Environmental data collection for ecological status assessment and monitoring of microplastics pollution in sediments in the Ave River estuary, Portugal

Ave River is considered one of the most polluted rivers in Portugal as its hydrographic basin is characterised by disorganised land use, including textiles, rubber, and plastics industries. During the MAELSTROM (Smart technology for MArinE Litter SusTainable RemOval and Management) project, the Ave estuary was chosen as a demo site for the installation of a technology to retain litter before it reaches the ocean. This study aims to characterize the estuary in a multidisciplinary way before the installation of this technology, evaluating hydrodynamic patterns, microplastics (MPs) concentration in sediments, and ecological status according to the European Water Framework Directive. The long-term CTD results showed that, for low river flow conditions, the estuary was dominated mainly by tides, with the presence of a permanent salt wedge. On contrary, higher river flows are the main drivers of the estuary during rainy seasons. This study highlighted for the first time that the river plume of the nearby Douro River may have an influence on the hydrodynamics of the Ave, pushing oceanic waters inward when its river flow is low. The presence of MPs in sediments of the Ave estuary was assessed for the first time. Seasonal variations in the abundance of MPs were observed, with a greater presence in autumn than in summer (more than two times), probably related to the higher input of MPs from riverine locations due to increased river flow. Most of the MPs identified were fragments, while the most common colours were blue and white. In addition, MPs smaller than 1 mm were dominant, suggesting a high bioavailability, potentially impacting on aquatic biota. Although the physical and chemical elements were rated “Good”, the ecological status of the estuary was rated “Poor” to “Moderate” due to the low abundance, species richness, and evenness of the benthic macroinvertebrate community, which was dominated by Capitella spp., an opportunistic species indicating organic enrichment.