This can be detailed or anecdotal, quantitative or qualitative. Over the course of years or decades. Anything that reports changes in the size of the canopy forming species.
You may find some clues about this question in Davies' papers such as :
DAVIES A.L 2004. Elfects of grazing and wave exposure on Ascophyllum over dffirent temporal scales.Thèse, Faculty of Science and Agriculture (School of Biology and Biochemistry), Queen's University of Belfast, 194 p. DAVIES A.J., JOHNSON M.P. & MAGGS C.A.2OO7 Lirnpet grazing and loss of Ascophyllum nodosum canopies on decadal time scales.Marine Ecology ProgressSeries, 339: 131-141.
On my site "famine chez les berniques" ("berniques" are limpets "Patella vulgata")
and my own papers available on Researchgate. (site in French and English, papers in French)