Local scale high frequency monitoring of seaweed strandings along an intertidal shore of the English Channel (Luc-sur-Mer, Normandy France) – Effect of biotic and abiotic factors
Résumé
Seaweed strandings on Luc-sur-Mer beach (Bay of Seine, English Channel, France) were monitored from March
2017 to October 2018 once or twice a week to investigate the wrack deposit dynamics linked to biotic and
environmental parameters. The extent of stranded seaweed, algal biomass and composition of algal wrack were
monitored through 99 field surveys. Forty-seven macroalgae taxa (14 Phaeophyta, 28 Rhodophyta, and 5
Chlorophyta) were identified in strandings. Almost 83% of the 35 taxa (8 Phaeophyta, 23 Rhodophyta, and 4
Chlorophyta) inventoried on the nearby intertidal rocky shore during the same period were also identified in the
wrack deposits, suggesting their local origin. Analysis of wrack composition revealed the dominance of sheet-like
species and annual algae mainly represented by Ulva in spring and summer, and by perennial and brown sea-
weeds in winter. The same stranding dynamics was observed in the two years with largest deposits in spring and
summer but strandings occurred earlier in 2017, due to contrasted environmental parameters in the two years.
Large wrack deposits (> 4.5 Ha) were mainly observed in spring when the wind speed was greater than 2.6 m.s