Temporal and spatial variation of seaweed biomass and assemblages in Northwest Portugal

Seaweed are ecologically important organisms playing a central structural and functional role in coastal ecosystems (providing habitat, mating and nursery grounds, food and refuge for several marine organisms). Additionally, their commercial potential has been rising since seaweed are directly or indirectly used by humans in many countries. This work aimed to evaluate the temporal and spatial variation of target species biomass and the composition of seaweed assemblages at an exposed rocky shore, in Northern Portugal. The targeted species were: Codium spp., Chondrus crispus, Mastocarpus stellatus, Osmundea pinnatifida and Chondracanthus acicularis. Two sampling sites were defined and in each one 18 haphazard placed quadrats (50 × 50 cm) were sampled: 9 in the low-shore and 9 in the mid-shore. Sampling was conducted periodically from April 2017 to January 2018, in six sampling occasions. Significant temporal variation was detected in the mean biomass values of Codium spp. and O. pinnatifida, at low-shore, where high abundance of these species in typically cold months could be related to the atypical warmer year 2017 in Portugal. C. acicularis revealed a significant spatial variation at both shore levels, which could be explained by different water channels orientations at the two sampling sites. The multivariate analysis showed that the effect of sampling occasion on the seaweed assemblage structure depended on site and revealed that some species, such as Ulva spp., apparently play an important role in seaweed assemblages of Belinho-Mar intertidal and highlights its ecological importance for these community. This study provides a baseline data for future work in similar areas, allowed the development of effective management plans as basis for seaweed exploitation guidelines. •Data on temporal and spatial variation of seaweed biomass from the coast of Portugal are scarce•Temporal and spatial variation of seaweed biomass could be due to the abiotic and biotic factors and species biology•Targeted species appear to be a “key” contributory elements in the temporal and spatial variation of seaweed assemblages•Multivariate analysis detected an interaction between the two evaluated factors, indicating a dependency among them