Reduction of carbon, alkalinity and nutrient fluxes in the southern Baltic Sea caused by dragging of otter trawl nets across the seafloor

Research article Biogeosciences, 22, 6727–6748, 2025 https://doi.org/10.5194/bg-22-6727-2025 © Author(s) 2025. This work is distributed under the Creative Commons Attribution 4.0 License. Reduction of carbon, alkalinity and nutrient fluxes in the southern Baltic Sea caused by dragging of otter trawl nets across the seafloor Pankan Linsy1 , Stefan Sommer1 , Jens Kallmeyer2 , Simone Bernsee2 , Florian Scholz3 , Habeeb Thanveer Kalapurakkal1,a , and Andrew W. Dale1 1 GEOMAR Helmholtz Centre for Ocean Research Kiel, Wischhofstraße 1–3, 24148 Kiel, Germany Helmholtz Centre for Geosciences, Telegrafenberg, 14473 Potsdam, Germany 3 Institute for Geology, Center for Earth System Research and Sustainability, Universität Hamburg, Hamburg, Germany a now at: Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research, 27570 Bremerhaven, Germany 2 GFZ Correspondence: Pankan Linsy () Received: 18 June 2025 – Discussion started: 27 June 2025 Revised: 16 October 2025 – Accepted: 17 October 2025 – Published: 13 November 2025 Abstract. Mobile bottom-contacting fishing (MBCF) represents a substantial anthropogenic disturbance, significantly disrupting seafloor integrity and altering oceanic carbon storage. In this study, we conducted a benthic trawling experiment on organic-rich muddy sediments in the Mecklenburger Bight, southern Baltic Sea, employing an otter trawl. Multiple trawl tracks were made to assess the temporal impact of bottom fishing on the benthic ecosystem over time scales ranging from days to weeks. Focus was on the wide area where the net footrope was dragged between the otter boards, rather than on much smaller area impacted by the trawl doors. This study constitutes the first comprehensive investigation to systematically monitor the effects of MBCF on benthic oxygen, carbon, alkalinity and nutrient fluxes using autonomous in situ lander measurements. Seafloor observations revealed a profound impact of trawling on seafloor morphology. Flux measurements, coupled with sediment data, indicated reductions in benthic fluxes of O2 , dissolved inorganic carbon (DIC), total alkalinity (TA), and nutrients + (PO−3 4 , NH4 , and H4 SiO4 ) within trawled areas compared to control sites. Additionally, observed decreases in organic carbon remineralization rates suggest that MBCF alters benthic respiration by disrupting key biogeochemical processes. Fluxes of O2 , DIC, and TA had not returned to baseline levels by the conclusion of the 16 d observation period, indicating prolonged disturbance effects, although natural temporal variations may have an influence. Despite substantial alterations to the benthic biogeochemical pathways, modeling suggests that the reduction in benthic DIC and TA fluxes exerts only a minor influence on CO2 release to the atmosphere compared to the potential impact of pyrite oxidation in resuspended sediment. 1 Introduction Mobile bottom-contacting fishing (MBCF) is one of the major anthropogenic activities that significantly affect the marine environment (Depestele et al., 2019; Hiddink et al., 2006; Oberle et al., 2016b; Olsgard et al., 2008; Pusceddu et al., 2014). Trawling involves catching benthic and demersal fish by towing nets along the seafloor using various devices. MBCF gear can penetrate the seafloor and dislocate the sediment to varying depths depending on the sediment type and gear used (Martín et al., 2014). Approximately 4.9 million km2 or 1.3 % of the global ocean is estimated to be trawled each year (Sala et al., 2021). The impacts of MBCF include the alteration of seabed morphology through the scraping, ploughing, and resuspension of surface sediments (Bruns et al., 2023; Oberle et al., 2016b). These processes ultimately disrupt benthic biogeochemical cycles (Allen and Clarke, 2007; Bradshaw et al., 2021; Bruns et al., 2023; Hale et al., 2017; Morys et al., 2021; Sciberras et al., 2016; Tiano et al., 2019; van de Velde et al., 2018). Additionally, MBCF has been shown to significantly reduce benthic fauna biomass (Bergman, 2000; Bergman and Meesters, 2020; Tiano et al., 2022) and, ultimately, change benthic Published by Copernicus Publications on behalf of the European Geosciences Union. 6728 P. Linsy et al.: Trawling induced-reduction of carbon, alkalinity and nutrient fluxes in the Baltic Sea community structures (Bradshaw et al., 2024; Kaiser et al., 2002; Pusceddu et al., 2014). The seafloor disturbance caused by trawling has been argued to disrupt oceanic carbon sequestration (Atwood et al., 2024; Epstein et al., 2022; Hiddink et al., 2023; Sala et al., 2021; Zhang et al., 2024) as marine sediments serve as a critical reservoir for long-term carbon storage (Burdige, 2007; LaRowe et al., 2020). The study by Sala et al., (2021) suggests that seafloor disturbances caused by trawling and dredging result in the annual release of 0.58–1.47 Pg of CO2 , largely due to increased remineralization of particulate organic carbon (POC). Atwood et al. (2024) estimated that 50 %–60 % of the CO2 released due to trawling is emitted into the atmosphere over a decade, accounting for approximately 0.34–0.37 Pg CO2 yr−1 globally. In particular, trawling in shelf seas has been shown to reduce POC by 29 % (Porz et al., 2024), with long-term losses equivalent to emissions of 3.67 Mg CO2 km−2 yr−1 , assuming complete mineralization of the disturbed POC (Zhang et al., 2024). However, a subsequent metadata analysis by Epstein et al., (2022) yielded mixed findings on the impact of trawling on POC stock, revealing no significant effect in 61 % of the 49 studies analyzed. Among the remaining studies, 29 % reported decreased POC levels associated with fishing activities, while 10 % observed an increase in POC. These results indicate that the impact of MBCF is site-specific and probably influenced by factors such as trawling frequency, type of fishing gear, local lithology, and towing intensity (De Borger et al., 2021; Depestele et al., 2019; Oberle et al., 2016a; Tiano et al., 2019). Thus, site-specific studies are necessary for studying the impact of MBCF in any given region (Stephens and McConnaughey, 2024). Bottom fishing is carried out over large swathes of the Baltic Sea (HELCOM, 2018), and approximately 36 % of the seafloor in the southwestern Baltic Sea is affected (DíazMendoza et al., 2025). A study by Van Denderen et al. (2020) revealed that trawling, together with bottom water hypoxia, has led to a 50 % reduction in benthic biomass in 14 % of the Baltic Sea region. The impact of MBCF on the benthos in the Baltic Sea has been the subject of numerous studies (Bradshaw et al., 2021, 2024; Bunke et al., 2019; Corell et al., 2023; Morys et al., 2021; Porz et al., 2023; Rooze et al., 2024; Schönke et al., 2022), but relatively few have examined the impact on benthic biogeochemistry (Bradshaw et al., 2021, 2024; Morys et al., 2021; Rooze et al., 2024; Tiano et al., 2024). Morys et al. (2021) conducted a controlled benthic dredging experime (...truncated)