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

Abstract

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 seaweeds 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−1 and the predicted tide height greater than 6.8 m. Most deposits were associated with WSW winds (38%) and SSW winds (26%) winds. These data will help develop a tool to assess the risk of algal bloom in sandy beach ecosystem. The knowledge acquired about the availability and composition of the macroalgae strandings will also help promote the use of this bio-resource which is under-exploited in Normandy.