Disruptive effect of artificial light at night on leaf litter consumption, growth and activity of freshwater shredders

Do Methodological Differences in Experiments with Stream Shredders Imply Variability in Outputs? A Microcosm Approach

Experiments are useful scientific tools for testing hypotheses by manipulating variables of interest while controlling for other factors that can bias or confuse the results and their interpretation. To ensures accuracy and reproducibility, experiments must have transparent and repeatable methodologies. Due to the importance of shredder invertebrates in organic matter processing, carbon cycling, and nutrient cycling, we tested experimentally the effect of different methodological approaches in microcosm experiments on the consumption and survival of shredders. We found that the shredder species, the presence or absence of the case, and the use or non-use of air-pumps in the microcosms did not affect shredder performance (i.e., consumption and survival). Furthermore, the type of water (stream or bottled) did not affect shredder performance. On the other hand, the amount of light had a negative effect on shredder performance, with constant light (i.e., 24 h) reducing shredder consumption and survival. Our results demonstrate that the use of different methodologies does not always result in changes in outcomes, thus ensuring comparability. However, luminosity is a critical factor that deserves attention when conducting microcosm experiments. Our findings provide valuable insights that can assist researchers in designing experiments with shredders from neotropical streams and conducting systematic reviews and meta-analyses.

Carry-over effect of artificial light at night on daytime mating activity in an ecologically important detritivore, the amphipod Gammarus pulex

Artificial light at night (ALAN) is a growing environmental problem influencing the fitness of individuals through effects on their physiology and behaviour. Research on animals has primarily focused on effects on behaviour during the night, whereas less is known about effects transferred to daytime. Here, we investigated in the lab the impact of ALAN on the mating behaviour of an ecologically important freshwater amphipod, Gammarus pulex, during both daytime and nighttime. We manipulated the presence of ALAN and the intensity of male-male competition for access to females, and found the impact of ALAN on mating activity to be stronger during daytime than during nighttime, independent of male-male competition. At night, ALAN only reduced the probability of precopula pair formation, while during the daytime, it both decreased general activity and increased the probability of pair separation after pair formation. Thus, ALAN reduced mating success in G. pulex not only directly, through effects on mating behaviour at night, but also indirectly through a carry-over effect on daytime activity and the ability to remain in precopula. These results emphasise the importance of considering delayed effects of ALAN on organisms, including daytime activities that can be more important fitness determinants than nighttime activities.

Effects of flow reduction and artificial light at night (ALAN) on litter decomposition and invertebrate communities in streams: A flume experiment

River ecosystems are heavily impacted by multiple stressors, where effects can cascade downstream of point sources. However, a spatial approach to assess the effects of multiple stressors is missing. We assessed the local and downstream effects on litter decomposition, and associated invertebrate communities of two stressors: flow reduction and artificial light at night (ALAN). We used an 18-flow-through mesocosm system consisting of two tributaries, where we applied the stressors, merging in a downstream section. We assessed the changes in decomposition rate and invertebrate community structure in leaf bags. We found no effect of ALAN or its interaction with flow reduction on the litter decomposition or the invertebrate community in the tributaries. Flow reduction alone led to a 14.8 % reduction in decomposition rate in the tributaries. We recorded no effect of flow reduction on the macroinvertebrates community composition in the litter bags. We also observed no effects of the spatial arrangement of the stressors on the litter decomposition and macroinvertebrate community structure downstream. Overall, our results suggest the impact of stressors on litter decomposition and macroinvertebrate communities remained local in our experiment. Our work thus calls for further studies to identify the mechanisms and the conditions under which spatial effects dominate over local processes.

Is part-night lighting a suitable mitigation strategy to limit Artificial Light at Night effects on the biological rhythm at the behavioral and molecular scales of the oyster Crassostrea gigas?

Artificial Light at Night (ALAN) is a fast-spreading threat to organisms, especially in coastal environments, where night lighting is increasing due to constant anthropization. Considering that ALAN affects a large diversity of coastal organisms, finding efficient solutions to limit these effects is of great importance but poorly investigated. The potential benefit of one strategy, in particular, should be studied since its use is growing: part-night lighting (PNL), which consists in switching off the lights for a few hours during nighttime. The aim of this study is to investigate the positive potential of the PNL strategy on the daily rhythm of the oyster Crassostrea gigas, a key species of coastal areas of ecological and commercial interest. Oysters were exposed to a control condition and three different ALAN modalities. A realistic PNL condition is applied, recreating a strategy of city policy in a coastal city boarding an urbanized bay (Lanton, Arcachon Bay, France). The PNL modality consists in switching off ALAN direct sources (5 lx) for 4 h (23-3 h) during which oysters are in darkness. Then, a PNL + skyglow (PNL + S) modality reproduces the previous one mimicking a skyglow (0.1 lx), an indirect ALAN source, during the direct lighting switch off, to get as close as possible to realistic conditions. Finally, the third ALAN condition mimics full-night direct lighting (FNL). Results revealed that PNL reduces some adverse effects of FNL on the behavioral daily rhythm. But, counterintuitively, PNL + S appears more harmful than FNL for some parameters of the behavioral daily rhythm. PNL + S modality is also the only one that affect oysters' clock and melatonin synthesis gene expression, suggesting physiological consequences. Thus, in realistic conditions, the PNL mitigation strategy might not be beneficial in the presence of skyglow, seeing worse for a coastal organism such as the oysters.

Light pollution of freshwater ecosystems: principles, ecological impacts and remedies

Light pollution caused by artificial light at night (ALAN) is increasingly recognized as a major driver of global environmental change. Since emissions are rapidly growing in an urbanizing world and half of the human population lives close to a freshwater shoreline, rivers and lakes are ever more exposed to light pollution worldwide. However, although light conditions are critical to aquatic species, and freshwaters are biodiversity hotspots and vital to human well-being, only a small fraction of studies conducted on ALAN focus on these ecosystems. The effects of light pollution on freshwaters are broad and concern all levels of biodiversity. Experiments have demonstrated diverse behavioural and physiological responses of species, even at low light levels. Prominent examples are skyglow effects on diel vertical migration of zooplankton and the suppression of melatonin production in fish. However, responses vary widely among taxa, suggesting consequences for species distribution patterns, potential to create novel communities across ecosystem boundaries, and cascading effects on ecosystem functioning. Understanding, predicting and alleviating the ecological impacts of light pollution on freshwaters requires a solid consideration of the physical properties of light propagating in water and a multitude of biological responses. This knowledge is urgently needed to develop innovative lighting concepts, mitigation strategies and specifically targeted measures. This article is part of the theme issue 'Light pollution in complex ecological systems'.

A novel approach for the assessment of invertebrate behavior and its use in behavioral ecotoxicology

Sublethal effects are becoming more relevant in ecotoxicological test methods due to their higher sensitivity compared to lethal endpoints and their preventive nature. Such a promising sublethal endpoint is the movement behavior of invertebrates which is associated with the direct maintenance of various ecosystem processes, hence being of special interest for ecotoxicology. Disturbed movement behavior is often related to neurotoxicity and can affect drift, mate-finding, predator avoidance, and therefore population dynamics. We show the practical implementation of the ToxmateLab, a new device that allows monitoring the movement behavior of up to 48 organisms simultaneously, for behavioral ecotoxicology. We quantified behavioral reactions of Gammarus pulex (Amphipoda, Crustacea) after exposure to two pesticides (dichlorvos and methiocarb) and two pharmaceuticals (diazepam and ibuprofen) at sublethal, environmentally relevant concentrations. We simulated a short-term pulse contamination event that lasted 90 min. Within this short test period, we successfully identified behavioral patterns that were most pronounced upon exposure to the two pesticides: Methiocarb initially triggered hyperactivity, after which baseline behavior was restored. On the other hand, dichlorvos induced hypoactivity starting at a moderate concentration of 5 μg/L - a pattern we also found at the highest concentration of ibuprofen (10 μg/L). An additional acetylcholine esterase inhibition assay revealed no significant impact of the enzyme activity that would explain the altered movement behavior. This suggests that in environmentally realistic scenarios chemicals can induce stress - apart from mode-of-action - that affects non-target organisms' behavior. Overall, our study proves the practical applicability of empirical behavioral ecotoxicological approaches and thus represents a next step towards routine practical use.

Controlled and driving mechanism of the SPM variation of shallow Brackish Lakes in arid regions

Suspended particulate matter (SPM) in the brackish Ebinur Lake of arid northwest China profoundly affect its water quality and watershed habitat quality. However, the actual driving mechanisms of the Lake's SPM changes remain unclear. Therefore, the purpose of this study is to explore the controlling factors driving the variability of SPM in the Ebinur Lake. This study constructed month-by-month SPM maps of Ebinur Lake based on time-series remote-sensing imageries and SPM inversion model. Thirty-four factors that might influence SPM changes were extracted, and the Partial Least Squares Structural Equation Modeling (PLS-SEM), suitable for complex relationships and factor interactions, was applied to identify the relative influence of each factor quantitatively. The results showed: (1) a clear increasing trend of SPM concentration in Ebinur Lake from 2011 to 2020; (2) that SPM changes were influenced by external and internal factors, explaining 48.2 % and 46.9 % of the changes, respectively; (3) that, to the external factors, meteorological factors exerted the greatest influence on SPM (relative contribution of 38.9 %); that, to the internal factors, water salinity imposed the greatest influence on SPM (relative contribution of 43.3 %); (4) that, among the meteorological factors, the measured variable Alashankou wind speed expressed the most significant positive effect on SPM (weighting coefficient of 0.894), and sulfate generated the strongest positive effect on SPM (weighting coefficient of 0.791) among the water salinity factors. Hence, the quantitative identification of drivers of SPM changes in Ebinur Lake could provide a new perspective to investigate the driving mechanisms of lake water quality in arid areas and inform their sustainable restoration and management.

Artificial light at night (ALAN) affects behaviour, but does not change oxidative status in freshwater shredders

Artificial light at night (ALAN) alters circadian rhythms in animals and therefore can be a source of environmental stress affecting their physiology and behaviour. The impact of ALAN can be related to the increased light level, but also to the spectral composition of night lighting. Previous research showed that many species can be particularly sensitive to the LED light, but it is unclear if they respond to its broad spectrum or specifically to the blue light wavelength. In this study, we tested whether dim ALAN (2 lx) differing in the spectral quality (warm white LED, blue LED, high-pressure sodium HPS light) modifies behaviour and changes oxidative status in two nocturnal freshwater shredder species: Dikerogammarus villosus and Gammarus jazdzewskii (Gammaroidea, Amphipoda). Our experiment revealed that ALAN, irrespective of its spectral quality, did not affect the oxidative stress markers in cells (the level of reactive oxygen species and lipid peroxidation). However, ALAN changed the gammarid behaviour in a species-specific manner, which can potentially reduce the fitness of the shredders. Dikerogammarus villosus avoided all types of light compared to darkness. Therefore, confined to the shelter, D. villosus may have fewer opportunities to forage and/or mate. Gammarus jazdzewskii was sensitive only to the narrow-spectrum blue light, but did not respond to the HPS and white LED light. Avoidance is a typical response of gammarids to natural light, thus the disruption of this behaviour in the presence of common ALAN sources can increase the predation risk in this species. To summarize, behavioural modifications induced by ALAN seem more pronounced than changes in physiology and can constitute the main driver of disturbances in the processing of organic matter in freshwater ecosystems by invertebrate shredders.

On the importance of concomitant conditions: Light and conspecific presence modulate prey response to predation cue

Assessment of individual costs of the anti-predator defence translating into changes in population parameters is meagre. This is because prey responses are likely to be modulated by additional factors, commonly present in the environment, but often neglected in experimental studies. To evaluate the effect of external factors on prey behaviour and physiology, we exposed amphipods Dikerogammarus villosus and Gammarus jazdzewskii to the predation cue of Perca fluviatilis in different densities and light conditions. Singletons of both species exposed to the predation cue in light modified their oxygen consumption (D. villosus: reduction, G. jadzewskii: increase) compared to their respiration in predator-free conditions. However , in the presence of conspecifics or in darkness, their respiration became insensitive to the predation cue. On the other hand, the swimming activity of prey was reduced in the presence of the predation cue irrespective of prey density and light conditions, but singletons were consistently more active than groups. Thus, external factors, such as conspecifics and darkness, constantly or periodically occurring in the field, may reduce the costs of predator non-consumptive effects compared to the costs measured under laboratory conditions (in light or absence of conspecifics). Moreover, we showed that behavioural and physiological parameters of prey may change differently in response to predation risk. Thus, conclusions drawn on the basis of single defence mechanisms and/or results obtained in artificial conditions, not reflecting the environmental complexity, strongly depend on the experimental design and endpoint selection and therefore should be treated with care.