The vast majority of the Earth’s ecosystem’s have been influenced and modified by human activities, especially freshwater ecosystems. As the preferred site for human activity and development, these ecosystems often accumulate the effects of activities within their catchments (Perkins et al., 2011) Research focused on the effects of human-induced chemical pollution, alteration to nutrient cycles and natural flows, invasive species, urbanization, and loss of riparian zones have dominated our understanding how we are impacting freshwater ecosystems for the past 40-50 years (Perkins et al., 2011). Conversely, the influence of artificial lighting on freshwater ecosystems has long been overlooked. An estimated 67% of Americans and 20% of people globally live in locations in which the Milky Way is no longer visible as a result of interference from artificial light sources (Perkins et al., 2011). While freshwater ecosystems only cover about 0.8% of the Earth’s surface, approximately 9.5% of all animal’s species and one-third of all vertebrates call these systems home (Perkins et al., 2011) Studies aimed at identifying how these ecosystems, and the organisms living in them, are influenced by artificial lighting is a growing priority among science. Improving our knowledge on how artificial light may modify community structure and ecosystem function within freshwater ecosystems could help guide our management and conservation strategies in the near future.
Figure 1: Artificial lighting in the United States as seen from space. https://www.audubon.org/conservation/project/lights-out.
One way in which aquatic ecosystems are impacted by artificial light is the disturbance in natural dispersal tendencies of both aquatic insects and certain species of fish. Aquatic insects move throughout aquatic ecosystems and the adjacent terrestrial environments as they transition from their larval to adult stages, providing essential nutrients and acting as a prey source in streams, rivers, and lakes (Meyer & Sullivan, 2013; Perkin et al., 2011). Perkin et al. (2011) identified three main ways in which artificial light may impact aquatic insect dispersal. The fist, know as the fixation of captivity effects, involves emergent adult insects located near lights flying directly to them. In this case the insects may be killed directly by the lights, or mortality may occur when these insects are unable to leave and die from exhaustion, predation, or heat. The second mechanism is termed the crash barrier effect, in which insect dispersal and migration are impeded by artificial light sources. In this case, nocturnal aquatic insects may actively avoid areas in which artificial light alters the visual environment, eliminating dispersal of these important aquatic subsidies. Lastly, insects from a large area may be attracted to a nearby light source, altering movement and predator-prey relationships in both aquatic and terrestrial systems found near freshwater bodies of water. However, these impacts are only hypothesized. Carefully designed research and experiments are needed to determine how these mechanisms may actually play out in disrupting aquatic insect dispersal. For example, studies have identified elevated artificial lighting as a means of diminishing invertebrate drift rates, while extending or improving fish foraging (Meyer & Sullivan, 2013). This gives a distinct advantage to invertivore fishes, potentially reducing population sizes of aquatic invertebrates through predation-induced mortality (Meyer & Sullivan, 2013). However, in some locations, attraction of terrestrial insects to the water as a result of increased reflection of light off the surface of water has been shown to increase terrestrial prey subsidies for stream fish and release predation pressure on benthic insects (Meyer & Sullivan, 2013). Incorporating these hypothesized impacts of artificial lighting on dispersal and movement of aquatic insects with studies of ecosystem functioning will allow us to tease apart just how how big of an impact enhanced lighting could have on our streams, rivers, and lakes.
Figure 2: Solid line shows natural light provided by the moon in a temperate region. Dashed line is light level measured in the Berlin on clear night. Dotted line is the light level in the center of Berlin on cloudy night. Figure depicts the ability of artificial lighting to eliminate natural variation in light levels (Perkin et al., 2011).
Fish, specifically those in which rely upon lighting cycles to cue migration, dispersal, or feeding, could also be disrupted by artificial lighting (Perkin et al., 2011). Studies have shown altered migratory timing of Pacific and Atlantic salmon species in the presence of artificial lighting. While many of these species typically wait until sunset or dusk to move throughout their systems, when exposed to artificial light, migration started at random times, impacting arrival times to breeding grounds (Perkin et al., 2011; Tabor et al., 2012). Differing light conditions may also impact predation of fish species. Many species of fish often wait to forge until the cover of night in order to avoid predation (Perkin et al., 2011). In systems where artificial lighting may eliminate darkness altogether, this protective cover may vanish. Tabor et al. (2012) found that predation mortality of sockeye salmon increased by 40% when they were exposed to artificial lighting. Impacts of artificial light on migration patterns and predation could prove to be extremely detrimental to species that are already struggling to survive in their altered natural environments.
Further enhancing our current knowledge beyond hypothesized impacts and single taxon studies by incorporating research addressing the influence of artificial lighting on food webs and ecosystem functioning will be crucial in conserving biodiversity in freshwater ecosystems. Of the many impacts that humans have on these ecosystems, artificial lighting lighting could modify dispersion and predation of both aquatic organisms and the terrestrial organisms that rely upon aquatic energy subsides to thrive in their natural environments (Meyer & Sullivan, 2013; Perkin et al., 2011). These influences could alter future population abundances and genetics within these ecosystems, especially when compounded with other human-induced stressors such flow modification or chemical pollution (Perkin et al., 2011). The need for management of these systems in the wake of human disturbance is essential and continued effort from all stakeholders involved in the use of freshwater ecosystems must be achieved in order to minimize our impacts on these environments.
References:
Meyer LA, Sullivan SMP (2013) Bright light, big city: influences of ecological light pollution on reciprocal stream-riparian invertebrate fluxes. Ecological Applications 23(6): 1322-1330.
Perkin EK, Holker F, Richardson JS, Sadler JP, Wolter C, Tockner K (2011) The influence of artificial lighting on stream and riparian ecosystems: questions, challenges, and perspectives. Ecosphere 2(11): doi.10.1890/ES11-00241.11.
Tabor RA, Brown GS, Luiting VT (2004) The effect of light intensity on sockeye salmon fry migratory behavior and predation by cottids in the Cedar River, Washington. North American Journal of Fisheries Management 24: 128-145.