Effects Of Light Pollution
Many species rely on natural light sources to set activity times for accomplishing activities like navigation and reproduction. Throughout the world, more than one billion people live within a hundred kilometers of a coastline. This causes many coastline marine ecosystems to be exposed to nighttime artificial light. (Figure 1, Davies et al. 2014). This addition of light alters the natural cycle of many species. The release of the hormone, melatonin, can be disrupted (Rodriguez et al. 2018). This hormone controls the circadian rhythm. In 2010, 354,760 kilometers of the world's coastlines were exposed to nightly artificial light pollution with Europe having the highest amount of coastline affected by light pollution (Davies et al. 2014).
Glare, skyglow, light trespass, and clutter make up light pollution. Glare is the undue brightness of a light source. Over-illumination is the use of too much light. Light clutter is having too many lights in a single location. Light trespass is the presence of light in unwanted areas. Skyglow is the increase in the brightness of the sky due to the scattering of light by water, dust and atmospheric gas molecules (Gaston et al. 2012). The sources of light pollution can be divided into two groups: temporary sources and permanent sources. Temporary sources include lights from shipping and light fisheries. Offshore oil platforms and land-based developments are examples of permanent sources. Light pollution can cause disorientation in many species that use natural light cues to navigate like sea turtle hatchlings. Bird strikes can also occur on artificially lit vessels at sea (Figure 3, Davies et al. 2014). The broadening spectrum of artificial light increases the opportunities for predators to find prey. For example, harbor seals have been spotted hunting salmon in illuminated areas (Depledge et al. 2012).
The International Convention for the Prevention of Pollution from Ships (MARPOL) currently does not recognize light pollution as a pollutant. There are efforts to quantify the extent of the threat, develop a sound knowledge base and design and implement protective measures. When mapping and modelling light pollution, the vertical and horizontal variability need to be taken into consideration. For example, turbidity affects the intensity of light. The overall impacts of light pollution can be reduced. The intensity and spectrum of artificial light can be controlled by using narrow band optical filters and LEDs. Long term databases can quantify light pollution and monitor the effectiveness of reduction measures. Since 1992, The U.S. Air Force Defense Meteorological Satellite Program Operational Linescan System has been collecting measurements of artificial light (Kamrowski et al. 2012).
The International Dark-Sky Association's Dark Sky Parks and the United Kingdom's Science and Technology Facilities Council's Dark Sky Discovery Sites are examples of national and international programs that focus on the importance of dark skies. By creating marine dark sky parks the effects of light pollution can be limited and the benefits to society, the environment, and the economic gains through nature tourism are maximized (Davies et al. 2014).
In Norfolk County, Massachusetts, Lake Waban is surrounded by residential and commercial areas as well as major highways. Within this lake the depth distribution of zooplankton is affected by light. When there is a full moon, the light decreases the movement of Daphnia (Moore et al, 2000). Moore et al. (2000) conducted a study to determine if the extent of zooplankton migration is affected by urban light sources. Three black enclosures and 3 clear enclosures that were closed at the top and open on the bottom were set up at 11.5 meters which is the deepest part of the lake. In the black enclosures, there was the greatest amount of movement of Daphnia (Figure 1, Moore et al. 2000). When there was light pollution, there was either no diel vertical migration of Daphnia or its movement was reduced to an undetectable distance. In clear lakes with low concentration of dissolved organic carbon and algae, the effects of light pollution should be the greatest (Moore et al. 2000).
The effects of light pollution on sea turtles is well studied because they are vulnerable to disorientation from artificial light near nesting areas. Sea turtle hatchlings rely on visual brightness to find their way back to sea. They head away from the dark, landward silhouettes and head towards the open, lower, brighter seaward horizon (Lorne and Salmon 2007). Upon the arrival at sea, sea turtles rely on wave direction and an internal magnetic compass (Thums et al. 2016). The presence of artificial light sources disrupts this process and the added time spent on the beach can cause exhaustion, dehydration, and increase the risk of predation. Tens of thousands of hatchlings die each year as a result of disorientation (Lorne and Salmon 2007).
To better understand how disorientation affects hatchlings, two tests can be conducted: short crawl tests and crawl duration tests. Short crawl tests determine if the ability of a hatchling to swim away from shore is affected by a brief crawl in the wrong direction. A crawl duration test determines whether or not the length of time turtles spend going in the wrong direction affects their ability to find the sea.A 2-hr misdirected crawl weakens the ability to crawl on straight paths to the sea. As the number of heavily lit beaches increases, the number of available dark nesting beaches will decrease. This means that there will be a greater concentration of nesting females on suitable beaches which leads to greater competition. Also, having to find new beaches may take hatchlings away from beaches that have favorable oceanographic conditions. The presence of ocean waves impacts the hatchling's ability to make it to sea. In the presence of waves, all hatchlings oriented away from the shore . When no waves were present, the hatchlings that started from the west were not oriented to the sea. Five swam parallel to the shore, one remained at the release site, one swam away from shore then reversed direction and returned to the release site and three others swam away from shore. This shows that when waves are absent, offshore orientation depends on the successful completion of a seaward-directed crawl (Figure 2, Lorne and Salmon 2007).
To better understand how light affects green sea turtle hatchlings at Wobiri Beach, North West Cape, Western Australia, miniature acoustic transmitters and passive receiver arrays were deployed. The transmitters would detach from each animal after one to two weeks. Acoustic receivers were set up in parallel rows beginning from the low tide mark. The tagged hatchlings were released under two conditions: ambient light and artificial light. The information gathered included the total time spent in the array, speed of each turtle and the bearing from where it was released to where it left the array. The results showed that one one hatchling returned to shore after arriving at the light but it finally left shore and the array. Ocean conditions differed on each night of this study. During the first night, there was a strong wind blowing from the west. The swell waves that usually break parallel to shore were disrupted. One the second night, there was little wind and waves. The turtles spent more time in the lit array on the first night. In the ambient treatment, hatchlings had similar movement patterns on both nights. In the artificial lit array, the hatchlings were strongly attracted to the light (Figure 3, Thums et al. 2017). Also shown by the black line in part d of this figure is one hatchling that returned to shore after reaching the light and then headed out to sea and left the array.
Six out the seven species of sea turtles nest along Australia's coastline. There are several factors that affect how bright a light appears to a turtle: the light intensity, wavelength and turtle spectral sensitivity (Kamrowski et al. 2012). Hatchlings are sensitive to wavelengths of light between violet and green (400-500 nanometers). There are ten nesting locations throughout Australia that are potentially at risk from light pollution (Figure 2, Kamrowski et al. 2012). In Western Australia, loggerhead turtles appeared relatively unaffected by light pollution. In the eastern Australian management unit, 22% of the nesting sites had light pollution exposure (Kamrowski et al. 2012). For green sea turtles, in eastern Australia, the risk of light pollution in the southern Great Barrier Reef stock was significantly higher than the northern stock. Olive ridley sea turtles were relatively unaffected by light pollution. Lastly, North West Shelf flatback sea turtles appeared to be exposed to significantly more light pollution than sites in eastern Australia (Kamrowski et al. 2000). This paper took a large spatial approach and the results can help in developing effective management tools for protecting sea turtles from artificial light.
The Florida Fish and Wildlife Conservation Commission has set guidelines for assisting disorientated hatchlings. According to these guidelines, if hatchlings fail to crawl seaward, they should be moved closer to the water or released in shallow water near the shore. These guidelines fail to address the proper locations for releases. These locations should be on a dark beach and different locations on the beach should be used each night because predators can learn where the hatchlings enter the water (Lorne and Salmon 2007).