1 3 Mar Biol DOI 10.1007/s00227-014-2579-3 ORIGINAL PAPER Impact of eutrophication and climate change on fish and zoobenthos in coastal waters of the Baltic Sea Martin Snickars · Benjamin Weigel · Erik Bonsdorff Received: 14 April 2014 / Accepted: 10 November 2014 © Springer-Verlag Berlin Heidelberg 2014 benthic-feeding fish exploring the deep waters. An inten- sification of these trends is likely in the future, as climate change scenarios suggest further temperature increase and salinity decrease. Introduction A large number of fish species utilize coastal waters during one or several life stages (Seitz et al. 2013). One reason for this is the high productivity of the benthic habitats, which are central for trophic links, as many fish species feed on zoobenthos in these shallow environments (Hüssy et al. 1997; Tomczak et al. 2009). However, multiple stressors pose pressures on growing portions of marine and coastal waters. For example, eutrophication and climate change are recognized as two major problems that alter coastal ecosys- tems (Fields et al. 1993; Cloern 2001; Harley et al. 2006), making these habitats vulnerable as they encounter the impacts of both. The effects of eutrophication on marine ecosystems are broad. Nutrient enrichment induces enhanced pelagic pri- mary production, leading to decreased Secchi (photic) depth and an elevated risk of low oxygen levels in the bottom water. These effects have many ecosystem consequences, affecting species across photic and aphotic habitats and trophic levels (Cederwall and Elmgren 1990; Bonsdorff et al. 1997; Conley et al. 2011). The response of zooben- thos to eutrophication often is nonlinear. Initially, biomass may increase due to higher food supply from increased pelagic primary production, but eventually the biomass begins to decrease, e.g., due to low oxygen levels in deep water, as the impact of eutrophication progresses (Pearson and Rosenberg 1978; Diaz and Rosenberg 2008). In coastal waters (10–30 m), oxygen has normally been considered Abstract Coastal waters are important in linking zooben- thos and fish, as many coastal fishes feed on benthic prey in these habitats. Major drivers, such as eutrophication and climate change, may alter this link, whereas shifts in the importance of these drivers may induce different responses in zoobenthos and fish, respectively, potentially changing productivity of coastal ecosystems. The aim of this study was to assess in which way abundance and distribution of benthic-feeding fish and biomass of zoobenthos have changed over time in response to eutrophication (Sec- chi depth) and climate change (temperature and salinity), respectively. This was done by analyzing gross changes in the responses over three decades (1983–2012) and across depth zones. Eutrophication and climate change caused different though specific impacts on fish and zoobenthos, respectively. Throughout the 1980s, increasing benthic- feeding fish abundance in shallow waters (<6 m) was pri- marily attributed to eutrophication (decreasing Secchi depth), implying increased system productivity. During the 2000s, the effect of eutrophication levelled out, whereas temperature caused contrasting development of fish abun- dance at different depth zones. Shallow waters had lower fish abundances during warm years compared with colder ones, while the abundance increased in deeper, aphotic waters (6–20 m). In the deep waters, zoobenthos showed a contrasting, declining trend in biomass, coinciding with the decrease in salinity during the 2000s. This suggests altered ecosystem productivity and potential food shortage for Communicated by F. Bulleri. M. Snickars (*) · B. Weigel · E. Bonsdorff Department of Biosciences, Åbo Akademi University, Artillerigatan 6, 20521 Åbo, Finland e-mail: msnickars@gmail.com