Ecological Indicators 10 (2010) 1162–1173 Contents lists available at ScienceDirect Ecological Indicators journal homepage: www.elsevier.com/locate/ecolind The influence of mesh size in environmental quality assessment of estuarine macrobenthic communities Thiago Couto ∗ , Joana Patrício, João M. Neto, Filipe R. Ceia, João Franco, João Carlos Marques IMAR - Institute of Marine Research, Department of Life Sciences, Faculty of Sciences and Technology, University of Coimbra, 3004-517 Coimbra, Portugal article info Article history: Received 9 November 2009 Received in revised form 28 March 2010 Accepted 30 March 2010 Keywords: Mesh size Macrofauna Ecological quality assessment Estuary Portugal abstract The Water Framework Directive (WFD) strengthened the need for environmental quality assessment with rapid and accurate results. Studies of estuarine benthic macrofauna communities often use 0.5-mm mesh sieves in samples processing. However, this represents a considerable increase in sampling and identification effort compared to the use of 1-mm mesh sieves. Therefore, it is relevant to determine if mesh size matters in environmental quality assessments. The objectives of this study were as follows: (i) to test whether sieves with different mesh sizes provided different environmental status assessments in transitional systems, (ii) to compare the performance of different ecological indicators based on data from 0.5- and 1-mm mesh sieves and (iii) to compare the costs involved in using these two mesh sizes. Data were collected in the fall of 2007 and winter of 2008 at four sampling stations located in the Mon- dego Estuary, Portugal. The relative performance of Margalef and Shannon–Wiener indices, AMBI—AZTI Marine Biotic Index, Pielou, Eco-Exergy and Specific Eco-Exergy indices was analysed. Additionally, the multimetric Benthic Assessment Tool (BAT) was applied. The samples from the 1-mm mesh sieve were processed 2.9 times faster than the samples from the 0.5-mm mesh sieves. As expected, the density, biomass and number of species retained in the 0.5-mm mesh sieve were significantly higher in both seasons than the density, biomass and number of species retained in the 1-mm mesh sieve. All indicators were significantly different for the two mesh sizes in at least one season. The Pielou index was signifi- cantly different for the two mesh sizes in both seasons. Most indices showed that the 0.5-mm mesh sieve captured more information from the study system. The first BAT analysis provided different Ecological Quality Status (EQS) assessments for the two mesh sizes. To use the EQS obtained from the 1-mm mesh sieve as a proxy for the EQS for the 0.5-mm mesh sieve, further modifications were done in terms of reference conditions and class boundary thresholds. Regarding the Mondego Estuary, the use of a 1-mm mesh sieve appeared to be advantageous on routine environmental quality assessment, giving unbiased results with relatively less effort. Nevertheless, the methodology needs further validation and additional tests. © 2010 Elsevier Ltd. All rights reserved. 1. Introduction The Water Framework Directive (WFD) requires EU Member States to assess the ecological quality (EcoQ) of their bodies of water based on different biological quality elements (phytoplank- ton, macroalgae and angiosperms, macroinvertebrates and fish communities) (EC, 2000). The key objective of WFD is to achieve good water status for all water bodies by 2015 (EC, 2000). To assess a given system for each element of biological quality, it is necessary to define specific undisturbed reference conditions for each element. These reference conditions must be known to determine any devia- ∗ Corresponding author. Tel.: +351 239 836386; fax: +351 266 760 912. E-mail address: thiagoc@student.zoo.uc.pt (T. Couto). tions in a system. Several metrics (e.g. M-AMBI-Multivariate AMBI, BAT; Pinto et al., 2009; Teixeira et al., 2009) have been developed to evaluate the ecological status of each water body as five classes (EQS): 1 = bad, 2 = poor, 3 = moderate, 4 = good and 5 = excellent (EC, 2000). Ecological indicators are often used to provide information about the status of an ecosystem. Certain species are consid- ered ideal ecological indicators because their presence or absence and relative dominance is correlated with ecosystem status (Abel, 1996). Several bioindicators are used in estuaries. Benthic commu- nities are usually considered good ecological indicators due to their low mobility, higher sensitivity to local disturbances and their abil- ity to efficiently assimilate the recent record of disturbances that may no longer remain as traces in the water column (Warwick, 1993). The ecological indicators that were chosen for this study 1470-160X/$ – see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.ecolind.2010.03.019