Effects of artificial light at night on the feeding behaviour of three marine benthic grazers from the Adriatic Sea are species-specific and mostly short-lived

Role of benthic fauna in the decomposition of Sargassum fusiforme litter

To explore the contribution of benthic fauna to mass loss during the decomposition of Sargassum fusiforme litter and their role in influencing chemical composition changes, a 48-day field decomposition experiment was conducted on S. fusiforme litter collected from Gouqi Island, East China Sea. Three types of mesh bags with different mesh size (2, 0.5, and 0.01 mm) were used to analyze the mass loss and chemical composition changes of the litter and their relationship with benthic fauna communities. The results indicated that benthic fauna contributed 14.83%-16.69% of the mass loss of S. fusiforme litter. Furthermore, benthic fauna facilitated the reduction of biochemical substances such as carbon, phosphorus, cellulose, and hemicellulose in the litter. A significant negative correlation was found between the number of taxonomic classes and the carbon and phosphorus content (p < 0.05), while the abundance of benthic fauna showed a significant negative correlation with carbon, phosphorus, lignin, cellulose, and hemicellulose content (p < 0.05). The number of taxonomic classes and abundance in the 2 mm mesh bags were also greater than those in the 0.5 mm bags. In conclusion, benthic fauna played a promotive role in the decomposition process of S. fusiforme litter, providing valuable insights into the natural decomposition mechanisms of large seaweed.

Artificial light at night reduces predation and herbivory rates in a nearshore reef

Artificial light at night (ALAN) is an escalating anthropogenic stressor that can affect ecological communities over a range of spatial scales by altering key ecological processes, such as predation and herbivory. Shallow subtidal reefs are highly diverse and productive habitats that are vulnerable to ALAN. We investigated rates of consumption by fish (predation and herbivory) under different light treatments (ALAN, dark and daylight conditions) using standardised bioassay methods, i.e. squidpops and Ulva pops in situ. We also used GoPros to record predator identity, number of strikes and time to strike in ALAN and daylight treatments. Contrary to previous studies, we found that predation and herbivory rates were significantly lower in ALAN treatments than in daytime and dark treatments. The highest predation and herbivory rates were observed in daytime treatments. The identity of predator species, time to strike and number of strikes also differed between daytime and ALAN treatments. Due to low light conditions, dark treatments were not filmed. Our findings suggest that ALAN can alter predation in unexpected ways, depending on the environmental conditions and species affected. Future coastal management strategies need to account for light pollution as a major stressor to preserve valuable ecological resources.

Artificial Light Increases Nighttime Prevalence of Predatory Fishes, Altering Community Composition on Coral Reefs

Artificial light at night (ALAN) is an anthropogenic pollutant that is intensifying and expanding in marine environments, but experimental studies of community-level effects are generally lacking. The inshore, shallow, and clear-water locations of coral reefs and their diverse photosensitive inhabitants make these ecosystems highly susceptible to biological disturbances; at the same time, their biodiversity and accessibility make them model systems for wider insight. Here, we experimentally manipulated ALAN using underwater LED lights on a Polynesian reef system to investigate the influence on localised nighttime fish communities compared to control sites without ALAN. We collected infrared video censuses of baseline communities prior to manipulation, which we repeated following short-term (mean of three nights) and prolonged (mean of 25 nights) exposures to ALAN. Short-term ALAN exposure did not induce any significant alterations to the nighttime fish community, but prolonged ALAN exposure increased nighttime species richness. Species compositions exposed to prolonged ALAN were more dissimilar from their baseline compared to control sites. The difference between community compositions at prolonged ALAN exposure and control sites was not apparent at the family level; instead, it was observed from the composition of trait guilds. Following prolonged ALAN exposure, more diurnal and nocturnal predatory species (piscivores, invertivores, and planktivores)-particularly those that are site-attached or mobile within reefs-were present in nighttime assemblages. Our experimental findings show that coastal ALAN could cause trophic imbalances and circadian disturbances in localised nighttime reef fish communities. Given that community-wide consequences were only apparent after prolonged ALAN exposure suggests that management of the duration of artificial lighting could potentially be used to reduce impacts on marine ecosystems.

Antagonistic Effects of Light Pollution and Warming on Habitat-Forming Seaweeds

Artificial Light at Night (ALAN) is an emerging global stressor that is likely to interact with other stressors such as warming, affecting habitat-forming species and ecological functions. Seaweeds are dominant habitat-forming species in temperate marine ecosystems, where they support primary productivity and diverse ecological communities. Warming is a major stressor affecting seaweed forests, but effects of ALAN on seaweeds are largely unknown. We manipulated ALAN (0 lx vs. 25 lx at night) and temperature (ambient vs. +1.54°C warming) to test their independent and interactive effects on the survival, growth (biomass, total-, blade- and stipe-length) and function (photosynthesis, primary productivity and respiration) on the juveniles of two habitat-forming seaweeds, the kelp Ecklonia radiata and the fucoid Sargassum sp. Warming significantly increased Ecklonia mortality; however, ALAN did not affect mortality. ALAN had positive effects on Ecklonia biomass, total and blade growth rates and gross primary productivity; however, warming largely counterbalanced these effects. We found no significant effects of warming or ALAN on Ecklonia photosynthetic yield, stipe length, net primary productivity or respiration rates. We found no effects of ALAN or warming on Sargassum for any of the measured variables. Synthesis. Our findings indicate that ALAN can have positive effects on seaweed growth and functioning, but such effects are likely species-specific and can be counterbalanced by warming, suggesting an antagonistic interaction between these global stressors. These findings can help us to predict and manage the effects of these stressors on seaweeds, which underpin coastal biodiversity.

Artificial light at night alters the locomotor behavior of the Mediterranean sea urchin Paracentrotus lividus

Artificial light at night (ALAN) is a recognized source of anthropogenic disturbance, although its effects on biological systems have not been fully explored. Within marine ecosystems, coastal areas are the most impacted by ALAN. Here, we focused on the Mediterranean sea urchin Paracentrotus lividus, which has a crucial role in shaping benthic ecosystems. Our objective was to investigate if ALAN affects the nocturnal locomotor behavior of P. lividus. A semi-controlled field study was conducted along a rocky shore near a promenade lit at night. Results suggested a potential impact of ALAN on the locomotor behavior of sea urchins. Individuals of P. lividus tended to move away from the light sources while its directions in dark conditions were uniform. Their locomotor performance, in presence of ALAN, was characterized by shorter latency time, lower sinuosity and higher mean speed at increasing light intensity, with potential cascading effect at the ecosystem level.

Artificial light at night and warming impact grazing rates and gonad index of the sea urchin Centrostephanus rodgersii

Artificial light at night (ALAN) is a growing threat to coastal habitats, and is likely to exacerbate the impacts of other stressors. Kelp forests are dominant habitats on temperate reefs but are declining due to ocean warming and overgrazing. We tested the independent and interactive effects of ALAN (dark versus ALAN) and warming (ambient versus warm) on grazing rates and gonad index of the sea urchin Centrostephanus rodgersii. Within these treatments, urchins were fed either 'fresh' kelp or 'treated' kelp. Treated kelp (Ecklonia radiata) was exposed to the same light and temperature combinations as urchins. We assessed photosynthetic yield, carbon and nitrogen content and C : N ratio of treated kelp to help identify potential drivers behind any effects on urchins. Grazing increased with warming and ALAN for urchins fed fresh kelp, and increased with warming for urchins fed treated kelp. Gonad index was higher in ALAN/ambient and dark/warm treatments compared to dark/ambient treatments for urchins fed fresh kelp. Kelp carbon content was higher in ALAN/ambient treatments than ALAN/warm treatments at one time point. This indicates ocean warming and ALAN may increase urchin grazing pressure on rocky reefs, an important finding for management strategies.

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.

The impacts of artificial light at night on the ecology of temperate and tropical reefs

Despite 22% of the world's coastal regions experiencing some degree of light pollution, and biologically important artificial light at night (ALAN) reaching large portions of the seafloor (greater than 75%) near coastal developments, the impacts of ALAN on temperate and tropical reefs are still relatively unknown. Because many reef species have evolved in response to low-light nocturnal environments, consistent daily, lunar, and seasonal light cycles, and distinct light spectra, these impacts are likely to be profound. Recent studies have found ALAN can decrease reproductive success of fishes, alter predation rates of invertebrates and fishes, and impact the physiology and biochemistry of reef-building corals. In this paper, we integrate knowledge of the role of natural light in temperate and tropical reefs with a synthesis of the current literature on the impacts of ALAN on reef organisms to explore potential changes at the system level in reef communities exposed to ALAN. Specifically, we identify the direct impacts of ALAN on individual organisms and flow on effects for reef communities, and present potential scenarios where ALAN could significantly alter system-level dynamics, possibly even creating novel ecosystems. Lastly, we highlight large knowledge gaps in our understanding of the overall impact of ALAN on reef systems. This article is part of the theme issue 'Light pollution in complex ecological systems'.