ECOSYSTEM FUNCTIONING IMPACTS OF THE INVASIVE SEAWEED SARGASSUM MUTICUM (FUCALES, PHAEOPHYCEAE) 1 Fátima Vaz-Pinto 2 Laboratory of Coastal Biodiversity, CIIMAR/CIMAR – Centro Interdisciplinar de Investigac ß~ ao Marinha e Ambiental, Universidade do Porto, Porto 4050-123, Portugal ICBAS – Instituto de Ci^ encias Biom edicas Abel Salazar, Universidade do Porto, Porto 4099-003, Portugal Celia Olabarria Departamento de Ecolog  ıa y Biolog  ıa Animal, Facultad de Ciencias del Mar, Universidad de Vigo, Vigo 36310, Spain and Francisco Arenas Laboratory of Coastal Biodiversity, CIIMAR/CIMAR – Centro Interdisciplinar de Investigac ß~ ao Marinha e Ambiental, Universidade do Porto, Porto 4050-123, Portugal Ongoing changes in natural diversity due to anthropogenic activities can alter ecosystem functioning. Particular attention has been given to research on biodiversity loss and how those changes can affect the functioning of ecosystems, and, by extension, human welfare. Few studies, however, have addressed how increased diversity due to establishment of nonindigenous species (NIS) may affect ecosystem function in the recipient communities. Marine algae have a highly important role in sustaining nearshore marine ecosystems and are considered a significant component of marine bioinvasions. Here, we examined the patterns of respiration and light-use efficiency across macroalgal assemblages with different levels of species richness and evenness. Additionally, we compared our results between native and invaded macroalgal assemblages, using the invasive brown macroalga Sargassum muticum (Yendo) Fensholt as a model species. Results showed that the presence of the invader increased the rates of respiration and production, most likely as a result of the high biomass of the invader. This effect disappeared when S. muticum lost most of its biomass after senescence. Moreover, predictability–diversity relationships of macroalgal assemblages varied between native and invaded assemblages. Hence, the introduction of high-impact invasive species may trigger major changes in ecosystem functioning. The impact of S. muticum may be related to its greater biomass in the invaded assemblages, although species interactions and seasonality influenced the magnitude of the impact. Key index words: biodiversity; ecosystem functioning; macroalgae; NIS; Sargassum muticum Abbreviations : NIS, nonindigenous species; PVC, polyvinyl chloride Natural diversity is being modified worldwide by changes such as species loss and biological invasions of NIS (Vitousek et al. 1997, Sala et al. 2000). Understanding the consequences of such changes on ecosystem functioning has become a key topic of ecological research (e.g., Worm et al. 2006, Byrnes et al. 2007, Airoldi and Bulleri 2011). The argument that biodiversity loss could lead to a reduction in global ecosystem functioning (i.e., interactions between biotic assemblages or with their abiotic environment) emerged as an issue in the early 1990s (e.g., Ehrlich and Wilson 1991, Naeem et al. 1994). Conversely, in some systems local species richness has increased significantly due to recent establish- ment of NIS, although the long-term consequences of these introductions are still debated (Sax and Gaines 2003). The spread of NIS has been consid- ered one of the strongest anthropogenic impacts on natural ecosystems by changing abiotic factors, com- munity structure, and ecosystem properties (Mack et al. 2000, Byers 2002, Ruesink et al. 2006). Life his- tory features of invaders may be key factors in deter- mining the fate and the impact of invasions. For instance, invasion by canopy-forming macroalgae (e.g., Sargassum muticum, Undaria pinnatifida) may influence the structure of understory assemblages by modifying levels of light, sedimentation (Airoldi 2003) or water movement (Eckman et al. 1989). Introduced species often exhibit novel features com- pared to native species and may have disproportion- ately high impacts on native ecosystem functioning (Ruesink et al. 2006). However, positive effects linked to the presence of invasive algal hosts have also been recorded in structuring host-epiphyte 1 Received 18 October 2012. Accepted 16 September 2013. 2 Author for correspondence: e-mail f_vazpinto@yahoo.com. Editorial Responsibility: K. Dunton (Associate Editor) J. Phycol. *, ***–*** (2013) © 2013 Phycological Society of America DOI: 10.1111/jpy.12136 1