Development of an estuarine quality index based on key physical and biogeochemical features

An index (EQUATION — Estuarine QUAlity and condiTION) is presented for integrated evaluation of estuarine quality, based on an aggregation of four different components: vulnerability, measuring the physical capacity of the system to react to change, water quality, which examines trophic status and eutrophication aspects, sediment quality, which looks at the sediments and benthic fauna, and trophodynamics, which addresses the quality and value of the top levels of the trophic web. The data requirements are reduced by the application of models and heuristic grading, and the four components are combined into a 1 (worse) to 5 (better) overall grade. The index was implemented as a decision support system, and it was tested on five different estuaries in the US and Europe. The test ecosystems were chosen to study a range of physiography, tidal regimes, organic loading and contamination by persistent pollutants. Scores ranged fom Low (grade 2) for the Elbe estuary to Excellent (grade 5) for Tomales Bay in California. Tests were also carried out on “concept” estuaries and using different scenarios for two of the chosen estuaries. The classification obtained is in agreement with other authors, and the methodology provides a useful synthesis of the basic descriptors of estuarine quality: physical aspects, water quality, benthos and higher trophic levels (including socio-economic aspects of fisheries).