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Abstract Harvesting is a direct and simple artificial method to regulate submerged macrophyte communities. However, there is still a gap in whether the succession of different seasonal submerged macrophytes can be achieved by harvesting. The morphological, physiological index, competition intensity of Hydrilla verticillata (H. verticillata) and underwater light under different harvesting intensities of Potamogeton crispus (P. crispus) was studied by a controlled experiment. Under moderate harvesting intensity (harvest 30% and 45% of plant height) of P. crispus, H. verticillata was the least inhibited, resulting in accumulating more Chl a and higher plant height. The underwater light created by moderate-intensity harvesting was favorable for H. verticillata to compete with P. crispus. Thus, moderate-intensity harvesting of P. crispus can promote the growth of H. verticillata. In practice, moderate harvesting intensity of P. crispus can be used to create conditions for the growth of summer species to achieve seasonal succession of submerged macrophytes and maintain the clear-water state of the lake.

At high latitudes, polar night is a prolonged period of seasonal darkness with the sun remaining below the horizon throughout the diel cycle for up to 177 days at the North Pole. Along with diffuse atmospheric light from the sun and the moon, bioluminescence is an in‐water light source that can facilitate ecological interactions in an otherwise dim‐light environment. At high latitudes during polar night, bioluminescence rather than sunlight represents a significant portion of the photons available in the pelagic. We investigated depths of transition zones (called the bioluminescence transition depth) in the pelagic light field during polar night, defining the transition of a light field dominated by atmospheric irradiance, to one dominated by bioluminescent point sources. We derived relationships between values of the transition depth, bioluminescence potential, surface irradiance due to atmospheric light and the diffuse attenuation coefficient. We conducted studies for two Svalbard, Norway fjords, as well as for offshore areas located in the shelf‐break, shelf‐slope and in the Arctic basin. Based on our results for two polar nights, the transition from underwater light dominated by atmospheric sources to that dominated by bioluminescence occurs between 10 and 40 m in two Svalbard fjords, and between 18 and 60 m for offshore areas. These transition depths may be of particular importance to understanding how bioluminescence structures planktonic communities both in polar regions and at lower latitudes. Plain Language Summary: The polar night is a period of continuous winter darkness north of ∼72.5°N latitude, and this period presents challenging light conditions for Arctic pelagic organisms. With the sun remaining below the horizon from one day at the Arctic Circle to 6 months at the North Pole, prolonged darkness limits light‐mediated predator prey interactions in the plankton. Bioluminescence is light produced by a photochemical reaction in organisms, and it is an in‐water light source that can facilitate ecological interactions in an otherwise dim‐light environment. We investigated depths of transition zones from a light field dominated by atmospheric irradiance, to one dominated by bioluminescent point sources, across a gradient from Svalbard fjords to the Arctic basin. Based on our results, the transition from underwater light dominated by atmospheric sources to that dominated by bioluminescence occurs between 10 and 40 m in two Svalbard fjords, and between 18 and 60 m for offshore areas. The light gradient occurring in these transition zones has ecological implications, including depth selection and predator‐prey interactions. This work provides another step in the difficult task of untangling the complex relationships among marine organisms and natural light. Key Points: We investigated depths of transition zones from one dominated by atmospheric irradiance to one dominated by bioluminescent sourcesWe derived relationships between transition depth, bioluminescence potential, surface irradiance and diffuse attenuation coefficientAt high latitudes during polar night, bioluminescence represents a significant portion of the photons available in the pelagic [ABSTRACT FROM AUTHOR]

Algal communities act as sensitive indicators of past and present climate effects on northern lakes, but their responses can vary considerably between ecosystems. Functional trait-based approaches may help us better understand the nature of the diverse biotic responses and their underlying ecosystem changes. We explored patterns in diatom (Bacillariophyceae) growth forms and species composition during the Neoglacial in two shallow lakes typical of subarctic regions, including a dark-colored woodland lake and a clear tundra lake. Sediment carbon and nitrogen elemental and isotope biogeochemistry and spectral indices were used to track broadscale changes in lake productivity, the inflow of organic carbon from land, and benthic substratum over the past three millennia. The biogeochemical indices tracked declines in land-lake connectivity as well as lake-water and sediment organic enrichment above and below the subarctic treeline driven by Neoglacial cooling. This broadscale environmental transition was intercepted by periods of elevated primary production associated with transient Neoglacial warm anomalies and, in particular, the twentieth century warming. Although the Neoglacial development of the lakes showed conspicuous similarities, diatom functional and taxonomic responses were not uniform between the lakes pointing to intrinsic differences in the development of benthic habitats and underwater-light regimes. Many of the observed biotic shifts aligned with expectations based on earlier research linking diatom functional traits to changing light and organic levels but the results also point to further research needs, particularly to better differentiate the individual and interactive effects of substratum and light. Despite distinct anthropogenic imprints in the biogeochemical record, the scale of human impact on the lakes' biota has not, as yet, been profound, but the changes are nonetheless clear when compared to the previous three millennia of natural lake development. [ABSTRACT FROM AUTHOR]

Source separation and decentralized domestic wastewater treatment represent effective strategies to enhance sewage treatment performance and facilitate water reuse economically. The Living Machine (LM) system has gained widespread adoption for decentralized sewage treatment. While underwater light source has been demonstrated to enhance the treatment performance of open aerobic reactors in LM systems, its influence on the treatment efficiency of a fully multistage LM system remains underreported. In this study, an underwater lamp-added LM system (ULLM) with eight reactors was constructed and investigated. The introduction of underwater light source obviously improved the removal capacity of chemical oxygen demand (COD) and NH 4 +-N, which was 96.1% and 61.6%, respectively. The diversity of algae, zooplankton, and aquatic animals was notably higher in the light-treated reactors than in the control group (CK) without underwater light source, and substantial alteration in the microbial community of the light-treated reactors was observed compared with CK reactors. At the phylum level, Proteobacteria and Nitrospirae enriched in the underwater light-treated reactors, while Bacteroidetes and Actinobacteria exhibited a decrease after light exposure. At the genus level, Nitrospira and Rhodanobacter were enriched in the ULLM system. Importantly, the prevalence of these two dominant genera was sustained until the final operational stage, indicating their potential key roles in enhancing wastewater treatment performance. The addition of underwater light source proves to be an effective strategy for augmenting the treatment efficiency of the multistage living machine systems, resulting in substantial improvements in pollutant removal. These findings contribute valuable insights into optimizing LM systems for decentralized wastewater treatment. • Underwater lamps were applied in a multistage living machine. • The removal rate of NH 4 +-N increased significantly. • TP removal performance was far more stable. • Bacterial and algal species composition were significantly changed after light exposure. • Prevalence of Nitrospira and Rhodanobacter sustained until the end stage in light treated system. [ABSTRACT FROM AUTHOR]

In ice-covered polar lakes, a narrow ice-free moat opens up in spring or early summer, and then persists at the edge of the lake until complete ice loss or refreezing. In this study, we analyzed the horizontal gradients in Ward Hunt Lake, located in the Canadian High Arctic, and addressed the hypothesis that the transition from its nearshore open-water moat to offshore ice-covered waters is marked by discontinuous shifts in limnological properties. Consistent with this hypothesis, we observed an abrupt increase in below-ice concentrations of chlorophyll a beyond the ice margin, along with a sharp decrease in temperature and light availability and pronounced changes in benthic algal pigments and fatty acids. There were higher concentrations of rotifers and lower concentrations of viruses at the ice-free sampling sites, and contrasts in zooplankton fatty acid profiles that implied a greater importance of benthic phototrophs in their inshore diet. The observed patterns underscore the structuring role of ice cover in polar lakes. These ecosystems do not conform to the traditional definitions of littoral versus pelagic zones but instead may have distinct moat, ice-margin, and ice-covered zones. This zonation is likely to weaken with ongoing climate change.

Underwater light is spatially as well as temporally variable and directly affects phytoplankton growth and competition. Here we systematically (following the guidelines of PRISMA‐EcoEvo) searched and screened the published literature resulting in 640 individual articles. We mapped the conducted research for the objectives of (1) phytoplankton fundamental responses to light, (2) effects of light on the competition between phytoplankton species, and (3) effects of climate‐change‐induced changes in the light availability in aquatic ecosystems. Among the fundamental responses of phytoplankton to light, the effects of light intensity (quantity, as measure of total photon or energy flux) were investigated in most identified studies. The effects of the light spectrum (quality) that via species‐specific light absorbance result in direct consequences on species competition emerged more recently. Complexity in competition arises due to variability and fluctuations in light which effects are sparsely investigated on community level. Predictions regarding future climate change scenarios included changes in in stratification and mixing, lake and coastal ocean darkening, UV radiation, ice melting as well as light pollution which affect the underwater light‐climate. Generalization of consequences is difficult due to a high variability, interactions of consequences as well as a lack in sustained timeseries and holistic approaches. Nevertheless, our systematic literature map, and the identified articles within, provide a comprehensive overview and shall guide prospective research. [ABSTRACT FROM AUTHOR]

Recent studies have indicated changes in the light climate of the North Sea. An overall reduction of water clarity over the twentieth century could be observed in measurements, and more recent analysis suggests that these trends continue. Inorganic sediment is often named one of the driving factors in these changes and it has been shown to locally increase. With 20 years' worth of satellite‐derived sediment data, we were able to conduct basin wide investigations of the temporal dynamics of the biogeochemical state of the North Sea. To identify the impact of inter‐annual and seasonal changes in sediment, we fed from two different remote sensing sources (GlobColour & IFREMER) into a 3D coupled hydrodynamic and biological model. The light scheme in the Carbon Silicon Nitrogen Ecosystem model (CoSiNE) was modified to account for sediment specific attenuation. We performed a total of five numerical experiments for the period of 2000–2017. The main two experiments were conducted using monthly averaged data. Additionally, as controls, one experiment with annually averaged and one with a 20 years average of sediment, as well as a fifth one without sediment were performed. Our model showed a clear relation between changes in sediment and water clarity. Phytoplankton biomass was reduced only in areas with high nutrient availability. Plain Language Summary: Studies using satellite or field data have shown that the North Sea has undergone decreases in water clarity and increases in sediment content, while simultaneously showing decreases in phytoplankton biomass over the past century. Declining water clarity would imply inhibited photosynthesis and phytoplankton growth. Phytoplankton itself reduces water clarity. Therefore, the coupling between the three quantities is complex. In this study, we used satellite‐derived sediment data of the years 2000–2017 from two different sources in a three‐dimensional model to account for sediment specific effects on underwater light. This way, we were able to determine how changes in sediment content affect water clarity and phytoplankton biomass. The two data sets show different long‐term behavior, with one showing increases and the other decreases. Changes in sediment directly corresponded to trends in water clarity. Long‐term changes in the light climate led to changes in biomass in areas of high nutrient availability. Key Points: Satellite‐derived sediment concentration data is fed into a model to study long‐term trends in water clarity over the period of 2000–2017Changes in sediment content correspond to changes in water clarity in the modelPhytoplankton biomass changes do not directly correspond to changes in water clarity in the model [ABSTRACT FROM AUTHOR]

The underwater light field in blackwater environments is strongly skewed toward the red end of the electromagnetic spectrum due to blue light absorption by colored dissolved organic matter ( CDOM). Exposure of phytoplankton to full spectrum irradiance occurs only when cells are mixed up to the surface. We studied the potential effects of mixing-induced changes in spectral irradiance on photoacclimation, primary productivity and growth in cultures of the cryptophyte Rhodomonas salina and the diatom Skeletonema costatum. We found that these taxa have very different photoacclimation strategies. While S. costatum showed classical complementary chromatic adaption, R. salina showed inverse chromatic adaptation, a strategy previously unknown in the cryptophytes. Transfer of R. salina to periodic full spectrum light ( PFSL) significantly enhanced growth rate (μ) by 1.8 times and primary productivity from 0.88 to 1.35 mg C · (mg Chl−1) · h−1. Overall, R. salina was less dependent on PFSL than was S. costatum, showing higher μ and net primary productivity rates. In the high- CDOM simulation, carbon metabolism of the diatom was impaired, leading to suppression of growth rate, short-term 14C uptake and net primary production. Upon transfer to PFSL, μ of the diatom increased by up to 3-fold and carbon fixation from 2.4 to 6.0 mg C · (mg Chl−1) · h−1. Thus, a lack of PFSL differentially impairs primarily CO2-fixation and/or carbon metabolism, which, in turn, may determine which phytoplankton dominate the community in blackwater habitats and may therefore influence the structure and function of these ecosystems. [ABSTRACT FROM AUTHOR]

When dealing with invasive fishes, permanent barriers may be best in preventing spread; however, they may not be feasible due to various costs and/or logistical constraints. Alternatively, various non-permanent barriers using electricity, light, sound, pressure, bubbles, and CO are being developed and deployed in efforts to limit the spread of aquatic invasive species or to achieve fish guidance and conservation. However, the effectiveness of these barriers is quite variable, and testing is often lacking for both target and non-target species. We conducted a series of laboratory trials to examine the effects of strobe light on behaviour of Common Carp, Brown Bullhead, and Largemouth Bass. In response to strobe lights, Common Carp and Brown Bullhead stayed significantly farther away compared to the control period and resumed their normal activity once the strobe light was turned off. This suggests that strobe lights may prove to be a useful fish deterrent in the field. Our results also highlight the importance of examining the response of both target and non-target species when evaluating fish deterrent technologies. [ABSTRACT FROM AUTHOR]

Light determines macrophyte distribution, community composition and biomass in shallow lakes. Therefore, it is vital to determine the critical underwater light climate thresholds for macrophyte degradation and recovery. In this study, we first proposed a novel index, defined as the ratio of euphotic depth ( Z eu ) to water depth (WD), as a measure of the underwater light supply for macrophytes. The underwater light environment in Lake Taihu (a large, shallow, eutrophic lake) was then characterized based on this index ( Z eu /WD) using field measurements collected from 2006 to 2013 (8 years × 4 seasons × 32 sites). The distribution of the macrophyte presence frequency (MPF, the number of investigations that identified macrophytes divided by the total number of investigations) was greater than 0.70 in Xukou Bay and East Lake Taihu over the 32 investigations, followed by the other sites distributed in East Lake Taihu. The proportion of macrophyte coverage increased with the increase in Z eu /WD. A significant relationship was observed between Z eu /WD and MPF for the 19 sites with macrophytes ( r 2 = 0.48, p < 0.001, n = 19). In the region with high nutrient concentrations and serious water pollution, better underwater light conditions are required for the growth of macrophytes. A Z eu /WD value of 0.80 can be regarded as the critical underwater light threshold for the growth of macrophytes in Lake Taihu. The region with Z eu /WD ranging between 0.57 and 0.80 was usually covered by sparse macrophytes; this region should be vital for macrophyte recovery and environmental management in Lake Taihu. The distribution of Z eu /WD was further obtained using MODIS satellite-derived Z eu from June to October in 2003 and 2013. Xukou Bay and Guangfu Bay in the southern part of Lake Taihu could be regarded as potentially crucial regions for the recovery of macrophytes from the perspective of underwater light and nutrient levels. [ABSTRACT FROM AUTHOR]