Impact of light pollution on aquatic invertebrates: Behavioral responses and ecological consequences

Artificial light at night (ALAN) is a growing global problem altering the behavior of organisms and thereby community composition and ecosystem processes. Research has mostly focused on terrestrial ecosystems, but a growing number of studies show that aquatic ecosystems are increasingly affected. Here, we provide a conceptual framework that describes how ALAN can influence aquatic ecosystems through effects on the behavior of ecologically important invertebrates. These organisms provide a range of critical ecological functions, from serving as food for other organisms to nutrient cycling and the translocation of energy and matter within and between ecosystems. In addition, we systematically searched the literature to assess the current state of the field and identify knowledge gaps. The literature search reveals that an increasing number of studies find light pollution to alter the behavior of aquatic invertebrates, such as their movements, habitat choice, and foraging behavior, but that the fitness consequences of these behavioral changes are largely unknown, as are their impacts on populations, communities, and ecosystems. Yet, assessing the consequences of behavioral changes for higher ecological levels is of vital importance given the central role of these invertebrates in ecosystems. Thus, more research needs to be directed to the ecological consequences of behavioral responses of aquatic invertebrates to light pollution. Overall, more effort should be made to assess the ecological consequences of behavioral responses to ALAN, and, importantly, how negative effects of light pollution could be mitigated. Light pollution is of growing ecological concern and influencing ecosystems through effects on the behavior of organisms. Aquatic ecosystems are increasingly exposed and an ecologically important group of organisms in these systems are invertebrates. Here, we discuss how artificial light at night alters the behavior of aquatic invertebrates and how this in turn influences ecosystem structure and function. Such an understanding of the mechanisms and pathways that underlie the effect of light pollution on aquatic ecosystems is needed if we are to develop efficient strategies to reduce negative effects of human-made lighting systems on ecosystems.


Introduction
The increased use of artificial light at night (ALAN) is of growing concern given its impact on biodiversity and the functioning of ecosystems (Gaston et al. 2013;Holker et al. 2010;Longcore and Rich 2004).The topic has received increased attention since the key publications by Longcore and Rich in 2004(Longcore and Rich 2004) and 2006(Rice and Longcore 2006), but has mostly focused on effects in terrestrial ecosystems (Sanders et al. 2020).However, a few early papers documented negative effects on aquatic organisms (Moore et al. 2001;Verheijen 1958) and recent studies show that aquatic ecosystems are increasingly exposed to artificial light (Davies et al. 2020;Holker et al. 2021).In particular, organisms in waters close to human settlement, transport networks, and industry are exposed, such as in rivers, ponds and coastal waters, but also organisms in deeper waters and offshore areas, through skyglow and light from vessels and offshore infrastructures such as oil rigs (Davies et al. 2014).The intensity of illumination in these areas varies spatially and temporally, with lower levels on seafloor than in shallow urban ponds (Davies et al. 2020), and depending on daily and seasonal light cycles (Gaston et al. 2017;Massetti 2020).The effect of this variation on organisms depends in turn on the environmental conditions that the species have adapted to over evolutionary time (Tuomainen and Candolin 2011).
Ecologically important organisms in these light-exposed habitats are aquatic invertebrates; they nourish higher trophic levels, decompose organic matter, recycle nutrients, and translocate energy and matter within and between habitats and ecosystems.They are often sensitive to artificial light because many of their behaviors are set by light cycles, such as daily activities and seasonal migrations (Gaston et al. 2017).Thus, artificial light at night has the potential to influence their behaviors and, hence, their ecological functions.
To assess the impact that light pollution has on aquatic invertebrates and their ecological functions, an understanding of the underlying mechanisms and pathways is needed, as well as of the influencing factors, such as life histories and interactions with other human-inflicted disturbances.Such knowledge is needed if we are to develop efficient management strategies to reduce negative effects of light pollution on aquatic ecosystems.
To facilitate research into this important field, we provide here a conceptual framework that illustrates the pathways and mechanisms through which light pollution influences the behavior of aquatic invertebrates and thereby their ecological functions (Fig. 1).We also assess the current state of the field by systematically searching the literature.We begin with giving an overview of the mechanisms and pathways through which artificial light influences the behavior of aquatic invertebrates.We then systematically search the literature to assess the current state of the field and to identify knowledge gaps.We continue with describing common behavioral responses of aquatic invertebrates to light pollution, illustrating with examples from the literature as well as expected effects based on ecological theory.We then move onto explaining the impact that behavioral responses can have on individual fitness, population dynamics, and community composition, and eventually on ecological conditions such as nutrient cycling and the translocation of energy and matter between disparate habitats.We end with discussing the research that is needed to advance this important field of research, and the knowledge that we need to acquire to develop efficient mitigation strategies.

Plastic responses and genetic adaptation
The first response of an animal to a change in the environment-such as light pollution-is often behavioral: the individual may alter its activity, habitat choice, food consumption etc. (Wong and Candolin 2015).These plastic responses are ultimately genetically determined and shaped by past environmental conditions and selection pressures (Sih et al. 2011;Tuomainen and Candolin 2011), but may be modified by experiences during lifetime and by transgenerational effects, i.e., when the experiences of parents, and possibly grandparents, influence offspring development and gene expression (Bell and Hellmann 2019).Given this dependence of responses on past conditions, behavioral responses to artificial light at night are often maladaptive, as species have not experienced similar conditions in their evolutionary past and, hence, are unlikely to have evolved adaptive reaction norms for responding to artificial nocturnal light (Robertson et al. 2013;Sih et al. 2011;Tuomainen and Candolin 2011).An example is the attraction of aquatic insects to streetlights when emerging from water bodies, which can drastically reduce their probability of survival (Manfrin et al. 2017(Manfrin et al. , 2018)).
When behavioral responses reduce the fitness of individuals and are maladaptive, new reaction norms are needed for the population to remain viable.Their evolution depends on a range of factors, such as the generation time of the species, the presence of genetic variation in the direction of selection, the rate at which new mutations arise, gene flow, and the size of the population (Bell 2017).Thus, species with longer generation time may not adapt fast enough to prevent population decline (Chevin and Lande 2010).Species with shorter generation time may be more likely, depending on the presence of genetic variation in the direction of selection, but little is currently known about the ability of aquatic invertebrates to adapt to light pollution.

Direct and indirect effects
Animals may respond to artificial light directly, or indirectly by responding to the reactions of other species or to the changes that the light causes to the abiotic environment (Wootton 1994).The responses can be immediate, such as when an individual changes its activity level or habitat use, or develop gradually over the lifetime of an individual, such as when physiological or morphological changes are needed, or a specific developmental stage needs to be reached before the behavior can be expressed (Buchanan and Partecke 2012;Wingfield 2013).
Indirect effects can in some instances be more profound than the direct effects, as species are linked to each other and to the environment through a myriad of interactions (Tylianakis et al. 2008).Thus, artificial light that alters the behavior of one species influences also the other species that it is linked with, and such effects can reach far beyond the directly affected species.Feedbacks among species and with the abiotic environment can further alter the ultimate effect of artificial light on species and result in complex changes to the community.

Influencing factors
The impact that artificial light has on aquatic invertebrates depends on a range of factors, such as the characteristics of the light and its temporal and spatial distribution, the transmission of the light within the water, the contrast to the background, water depth, and the structure of the habitat above and within the water body, as discussed in several recent publications (e.g., Davies et al. 2014;Tidau et al. 2021) (Table 1).In particular, the ongoing switch from narrow spectrum lamps to broad-spectrum LED lamps is expected to amplify negative effects of artificial light on organisms, as the LED light resembles natural day light more strongly than light from older lamps (Longcore et al. 2018).
Both direct illumination and sky glow from urban areas influence light conditions (Kyba et al. 2017).The impact can be further affected by habitat choice and the activity time of species; species in surface waters and more open habitats, and those emerging from hiding when night falls, are often more sensitive to light at night than burrowing species or species shielded by vegetation (Lynn et al. 2021).The responses can also depend on the circadian rhythm of the species, as diurnal species often extend their activity period and reduce the time for rest, while nocturnal species may show the opposite pattern (Sanders et al. 2020).
How a species responds to artificial light at night depends also on its visual system.This can vary from simple perception of the presence of light to the possibility of perceiving colors (Gaston et al. 2013).Aquatic species in dark habitats, such as in deeper waters, are often highly sensitive to artificial light (Gal et al. 1999).Some species are also able to perceive wavelengths that we humans cannot, as well as light polarization, which needs to be considered when assessing the effects of light pollution on animals (Marshall 2017;Marshall and Cronin 2011).

Review of the literature
To review the literature on the impact of artificial light on aquatic invertebrates and their ecological functions, we used the guidelines set out in the Collaboration for Environmental Evidence (CEE) and in RepOrting standards for Systematic Evidence Synthesis (ROSES) (Berger-Tal et al. 2019;Haddaway et al. 2018).We searched Web of Science using 'All databases' option with searches starting from 1945, using the search string (("artificial night light*" OR "night-time light*" OR "nocturnal illumination" OR "outdoor light*" OR "artificial light" OR "light* at night*" OR "anthropogen* light*" OR "urban light*" OR "light pollution*" OR "nightlight*" OR "man made light*" OR "street light*" OR streetlight* OR "street lamp*" OR streetlamp* OR "sky glow*" OR "skyglow*") AND (aquatic OR marine OR limnetic OR sea OR pelagic OR river* OR stream* OR lake* OR pond* OR We screened the publications for inclusion criteria: (1) test for effects of artificial light at night on the behavior of one or more aquatic invertebrates either in the field or in the laboratory (excluding constant day and night light); (2) have a control in the form of before/after study or paired sites/ treatments or use different light intensities or spectrum.This Life-history Distortion of cues for timing of seasonal life-history events (Gaston et al. 2017) Habitat choice Reduced emergence from burrows to feed by the sandy beach amphipod Americorchestia longicornis (Lynn et al. 2021) resulted in 76 publications that we categorized according to (1) the habitat of the species: benthos, plankton, nekton, intertidal, and species with terrestrial stages, (2) the behavior recorded: antipredator (including boldness), attraction to light, emergence (from water or from hosts), foraging, movements (including migration and habitat choice), and social behavior (including competition and reproduction), (3) whether the behavioral response was adaptive, maladaptive, or if adaptedness was unknown, and (4) the ecological level at which effects where recorded: individual, population, community, or ecosystem.The literature search is not exhaustive, but to keep the search objective and repeatable, we included only studies detected with the search string.
Inspecting the results, we find research effort to vary among taxa: insects with terrestrial stages and benthos have received most attention, while less attention has been paid to zooplankton (Fig. 2).Thus, the scope of the research should be broadened to more often consider species in the open water, as these are increasingly exposed to light pollution and important components of food webs (Davies et al. 2014).Attention also differs among behaviors; most research has focused on movements and attraction to artificial light, while antipredator and social behaviors, including reproductive behaviors, have gained less attention.Yet, these behaviors are critical determinants of fitness and influence population dynamics.Most worryingly, few studies have assessed the adaptive value of the recorded behavioral responses, i.e., whether the responses influence fitness components such as survival, growth, and reproduction.As a consequence, none of the included studies were able to determine the impact of the recorded behavioral responses on population dynamics.A few studies have documented effects of light pollution on community composition, but the mechanisms behind the effects are unknown, i.e., whether the changes in composition are related to changes in dispersal, mortality, or reproductive success of individual species.Information on indirect effects of artificial light, through changes in species interactions, are scarce, although these effects can be larger than the direct effects (Tylianakis et al. 2008).Finally, the consequences of behavioral responses for ecosystem processes are notable poorly known, such as decomposition, nutrient cycling, and the transmission of energy and matter within and between habitats and ecosystems.When conclusions are drawn, these are usually circumstantial.In addition, the duration of performed investigations is often short, such as short-term exposures of individuals to light in the lab or in the field, while longer term and gradually developing changes in behavior and their consequences for individual and higher ecological levels are unexplored.Thus, while our knowledge of the effects of light pollution on the behavior of aquatic invertebrates has been steadily growing during the last decades, worrying little is known about the consequences of these behavioral responses for populations, communities, and ecosystems, and especially so in the longer term.

Behaviors sensitive to artificial light
In the next sections, we discuss how artificial light at night is influencing, and expected to influence, various behaviors of aquatic invertebrates, drawing on examples from the literature and general ecological theory (Table 2).Behaviors sensitive to artificial light at night are expected to differ between day and night active species: sensitive behaviors in day active species are likely to be those that depend on vision, such as foraging, while sensitive behaviors in night active and crepuscular species are more likely to be behaviors that increase visibility to predators, such as general activity.

Biological timings
Many biological activities are set by internal clocks that are synchronized by natural light cycles, such as circadian, circalunar, and circannual clocks (Gaston et al. 2017).Circadian cycles are easily disrupted by artificial light, such as the nightly migration by zooplankton to surface waters (Cohen and Forward 2009), and the emergence of animals from their daily hiding places when night falls (Duarte et al. 2019;Torres et al. 2020).
Behaviors that are set by lunar cycles can similarly be altered by artificial light.An example is the synchronization of massspawning by corals that is set by moonlight and, hence, can be disrupted by artificial light (Lin et al. 2021).Artificial light can also alter seasonal activities-such as yearly migrations and the timing of reproduction-by extending the day and giving the impression of spring arriving earlier or autumn later.Such changes can in turn cause mismatches with other environmental conditions, such as with temperature or activities of other organisms.For instance, changes in the appearance of zooplankton in spring because of artificial light can cause mismatches with phytoplankton blooms, which in turn can alter the population dynamics of the zooplankton and, hence, the species that depend on the zooplankton as food (Asch et al. 2019).

Habitat choice and movements
Night-time illumination can alter the habitat choice of aquatic invertebrates through attraction (positive phototaxis) or repulsion (negative phototaxis) (Longcore and Rich 2004).Many nocturnal aquatic invertebrates are positively phototactic and artificial light can consequently cause an evolutionary trap, i.e., that they are attracted to the light because past selection has tuned the species to respond to light in such a way (Robertson et al. 2013).Light can also promote the emergence of aquatic insects from water bodies and influence their behavior after emergence.For instance, streetlights close to the water can double the number of insects emerging and draw them to artificially illuminated patches rather than to the surrounding habitat (Manfrin et al. 2017).
Negatively phototactic invertebrates avoid in turn illuminated areas.For instance, the sandy beach amphipod Americorchestia longicornis, which hides in the sediments in the day and emerges during the night to feed, is less inclined to emerge when exposed to artificial light, and when it emerges, it restricts its tidal distribution range if exposed to the light (Lynn et al. 2021).Species that use visual cues in their habitat choice can also have the choices altered by artificial light (Matsumura and Qian 2014).For instance, the larvae of several coral species select the depth for settlement depending on how the incoming light influences the appearance of the habitat (Mundy and Babcock 1998).
Artificial light has frequently been found to influence migration and dispersal.For instance, streetlights along streams reduce the drift of aquatic insects by increasing the time the insects spend hiding in the leaf litter or in the interstitial spaces between cobbles (Henn et al. 2014;Perkin et al. 2014).Zooplankton alter in turn their vertical position when exposed to night-time light (Berge et al. 2020).An example is light from bypassing ships that alters the distribution of arctic zooplankton during polar nights (Ludvigsen et al. 2018).Artificial light can also form barriers to dispersal and disrupt habitat connectivity, such as the illumination of only a part of a shore (Bennie et al. 2014), or disrupt navigation, as found for the sandhopper Talitrus saltator that uses moonlight as a compass during tidal migrations up and down the shore (Ugolini et al. 2005).

Foraging
Both foraging activity and foraging success can be sensitive to artificial light.Night active species may reduce their foraging activity, while day active species may extend foraging into the night, or initiate foraging earlier in the morning.Light can also improve the ability of individuals to orient in a habitat and increase the foraging success of both diurnal and crepuscular species (Longcore and Rich 2004).
Changes in foraging behavior can, in turn, influence the amount of food consumed or the nutritional value.For example, the snail Lymnea stagnalis increases its feeding rate under artificial light (Mondy et al. 2021).The light can also enhance foraging activity of herbivores by improving the abundance or palatability of primary producers, as found in terrestrial environments (Bennie et al. 2015).On the other hand, light sensitive species may reduce their foraging activity, as found for the night active sandy beach amphipod Orchestoidea tuberculate (Luarte et al. 2016) and the inshore mollusc Concholepas concholepas (Manriquez et al. 2021).Cannibalistic interactions can also be influenced by artificial light.An example is the intertidal burrowing crab Neohelice granulate where adults increase their cannibalism of juveniles when exposed to artificial light, probably because the juveniles are less able to escape the cannibalistic adults in lit environments (Nunez et al. 2021a).
Night-time light that induces stress and enhances energy demands increases again the need to feed (Swaddle et al. 2015).For instance, the dogwhelk Nucella lapillus increases its foraging activity while reducing its refuge-seeking behavior when exposed to night-time light (Underwood et al. 2017).

Predator avoidance
Artificial light at night can increases predation risk for night active species by eliminating the presence of darkness as a refuge.For instance, gammarids suffer higher predation pressure from fish in lit areas (Czarnecka et al. 2019).Similarly, insects emerging from water bodies experience higher predation pressure from terrestrial predators (Eisenbeis 2006).An increased risk of predation can in turn reduce the willingness of individuals to engage in fitness related activities, such as foraging and reproduction.For example, two crayfishes, the New River crayfish Cambarus chasmodactylus and the spiny stream crayfish Orconectes cristavarius, spend more time in shelter when exposed to night-time light (Fischer et al. 2020).
For day active species, artificial light at night can reduce predation risk by improving the ability to detect predators, or reducing the activity of their predators.On the other hand, the shortening, or even disappearance of the night, can cause stress and decrease physical condition or vigilance and, hence, make individuals easier prey.Little is, however, known about such effects for aquatic invertebrates.
The ability of species to melt into the background-camouflage -can be affected by artificial light and influence predation risk.A potential example is cephalopods that avoid predators by adjusting their body patterns to the immediate environment, the efficiency of which could be influenced by artificial light (Mathger et al. 2009).Bioluminescence signals that are used as defenses against predators can in turn be masked by artificial light and increase predation risk (Haddock et al. 2010).Ostracods, for instance, reduce bioluminescent signaling under artificial light, which could reduce their predation risk (Gerrish et al. 2009).

Reproduction
Artificial light can influence reproductive behaviors through effects on hormones, physical condition, general activity, and 104 Page 8 of 15 the possibility to find and evaluate potential mates (Buchanan and Partecke 2012;Candolin and Wong 2019;Elgert et al. 2020).Illumination often needs to be below a critical threshold before night active individuals engage in reproductive activities, while day active invertebrates may extend their reproductive activities into the night (Davies et al. 2014).Artificial light that decreases visibility and the background contrast of mate choice signals, such as bioluminescent signals, can in turn reduce the ability to locate and assess mates (Candolin 2019a;Gerrish et al. 2009;Rosenthal and Stuart-Fox 2012).
Artificial light that hampers the perception of natural light cycles may in turn disrupt the synchronization of reproductive activities.For instance, artificial light disturbs the moonlight dependent synchronization of gamete release in two coral species, Acropora millepora and A. digitifera (Ayalon et al. 2021b).Artificial light can also alter hormonal levels and thereby influence seasonal reproductive activities (Buchanan and Partecke 2012).For instance, increased perceived length of the day stimulates the production of an egg laying hormone in the pond snail Lymnaea stagnalis (Kitai et al. 2021).
The localization of oviposition sites by aquatic insects with terrestrial adult stages are again distorted by artificial light, as the insects use horizontally polarized light reflected off water bodies for finding suitable sites (Horvath et al. 2009;Schwind 1991).Instead, the insects may oviposit on human constructed surfaces that reflect horizontally polarized light, such as asphalt, bridges, windows, and cars (Egri et al. 2019;Kriska et al. 2008;Szaz et al. 2015).

Competition
Both intra-and interspecific competition can be affected by artificial light at night.Increased visibility can intensify agonistic interactions among individuals, or induce stress that reduces competitive ability (Navara and Nelson 2007).For instance, two crayfishes, Faxonious rusticus and F. virile, spend more time fighting when exposed to night-time light (Jackson and Moore 2019).Increases in general activity can in turn deplete energy reserves and intensify competition for food, while decreased activity may reduce encounters both within and between species and, hence, agonistic interactions.Changes in the efficiency of visual signals reflecting competitive ability can further influence competitive interactions, as documented for fishes (Candolin et al. 2014(Candolin et al. , 2016;;Rosenthal and Stuart-Fox 2012), and similar effects may occur in aquatic invertebrates.
Competition among species is especially likely to be influenced by artificial light, as only one of the species needs to be affected.In addition, the impact of artificial light can be mediated by effects on a third species, such as altered abundance of the prey that two species are competing for (Hoover and Tylianakis 2012).Such alterations may be common but their effects on aquatic invertebrates still need to be documented.

Host-parasite interactions
Host-parasite interactions (including host-pathogen interactions) are ubiquitous and changes in the behavior of one of the species can influence the other species (Budria and Candolin 2014).Artificial light is especially likely to alter their encounter rate through effects on visibility, movements, and population sizes.For example, the encounter rate of trematodes with their ultimate hosts depends on light conditions that determine the release of the infective stages from their intermediate hosts, molluscs, and also their subsequent survival and infectivity (Pietrock and Marcogliese 2003).Similarly, changes in light conditions that alter daily or seasonal activity patterns of either hosts or parasites can alter their encounter rate and thereby infection rate.Yet, such effects remain to investigated for aquatic invertebrates.
The impact of artificial light on infectivity and resistance to infections can be mediated by impact on physiological processes, such as induce stress that reduces immunocompetence, or alter the abundance and quality of food and thereby influence physical condition (Lopez and Duffy 2021).Changes in infectivity and resistance can, in turn, alter the abundance of the interacting partners and their encounter rate, which can cause further changes to the host-parasite interaction.Such potential effects would deserve more attention given the profound effect that infections can have on population viability.

Mutualism and commensalism
Mutualistic and commensalistic interactions are essential components of species interaction webs, which contribute to maintain biological biodiversity and ecosystem stability (Stachowicz 2001).These interactions are likely to be sensitive to light pollution because of their high degree of specialization; species may not be able to switch to another partner if their main partner is negatively affected by light pollution (Hoover and Tylianakis 2012).An example is the symbiosis between corals and endosymbiotic algae that is sensitive to artificial light through negative effects on the photosynthetic performances of the algae (Ayalon et al. 2021a).Similarly, the mutualistic relationship of corals with fishes is likely to be sensitive to artificial light, as the light influences the survival and growth of coral reef fishes, as demonstrated for the orange-fin anemonefish Amphiprion chrysopterus (Schligler et al. 2021).
Mutualistic and commensalistic interactions can also be indirectly affected by artificial light, similarly to other interactions.For example, light pollution that attracts larger predatory fishes can increase predation pressure on smaller species (Becker et al. 2013), which in turn can alter their mutualistic and commensalistic relationships with other organisms.However, little is currently known about the impact of artificial light on mutualistic and commensalistic interactions of aquatic invertebrates, despite their expected ubiquity and important ecological function.

Impact of behavioral responses on individual fitness and population growth
Behavioral responses to artificial light that influence the fitness of individuals-their survival and reproductive success-can influence population dynamics (Fig. 1).The review of the literature reveals that research has so far focused on documenting behavioral responses of aquatic invertebrates to artificial light, while their fitness consequences have remained largely unknown (Fig. 2).Yet, behavioral responses can have profound fitness consequences, as suggested by a few studies.For instance, reduced foraging activity by night active amphipods and crabs reduces their growth rate, which could negatively impact their survival and reproduction (Czarnecka et al. 2021;Luarte et al. 2016;Nunez et al. 2021a, b).Increased activity under light pollution can in turn increase visibility to predators, as well as cause the allocation of energy away from growth and reproduction.
Another common behavioral response that is likely to influence fitness is the attraction of aquatic insects to artificial light.The aggregation of insects with terrestrial life stages around light sources increases their visibility to predators and reduces their foraging activity while increasing energy expenditure (Bolton et al. 2017;Eisenbeis 2006;Owens and Lewis 2018).Moreover, artificial light that hampers their detection of suitable oviposition sites when returning to water to reproduce can reduce reproductive success (Horvath et al. 2009;Kriska et al. 2008).Artificial light that obscures lunar cycles can again disrupt the synchronizing of mass spawnings in species that depend on the lights for the timing of the event, such as corals (Ayalon et al. 2021b).
Changes in individual fitness can, in turn, influence the size, structure and distribution of the population (Gaston and Bennie 2014;Pelletier and Garant 2012).Yet, we currently lack knowledge of such population consequences.Most evidence is circumstantial, such as the recent decline of the giant water bug Lethocerus deyrolli, which could be related to its attraction to artificial light, such as streetlight, which increases predation risk (Yoon et al. 2010).

Impact of behavioral responses on community composition
Changes in the behavior of one species influence other species via the network of species interactions (Hoover and Tylianakis 2012;Sanders and Gaston 2018).The impacts can be traitor density-mediated, with changes in behavior, or in density because of behavioral responses, influencing directly and indirectly linked species (Fig. 1) (Werner and Peacor 2003).
A change in species interaction can initiate a cascade of effects that ripple through the ecological network of species interactions (Tylianakis et al. 2008).The cascade of effects can influence species that are not directly affected by the artificial light, while feedback responses and time lags can cause complex shifts to the species community (Bartley et al. 2019).Despite the obvious importance that species interactions play in determining community composition, the consequences of behavioral responses of aquatic invertebrates have received little attention, as revealed in our literature search (Fig. 2).Only a few studies have investigated the impact of light pollution on the composition of aquatic communities, and the contribution of altered behavior and species interactions are unexplored (Bolton et al. 2017;Davies et al. 2015;Garratt et al. 2019;Martin et al. 2021;Meyer and Sullivan 2013).Hence, the pathways and mechanisms behind community changes in light polluted areas are largely unknown.

Ecological ramifications
Behavioral responses to light pollution could influence ecosystem processes, such as nutrient cycling, bioturbation, carbon sequestering, primary production, and the flow of energy to higher trophic levels (Fig. 1) (Candolin 2019b;Palkovacs and Dalton 2012;Wilson et al. 2020;Zapata et al. 2019).Even habitats not directly exposed to the light could be affected through the movements of species.For instance, changes in the diel vertical migration of zooplankton is likely to alter the transport of energy and matter between surface and deeper water, as the zooplankton consume nearsurface phytoplankton during the night and defecate fecal pellets in deeper water during the day (Hays 2003).Changes in their migration could also influence water clarity through altered predation on phytoplankton, as well as the transfer of energy and matter to higher trophic levels (Hays 2003).
Changes in diel movements of benthic species could similarly influence the transfer of energy and matter between habitats and also to higher trophic levels.An example is the change in diel movements of crabs and shrimps between seagrass beds and adjacent habitats, which may alter not only the transfer of energy and matter between these habitat but also the movements of their predators (Martin et al. 2021).Similarly, changes in the drift of invertebrates, and the choice of settlement habitat, can influence the flow of organic matter within water bodies, as well as the food availability to drift-feeding fishes, including game fishes such as salmonids (Baxter et al. 2005).
Changes in grazing or detritus feeding can in turn influence the recycling of energy and matter and thereby the biomass of primary producers and the length of food chains.An example is the change in detritus and litter feeding by freshwater amphipods exposed to artificial light, as this can 104 Page 10 of 15 alter the decomposition of organic matter and thereby the cycling of carbon and nutrients (Czarnecka et al. 2021).
Regarding the connection between aquatic and terrestrial ecosystems, increased emergence of aquatic insects from illuminated water bodies is likely to influence the flux of energy and matter between the systems (Manfrin et al. 2018;Perkin et al. 2011).Increased emergence could also influence the distribution and abundance of their terrestrial predators, such as bats, birds, and lizards, such as the concentration of aquatic insects around terrestrial artificial light sources (Baxter et al. 2005;Manfrin et al. 2017;Parkinson et al. 2020;Sullivan et al. 2019).Similarly, the aggregation of invertebrates in lit areas within aquatic ecosystems could influence the abundance and distribution of their predators.For instance, the recent shift towards smaller insects in light polluted habitats could be a consequence of the aggregation of fishes that prey on larger insects in these illuminated patches (Meyer and Sullivan 2013).Thus, while an increasing number of studies suggest that behavioral responses to artificial light could influence ecological processes, surprisingly little direct evidence exists (Fig. 2).

Interactions between light pollution and other disturbances
How behavioral responses to artificial light interact with other human-caused disturbances is generally poorly known (Swaddle et al. 2015).This is especially the case for aquatic ecosystems where the impact of light pollution has only recently received more interest.Interactions between disturbances can be additive or interactive (synergistic or antagonistic) and increase or lessen the effect of each stressor (Cote et al. 2016).Considering the number of anthropogenic disturbances that are currently impacting aquatic ecosystems, such interactions are likely to be common and could have profound effects on individuals and their ecological functions.
Studies on terrestrial ecosystems show that light pollution often interacts with noise pollution in influencing the behavior of individuals (Swaddle et al. 2015).Similar interactive effects could occur in aquatic ecosystems, as these are increasingly exposed to human-made noise.Climate change is another factor likely to interact with light pollution, particularly through effects on metabolism, stress levels, and the habitat choice of animals.An example is the tropical copepod Pseudodiaptomus incisus that reduces offspring production when light pollution is combined with rising water temperature, probably because the light increases stress levels (Nguyen et al. 2020).Interactions with rising temperature could also promote primary production and thereby influence the behavior of invertebrates, such as the foraging activity of grazers.The effect could be further amplified by anthropogenic eutrophication that also promotes primary production.Thus, many anthropogenic disturbances could be interacting in influencing the behavior and ecological function of invertebrates.
The invasion of foreign species is a growing ecological problem whose impact on ecosystems could be further enhanced by light pollution, particularly through negative effects on native species.Artificial light that reduces the abundance or competitive ability of native species could favor the invading species, and the effect could be further escalated by habitat degradation that alters the habitat choice, activity level, or physical condition of native species (Komine et al. 2020;Perkin et al. 2011;Thomas et al. 2016).
Finally, light is an important zeitgeber of many activities and interaction with disturbances in other zeitgebers, such as in temperature or water flow, could amplify the negative effects on organisms (Perkin and Wilson 2021).Such interactive effects could have profound implications for ecosystems given the importance of the correct timing of many activities for the fitness of individuals and the viability of populations.

Knowledge gaps and recommendations for future research
The review of the literature reveals that research has so far focused on examining the effects of artificial light on the behavior of aquatic invertebrates, while the consequences of these behavioral responses for populations, communities, and ecosystems are poorly known (Fig. 2).Yet, aquatic invertebrates are critical components of aquatic ecosystems that provide essential services, such as nutrient cycling, supporting higher trophic levels, and translocating energy and matter between habitats and ecosystems.Thus, changes in their behavior could have far-reaching consequences for ecosystem structure, function, and stability.Research is particularly needed on species that are known to play, or suspected to play, key roles in maintaining ecosystem function.
Below, we highlight a few areas especially in need of more research if we are to improve our knowledge of the impact that light pollution has on aquatic invertebrates and their ecological function, as well as the management actions needed to reduce negative effects on ecosystems.

Spatial effects of light pollution
Effects of artificial light could vary across areas and habitats depending on natural light cycles, intensities, and spectra.Regarding latitudes, more stable daily light cycles at lower latitudes could make species more sensitive to changes in these cycles, while the lower light intensities at higher latitudes could cause species to be sensitive to even low levels of artificial light.Similarly, Page 11 of 15 104 water depth and light penetration are likely to influence the effects of light pollution: species in deeper water or more turbid water could be more sensitive to artificial light, given the naturally lower light intensities.Furthermore, spatial variation in light conditions within habitats could form barriers to movements and gene flow and influence migrating and dispersing species, while more sessile species may be unable to escape the light.

Temporal effects of light pollution
The time of the day or season when species are exposed to artificial light, and the duration of the exposure, are likely to influence responses.Crepuscular species are likely to be sensitive to light at twilight, while nocturnal species may be able to switch their activity times to later in the night (if light levels are lower then).The effect of artificial light may also depend on when during the life cycle they are exposed, and during which activities.For instance, exposure early in life could have more severe consequences than exposure later in life, while exposure during migration or when initiating reproductive activities could be more detrimental than during other times of life, given the profound impact these behaviors have on fitness.

Potential for genetic adaptation to light pollution
Information is scarce on the ability of species to adapt to light pollution.A terrestrial moth Yponomeuta cagnagella shows indications of adaptating to light pollution in terms of reduced flight-to-light behavior (Altermatt and Ebert 2016), but the degree to which aquatic invertebrates have been able to adapt to light pollution is unknown.Species are expected to differ in their potential depending on life history, population size, gene flow, and past light conditions and the evolutionary history of the population.The quality of the artificial light could further influence the potential to adapt, as light conditions that more strongly resemble day light-such as light from modern broadspectrum LEDs-could be more difficult to adapt to than the more yellow light from older lamps.
Influence of light pollution on species interactions Species interactions determine ecosystem structure and function and more attention needs to be paid to the impact of light pollution on these interactions.Species with obligate interactions with only one or a few species are likely to be most sensitive to light pollution, as they cannot switch to other species if their main interaction partners are disturbed by the light.Species with higher flexibility could be more resilient, such as many invasive species.

Interactions among disturbances
More research is needed on how light pollution interacts with other anthropogenic disturbances, such as climate change, chemical pollution, noise, harvesting and fishing, and the invasion of new species.These other disturbances and stressor can amplify or curtail the effects of light pollution; failure to consider these interactions may hide the ultimate impact of light pollution on organisms and ecosystems.
Mitigation strategies To limit the negative effects of light pollution on ecosystems, a range of solutions have been proposed, such as dimming or shielding light sources, limiting the illumination to desired areas and times, using motionsensing lights that turn on and off as needed, and avoiding the use of light sources that resemble natural daylight, such as broadspectrum LEDs.The effectiveness and applicability of these measures have mainly concerned terrestrial ecosystems.Thus, efficient mitigation strategies need to be developed and evaluated also for aquatic ecosystems.In this endeavor, more information is particularly needed on the impact of the quality of artificial light and the time exposed.

Conclusions
The global spread of artificial light is eroding the natural night-time environment, which influences the behavior of both night and day active species.Given that behavior underlies interactions both within and between species, and with the abiotic environment, and these interactions determine biodiversity and ecological processes, assessing the impact of light pollution on behavior is of utmost importance.By systematically assessing the literature, we have shown that much evidence exists that light pollution influences various behaviors of aquatic invertebrates, from foraging to reproduction and the ability to avoid predators.Yet, little information exists on the fitness effects of these behavioral changes, or their consequences for populations, communities, and ecosystem and, hence, for biodiversity and the functioning of aquatic ecosystems.Thus, we call for more research into the ecological consequences of behavioral responses of aquatic invertebrates to artificial light at night.Aquatic invertebrates are key components of ecosystems and changes in their ecological functions could have far-reaching consequences for ecosystem structure, function, and stability.To facilitate the research, we have provided a conceptual framework that indicates the pathways and mechanisms through which light pollution can influence the behavior of aquatic invertebrates and thereby higher ecological levels.In addition, we have outlined the major gaps in our knowledge and the research that is needed to advance the field.Light pollution is fairly easily reversed and it leaves behind no residual effect, but the recovery may still take time depending on connectivity between habitats and colonization rates.Yet, negative effects of artificial light can be mitigated if we put our minds to it.

Fig. 1 A
Fig. 1 A conceptual framework of the impact that artificial light can have on aquatic ecosystems through effects on the behavior of invertebrates.Changes in the behavior of individuals, such as habitat choice or foraging, can influence their population dynamics and inter-

Fig. 2
Fig.2A search of the literature on studies on the impact of artificial light at night on the behavior of aquatic invertebrates and (A) whether the responses were adaptive at the individual level, and (B) the high-

Table 1
Factors that can influence the impact of artificial light on the behavior of aquatic invertebrates, and examples of such effects Skyglow differs from direct illumination in that the magnitude of irradiance is generally much lower (Underwood et al. 2017)a et al. 2011))SeasonImpact of artificial light is larger during darker months (Massetti 2020) Latitude Impact of artificial light strongest at high latitudes and in intertropical zones(Secondi et al. 2020)Organism Vision Light sensitive species adapted to dim light conditions are most sensitive(Tidau et al. 2021)Activity time Reduced activity and increased sheltering behaviour by the crayfishes Cambarus chasmodactylus and Orconectes cristavarius(Fischer et al. 2020)Evolutionary history Artificial surfaces reflect polarized light that attracts dragonflies and mayflies(Kriska et al. 2009)Experience Earlier experience of artificial light influences the foraging and anti-predator behavior of the dogwhelk, Nucella lapillus(Underwood et al. 2017)

Table 2
Examples of behaviors that are influenced by light pollution