Biomass, Taxonomic and Morpho-Functional Structure and Annual Dynamics of Stranded Macroalgae in a Shellfish Estuarine Bank of NW Iberia

This study provides a comprehensive assessment of the biomass and taxonomic and morpho-functional composition of stranded macroalgae and seagrasses on a highly productive shellfish bank in NW Iberia (Galicia), frequently affected by green tides, based on a year-long monitoring program and multivariate analyses. Macroalgal biomass showed marked seasonal and spatial variability, with the highest values recorded in spring. Chlorophyta dominated stranded biomass, particularly Ulva lacinulata, which accounted for over 63% of the total. Thin foliose morphotypes also made a major contribution to biomass across seasons and sampling locations. Linear mixed-effects modelling revealed a significant interaction between transect and intertidal level, indicating that spatial biomass patterns varied across the intertidal gradient. An interaction between season and intertidal level was also detected, although its statistical support was less consistent in sensitivity analyses. The absence of a consistent vertical zonation pattern suggests that stranded biomass distribution may be influenced by detachment, transport and deposition processes, rather than solely reflecting local macroalgal production. These findings highlight the importance of considering taxonomic and morpho-functional groups composition, spatial variability and hydrodynamic context when monitoring and managing valuable estuarine environments affected by green tides.