Abstract

In recent decades, the use of artificial nocturnal illumination has rapidly increased worldwide, imposing an increase of nocturnal light levels and a disruption of natural cycles of light and dark that have been stable over geological and evolutionary time scales. This wide-spread alteration of the natural light regime by artificial light at night (ALAN) is contributing to global environmental change and raises concerns about the potentially adverse effects on organisms and processes in illuminated ecosystems. Simultaneously, a global shift in outdoor lighting technologies from yellow high-pressure sodium (HPS) to white light-emitting diode (LED) light is taking place, changing the spectral composition of nocturnal illumination. Mounting evidence suggests that ALAN affects microorganisms, plants and animals in both aquatic and terrestrial ecosystems. Light is a major source of energy and an important environmental cue for primary producers that influences and to a large extent drives their growth, production and community structure. Freshwaters are increasingly illuminated at night, as they are often located near the human population centers. Despite this, the impacts of artificial nocturnal illumination in freshwater ecosystems are still largely unknown. In particular, effects on aquatic primary producers in urban and sub-urban rivers and streams have hardly been addressed. This thesis aimed to investigate effects of artificial nocturnal illumination on biomass and community composition of communities of benthic primary producers in freshwaters, the periphyton. The presented work is based on manipulative field studies performed in two contrasting freshwater systems whose periphyton communities are characterized by different species. The first study was performed in a stream-side flume system on a sub-alpine stream and the second in a lowland agricultural ditch. I found that two to 13 weeks of exposure to LED light at night decreased the biomass of periphyton in both aquatic systems. In stream periphy-ton, the decrease in biomass was observed for periphyton in early developmental stages (up to three weeks), but not that in the later developmental stages (four to six weeks). The effects of LED on community composition were found only in stream periphyton, where it increased the proportion of the dominant autotroph group, the diatoms and decreased the proportion of cya-nobacteria in early developmental stages, but indicated a decreased proportion of diatoms and an increased proportion of cyanobacteria in the later developmental stages. I found that LED light at night altered pigment composition and quantitative taxonomic composition in stream periphyton in later developmental stages and that several diatom and chrysophyte taxa, both autotrophic and heterotrophic, responded to ALAN by increasing or decreasing in abundance in a taxon-specific manner. LED did not affect periphyton community composition in lowland agricultural ditch, likely because periphyton was composed of different species. All effects of LED light were different between the seasons presumably due to seasonal differences in community composition and environmental variables. I did not find any evidence that HPS-light affects either biomass or community composition of periphyton. Differential effects of the two light sources are likely a result of differences in their spectral composition, in particu-lar the high proportion of blue light emitted by LED but not by HPS. This thesis provides, for the first time, evidence that LED light at night can profoundly affect benthic primary producers and periphyton communities in freshwater systems by reduc-ing their biomass and altering community composition. Systems dominated by periphyton in its early developmental stages, such as streams prone to physical disturbances, are likely to be more sensitive to ALAN compared to systems with stable flow conditions based on the results presented. Periphyton plays a fundamental role in productivity, nutrient and carbon cycling and food supply for higher trophic levels in small, clear waters; its position in the base of aquatic ecosystems suggests that the alterations induced by ALAN may have important con-sequences for ecosystem functions. This should be considered when developing lighting strat-egies for areas close to freshwaters in order to mitigate potentially adverse effects of nocturnal artificial illumination on aquatic ecosystems.