Decadal-scale loss of an intertidal algal bed: combining remote sensing and in situ mappingBiogenic Reefs

Rodney Forster¹

¹University of Hull, UK

Intertidal macrophyte populations are undergoing contractions in range across large areas of southern European coastlines, in particular the western coasts of Spain and Portugal. Here, I record a loss in surface area of macroalgal (Fucus/Ascophyllum) coverage of more than 90% for the period from 1980 to present for a rocky shore in Morbihan, Brittany. I first noticed an up-shore shift in the lower limit of the Fucus canopy in the mid-2010s, as the character of a regular bass fishing spot began to change. I started recording the outline of the main macroalgal bed using a handheld GPS in 2018, at which point the retreat of Fucus and replacement by rock oysters was well established. Annual GPS and photographic surveys of the remaining macroalgal bed have been done every year since 2018. The study has been extended back in time through the use of early archive Landsat red/infrared colour images from the 1970s to 1990s, then more recent multipsectral Landsat 7/8 and Sentinel-2 images are used in combination with high-resolution Lidar data to calculate rates of change at different shore heights. The cause of the loss is an open question:- most likely it is a combination of multiple pressures from more frequent land and marine heatwaves, human trampling and extraction, and increased grazing facilitated indirectly by the rock oyster matrix.