RESEARCH LETTER Persistence of Biogeochemical Alterations of Deep-Sea 10.1029/2020GL091279 Sediments by Bottom Trawling Key Points: S. Paradis1,2,3 , M. Goñi4 , P. Masqué1,2,5,6 , R. Durán3 , M. Arjona-Camas3,7 , • B ottom trawling disturbance on the A. Palanques3 , and P. Puig3 deep seafloor depletes organic matter through erosion and degradation, 1 Institute of Environmental Science and Technology, Universitat Autònoma de Barcelona, Bellaterra, Spain, 2Physics hampering its preservation • A 2-month seasonal trawling closure Department, Universitat Autònoma de Barcelona, Bellaterra, Spain, 3Marine Sciences Institute, Consejo Superior is insufficient to restore the seafloor de Investigaciones Científicas, Barcelona, Spain, 4College of Earth, Ocean, and Atmospheric Sciences, Oregon State and it is estimated that it would take University, Corvallis, OR, USA, 5School of Natural Sciences, Centre for Marine Ecosystems Research, Edith Cowan decades to recover University, Joondalup, WA, Australia, 6International Atomic Energy Agency, Principality of Monaco, Monaco, • Bottom trawling causes organic 7 carbon losses from the seafloor Department of Earth and Ocean Dynamics, University of Barcelona, Barcelona, Spain comparable to those generated on land by tillage of agricultural fields Abstract Bottom trawling grounds have been expanding to deeper areas of the oceans since the mid- XXth century, and mitigating strategies aimed to protect fish stocks, such as temporal trawling closures, Supporting Information: have recently been implemented. Here we investigated the biogeochemical properties of sediment from • Supporting Information S1 a deep-sea trawling ground in Palamós Canyon (NW Mediterranean) to assess the effects of a 2-months trawling closure in the recovery of sedimentary organic matter. In comparison to untrawled areas, Correspondence to: the continuous erosion and sediment mixing in trawling grounds led to coarser reworked sediments S. Paradis, sarah.paradis@uab.cat impoverished in organic carbon (∼30% loss) and promoted the degradation of labile compounds (52–70% loss). These impacts persisted after the temporal trawling closure, highlighting that this management Citation: strategy is insufficient to restore the seafloor. Considering the continuous expansion of bottom trawling Paradis, S., Goñi, M., Masqué, P., grounds, this activity could have significant and irreversible biogeochemical impacts on ocean margins at Durán, R., Arjona-Camas, M., a global scale, hampering their carbon burial capacity. Palanques, A., & Puig, P. (2021). Persistence of biogeochemical alterations of deep-sea sediments by Plain Language Summary Bottom trawling grounds have been expanding to deeper areas bottom trawling. Geophysical Research of the oceans since the mid-XXth century, causing substantial effects to fish populations. In order to Letters, 48, e2020GL091279. https://doi. reduce this impact, new management strategies have been introduced, such as establishing temporal org/10.1029/2020GL091279 trawling closures to allow fish populations to recover. Bottom trawling not only poses a threat to fish Received 23 OCT 2020 populations but also to the seafloor, since the continuous contact of trawling gear on the bottom erodes Accepted 3 DEC 2020 large volumes of sediment and reduces the amount and quality of organic matter. In this study, we assessed the effect of a 2-months trawling closure in deep-sea bottom trawling grounds in Palamós Canyon (NW Mediterranean) by collecting sediment samples in trawled and untrawled areas during different seasons. Our results show that sediment samples in the trawled site had substantially lower organic matter contents than in the untrawled site, and the remaining organic matter had poor nutritional quality. These impacts persisted after the temporal trawling closure, highlighting that this management strategy is insufficient to restore the seafloor. Considering the continuous expansion of bottom trawling grounds, this activity could have significant and irreversible impacts in organic matter available for benthic species in ocean margins at a global scale. 1. Introduction Continental margins are the continuum between the land and the ocean, and they are responsible for the majority of both burial and remineralization of organic carbon (OC) in the ocean (Dunne et al., 2007). How- ever, biogeochemical cycles of organic matter in continental margins are altered by several anthropogenic © 2020. The Authors. activities (Bauer et al., 2013). Amongst those, bottom trawling is considered to be the most harmful and This is an open access article under has the potential to alter the biogeochemistry of continental margins at a global scale given its widespread the terms of the Creative Commons extension, high frequency, and direct impacts on the seafloor (Eigaard et al., 2017; Ferguson et al., 2020; Attribution-NonCommercial License, which permits use, distribution and Hayes et al., 2018; Keil, 2017). reproduction in any medium, provided the original work is properly cited and The continuous contact of bottom trawling gear on the seafloor causes a variety of physical impacts (Martín is not used for commercial purposes. et al., 2014b; Oberle et al., 2018). In muddy seafloor environments, trawling gear can create long-lasting PARADIS ET AL. 1 of 12 Geophysical Research Letters 10.1029/2020GL091279 trawl marks, overturn sediment, and cause resuspension (O'Neill & Ivanović, 2016; Palanques et al., 2014). Resuspended sediments by bottom trawling is often exported by ambient currents, leading to significant erosion in fishing grounds (Oberle et al., 2016a). These disturbances modify the biogeochemistry of marine sedimentary environments, but the effects on the carbon stock stored in the seafloor is still uncertain (Leg- ge et al., 2020) due to the variable alterations on organic matter content and remineralization (Ferguson et al., 2020; Hale et al., 2017; Palanques et al., 2014; van de Velde et al., 2018). Many of these previous studies were carried out in muddy continental shelves, which are areas with high sediment and organic matter accumulation rates from both terrestrial and marine origins (Gordon & Goñi, 2004; Hedges & Keil, 1995) that can abate the physical and biogeochemical alterations caused by bottom trawling on the seafloor. Additionally, shallow environments are frequently impacted by natural processes, such as storm waves, which can resuspend similar volumes of sediment and organic matter com- pared to bottom trawling (Ferré et al., 2008; Pusceddu et al., 2005). However, the growing demand of fish and the depletion of shallow-water fishing stocks have driven the expansion of bottom trawling grounds to deeper environments since the 1950s, fueled by technological advances and subsidies. Bottom trawling now generally occurs at >500 m water depth and can extend to more than 1,500 m (Watson & Morato, 2013). Since new and deeper areas are continuously being explored, there is an increasing urgency to understand the impacts of bottom trawling on deep-sea environments. Deep bottom trawling grounds are usually concentrated near topographic features such as seamounts and submarine canyons, which act as ecological hotspots of many commercial species (Clark et al., 2007; Fer- nandez-Arcaya et al., 2017). The impacts of bottom trawling in these environments are intensified due to the low productivity and growth rates of deep-sea species, which has led to a rapid collapse of deep-sea fisheries (Norse et al., 2012). The growing concern of stock overexploitation has stimulated the implementation of management strategies, such as temporal trawling closures to allow stock rebuilding and the recruitment of juveniles (Armstrong et al., 2013). Bottom trawling impacts on deep-sea sedimentary environments are also accentuated due to the low sedimentation rates and organic matter fluxes, producing comparatively enhanced erosion and depletion of OC in sediments (Martín et al., 2014a). However, neither the mecha- nisms causing the reduction of organic matter nor the efficacy of seasonal trawling closures have been fully addressed. The Mediterranean Sea supports important deep bottom trawling fisheries, mostly located in the Western basin, where virtually the whole slope is covered by bottom trawling grounds (Figure 1a). Studies aimed to understand the impacts of deep bottom trawling in this region have been centered in the vicinities of Palamós Canyon (Figure 1b), where intense bottom trawling occurs along its flanks, targeting the coveted red-and-blue deep-sea shrimp Aristeus antennatus (Bjørkan et al., 2020; Gorelli et al., 2016). The studies addressing the trawling-induced seafloor erosion and the associated impacts on the sedimentary organic matter (Martín et al., 2014a; Pusceddu et al., 2014; Sañé et al., 2013) mainly focused on the spatial varia- bility in the canyon, observing comparable impacts on both canyon flanks. However, these studies did not evaluate the temporal changes in organic matter composition. Moreover, a 2-months trawling closure from early January to early March was recently established since 2013 to avoid the risk of overexploitation due to the continuous increase in fishing effort over the last decades (Bjørkan et al., 2020). This scenario provided the ideal setting to assess the effectiveness of seasonal trawling closures to restore deep-sea sedimentary environments. To achieve these objectives, sediment cores were collected from representative sites in the northern trawled canyon flank and in the southern untrawled canyon flank during different seasons (Fig- ures 1b and S1), and at similar depths and distance from the coast to minimize the potential effects of spatial variability, since these are the main spatial factors controlling the composition and magnitude of downward particle fluxes within the canyon (Palanques et al., 2005; Martín et al., 2006, 2007). Several biomarker com- pound classes yielded from the oxidation of CuO were used to assess the quantity, composition, source, and degree of degradation of sedimentary organic matter (e.g., Goñi et al., 2013; Tesi et al., 2012) during trawling and closure periods. PARADIS ET AL. 2 of 12 Geophysical Research Letters 10.1029/2020GL091279 Figure 1. Bottom trawling grounds (red) on continental slopes of the Mediterranean Sea and surrounding margins (a). Bathymetric map of Palamós Canyon and cumulative bottom trawling fishing effort computed from February 2017 to March 2018 (b). Circles indicate the sampling locations of sediment cores from the trawled (red) and untrawled (green) sites. The location of the Cap de Begur buoy is shown by a yellow star. PARADIS ET AL. 3 of 12 Geophysical Research Letters 10.1029/2020GL091279 2. Materials and Methods 2.1. Sampling Sampling was conducted during four cruises onboard the R/V García del Cid at contrasting seasons: during the temporal trawling closure in February 2017, during the trawling season in June and October 2017, and toward the end of the trawling closure in March 2018 (Figure S1 and Table S1). Sediment cores were collect- southern flank at ∼500-m depth (Figure 1b). When possible, duplicate sediment cores were collected from ed using a K/C Denmark multicorer on the trawled northern flank of Palamós Canyon and on the untrawled independent multicore deployments to account for spatial heterogeneity, although this was only possible during some cruises due to time restrictions and sea state conditions. Bad weather conditions in February 2017 only permitted the retrieval of one sediment core in the trawled site, whereas the untrawled site could not be sampled. From each deployment, the sediment core with the best preservation of the surface-water interface was sliced at 1-cm intervals on deck and stored at −20°C until lyophilization in the laboratory. 2.2. Analyses Concentrations of 210Pb were determined through alpha spectroscopy of the decay product 210Po following the method described by Sánchez-Cabeza et al. (1998). Average sedimentation rates were calculated us- ing excess 210Pb concentrations over accumulated dry mass with depth following the Constant Flux: Con- stant Sedimentation model (Krishnaswamy et al., 1971). Concentrations of 226Ra were obtained through the gamma ray emissions of its decay product 214Pb using calibrated geometries in a well-type high-purity germanium gamma detector. Concentrations of 234Th (t1/2 = 24.1 days) were measured by gamma spectrom- etry through its emission line at 63 keV within one half-life since sampling, restricting measurements of sediment cores to the upper 3–6 cm. Samples were remeasured after all excess 234Th had decayed to obtain 238 U concentrations and quantify excess 234Th concentrations. Gamma measurements also permitted the quantification of the short-lived cosmogenic radionuclide 7Be (t1/2 = 54 days; 478 keV) and the artificial radionuclide 137Cs (t1/2 = 30 years; 662 keV). Dry bulk densities of sediment cores were calculated by dividing the net dry weight, after correcting for salt content, by the sample volume. Sediment grain size was measured using a Horiba Partica LA-950V2m particle-size analyzer by first oxidizing 1–4 g of dry sediment with H2O2 (20%) and disaggregating it using P2O7− (2.5%) left overnight. Elemental analysis of total carbon, organic carbon (OC), and total nitrogen (TN) contents were carried out with an elemental analyzer (Costech ECS Analyzer-4010), following the proce- dure described by Nieuwenhuize et al. (1994). Inorganic carbon, quantified as the difference between total carbon and OC, was converted to CaCO3 contents using the molecular mass ratio of 100/12. Dry bulk densi- ties, sediment grain size, and elemental analyses were performed in the upper 10 cm of all sediment cores. Sedimentary OC yields a suite of biomarkers upon alkaline CuO oxidation that can be grouped into lignin-derived phenols (vanillyl, syringyl, and cinnamyl phenols), cutin acids, benzoic acids, p-hydroxyben- zenes, amino acid-derived products, di-carboxylic acids, and fatty acids (Table S2). These biomarkers have distinct chemical precursors and its origin can be classified asterrestrial and heterogenous, the latter as a result of a combination of both terrestrial and marine sources (Table S2). CuO oxidations were conducted at specific depths (0–1, 2–3, 4–5 cm) in all sediment cores following the method described by Goñi and Mont- gomery (2000). Individual reaction products (Table S2) were quantified through selective ion monitoring by gas chromatography-mass spectrometry using a Hewlett Packard 6890GC fitted with a HP5973 mass selective detector as previously described by Goñi et al. (2009). 2.3. Statistical Analysis Permutational two-way analysis of variance was employed to assess whether there were statistical differ- ences of all parameters between sites (trawled, untrawled), sampling season (June 2017, October 2017, and March 2018), and the interaction between site and sampling season, using 9,999 permutations. Since no untrawled core was collected in February 2017, the trawled sediment core was excluded from this analysis. This statistical analysis was performed for grain size, CaCO3, OC, TN, and OC/TN in the upper 10 cm of all PARADIS ET AL. 4 of 12 Geophysical Research Letters 10.1029/2020GL091279 Figure 2. Inventories of excess 210Pb (a), 137Cs (b), excess 234Th (c), and 7Be (d) for trawled (red) and untrawled (green) sites. Arrows above excess 210Pb and excess 234Th inventories indicate that supported concentrations were not reached, thus values must be regarded as lower estimates. Inventories for 137Cs correspond to the upper 2.5 g·cm−2 cumulative dry mass of each core to allow comparison. Be-7 was only detected in the top-most sediment of the February 2017 trawled core. Replicate cores are indicated by A and B codes. sediment cores, and for CuO products in sections 0–1, 2–3, 4–5 cm of all sediment cores, including duplicate cores. Principal components analysis (PCA) was performed on dry bulk density, grain size (proportion of sand, silt, and clay), as well as concentrations of OC, TN, CaCO3, and CuO products in sections 0–1, 2–3, 4–5 cm of all sediment cores. To identify the variables that mostly determine trawled and untrawled samples, a linear discriminant analysis (LDA) was performed on the results of the PCA to find the boundary line that best separates these two classes. Both PCA and LDA were performed using Python's scikit-learn module (Pedregosa et al., 2011). Methods employed to obtain information on fishing effort and environmental conditions are provided in the Texts S1 and S2. 3. Results and Discussion 3.1. Bottom Trawling Erodes the Seafloor Sediment cores collected on the trawled site were generally depleted in excess 210Pb and 137Cs inventories in comparison to sediment cores collected on the untrawled site (Figures 2a and 2b), indicating a high degree of erosion by trawling activities. Two of the trawled sediment cores, February 2017 (TR-A) and March 2018 (TR-B), presented a deep penetration of excess 210Pb and higher inventories than the remain- ing trawled cores (Figures 2a and S2), a trait attributed to either intense sediment mixing by trawling gear, and/or the lateral displacement of sediment observed in trawl marks (Martín et al., 2014a; Paradis et al., 2017, 2018). In contrast, all untrawled sediment cores collected in the southern flank had high excess 210Pb inven- tories (Figure 2). Their surface layers presented altered excess 210Pb concentrations (Figure S2), which could indicate intense mixing or an increase in sedimentation rates associated with the lateral transport PARADIS ET AL. 5 of 12 Geophysical Research Letters 10.1029/2020GL091279 Figure 3. Multivariate analyses of sediments from trawled and untrawled sites. (a) PCA of sedimentological parameters (dry bulk density, grain size), elemental (CaCO3, OC, and TN) and CuO oxidation products in trawled (red) and untrawled (green) samples. The dashed line indicates the LDA boundary line separating trawled and untrawled samples. (b) LDA coefficients of all parameters. Abbreviations: VP, vanillyl phenols; SP, syringyl phenols; CP, cinnamyl phenols; CA, cutin acids; PB, p-hydroxybenzenes; BA, benzoic acids; AA, amino acids; DA, di-carboxylic acids; FA, fatty acids. of resuspended sediment from trawling grounds (Martín et al., 2014c). Below these altered layers, ex- ponentially decreasing excess 210Pb concentration profiles allowed to estimate sedimentation rates of 0.37–0.85 cm yr−1 (Figure S2 and Table S2), in agreement with values previously reported in the area (Martín et al., 2014a). With the exception of the sediment cores from February 2017 (TR-A) and March 2018 (TR-A), trawled sed- iment cores generally had comparable excess 210Pb inventories and penetration depths during all sampling seasons (Figures 2a and S2), consistent with measurements from a sediment core collected in the same site in 2011 (core NF-4 in Martín et al. (2014a)). These similarities suggest that the rate of erosion in the north- ern flank of Palamós Canyon is relatively constant and is compensated by the rate of sediment accumula- tion in the area (0.33 and 0.63 g cm−2 yr−1; Table S3). Hence, we can estimate that the rate of erosion ranges between 3 and 6 kg m−2 yr−1, which, if extrapolated to the entire northern flank trawling ground (42 km2), would amount to up to 125–250 × 103 tones of sediment eroded annually. Despite the almost-daily trawling frequency on this fishing ground (Figure S1c), which erodes surface sed- iments, excess 234Th inventories were generally similar in trawled and untrawled sites in each sampling period, with the exception of the thoroughly mixed March 2018 TR-A core (Figure 2c). Trawled sediment cores usually had shallower penetration depths and higher superficial excess 234Th concentrations in com- parison to untrawled sites (Table S2 and Figure S2). This fact suggests that some of the sediment resuspend- ed by bottom trawlers is redeposited on fishing grounds, scavenging excess 234Th from the water column in the process. This continuous resuspension and partial redeposition of sediments causes grain-size sorting through the selective advection of fine-grained sediment, whereas coarser sediment is preferentially rede- posited. This process progressively increases dry bulk densities as well as the sand content of the trawled site (Figure S3), causing bed armoring effects (Martín et al., 2014a). PARADIS ET AL. 6 of 12 Geophysical Research Letters 10.1029/2020GL091279 3.2. Bottom Trawling Hampers the Preservation of Organic Matter Chronic and intense bottom trawling activities not only altered sedimentological properties, but also mod- ified the biogeochemical composition of sediments through the relative enrichment of trawling grounds in CaCO3 and the reduction of OC, TN, and biomarkers (Figures S4 and S5). Based on the results of the PCA, it is apparent that sediments in bottom trawling grounds have lower sedimentary organic matter content (Figure 3). The LDA coefficients highlight a particular impoverishment of labile biomarkers, such as amino acid-derived and fatty acid-derived compounds in trawled sediments, and a predominance of refractory compound classes such as p-hydroxybenzenes and vanillyl phenols (Figure 3b). Predominance of refractory terrestrial compounds in trawled samples was mostly driven by the surficial sediments of the trawled core retrieved in February 2017 after a Ter River flood (see Section 3.3 for more detail). Bulk concentrations of elemental OC and TN in the trawled site were ∼30% lower than in the untrawled site (Figure 4a). Similar differences were observed for the more refractory compound classes such as lignin phenols (vanillyl, syringyl, and cinnamyl phenols) and both p-hydroxybenzenes and benzoic acids (−24% to −33%). In contrast, the more labile compound classes such as di-carboxylic acids, fatty acids, amino acid-de- rived products, and cutin acids displayed considerably greater depletions (−52% to −70%) in the trawled site relative to the untrawled site consistent with enhanced losses (Figure 4a and Table S4). The lower bulk organic matter contents in the trawled site could be due to winnowing of fine-grained sediments, which are typically characterized by higher organic matter content (e.g., Mayer, 1994; Smeaton & Austin, 2019; see Figure S6). After normalizing concentrations to clay and silt content, the differences between trawled and untrawled sites for the most refractory compound classes became insignificant (Fig- ure 4b and Table S5), suggesting that removal of fine-grained sediments by winnowing is responsible for the losses of these compounds (Keil et al., 1998). However, even after normalizing by grain size, the trawled site cores still had significantly lower contents of OC, TN, and labile compounds than the untrawled cores (Figure 4b and Table S5), indicating that winnowing of organic matter-rich fine-grained sediment is not the only mechanism accounting for the lower organic matter content in the trawled site. In fact, the compositional differences of sedimentary organic matter between trawled and untrawled sites obtained after normalizing biomarker concentrations to OC contents show that organic matter from both sites had comparable contents of the more refractory compounds, whereas the trawled site was still signif- icantly impoverished in labile biomarkers (−31% to 69%; Figure 4c and Table S6), suggesting a preferential degradation of these most reactive compounds. This degradation could be driven by greater sediment mix- ing by bottom trawling gear (Ferguson et al., 2020; van de Velde et al., 2018), which would be enhanced by increasing the permeability of the seafloor with coarser sediment (e.g., Keil, 2017; Rao et al., 2007). Additionally, the shift of benthic community structure in this trawling ground to more opportunistic life strategies (Pusceddu et al., 2014) points to a rapid and efficient degradation of nutritious organic matter, as observed in other deep-sea trawling grounds (Paradis et al., 2019). The lower contents of all organic matter compound classes in trawled sediment cores indicate that bottom trawling removes the organic carbon stock from trawled continental slope sediments, through the erosion of organic matter-rich fine-grained sediment and the enhanced degradation of the more labile compounds. 3.3. Bottom Trawling Causes Persistent Impacts on the Seafloor Despite the statistically significant seasonal variations of certain biomarker contents, the impacts caused by bottom trawling generally persisted throughout the year, with a substantial impoverishment of organic matter in the trawled site in comparison to the untrawled site (Figure 4 and Tables S4–S6). Greater arrival of fresh sediment to the seafloor were recorded in February and June 2017, as evidenced by their high excess 234 Th inventories (Figure 2c). Enhanced particle export during the spring phytoplankton bloom in May 2017 (Figure S1c) led to a slight increase in OC and TN contents in surficial sediments of both sites in June 2017 (Figure S4). On January 23, 2017, a strong storm produced significant wave heights greater than 6 m (Figure S1a) and a torrential Ter River discharge of 130 m3 s−1 (Figure S1b). These type of winter storms, when combined with flash floods, can transport large volumes of particulate organic matter to deeper parts of the margin (Martín et al., 2006; Sánchez-Vidal et al., 2013). This event supplied high concentrations of PARADIS ET AL. 7 of 12 Geophysical Research Letters 10.1029/2020GL091279 Figure 4. Relative difference in concentrations of organic matter compounds in trawled cores versus untrawled cores during the different sampling periods for bulk concentrations (a), after normalizing for fine-grained sediment (b), and after normalizing for OC contents (c). Dots indicate the average differences and the floating bars represent errors derived from standard deviations. Asterisks denote statistically significant differences between trawled and untrawled sites (Tables S4, S5, and S6): ***P < 0.001, **P < 0.01, *P < 0.05, and n.s., not significant. the short-lived radionuclide 7Be (Figure 2d) that were detected in surficial sediments two weeks later in the February 2017 core, as well as high concentrations of OC, TN, and biomarkers of terrestrial origin (i.e., cinnamyl phenols and cutin acids from nonwoody plant tissues and soil derived benzoic acids) (Figure S5). Despite the arrival of fresh organic matter by these natural processes, the contents of most biomarkers in the trawled site were always significantly lower than in the untrawled counterpart, a condition that pre- vailed even at the end of the trawling closure in March 2018 (Figure 4). These results reveal that, although a 2-months seasonal trawling closure could allow fish stocks to rebuild (Martín et al., 2019), it is insufficient to restore the quantity and quality of organic matter in sediments of deep-sea trawling grounds. Indeed, with sedimentation rates of 0.37–0.85 cm yr−1 (Table S3), a trawling break of 2 months would only allow the accumulation of approximately 0.05–0.15 cm of sediment, which PARADIS ET AL. 8 of 12 Geophysical Research Letters 10.1029/2020GL091279 cannot lead to measurable recoveries in these disturbed environments. With such low sedimentation rates, it would take between 10 and 30 years to accumulate at least 10 cm of sediment and slightly restore the sea- floor. This period could actually be longer considering that trawled deep-sea sites have a greater dependence on the arrival of fresh and nutritious organic matter, which is rapidly consumed upon deposition (Paradis et al., 2019). The slow rate of recovery of the sedimentary structure of the seabed and associated organic matter in trawled areas may inhibit the restoration of benthic communities at such depths (Clark et al., 2019; Goode et al., 2020). It is hypothesized that deep-sea environments would need around half a century to naturally recover from the damage exerted by bottom trawling (Da Ros et al., 2019), whereas shallower environments may present recoveries in a few weeks (Ferguson et al., 2020). Hence, the loss of organic carbon from trawl- ing grounds on continental slopes, which are considered preferential carbon burial areas due to their large extension and infrequent natural disturbances (de Haas et al., 2002; Gordon & Goñi, 2004), may become irreversible. The harmful effects of bottom trawling activities have been compared to forest clear-cutting on land (Wat- ling & Norse, 1998), but the effects of bottom trawling on the seafloor may be more akin to the effects of con- tinuous plowing by intensive agricultural practices (Puig et al., 2012). In agricultural fields, tillage on land is responsible for 66–90 Gt of organic carbon lost due to accelerated erosion and mineralization (Lal, 2004), tensive bottom trawling activities. Using the ∼4.4 million km2 spatial coverage of world continental slope and our results indicate that a similar and comparable effect occurs on the seafloor due to chronic and in- these fishing grounds are estimated to store ∼20 Pg of OC in the upper meter. Assuming that ∼30% of OC trawling grounds (Puig et al., 2012) and the global distribution of sediment OC stock (Atwood et al., 2020), preserved in sediments is lost by bottom trawling, ∼60 Gt of OC could have been removed from deep-sea trawling grounds, considering only the upper-most centimeter of the seabed. These estimates could be even greater taking into account that bottom trawling gear often penetrates deeper into the sediment (Oberle et al., 2016b). However, bottom trawling continues to intensify and expand to new unexploited grounds, with little regulation on the type of gear used and with the implementation of ineffective management strat- egies, such as seasonal trawling closures. Considering the wide extension of bottom trawling grounds and their continuous expansion to deeper areas, the disturbances by bottom trawling could be causing extensive and long-lasting impacts on the seafloor, leading to the release of vast amounts of carbon stored in deep-sea sediments. While more studies are needed to fully understand its implications and the magnitude of these impacts as a driver for global change, our findings are a clear indication of the severity and potential global Acknowledgments implications of this fishing practice. The authors would like to thank the crew of the R/V García del Cid, as well as Elena Martinez, Neus Maestro, and Silvia de Diago, who helped collect the Data Availability Statement samples. The authors are also grateful to Miquel Junyent for his help program- Data sets for this research are available in Paradis et al. (2020) at https://ddd.uab.cat/record/225679. ming the fishing effort algorithm and to the Spanish Ports Authority for the ac- cess to AIS data. This study was funded by the “Assessment of Bottom-trawling References Impacts in the Deep-sea Sediments” (ABIDES) Spanish Research Project Armstrong, M. P., Dean, M. J., Hoffman, W. S., Zemeckis, D. R., Nies, T. A., Pierce, D. E., et al. (2013). 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