1. French Seagrass indicator - Atlantic and Channel CW & TW
NEA GIG
Scientific team
Isabelle Auby, Ifremer Arcachon
Hélène Oger-Jeanneret, Ifremer Nantes
Pierre-Guy Sauriau, CNRS, LIENSs
Christian Hily, CNRS, LEMAR
Laurent Barillé, University of Nantes

2. Index General information Description of the method
Necessary information
Reference conditions
Indicators description
How to apply the index

3. 1. General information Acronym: SBQ CW & TW
Method name: Seagrass Bed Quality
Original name: Qualité des herbiers de zostères dans les masses d’eau côtières et de transition
WISER ID: 351
GIG: NE Atlantic
Category: Coastal and Transitional Water Bodies
BQE: Angiosperms (Seagrass)
Country: France
Scientific literature :
Foden J., Brazier D.P. (2007). Angiosperms (seagrass) within the EU water framework directive : A UK perspective. Marine Pollution Bulletin 55, (2007) 181–195

4. 2. Description of the method
3 metrics have been selected to build the French indicator for angiosperms (seagrass bed quality) in the Atlantic/Channel CW & TW :
Taxonomic composition : Changes in the number of species over time (« only » 2 species are considered : Zostera noltii and Zostera marina)
Spatial extension of seagrass bed : state of the current extension compared to the largest known historical extension (after the 1930’s epidemic that decimated Zostera marina).
Density of seagrass bed (abundance, biomass or % cover) : state of the current density from the highest known historical density
Z. noltii (intertidal)
Z. marina (intertidal & subtidal)

5. 3. Necesary information : spatial extension of seagrass beds
In all cases, mapping of seagrass beds is carried out when their development is maximal aboveground (late summer).
Frequency chosen for these measurements : once every 6 years (duration of a management plan).
Methods for mapping of intertidal seagrass beds : 2 situations
Small seagrass beds (until a few km2) : mapping is carried out using field observation (with a GPS tracking), which are directly integrated into a GIS.
Major seagrass beds : mapping is carried out from satellite imagery or aerial orthophotographies, and in all cases completed by field validation (especially in WB with macroalgae and/or Spartina islands)
Methods for subtidal seagrass beds
When water transparency allows it : mapping is carried out from aerial photography
Otherwise: indirect methods are used (optical methods using video cameras or acoustic methods using side-scan sonar and/or single-beam sonar)

6. 3. Necesary information : spatial extension of seagrass beds
Field observation with GPS tracking : small intertidal seagrass beds & field validation for major intertidal seagrass beds
Aerial photography : major intertidal seagrass beds and subtidal seagrass beds (Z. marina) when water transparency allows it

7. 3. Necesary information : stational approach
In all cases, sampling is carried out when development of Zostera is maximal aboveground (late summer).
Frequency for these measurements is once every 3 years.
For intertidal Z. noltii, seagrass coverage is visually determined (% of area covered with seagrasses on a 100 m* 100 m area/station).
Then, sampling is carried out with 9 cores/replicates per station
Total sampled area :
63,62 cm² for Z. noltii
0,08 m² for Z. marina
On each sample : shoot density and biomass (aboveground and below ground) are determined.
© PG Sauriau

8. 4. Reference conditions
Seagrass (Zostera) beds on French coasts (Channel & Atlantic) differ in terms of extension, density/biomass and number of seagrass species present in the considered water body.
These parameters depend on a combination of geographical, edaphic, bathymetric and hydrodynamic factors, that are specific for each water body.
For this reason, reference conditions are defined for seagrass beds of each water body : they correspond to the highest known values during the period for which historical informations are available.
They are determined on the basis of historical data when they exist, otherwise by expert judgment.
These choices require the collation and analysis of all available historical information on seagrass beds of water bodies. When such information is unavailable, expert judgment or modeling (work on progress) is used.

9. 5. Indicators description
Composition metric (Zostera noltii & Zostera marina)
Status
High
Good
Moderate
Poor/Bad
Composition
Species emerged or no loss of any species
Loss of 1 species :
Zostera marina
Loss of 1 species : Zostera noltii
Loss of 2 species
EQR 1
0,7
0,5
Density metric and Extension metric
Etat
High
Good
Moderate
Poor
Bad
Density (loss)
Extension (loss)
0-10%
11-20%
21-30%
31-50%
> 50%
EQR
1-0,8
0,78-0,6
0,59-0,5
0,49-0,3
0,295-0
Class limits were derived from the first intercalibration exercise ( ) and adapted from French expert judgment

10. 6. How to apply the index
The average of the three EQR obtained for each of the three metrics (composition, spatial extent and density) is calculated.
Then the ecological status is assigned as follows :
Ecological status
EQR
High
0,8–1,0
Good
0,6–0,79
Moderate
0,4–0,59
Poor
0,2–0,39
Bad
0,0–0,19