ORIGINAL ARTICLE Zoobenthos as an environmental quality element: the ecological significance of sampling design and functional traits Katri Aarnio, Johanna Mattila, Anna To ¨ rnroos & Erik Bonsdorff Department of Biosciences, Environmental and Marine Biology & Huso ¨ Biological Station, A ˚ bo Akademi University, Turku, Finland Introduction Eutrophication is one of the most severe environmental issues in the Northern Baltic Sea, affecting both pelagic and benthic environments, in shallow and deep areas (Elmgren 1989; Bonsdorff et al. 1997; HELCOM 2009). Zoobenthos is widely used as an indicator of change in environmental conditions, as the organisms are relatively stationary and several species live for many years. Thus changes in environmental conditions are reflected in zoobenthos as altered community parameters. Increased disturbance of coastal areas leads to changes in species number and the composition of assemblages, as well as in their abundance and biomass (Pearson & Rosenberg 1978; Cederwall & Elmgren 1980; Diaz & Rosenberg 1995; Norkko & Bonsdorff 1996; Bonsdorff & Pearson 1999; Perus & Bonsdorff 2004). In the Northern Baltic Sea, the number of benthic spe- cies is low primarily due to brackish water conditions (salinities vary between 3 and 7 psu). Species are either of marine or limnic origin and they live at the limits of their physiological tolerance (Bonsdorff 2006). Due to the low Keywords Brackish water benthic index; biological traits analysis; ecological status; sampling methodology; water frame directive; zoobenthos. Correspondence Katri Aarnio, Department of Biosciences, Environmental and Marine Biology & Huso ¨ biological station, A ˚ bo Akademi University, FI-20500 Turku, Finland. E-mail: katri.aarnio@abo.fi Accepted: 25 November 2010 doi:10.1111/j.1439-0485.2010.00417.x Abstract The EC Water Frame Directive (WFD) states that all coastal water bodies must achieve ‘good ecological status’ by the year 2015. A range of different classifica- tion methods have been developed and used to define ecological status to sup- port the WFD. The aim of this study was to compare the effects of using two different mesh sizes of sieve (1.0 and 0.5 mm) on zoobenthic assemblages and on the ecological status of benthic macrofauna (using the Brackish water benthic index, BBI) in three ecologically distinct archipelago areas (Inner, Mid- dle and Outer) in the A ˚ land Islands, Northern Baltic Sea. We performed a bio- logical trait analysis (BTA) to evaluate differences in the functional (trait) diversity of macrofauna collected using different mesh sizes and estimate the ecological relevance of mesh size. The results showed that sieve mesh size had significant effects on the recorded number of species, abundance, and total bio- mass of the zoobenthos. Small-bodied species and juveniles (e.g. Macoma balth- ica) were not observed when using a 1.0-mm mesh. The ecological status (sensu WFD) was only slightly affected by the mesh size, and all areas had good or high ecological status. BTA showed a difference in trait composition when using 0.5- or 1.0-mm mesh, particularly in the Outer area, where the propor- tion of small-sized species was high. Our results highlight how biological traits, in addition to species number and biomass, can play a key role when analyzing ecosystem structure for assessment and classification of coastal ecosystems. We show that combining traditional monitoring for the EU WFD with a functional analysis strengthens our ability to interpret environmental quality, and thus increases the precision of our advice for management purposes. Marine Ecology. ISSN 0173-9565 58 Marine Ecology 32 (Suppl. 1) (2011) 58–71 ª 2010 Blackwell Verlag GmbH