>b/b<: distribution and levels of contamination on the benthic biodiversity

Human activities have increasingly impacted natural ecosystems, driving rapid environmental changes. Climate change, pollution, and biodiversity loss are now recognized as interlinked issues, being referred to as the Triple Planetary Crisis. Within this concept, marine and coastal pollution by various hazardous substances has received significant attention over the past decades due to its significant effects on ecosystems, biodiversity, and human populations. Sandy beaches, which represent about one-third of the world\'s ice-free shorelines, are particularly vulnerable. These environments provide unique ecosystem services and are intensively frequented for recreation, tourism, fishing and other human uses. Given their importance, evaluating and monitoring the quality of sandy beaches especially through the analysis of contamination levels is crucial to support management and decision-making. This study focused on the Northern Coast of São Paulo State (NCSP) in southeastern Brazil, a region with diverse morphodynamic and hydrodynamic conditions, varied human uses, and beaches within different categories of conservation status. The general goal was to assess the spatial distribution of contamination along the NCSP beaches, using representatives of the beach macrofauna, exploring their potential as biomonitoring tools and the risks posed by these contaminants to beach quality and human health. In Chapter 1, the gastropod Hastula cinerea was used to assess organotin contamination through imposex incidence, detected on 14 of the 23 studied beaches, including one within a no-take protected area. This marked the first spatial analysis employing H. cinerea as indicator for organotin contamination. Chapter 2 evaluated metal(loid) composition in the clams Donax hanleyanus and Tivela mactroides, revealing species-specific differences and contamination hotspots that varied between analyzed elements. Notably, concentrations of arsenic (As) and lead (Pb) in some samples exceeded the safety limits established by ANVISA and international standards, posing potential risks to human health. In Chapter 3, low concentrations of organic compounds (polycyclic aromatic hydrocarbons and persistent organic pollutants) were found in D. hanleyanus and T. mactroides, with all samples below international safety thresholds. Nonetheless, few hotspots were identified, indicating the need for continued regulations and monitoring. Chapter 4 identified similar patterns of microplastic accumulation in D. hanleyanus and T. mactroides. A widespread microplastic occurrence across beaches and species was observed, highlighting the diffuse and persistent nature of this pollutant. This thesis emphasized the value of macrofauna species in contamination assessments and environmental monitoring on beaches. The exposure to these contaminants may affect the local biota and trophic interactions, leading to ecological impacts and risks to ecosystem health. Further research is needed to investigate the ecotoxicology of these contaminants and their interaction and distribution with environmental factors in beach environments. The findings provide baseline data for future studies in the region, offering valuable insights for identifying contamination hotspots, developing biomonitoring strategies, and supporting decision making for conservation and management efforts on sandy beaches.