Marine Pollution Bulletin 186 (2023) 114418 0025-326X/© 2022 Elsevier Ltd. All rights reserved. Economic losses related to the reduction of Posidonia ecosystem services in the Gulf of Gabes (Southern Mediterranean Sea) Radhouan El Zrelli a, * , Ahlem Hcine b , Lamia Yacoubi c , Ruben H. Roa-Ureta d , Nicola Gallai e , Sylvie Castet f , Michel Gr´ egoire f , Pierre Courjault-Rad´ e f , Lotf Jilani Rabaoui c, g a SADEF Agronomy & Environment, 30 Rue de la Station, 68700 Aspach-le-Bas, France b University of Sfax, Faculty of Economics and Management of Sfax, Research Laboratory in Competitiveness, Commercial Decisions and Internationalisation (CODECI), Sfax, Tunisia c University of Tunis El Manar, Faculty of Science of Tunis, Laboratory of Biodiversity and Parasitology of Aquatic Ecosystems (LR18ES05), University Campus, 2092 Tunis, Tunisia d Independent Consultant, Bilbao, Spain e LEREPS, ENFA, Universit´ e F´ ed´ erale Toulouse Midi-Pyr´ en´ ees, Toulouse Cedex F31042, France f G´ eosciences Environnement Toulouse (GET), Universit´ e de Toulouse, UMR 5563 CNRS/UPS/IRD/CNES, 14 Avenue Edouard Belin, 31400 Toulouse, France g National Center for Wildlife, Riyadh, Saudi Arabia A R T I C L E INFO Keywords: Mediterranean Gulf of Gabes Posidonia meadows Phosphogypsum Ecosystem services Negative production externalities ABSTRACT In the early XXth century, the Gulf of Gabes in SE Tunisia used to host the most extended Posidonia oceanica seagrass beds in the Mediterranean basin and was a highly productive hotspot of benthic species. Since the 70’s, >500 million t of wet toxic phosphogypsum discharges from a fertilizer industrial complex have led to the gradual loss of ~90 % of its initial surface. This drastic shrinkage is accompanied by signifcant value losses originated from the direct and indirect-use services of which the most important ones are small scale fsheries and carbon storage function. Using market valuations of a number of services we estimate economic losses at 105 million € in 2014 (~915€/ha), i.e., around 115 % of the added value of the gabesian fertilizer factories for the same year. Value losses should increase in the near future in relation with the COP26 agreements which boosted the open carbon credit market. Without actions to reduce negative production externalities caused by the fertilizer industry in the Gulf of Gabes it would not be possible to recover Posidonia ecosystems in this region leading to further economic, ecologic, and cultural losses. 1. Introduction As recently pointed out by the United Nations Environment Program (UNEP), seagrasses are the “forgotten ecosystem” although they are among the most productive natural habitats on land or sea, providing critical ecosystem services (ESs) to human well-being (UNEP, 2020). Worldwide economic value estimation of ESs provided by seagrass ranged from ~33,000 $/ha/yr (2021 USD rate) in the late 90’s (Cos- tanza et al., 1997), to 40,000 $/ha/yr (2021 USD rate) in 2011 (Cos- tanza et al., 2014). Furthermore, seagrasses are effcient to sequester CO 2 through photosynthesis and store organic carbon (Duarte et al., 2005). Seagrass ecosystems come in second place among the most eff- cient natural mechanisms of reabsorbing carbon (Duarte et al., 2013). Although they cover <0.2 % of the ocean, they constitute 10 % of the ocean’s ability to store carbon, twice as much carbon per km 2 as forests do on land and for much longer (Fourqurean et al., 2012). Nevertheless, despite these high carbon sink qualities and the high estimated value losses, seagrasses are still declining worldwide at an alarming rate due to human impacts such as industrial pollution, eutrophication, and me- chanical disturbances (Unsworth et al., 2014; El Zrelli et al., 2020; Green et al., 2021). The potential of seagrasses to store carbon is now starting to gain attention with the recognition of blue carbon ecosystems and their po- tential to contribute to climate change mitigation (Duarte et al., 2005; Fourqurean et al., 2012). The 2021 United Nations Climate Change conference (COP26) established a new operating framework for regu- lating carbon markets (Smith et al., 2022) that may encourage countries to earn credit via seagrasses and the carbon they store. This paper * Corresponding author. E-mail address: radhouan.elzrelli@gmail.com (R. El Zrelli). Contents lists available at ScienceDirect Marine Pollution Bulletin journal homepage: www.elsevier.com/locate/marpolbul https://doi.org/10.1016/j.marpolbul.2022.114418 Received 30 June 2022; Received in revised form 15 November 2022; Accepted 23 November 2022