1 3 Mar Biol (2014) 161:2395–2405 DOI 10.1007/s00227-014-2514-7 ORIGINAL PAPER Single and joint effects of regional- and local-scale variables on tropical seagrass fish assemblages Elisa Alonso Aller · Martin Gullström · Floriaan K J Eveleens Maarse · Michaela Gren · Lina Mtwana Nordlund · Narriman Jiddawi · Johan S. Eklöf Received: 13 March 2014 / Accepted: 30 July 2014 / Published online: 14 August 2014 © Springer-Verlag Berlin Heidelberg 2014 local scale, seagrass biomass had a positive (but weaker) influence on fish density. However, the positive effect of seagrass biomass decreased with increasing level of human development. In summary, our results highlight the importance of assessing how multiple local and regional variables, alone and together, influence fish communi- ties, in order to improve management of seagrass ecosys- tems and their services. Introduction Seagrass beds are among the most productive and diverse ecosystems on Earth (Duarte and Chiscano 1999) and are of great importance to human wellbeing (Cullen-Unsworth et al. 2014). They support abundant and diverse fish assem- blages (Gell and Whittington 2002) and typically have a higher density and diversity of fish than non-vegetated areas (Pollard 1984; Edgar et al. 1994; Eklöf et al. 2006). In tropical areas, seagrass beds are commonly found in close proximity to other shallow-water habitats, such as mangrove forests, coral reefs, mud and sand flats (Ogden 1988; Olds et al. 2012). These habitats are connected by an array of physical, chemical and biological processes, form- ing the spatially diverse ‘tropical seascape’ (Ogden 1988; Pittman et al. 2011). Numerous fish species are known to migrate between seagrass beds and other shallow-water habitats, either on a daily basis (Unsworth et al. 2007) or through ontogenetic migrations (Dorenbosch et al. 2006a; Berkström et al. 2012). Consequently, seagrass beds are an essential habitat for many fish species that spend most of their life in other habitats (Parrish 1989; Nagelkerken et al. 2000; Gullström et al. 2012). A number of variables are known to influence sea- grass-associated fish assemblages. On a local patch scale, Abstract Seagrass beds are highly important for tropi- cal ecosystems by supporting abundant and diverse fish assemblages that form the basis for artisanal fisheries. Although a number of local- and regional-scale vari- ables are known to influence the abundance, diversity and assemblage structure of seagrass-associated fish assem- blages, few studies have evaluated the relative and joint (interacting) influences of variables, especially those act- ing at different scales. Here, we examined the relative importance of local- and regional-scale factors structur- ing seagrass-associated fish assemblages, using a field survey in six seagrass (Thalassodendron ciliatum) areas around Unguja Island (Zanzibar, Tanzania). Fish density and assemblage structure were mostly affected by two regional-scale variables; distance to coral reefs, which positively affected fish density, and level of human devel- opment, which negatively affected fish density. On the Communicated by D. Goulet. Electronic supplementary material The online version of this article (doi:10.1007/s00227-014-2514-7) contains supplementary material, which is available to authorized users. E. Alonso Aller (*) · M. Gullström · F. K. J. Eveleens Maarse · M. Gren · L. M. Nordlund · J. S. Eklöf Department of Ecology, Environment and Plant Sciences, Stockholm University, 106 91 Stockholm, Sweden e-mail: elisa.alonso-aller@su.se L. M. Nordlund Western Indian Ocean – Community, Awareness, Research, and Environment (WIO CARE), P.O. Box 4199, Zanzibar, Tanzania N. Jiddawi Institute of Marine Sciences, University of Dar Es Salaam, P.O. Box 668, Zanzibar, Tanzania