Mechanistic knowledge on drivers of benthic community traits is key to predict the effects of global change, particularly for the Arctic, where climate change is paramount while its causalities on biota have been scarcely studied. Manipulative field experiments were conducted in two intertidal sedimentary sites (Thiisbukta and Longyearbyen) on Svalbard (Norway) from May to August 2017. At both sites, a complete randomized block design was used to assess the effects of detached seaweed presence on species composition, and biomass of soft-bottom communities. Species composition was significantly distinct between both sites, with the communities of Thiisbukta being more divers, more populated, and more productive in terms of biomass. Here, the presence of detached seaweeds had no effect on any of the tested community responses. In Longyearbyen, however, the number of individuals and biomass were both significantly lower in plots covered by detached seaweeds than in un-manipulated controls. We conclude that detached seaweeds may negatively affect the structure and productivity of Arctic soft-bottom communities, but this response is strongly site-dependent as the more diverse community appeared resistant to the experimental manipulations.