The dataset originates from the project "Cartography and Biomass Assessment of the Alga Gelidium sesquipedale along the Coast of Cantabria", whose main objective was to determine the spatial distribution and abundance of Gelidium sesquipedale (now regarded as Gelidium corneum) along the Cantabrian coastline. The ultimate aim was to evaluate the commercial potential of the algal beds formed by this species and to provide baseline information to support the sustainable management of this resource. \nThe study area encompassed the entire coastal zone of Cantabria, divided into two regions surveyed in 1990 and 1991. Based on nautical charts, the position and orientation of 125 transects (sampling strips perpendicular to the coastline) were established at approximately 1000-meter intervals. These transects were grouped into 21 homogeneous sectors (in terms of orientation, substrate, slope, etc.) covering the full coastal extent. Along each transect samples were collected at several depths, from the shallowest nearshore zone to the deepest limit of Gelidium distribution, at 4-meter depth intervals. At each depth, two algal samples were collected from areas of maximum density (100 percent cover), using a 50 x 50 cm quadrat (2500 cm2). Additionally, the percentage cover of Gelidium surrounding each sampling point was recorded to allow for correction of biomass estimates representative of the sampled area. The total algal biomass per sector was calculated following the method described by Mann (1972) and Niell and Soneira (1976).