Beach sand as a biofilter: enzymatic activity and organic matter relationships in oligotrophic sites differently influenced by anthropogenic pressures and morphodynamism

Sandy beaches provide a number of ecosystem services, such as the breakdown of organic matter (OM), nutrient mineralisation and water filtration. These environments may be considered as biofilters, able to regulate the transport and transformation of OM from land to sea and vice-versa, due to the porous structure of the sediment and the colonisation by microorganisms. The efficiency of OM recycling performed by enzymatic activities (EAs) was evaluated in three beaches of the NW Mediterranean Sea, characterised by different anthropogenic pressure (urbanisation), and morphodynamic characteristics (exposition to waves). The sampling was focused on the area that includes the swash zone and the wet emerged beach. Urbanisation significantly influenced EAs, leading to a decoupling between the lipolytic EAs and the lipid content of sediment. Natural and anthropogenic lipids, showing higher turnover times than other OM (proteins), accumulated in the sediment, notwithstanding the very high lipolytic activities. In the sheltered side of the urbanised sites, significant correlations between EAs, sedimentary OM and seawater inorganic nutrients were recorded, confirming the biofilter activity. In the exposed sites, the wave action increased the exchanges with the sea by water flushing inside the sediment, and EAs were related mainly to salinity. Especially in the least urbanised beach, coarse texture, higher pendency and heterogeneity of the sediment decreased the biofilter activity, allowing photosynthetic producers to increase their biomass. In this case, the beach hosted production of fresh OM, contributing to fulfil the trophic web requirements.