Marine Pollution Bulletin 170 (2021) 112675 0025-326X/© 2021 Elsevier Ltd. All rights reserved. Floating marine litter detection algorithms and techniques using optical remote sensing data: A review Konstantinos Topouzelis a, * , Dimitris Papageorgiou a , Giuseppe Suaria b , Stefano Aliani b a Department of Marine Science, University of the Aegean, University Hill, 81100 Mytilene, Greece b Institute of Marine Sciences (ISMAR) National Research Council (CNR), Forte S. Teresa, 19032 Pozzuolo di Lerici Lerici, SP, Italy A R T I C L E INFO Keywords: Marine litter Satellites Remote sensing ABSTRACT Floating Marine Litter (FML) are mainly plastics or synthetic polymers that foat on the sea surface after being deliberately discarded or unintentionally lost along beaches, rivers or marine environments. In recent years, much focus has been placed on locating, tracking and removing plastic items in both coastal areas and in the open ocean. The use of high-resolution multispectral satellite images for such purpose is very promising, since satellite images can systematically monitor much larger areas in comparison to the traditional in situ observa- tions. This paper contains a literature review of the published research regarding the optical remote detection of foating marine debris and the proposed associated methodologies. The main aim of this review is to compile all available information on detection methodologies, providing at the same time valuable insights into the different approaches used for foating marine litter monitoring. First, a brief introduction into the theoretical basis of a spaceborne foating marine litter detection system is given. Next, published articles, or relevant research work have been compartmentalised, for analysing the proposed procedures and assisting in a further assessment of their methodological frameworks. Lastly, conclusions and bottlenecks of the existing knowledge on marine litter detection from space are derived. Although the remote detection of foating marine litter is currently limited by inherent restrictions of the available satellite sensors specifcations, we highlight how the methodological pro- cessing chain can signifcantly affect the future accuracy of plastic detection from space. 1. Introduction Marine litter consists of solid materials that have been made or used by people and deliberately discarded or unintentionally lost in in marine and coastal environments, such as wood, metals, glass, rubber, textiles, paper and plastics (UNEP, 2005). Due to its transboundary nature, all the world's oceans and seas, including some of the most remote corners of our planet, now contain substantial amounts of marine litter (Worm et al., 2017). Marine plastics are of particular interest due to the fact that in the last 50 years, plastic production has increased more than 22-fold while the global recycling rate of plastics in 2015 was only estimated at 9% (Geyer et al., 2017). As a result, plastic now accounts for more than 80–90% of all litter found in the marine environment, and it is estimated that in 2016 alone, between 19 and 23 million metric tons of plastics (i.e. 11% of plastic waste generated globally), entered aquatic ecosystems globally, with annual emissions projected at up to 53 million metric tons per year by 2030 under a business-as-usual scenario (Borrelle et al., 2020). The slow rate of degradation of plastics, combined with a steady increase in the production of single-use items over the last 50 years, has led to a remarkable accumulation of plastic debris at the sea-surface, on the sea-foor and in coastal environments worldwide (Chamas et al., 2020; Smail et al., 2019). This continuous accumulation of plastic has now become a major environmental issue posing a complex multi- dimensional challenge. Plastic debris items in the oceans vary widely in terms of size, shape or chemical composition. Large plastic products and thousands of plastic items can break down into smaller pieces, called microplastics, through a combination of photodegradation, oxidation and mechanical abrasion (Andrady, 2011; Suaria et al., 2016). But in many instances, larger plastic items (i.e. macroplastics) can persist for decades in the ocean especially when shielded from UV radiation under water or in sediments (Gregory and Andrady, 2003), accounting for a large portion of the global mass of foating debris (Lebreton et al., 2019). The UN Sustainable Development Goals recognized the importance of plastic pollution, establishing a target specifcally related to marine litter (SDG target 14.1 and the associated indicator 14.1.1b, “plastic * Corresponding author. E-mail address: topouzelis@marine.aegean.gr (K. Topouzelis). Contents lists available at ScienceDirect Marine Pollution Bulletin journal homepage: www.elsevier.com/locate/marpolbul https://doi.org/10.1016/j.marpolbul.2021.112675 Received 4 February 2021; Received in revised form 24 April 2021; Accepted 23 June 2021