting marine critters!

Artificial light is made up of different colors and frequencies than the natural rhythm of moonlight. Artificial light has already been shown to disrupt terrestrial ecosystems, such as causing some species of birds to sexually mature faster. In this study, Davies et al., (2014) seek out to better understand the ecological impacts artificial light has in the ocean. Their aim was to demonstrate and emphasize why more light pollution studies are needed by compiling and analyzing previous studies on this issue.

Potential Impacts and Threats

 

Figure 3. Known and potential impacts of artificial light pollution on marine ecosystems. (a) Suppression of zooplankton diel vertical migration by artificial skyglow. (b) Bird strikes on lit ships at night. (c) Extension of visual foraging behavior in coastal wading birds into the night. (d) Disruption of settlement site selection in sessile invertebrate larvae. (e) Aggregation of fish under pier lights leads to intensified predation. (f) De-synchronization of broadcast spawning from lunar phase (corals releasing gametes). (g) Displacement of nesting sea turtles from artificially lit nesting areas. (h) Disorientation of seaward migration in sea turtle hatchlings by street lights.

Figure 3. Potential impacts of artificial light pollution on marine ecosystems. (a) Suppression of zooplankton vertical migration  (b) Bird strikes on lit ships at night. (c) Increased visual foraging of coastal birds into the night. (d) Disruption of settlement of  sessile invertebrate larvae. (e) Aggregation of fish under pier lights leads to more predation. (f) De-synchronization of broadcast spawning . (g) Displacement of nesting sea turtles from artificially lit nesting areas. (h) Disorientation of baby sea turtle by street lights.

Many marine species use moonlight and starlight for navigation, reproduction, and to find food (or hide from things trying to make them dinner!). Scientists have postulated that artificial light could impact many organisms that depend on the natural “lunar clock” for their survival. Davies et al. have analyzed a few of the known or expected threats to the ocean.

Disorientation:

Light pollution has already been shown to disorient seabirds and sea turtles. For example, lights from coastal areas can delay newly hatched sea turtles from finding the sea for the first time. Similarly, fishing techniques that use artificial lights to attract marine creatures, such as squid, can affect the survivorship and behavior of not only the squid, but other fish and marine critters that are also attracted to the lights.

Although the research is unclear, scientists have speculated that one of the most important nightly migrations in the ocean could be altered. Many species of zooplankton use light as their cue to vertically migrate upwards at night (for many reasons such as avoiding predators). Changes in this nightly migration could impact the carbon cycle by changing how much fecal pellets are exported out of the surface.

Reproduction

Many marine critters use celestial cues to spawn, or release their gametes. For these organisms, they can use a certain celestial event to synchronize their spawning in order to optimize their reproductive success. A famous example is the palolo worm. The cue for this species to broadcast spawn is the third-quarter moon in October. Artificial lights could disrupt spawn events such as this one.

 

Another example of a light pollution threat deals with sessile (non-moving) invertebrates. Some larval species will decided where to “settle” for their adult lives depending on the amount of light. Settling is an area with a lot of light help prevents competition from above water photosynthesizers. However, white light has been measured to be up to four orders of magnitude dimmer under urban glow, which could limit the amount of areas these invertebrates will settle in.

Predation

Figure 4: The bioluminescent comb jelly, Beroe cucumis, occurs at depths that can be penetrated by artificial light.

Figure 4: The bioluminescent comb jelly, Beroe cucumis, occurs at depths that can be penetrated by artificial light.

Many marine creatures evolved to have specific coloration patterns to avoid being seen. This tactic is used to both hide from predators and to sneak up on prey. Artificial lights, overall, make affected regions brighter and thus harder for organisms to hide. A land-based example of how artificial lights change predations deals with moths. A previous study has shown that more moths are eaten by bats because they aggregate to street lights. Similarly, in the ocean, one study demonstrated that the amount of salmon eaten by harbor seals decreased when the lights of a bridged where turned off in British Columbia, Canada.

Another problem deals with bioluminescent. The lovely “glow” of some organisms like the comb jelly are used to confuse predators and lure prey (Figure 4). Unfortunately, LED lights have similar light spectrum to most bioluminescent frequencies, posing a potential threat to this survival method.

What’s Next?

 Now that many of the potential and known threats have been identified, logically the next step is to figure out the extent of these threats. In other words, which organisms are in the most danger?, which regions are likely to have the most problems?, and what could these changes mean for the ecosystems of the ocean at large?

The first thing that research scientists need to discover is which species are most vulnerable to light pollution. Once these species are identified, the challenge becomes monitoring them. Presently, scientists have been using remote sensing techniques, such as satellites, which are not ideal since measurements can only be made during clear nights. One proposal, although tricky, is to establish long-term monitor stations on light pollution sources, such as oil platforms.

A need for light pollution studies is the development of a sound knowledge base. While a few exist, more works still needs to be done. For example, the British Antarctic Survey currently logs all bird strikes (when birds collide with ships). This type of survey has helped identify “hotspots” where bird populations are most affected. This change in bird behavior and population could have an unknown ecosystem cascade. In other words, what affects do these bird strikes have on the marine organisms below? Could less birds mean less predation pressure on some species, etc?

At the present, light pollution is still a very new concept and is not recognized by most policy-makers. However, new initiatives, such as “marine dark sky parks,” are being created and proposed to preserve dark regions. In order to get more policy-makers in-board, researchers need to have a better understanding of what light level is acceptable before major ecological effects occur.

 Significance

Light pollution is a global issue that will only increase in the future. At the present, however, not enough research has been done to understand the biological and marine ecological consequences of artificial light pollution. The author’s suggest that this lack of knowledge should encourage coastal environments and boats to take precautionary measures and to promote dark sky initiatives. Hopefully, review papers such as this one will spark researchers to solve many of these unanswered questions.

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