Light is an elementary prerequisite for the existence of life. It acts as a source of energy, as well as a ‘zeitgeber’ for regulating biological rhythms of organisms, whose internal clocks are regulated by the natural transition and duration of day and night. A number of behaviours and processes are in sync with the time of the day and the season. Due to the increasing use of artificial light, the majority of the earth’s surface has been deprived of this natural day-night rhythm in recent decades.
Artificial light disrupts the internal clock
Atmospheric particles cause urban artificial light to scatter and reach distant regions. Even the most remote areas with little artificial light can be affected by light pollution. As a result, the night sky is lighter than in a natural full moon night in many places. Increasingly bright nights do not only disturb astronomical observations – artificial light at night is also becoming a problem for flora and fauna. It is known to have a negative impact on insects, reptiles, birds, mammals and human beings. The day-night rhythm and seasonal changes in daylight duration influence many natural behaviour patterns of animals, such as food seeking, courtship and mating, migration and hibernation. If a phase of natural darkness is lacking or if its duration is changed by artificial light, all of these processes regulated by daily or seasonal periods may be impaired.
Increasing light pollution in urban areas does not only affect terrestrial organisms. Aquatic ecosystems react very sensitively to light, and are often severely affected by light at night, since mankind has always settled in the vicinity of water. For example, the productivity of freshwaters changes, as do predator-prey relationships. As yet, little research has been conducted into how increasing levels of light at night affect aquatic organisms and their interactions among each other and with their environment.
In fish, for example, most physiological and behavioural-biology processes are governed by daily or seasonal dynamics. Artificial light blurs the boundary between day and night, and therefore interferes with the physiological functions and behaviour of fish. It is already known that artificial light affects the growth and development of fish, and can even disrupt the spawning migration of diadromous (migratory) fish.
Melatonin – a hormone that responds to rhythm and colour
How, then, are these biological rhythms regulated in fish? Just like in the case of human beings, the hormone melatonin is mainly responsible. In fish, however, it is mainly the pineal gland in the brain, which receives light signals, transforms them into rhythmic hormonal signals and introduces them to the blood circulation. Melatonin is mainly produced at night; concentrations are low in the daytime. In this way, the melatonin balance always reflects the dominant photoperiod and constantly provides fish with information about the season and time of day. Night light suppresses the nocturnal increase in the melatonin level, which may disrupt processes that follow this hormonal rhythm.
In my thesis, I wanted to find out how light pollution, i.e. low levels of artificial light at night, affect two of the most common fish species in our freshwaters, the perch (Perca fluviatilis) and the roach (Rutilus rutilus). The aim was to clarify whether the melatonin rhythm is disturbed, to determine the threshold value, and to establish which light colour caused the disturbance. Such a disturbance can have a crucial impact on reproductive physiology, and may therefore be the most important parameter concerning the continued existence of a species.
I performed laboratory experiments to investigate the influence of different light intensities and colours on the melatonin rhythm and on various reproductive hormones in perch and roach. The laboratory studies demonstrated that the melatonin rhythm is even suppressed with the lowest intensity of 1 lx of white light. As a comparison, the light of a full moon is around 0.3 lx. All of the tested light intensities led to a considerable reduction in the nocturnal concentration of melatonin in the water in which the fish were kept. This suggests that light pollution can potentially disrupt the biorhythm of fish. It was also shown that the threshold value for the suppression of melatonin must be below 1 lx for both species. Considering that nocturnal light intensities of around 1 lx have already been measured in several urban freshwaters, these results are worrying. In the case of night lighting using different light colours, melatonin production was suppressed by all three light colours (blue, red and green) in both the perch and the roach compared to a control group. However, blue light had a smaller inhibiting impact in perch. In contrast, blue light in particular has a strong inhibiting effect on melatonin production in humans and many animals. These species-specific differences with regard to sensitivity to intensities and colours make it difficult to offer sweeping recommendations for action concerning the right use of light.
Reproduction only in the dark?
Light is also an important factor when it comes to reproduction. Artificial light and altered photoperiods have already been used for some time in aquaculture to delay sexual maturation in fish. In this way, fish farmers can enhance the growth of fish or stimulate reproduction outside the actual reproductive period. High light intensities are usually used to achieve this. Can, then, light pollution, i.e. light with a very low intensity, also influence reproduction?
The results of my laboratory experiments suggest that the reproductive mechanisms of fish may likewise react to very low levels of light at night. However, there appears to be a certain light-sensitive period of time in the year in which this is the case. Since laboratory experiments are usually unable to reproduce natural surroundings adequately, I additionally carried out field experiments.
These experiments demonstrated that the reproduction of perch and roach in the month of August can be influenced by the suppression of reproductive hormones using artificial light. This period corresponds to the start of the reproductive cycle, referred to as the preparatory phase. Such a phase is also known in other non-native species of fish. However, the reproduction of different species of fish takes place within different time frames, depending on their mode of life and habitat. For this reason, it is likewise difficult to establish a universally applicable regulation and recommendations for the use of light at night.
Nevertheless, it is possible to state conclusively that light pollution influences biological rhythms of fish. In this respect, the suppression of the melatonin rhythm could have many effects that have not been adequately studied so far. For example, the consequences for the immune system, growth, development and behaviour may not only affect individual fish, but also whole populations. Species-specific differences in sensitivity to artificial light at night and the associated effects on reproductive biology in fish could lead to shifts in biological niches and altered predator-prey relationships, and could therefore have an impact on species communities and entire ecosystems.
About Anika Brüning
Anika Brüning studied Biology at the Freie Universität Berlin and wrote her diploma thesis on the topic of light pollution and its effect on fish. She wrote her PhD thesis entitled “Spotlight on fish: the biological impacts of artificial light at night” at the Leibniz-Institute of Freshwater Ecology and Inland Fisheries (IGB) and successfully defended it in 2016. She is currently writing a proposal for a postdoc project at IGB.