Setting the maximum ecological potential of benthic communities, to assess ecological status, in heavily morphologically-modified estuarine water bodies.

Investigations on setting benthic macroinvertebrates reference conditions in natural waters have increased recently. Under the European Water Framework Directive, importance is given to research in morphological heavily-modified water bodies (HMWBs), which are very common in countries with high human pressure. However, research has not been undertaken on setting the maximum ecological potential (MEP), as a reference in HMWB. The objective of the present investigation is to set the MEP of two metrics (diversity and richness), used in assessing the ecological status in different benthic indices. The Oiartzun estuary (Basque Country) is used as a case study, which changed morphologically in the 19th Century, following harbour construction. Data obtained from 1874 and the present were used to model changes in currents, water residence time, salinity, volume, and intertidal area. Benthic macroinvertebrate data, from 1995 to 2011, were used to predict 19th Century and present MEP. Changes in the estuary were described: loss of all of the intertidal areas; doubling of the volume; residence time, changing from 2 to 95 days; current velocity reduced by 50%; salinity increase. All these factors have led to changes in the benthic communities and the structural variables. Predicted richness and diversity, for 1874, were lower (48-76%) than those at present. Taking into account the differences between natural and modified waters, it is proposed to utilize 75% of the natural reference conditions, as the MEP values for Basque HMWB.