The Water Framework Directive (WFD; 2000/60/EG) aims to achieve a good ecological and chemical quality status of the European waters (Rivers, Lakes, coastal- and transitional waters). The quality status is determined based on the evaluation of different quality elements, e.g. macro-invertebrates. Macro-invertebrates are good indicators for detecting anthropogenic impacts and ecological degradation. The Belgian Coastal water body (< 1 nautical mile) is a small area, but this environment is highly influenced by the inputs from 4 harbors and 2 rivers (Ijzer, Scheldt), coastal defense works (dredging, sand suppletion) and fisheries. To evaluate the ecological quality status of this area, a WFD monitoring program was implemented in 2007 and the BEQI (Benthic Ecosystem Quality Index) (www.beqi.eu) is used as macro-invertebrate indicator tool. The ecological quality status has to be reported in the River Basin Management Plan of a water body, which is characterised by a reporting cycle of six years (2009-2015). The current WFD monitoring program for macro-invertebrates is designed to evaluate its ecological quality status with a good confidence on an adequate spatial and temporal scale. A good spatial coverage of the samples per habitat in a water body is very important for a water basin assessment (habitat approach), due to the small scale spatial variation of the benthic characteristics. Therefore, samples were randomly taken at 9 or 11 locations, representative for one or more habitat types ( Abra alba, Nephtys cirrosa and Macoma balthica habitat), in the water body Belgian Coast. A temporal coverage of the samples is important due to the fact that the benthic characteristics show strong year-to-year variations. Therefore, a yearly monitoring is executed instead of monitoring once over the evaluation period of 6 years. Currently, we are halfway of the first monitoring cycle (2007-2012). Finally to obtain a good confidence, a high number of samples has to be taken to reduce the chance of misclassification in the ...