Marine life under plastic threat: A systematic review of systematic reviews.

Despite rapid advances in research on plastic pollution in marine ecosystems, inconsistencies in reported effects continue to hinder a clear understanding of its mechanisms and impacts. Here, we present a novel and integrative approach, namely a systematic review of existing systematic reviews (SRs) and meta-analyses (MAs), to assess the available evidence and identify potential factors underlying conflicting results concerning the ecotoxicological, ecophysiological, and functional effects of plastic exposure across diverse marine taxa at multiple levels of biological organisation. A total of 34 SRs/MAs, published between 2018 and 2024 and synthesizing 1176 primary studies, were included. Our findings reveal substantial heterogeneity in observed effects, driven by taxonomic and plastic-type variability, and experimental conditions. Notably, while the overlap among SRs/MAs was minimal (Corrected Covered Area = 1.8 %), indicating limited redundancy but strong thematic specialization of research on marine plastic impact, methodological quality was generally poor, with 74 % of SRs/MAs classified as "critically low" according to the AMSTAR 2 framework. By systematically identifying knowledge gaps and key limitations (i.e., heterogeneous methodologies, strong taxonomic and plastic-size biases, and low ecological realism), this novel approach lays a valuable foundation for guiding future research directions. In addition, it highlights the need for harmonized procedures and enhanced methodological rigour to inform evidence-based policy decision on marine plastic pollution.