Alexandra Marie-José Carriere, Nathalie Reynaud, Aurore Gay, Jean-Marc Baudoin, Christine Argillier

1. Hydromorphology provides a physical framework for aquatic biocenoses. Its condition directly affects the quality of habitats available for fauna and flora and its assessment is therefore required by the EU Water Framework Directive (WFD) to assess the ecological status of lake water bodies.
2. In this study, we developed an index of Lake HYdroMOrphology (LHYMO) to provide a quantitative assessment of the hydromorphological status of natural and non-natural lakes that can be used for the implementation of the WFD and is consistent with CEN standards.
3. This new index addresses the shortage of methods describing adequately hydromorphological alterations on lakes on both hydrology and morphology. LHYMO includes nine metrics related to morphological WFD quality elements (QE) and six metrics related to hydrological WFD QE, all of which considered to support biological elements. The reference conditions were defined for each metric using an original approach: the degree of alteration is measured in relation to the natural characteristics of each lake, relative to a state that ‘would be expected in the absence of disturbances’.
4. Besides its use for regulatory purposes, this index is also an interesting tool to monitor the efficacy of hydromorphological restoration projects or to help target effective conservation measures on lakes.
5. Application of this index to 72 French lakes provides the first quantitative and homogeneous assessment of the hydromorphological quality of lakes over a whole territory and gives the first overview of the hydromorphological status of lakes in France, with classification into five classes ranging from ‘high’ to ‘bad’.
6. LHYMO is already operational for large French lakes as it relies mostly on reference datasets available at a national scale, and may also be used in a wider scope through gathering or completing data from other sources.