Marine environments surrounding cities are unusually bright and noisy as a result of light and sound pollution. Recent research suggests this impacts marine organisms in some surprising ways.
Effects of light and sound:
|Towards the light|
|Adapting to illuminated seascapes|
- Artificial lighting in urban areas disorients sea turtle hatchlings, resulting in high mortality.
- Over time, this has led to fewer sea turtle nests in urban areas, however nesting populations can still be found in several major cities.
- While ‘turtle-friendly lighting’ is already being marketed by several manufacturers, it’s currently unclear whether these are effective.
Mother sea turtles are stalwart about where they lay their eggs – only their only place of birth will do, even if that place has since grown into a bustling metropolis. Once they lay their eggs, they have no further contact with their offspring.
After two months of incubating in their sandy nest, baby turtles hatch from their eggs, dig to the surface, and must quickly find their way to the sea to avoid getting gobbled up by hungry shore birds and other predators.
In those few crucial moments, hatchlings orient themselves with light and sprint towards the lightest point visible, which on undeveloped beaches is the horizon over the sea.
But on urban beaches, this instinct leads them astray. Bright lights of the city can take them inland, into hardscapes and across roads, where they fall into sewer gutters and encounter cars and predators. Those that survive the dangerous obstacles of the city eventually run out of energy reserves and become too exhausted to continue.
Over time, mortality of juvenile turtles from light pollution has led to decreases in the number of nests on developed beaches and contributed to declines in sea turtle populations. While ‘turtle-friendly lighting’ is already being marketed by several manufacturers (example), their effectiveness has yet to be thoroughly tested. In at least some cases, they do little to improve survivorship, but other studies hold out hope that they could improve sea turtle conservation in urban areas.
- The many known negative impacts of light pollution begs the question of whether illuminated seascapes in urban areas are exerting selective pressure on marine organisms.
- Thus far, studies addressing this question have been limited.
Light pollution affects a whole suite of marine organisms beyond turtles, as detailed in an excellent review by Thomas Davies and colleagues at the University of Exeter.
A conceptual figure from their paper summarizes known impacts, which relate to navigation, reproduction, larval settlement, relationships between predators and their prey, and communication:
Populations of marine organisms appear to persist in illuminated seascapes, despite numerous ways in which artificial light is thought to threaten their survivorship may be threatened by artificial light.
Could artificial light be acting as a selective force in urban areas?
It’s unclear at this stage.
We might expect evolutionary responses to light pollution, particularly in the tropics, as noted by Travis Longcore and Catherine Rich in a 2004 paper: light pollution “is more likely to affect tropical species adapted to diel patterns with minimal seasonal variation than extratropical species adapted to substantial seasonal variation.”
However, adaptations to light pollution among marine organisms have yet to be thoroughly studied, and empirical evidence is lacking. Hopefully this will be explored further soon.
- Noise pollution can impact how marine organisms process and respond to visual and chemical stimuli.
- This has primarily been studied through tank experiments with species that are not reliant on acoustic communication, such as damselfish and cuttlefish.
- These species may be less adept at avoiding predators and capturing prey in noisy environments near cities.
Noise pollution doesn’t only affect organisms that vocalize. It also impairs the ability of some marine life to process visual and chemical stimuli. This in turn may make them more vulnerable to predation and less effective at acquiring food and communicating with one another.
For instance, Ambon damselfish (Pomacentrus amboinensis) are small fish that live in the Western Pacific and make a tasty treat for predators like the dusky dottyback (Pseudochromis fuscus). Dusky dottybacks are stinky, and damselfish learn to run and hide when they smell a dottyback coming. However, in a recent study by Maud Ferrari and colleagues, young damselfish that learned to fear dusky dottybacks in the presence of boat noise later forgot to be afraid when exposed to dottyback odor, while those who learned in the absence of boat noise ran for cover.
Cuttlefish use visual signaling to communicate with one another, deter predators, and confuse prey. Elastic pigment sacs called chromatophores allow them to change the color of their skin, and they undergo impressive transformations in the blink of an eye:
This skill is important for capturing prey and deterring predators. Yet, cuttlefish appear to use it almost spastically when subjected to noise pollution. In a 2014 study by Hansjoerg Kunc and colleagues, the common cuttlefish (Sepia officinalis) was found to change its color more frequently while subjected to boat noise. As Kunc et al. note, this suggest “that anthropogenic noise has a marked effect on the behavior of species that are not reliant on acoustic communication.”
Further work is needed to confirm these findings in wild populations and assess impacts on survival. However, they suggest that noisy environments near cities may have consequences even for non-vocalizing species, by challenging their ability to avoid predators and inciting energy expenditure that would otherwise go towards foraging and other activities that ensure survival.