Sandy Beaches as Biogeochemical Hotspots: The Metabolic Role of Macroalgal Wrack on Low-productive Shores
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Sandy beaches, which represent the most common type of land–sea interface, harbor distinctive biotic communities and regulate the flow of energy between marine and terrestrial ecosystems. Accumulations of sea wrack on sandy beaches are of crucial importance for recycling beach nutrients and for regulating trophic connectivity and coastal functioning. We investigated the role of beaches as biogeochemical hotspots by examining the metabolic activity in accumulations of different species of wrack on two exposed beaches affected by different levels of human pressure. Experimental wrack patches provided large amounts of different sedimentary nutrients over time due to remineralization of the algae. Unsurprisingly, the variation in the nutrients present in the beach sediments was related to the species of wrack considered. Macroalgal wrack was metabolically very active and supported high respiration rates represented by intense CO2 fluxes. Importantly, we demonstrated that the wrack metabolic rate differed significantly depending on the algal species considered. Different macrofauna and bacterial assemblages were identified in the different wrack patches and on the different beaches. We suggest that human activities such as beach grooming can modify the wrack-associated communities, thus contributing to the variability in the biogeochemical processes and metabolic rates. Significant changes in the type and amount of wrack deposited on beaches can change fundamental processes related to the marine-terrestrial transfer of nutrients and energy and to the marine-atmospheric transfer of CO2 emissions, with ecological consequences for nearshore environments.
Keywords:bacterial assemblages benthic macrofauna CO2 emissions metabolic hotspots non-native species nutrient inputs
We thank B. Araujo, P. de Pedro, and L. Gestoso for field and laboratory assistance. We also thank F. Barreiro, for providing some of the photographs included in this paper, and P. Lucena-Moya for comments in an early version of this manuscript. The constructive comments of two anonymous reviewers and the handling editor improved the final version of this manuscript. This study was funded by the IACOBUS European cooperation program, the Portuguese Foundation for Science and Technology (SFRH/ BPD/ 87042/ 2012) and the Galician Government (GRC2013/0049). IFR is supported by strategic research funding for collaboration between the University of Helsinki and Stockholm University.
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