Monitoring litter pollution and zooplankton community in the Ave River estuary, Portugal.

Litter pollution is a growing concern that threatens both ecosystems and human health. It is composed of a wide variety of items from different sources, with plastics being the most abundant material. Rivers and estuaries act as pipelines, channelling litter to the ocean, making data collection on these systems crucial to understand how these pollutants enter the ocean. As part of the MAELSTROM (smart technology for MArinE Litter SusTainable RemOval and Management) project, this study aims to: (i) characterise floating macro litter (FML) and microplastics (MPs) in the Ave River estuary (Portugal), according to international guidelines, (ii) assess, for the first time, the zooplankton community, investigating the presence of MPs in it, and (iii) engage stakeholders to tackle marine litter. Spatio-temporal distribution of FML and MPs was evaluated prior to the installation of a technology, which can retain the litter and prevent it from entering the ocean. FML was observed at all sites and seasons, with the highest frequencies in autumn, possibly due to increased runoff, and at sites closer to the estuary mouth, where styrofoam pieces were the main items observed. All the water samples, collected at two different depths, contained MPs (16.54 ± 1.24 items m 3). Blue fibres and transparent fragments were dominant in summer and autumn respectively, with a mean size of 1.243 ± 0.008 mm. Zooplankton were more abundant in summer and distributed along the salinity gradient, dominated by copepods and meroplankton larvae. In contrast, cladocerans and copepods prevailed in autumn, with a salinity-related decrease in abundance. MPs within zooplankton were mostly small fragments (0.071 ± 0.008 mm) found in both copepods and cladocerans. These results highlight the urgent need for management actions and contribute to the spatio-temporal litter monitoring in estuaries and other coastal ecosystems and its impacts on biota.