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In the Arctic, rising seawater temperatures and increasing underwater light caused by reductions in sea ice cover are expected to change the structure of arctic marine communities. Substantial, sometimes sudden, increases in macroalgal productivity and biomass have already been observed in arctic rocky bottom communities. These macroalgal responses have been attributed to increasing temperature and light, but the relative importance of the suggested drivers of change has not yet been assessed. In this study, we used a mechanistic competition model to unravel the effects of temperature and light on benthic community structure and algae dominance, focusing on key algae species: red calcareous algae and macroalgal fronds. We find that light is the primary driver of increases in macroalgal coverage, whereas increased seawater temperature plays a secondary role. Shifts leading to macroalgae dominated communities may be mediated by competitive interactions, and are likely due to three light-related processes: earlier sea ice break-out at high latitudes can result in an exponential increase in the cumulative amount of light that enters the water column during a year; threshold effect in light requirements for algal growth; and light requirements of calcareous algae being substantially lower than those of macroalgae. With continued warming, our modeling results suggest that reduced sea ice coverage and increased light availability will favor dominance of macroalgae, which due to their key ecological role are expected to alter the structure and functioning of arctic rocky bottom ecosystems. The Arctic is currently warming at more than twice the average rate compared to lower latitudes (Overland et al. 2014). Rising sea surface temperatures are accompanied by rapidly decreasing sea ice cover (Comiso et al. 2008; Stroeve et al. 2011), allowing more light to enter the aquatic realm (Varpe et al. 2015). Changes in temperature and light conditions induce large alterations in species’ composition and abundances in the Arctic (Wassmann et al. 2011; Fossheim et al. 2015), and are predicted to promote dominance of macroalgae in shallow benthic ecosystems (Krause-Jensen et al. 2012; Clark et al. 2013; Krause-Jensen and Duarte 2014; Olesen et al. 2015). Supporting these predictions, 1.6 to 8-fold increases in biomass and coverage of erect macroalgae (Weslawski et al. 2010; Kortsch et al. 2012; Bartsch et al. 2016), and climate-driven increases in macroalgal growth (Marbà et al. 2017), have been documented in the Arctic during the last decades. Such changes have the potential to alter energetic pathways and reshape the habitat of other benthic organisms (Jones et al. 1994; Bruno and Bertness 2001). In the Arctic, growth and persistence of benthic macroalgae is constrained by low water temperatures (< 5 C) and the lowlight regime, as well as by mechanical ice scouring in shallow waters (Peck et al. 2009; Gómez et al. 2011; Krause-Jensen et al. 2012; Olesen et al. 2015; Bartsch et al. 2016; Marbà et al. 2017). Changing dominance patterns of macroalgae along depth transects in the Arctic (Teichert 2013) and high seaweed mortality in annually ice-covered communities (Clark et al. 2015) especially emphasize the importance of light on benthic algae composition. However, possibly because of the strong correlation between water temperature, sea ice and underwater light, the relative importance of the climatic pressures (temperature and light) suggested to be responsible for *Correspondence: [email protected] Present address: Institut de Ciència i Tecnologia Ambientals, Universitat Autònoma de Barcelona, Cerdanyola del Vallès, Spain This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. Additional Supporting Information may be found in the online version of this article.