One of the fastest growing areas of concern among scientists is the ever increasing quantity of plastic litter finding its way into aquatic environments. In 2014 alone, approximately 311 million metric tons of plastic were produced, with that number steadily increasing each year (Jonanovic, 2017). Of the plastic produced on a global scale, up to 10% of it ends up in aquatic environments. Specifically, by 2025 the mass ratio of plastic to fish in the oceans is predicted to be 1 to 3 and by 2050 it is expected that plastic will likely surpass fish stocks in the ocean by weight (Jonanovic, 2017). Microplastics, or plastic particles smaller than 5 mm derived from the break down of larger debris or entering the environment directly as microscopic fragments, has been estimated at over 5 trillion floating particles at a weight of 250,000 tons (Sussarellu et al., 2016). While much of the attention of scientists and the public eye have focused on the entanglement in and ingestion of macroplastics by vertebrates, the biological impacts of microplastics on marine organisms is only just now emerging (Wright et al., 2013). The aim of this post is to provide a summary of some of the current research completed looking at the deleterious impacts of microplastics on marine life.

Figure 1: Microplastics collected from the Chesapeake Bay Watershed . https://www.flickr.com/photos/chesbayprogram/16999300502

Given the abundant nature and small dimensions of microplastics, ingestion by marine filter-feeders is of growing cause for concern. Filter-feeding species ingest large volumes of water, and subsequently, large quantities of particles potentially leaving them more susceptible to microplastic pollution. Sussarellu et al. (2016) experimentally exposed reproductively active Pacific oysters (Crassotstrea gigas) to microplastics for two months to access physiological impacts on both adults and offspring. The results of this study demonstrated that adult oysters altered their energy allocation from reproduction to structural growth and maintenance when exposed to microplastics. These alterations lead to reproductive impairment and decreased survival and growth of offspring (figure 2).

Figure 2: Comparison of larval growth of Pacific oysters between control group and those exposed to microplastics. Suppressed growth of larval oysters exposed to microplastics evident (Sussarellu et al., 2016).

Along with filter feeders, indiscriminate deposit-feeders (aquatic organisms that feed on organic matter that has settled on the sea floor), could be negatively impacted by microplastic pollution. Wright et al. (2013) found that lugworms (Arenicola marina) exposed to natural sediments infused with 5% microplastics depleted energy reserves by up to 50%.  The time taken to egest (discharge undigested material) ingested material was 1.5 times longer in lugworms exposed to microplastics compared to control worms. Unfortunately, these two studies represent some of the only research looking at the ecological impacts of microplastic ingestion by filter- and deposit-feeders.

While often considered wholefully insufficiently investigated, recent discoveries regarding the potential negative impacts of microplastic ingestion by fish are emerging throughout the scientific realm (Jovanovic, 2017). Fish could ingest microplastics both intentionally and unintentionally during all stages of development. Laboratory studies focused on on fish ingestion of microplastics have demonstrated varying physiological impacts. Reduced food intake resulting from microplastic gut blockage could lead to decreased energy availability in fish (Mazurais et al., 2015). Rochman et al. (2013) found that marine fish ingesting microplastics sorbed with environmental contaminants bioaccumulate the pollutants and suffer liver toxicity. In the study, fish exposed to microplastics and sorbed contaminants expressed high levels of stress and hepatic (liver) inflammation. These studies indicate the potential for negative physiological impacts on fish populations, but further research is needed to validate these results in the field (Steer et al., 2017).

With potentially devastating physiological implications of microplastic ingestion evident, concentrated efforts on disentangling the ecological impacts of microplastic ingestion on our aquatic ecosystems remains vital. While the elimination of plastic use by humans may be unachievable and the removal of existing microplastics all but impossible, we have a moral obligation to reduce our plastic usage to help alleviate future negative impacts on all aquatic organisms.

Figure 3: https://www.photolib.noaa.gov/htmls/reef2129.htm

References:

Jovanovic B (2017) Ingestion of microplastics by fish and its potential consequences from a physical perspective. Integrated Environmental Assessment and management 13(3): 510-515.

Mazurais D, Ernande B, Quazuguel P, Severe A, Huelvan C, Madec L, Mouchel O, Soudant P, Robbens J, Huvet A, Zambonino-Infante J (2015) Evaluation of the impact of polyethylene microbeads ingestion in Eurpoean sea bass (Dicentrarchus labrax) larvae. Marine Environmental Research 112: 78-85.

Rochman CM, Hoh E, Kurobe T, Teh SJ (2013) Ingested plastic transfers hazardous chemicals to fish and induces heptic stress. Scientific Reports 3(3263): doi.10.1038/srep03263.

Steer M, Cole M, Thompson RC, Lindeque PK (2017) Microplastic ingestion in fish larvae in the western European Channel. Environmental Pollution 226: 250-259.

Sussarellu R, Suquet M, Thomas Y, Lambert C, Fabioux C, Pernet MEJ, Goic NL, Quillien V, Mingant C, Epelboin, Corporeau C, Guyomarch J, Robbens J, Paul-Pont I, Soudant P, Huvet A (2016). Oyster reproduction is affected by exposure to polystyrene microplastics. PNAS. U.S.A. 113, 2430–2435.

Wright SL, Rowe D, Thompson RC, Galloway TS (2013) Microplastic ingestion decreases energy reserves in marine worms. Current Biology 23(23): 1031–1033.

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.