Spatio-Temporal Variability of Wrack Along the Northern Portuguese Sandy Beaches

Abstract

Wrack accumulations are a source of nutrients on sandy beaches and host invertebrate species, increasing local biodiversity. The distribution of these important accumulations varies in space and time depending on the environmental conditions and the composition of species that are in the wrack. Species composition and spatiotemporal variability in wrack accumulations remain poorly understood in some regions. In this study, we evaluated the spatiotemporal variability in wrack biomass and diversity on four sandy beaches in northern Portugal over the course of 1 year. The results revealed that both the diversity and biomass of wrack varied temporally depending on the beach and tidal level where it was collected. Wrack biomass was about 20 to 35 times greater at higher than at the low tide level and was more abundant (87%) and twice as diverse at the two northernmost beaches of the study, probably due to a greater proximity to rocky shores. Large brown temperate and boreal macroalgae species such as fucoids (Fucus spp., Cystoseira spp., and Ascophyllum nodosum) and the kelp Saccorhiza polyschides were the most abundant taxa in the wrack. Fucoids were in the wrack throughout the year, while kelps predominated in the warmer months, generating 8% more biomass than that produced by fucoids throughout the year. However, since large brown macroalgae species are reducing their size and recruitment due to global warming in the north of the Iberian Peninsula, the biomass and diversity of wrack in northern Portugal could be limited in the future, compromising crucial functions of wrack in ecosystems.