The BENTIX Index


The BENTIX index, developed in HCMR by Simboura & Zenetos, 2002 is nowadays one among the most common ecological quality indices used worldwide and especially in the Mediterranean Sea for the assessment of soft bottom benthic macroinvertebrates’ communities quality status. The original publication (search up to October 2019) has been cited 976 times under the ISI Web of Knowledge: and 553 times in: The method has been applied mostly in the Mediterranean Sea but also in coastal waters and estuaries of European Atlantic waters, the Black Sea, South America, India, China, Iranian estuaries, Malaysia. It was developed for the purposes of the European Water Framework Directive 2000/60/EC (WFD) but is also proposed for the assessment of GES (Good Environmental Status) for soft bottom benthic habitats under the MSFD (2008/56/EC) and specifically for the Descriptors of Biodiversity (D1) and seafloor Integrity (D6).

Theoretical background

The index is based on the model of benthic communities’ response to organic enrichment or pollution; however benthic organisms react in the same way to other forms of pollution as well, as to sediment disturbance due to spoiling or dredging etc. Under anthropogenic stress, tolerant benthic species increase in numbers or density adopting the r-strategy of rapid reproduction, or simply resist to disturbances, while on the other hand, the sensitive species retreat. Moderate to high prevalence of opportunistic or tolerant species indicate some degree of environmental pressure.

Formula and classification scheme

The BENTIX index uses the relative contribution of generally tolerant and sensitive taxa, recombining the five ecological groups (GI-GV) described by Hily (1984), Glémarec (1986) and Grall and Glémarec (1997) and used in the AMBI index, weighting them according to the ratio of their occurrence in the benthic fauna. The metric was designed for the Mediterranean coastal waters and renders a five-step numerical scheme for the classification of benthic communities.

The selection of the weight coefficients in the Bentix formula is not random and it is based on the realization that the probability of a benthic species picked up randomly, to be tolerant to stress is 3:1. This ratio is multiplied by 2 to create a scale ranging from 2 to 6. The ‘sensitive’ taxa group GS, including all sensitive (GI) and indifferent (GII) species is weighted by 6 and the ‘tolerant’ taxa group GT, including all tolerant (GIII), first (GV) and second order opportunistic species (GIV) are equally weighted by 2.

The boundaries of the method were delimited following the paired metric concept included in the IC boundary setting protocol (EC, 2003). Comparison (Simboura & Argyrou, 2010) with other biotic indices’ models along pressure gradients have shown that in the Eastern Mediterranean (Bentix model), the GIII group of tolerant species is shifted towards more disturbed situations compared to the AMBI model and is equally important to the opportunistic groups (GIV, GV) in polluted areas.

The Bentix index formula and classification scheme

The BENTIX Index

At the border of good to high status, the sensitive group accounts roughly for more than 60% or more than two-third of the fauna, while the tolerant group as a whole (tolerant plus opportunists) accounts for less than 40% or less than one-third of the fauna. At the border of good to moderate status, the sensitive group accounts roughly for less than 40% or less than one-third of the fauna, while the tolerant group as a whole (tolerant plus opportunists) accounts for more than 60% or more than two-third of the fauna. It is important to stress here that for purely muddy habitats with fine (silt and clay particles over 90%) where the benthic fauna is normally dominated by some tolerant species, a refinement of the H/G (4.5) and G/M (3.5) boundaries is recommended as H/G: 4 and G/M: 3.


Comparisons with other indices used in the Mediterranean Sea such as AMBI (Borja et al., 2000), M-AMBI (Muxica et al., 2007), BOPA (Dauvin & Ruellet, 2007), and MEDOCC (Pinedo et al., 2014) showed an acceptable agreement with all indices but a higher with MEDOCC and M-AMBI (Occhipinti et al., 2009; Simboura & Argyrou, 2010; GIG, 2013; Subida et al., 2012; Van de Bund et al., 2008). The index has been successfully intercalibrated with other metrics within the Mediterranean geographical intercalibration group (MedGIG) (GIG, 2013), approved by ECOSTAT and established as a national method in Greece and Cyprus for the classification of benthic communities under the WFD.


It has been successfully applied for the classification of coastal water bodies of Greece throughout the first monitoring cycle (Simboura et al., 2015, 2016). It has also been tested over a wide variety of geographical areas and soft bottom benthic communities’ types against various anthropogenic pressures such as eutrophication and organic pollution, mining residues, aquaculture and other pressures (Simboura & Argyrou, 2006; Simboura & al., 2005; 2007; Simboura & Reizopoulou, 2008; Pavlidou et al., 2015). In order to include also structural components of benthic communities for the purposes of the Marine Strategy Framework Directive 2008/56/EC (MSFD), a formula has been developed combining the Bentix index with diversity indices using specific reference values for different ecotypes (Simboura et al., 2015).

The software

A freeware programme Bentix Add-In (version 1.1.) for calculating the Bentix index is downloadable from HERE

The software provides:

1. The scoring of a total of 1250 benthic species (and their corresponding taxonomic code), assigning the score 1 to species belonging to the general ecological group GS (sensitive) and the score 2 to species belonging to the general ecological group GT (tolerant). This scoring list is used by the software to automatically assign the user’s species list to one of the ecological groups (EG) GS and GT. It is important to stress that the user may extend (scoring more species) or modify (change a species score) the reference Bentix scores list used by the method and install it (see “Changing species’ scores list”).

2. The calculation of the Bentix index values for a given benthic data set and the resulting five scale WFD ecological quality status (EQS) classification assessed by the Bentix index.

3. For each station or replicate the calculation of the total number of species, the total number of specimens, the percentage abundance and the total abundance of the species belonging to ecological groups GT and GS.

4. The plotting of the Bentix values at each station and the corresponding EQS classification in color as required by WFD.

5. The histograms of the percentage abundance of the species belonging to the ecological groups GS and GT and the percentage abundance of the non-assigned species at each data set station or replicate.

The classification boundaries are the general boundaries of the method except for the case when the user indicates in the “settings” that the data correspond to the “muddy” habitat type in which case the modified boundaries for the high to good (H/G) and the good to moderate (G/M) classes are used. The input file (see file format) may be in the form of a simple species (vertical) to stations or replicates (horizontal) matrix. The calculation of the Bentix values is considered of low validity (low level of confidence) if the number of species is 3 or less, the number of specimens is 6 or less, the percentage of ignored species is 7% or higher or the percentage of non-assigned species is 20% or higher. In these cases, a warning appears in the results sheet.


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Simboura, N., E. Papathanassiou & D. Sakellariou, 2007. The use of a biotic index (Bentix) in assessing long term effects of dumping coarse metalliferous waste on soft bottom benthic communities. Ecological Indicators, 7(1): 164-180.

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Subida, M.D., P. Drake, E. Jordana, B. Mavrič, S. Pinedo, N. Simboura, J. Torres, F. Salas. 2012. Response of different biotic indices to gradients of organic enrichment in Mediterranean coastal waters: implications of non-monotonic responses of diversity measures. Ecological Indicators 19 (2012) 106–117.

Pavlidou, A., N. Simboura, Rousselaki, M. Tsapakis, K. Pagou, P. Drakopoulou, G. Assimakopoulou, H. Kontoyiannis & P. Panayotidis, 2015. Methods of eutrophication assessment in the context of the water framework directive: Examples from the Eastern Mediterranean coastal areas. Continental Shelf Research (2015), 108:156-168.

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