The biomass indicator is the 90th percentile values of chlorophyll a concentration measured monthly between March and October (Soudant et al., 2010). The abundance index is based on the frequency of blooms. A bloom is set on the French coast of the English Channel / North Sea as a concentration greater than 100 000 or 250 000 cells per litre, for a given taxon in a sample (Soudant et al., 2010). The measured frequency of blooms is then compared to the natural frequency found in the region, here equal to two months of blooms in the twelve months of a year (one bloom in spring and another in autumn).

The evaluation results for these two indices are constructed from data Quadrige² over the period 2005-2010 for coastal water bodies.

The oxygen balance appears among the elements of physicochemical quality selected for the classification of ecological state. The metric used is the 10th percentile. It is calculated on monthly data acquired in summer (June-September) and during six years at the bottom and the sub-surface of the water column. Methods of sampling and analysis are detailed in the reports Daniel (2007) and Aminot and Kerouel (2004). As the concentration of dissolved oxygen is the only parameter used, this index is also the indicator for the quality element. The reference value (= value of very good condition) for the oxygen balance is 8.33 mg/l. The data are taken from the database Quadrige2 dated 2 May 2011.

Benthic macroinvertebrates are excellent integrators and indicators of the general state of the environment and because of certain sensitive organisms can help to identify and quantify the anthropogenic pressures exerted on these water bodies . They can be good indicators of environmental enrichment in organic matter. The metrics for this element of quality, helping to define the ecological state, are the level of diversity and abundance of taxa and all taxa sensitive to disturbance.

The indicator used for the qualification of coastal water bodies is the M-AMBI (Desroy et al., 2009 and Desroy et al., 2010). It is based on:

- The AMBI indicator, itself based on the recognition in the settlement of five ecological groups of differing polluo-sensitivities, as proposed by Hily (1984).

This index is based on the weighting of each ecological group by a constant which represents the level of disturbance with which the species are associated.

- Richness or number of species having at least one individual to the station

- The diversity index of Shannon-Weaver