Large-scale effects of green tides on macrotidal sandy beaches: habitat-specific responses of zoobenthos

Sandy beaches are highly dynamic ecosystems mainly driven by physical variables, such as tidal regime. These ecosystems support numerous essential ecological functions and contain a distinctive biodiversity, but are threatened by increasing direct or indirect anthropogenic pressures, among which are green tides composed of free living Ulva spp. Studies that have been conducted to understand the effects of macroalgal mats on coastal sediment communities have mostly addressed responses in atidal or microtidal systems, and are often single-site assessments. Using large-scale field surveys across 13 macrotidal sandy beaches of two types (exposed and semi-exposed) distributed along 2,700 km of coast for 7 years (REBENT program, Brittany, France), we analyzed responses of zoobenthos to the presence of green tides in relation to tidal range, exposure, sediment characteristics, air and seawater temperature, precipitations, wind and salinity. Despite the high variability existing between two distinct categories of sandy habitats and also between macrotidal beaches within the study area, differences in macrofaunal community structure arose from the presence of green tides: mean abundance and species richness of macrozoobenthic invertebrates were higher where green tides occurred. Moreover, macrobenthic assemblages in the two beach-categories respond differently to eutrophication seen as green tides. Surprisingly, the effects of the presence of Ulva mats were stronger at exposed sandy beaches than at semi-exposed beaches. Our study also highlights species-specific responses: herbivorous marine invertebrates and suspension feeders were favored by the presence of Ulva mats, whereas large sub-surface deposit feeders and bivalve drifters which surf up and down the shore with the tides were negatively affected by green tides