The study area is located in the north-west Baltic Proper (Figure 1). The area is quite sheltered from all wind directions by the mainland and numerous surrounding islands. The seabed where the trawling was carried out comprises unconsolidated gyttja clay sediments at 20–35 m depth (Jakobsson et al., 2020; Supplementary Figure 2). The area is non-tidal and water currents are generally slow.

A small (12 m × 4 m) commercial trawler with an otter trawl was used (Figure 2) to create a single trawl track. The two metal trawl doors, 110 cm high and 160 cm long, weighing c. 230 kg each, were connected by 50 m sweeps to the headline and groundgear which comprised a row of rubber discs. The net was 65 m long and kept open vertically by six groups of three floats attached approximately half way between the headline and the codend.

The bathymetry of the area was acoustically mapped with a multibeam echo sounder (EM2040, see below) in May 2018. Immediately prior to the experimental trawling (22 October 2018), and along the planned trawl track, two CTD (Conductivity, Temperature, Depth) and water sample profiles were taken, and an Acoustic Doppler Current Profiler (ADCP; Workhorse Mariner, 600 kHz) transect taken to ascertain water current speed and direction. The trawl was then set at 58°50.8215′ N, 017°41.8905′ E, towed at a normal fishing speed of c. 2.5 knots in a SW to NE direction, and hauled at 58°49.6027′ N, 017°39.5284′ E (Figure 1), with an open codend. Stockholm University’s R/V Electra followed behind the fishing boat, performing acoustic geophysical mapping of the seafloor and water column along the whole length of the trawl track with the multibeam echo sounder (Figure 1 and Supplementary Table 1). A distance of c. 300 m was maintained to ensure that the acoustic instruments measured just behind the end of the trawl net. The geophysical mapping methods are described in detail in section “Geophysical Mapping, Processing, and Analyses” and Supplementary Section 2.1).

Further CTD casts and water sampling were done 2 h, 20 h, 3 days, and 4 days after trawling (Figure 1 and Supplementary Table 2) to measure the spread of the trawl-suspended sediment. Sediment samples were taken using a 4-core multicorer, 1 and 2 days after trawling, within the trawl track and c. 100 m away from the trawl track (Figure 1 and Supplementary Table 3). Bottom water current direction and speed were determined with ADCP along two transects 20 h and 3 days post-trawling (Supplementary Section 3.1). Four days after trawling, sub-bottom profiles were acquired with a penetrating echo sounder along the trawl track to characterize the underlying sediments (Supplementary Section 2.4).

High-resolution bathymetric and water column data were acquired using R/V Electra’s hull-mounted Kongsberg EM2040 0.4° × 0.7°, 200–400 kHz, multibeam echo-sounder. Position, heading and attitude (heave, pitch and roll) data were received from a Kongsberg-Seatex Seapath 330+ Global Navigation Satellite System (GNSS), making use of both GPS and GLONASS satellites and an attached Seatex MRU5+ motion and reference sensor. The Seapath 330+ capacity of applying real-Time Kinematic (RTK) positioning was used and corrections were imported from SWEPOS¹. The Seapath 330+ reported maximum ellipses of Estimated Position Error (EPE) on the order of 2–5 cm during the surveys. An AML sound velocity probe mounted near the multibeam transducers gave continuous readings of surface sound speed. In addition, a Valeport Mini SVP (Sound Velocity Profiler) was used to acquire sound speed profiles in the survey area.

Sub-bottom profiles were acquired using the hull-mounted Kongsberg Topas PS40, 24 channel, parametric sub-bottom profiler operating with primary and secondary frequencies of 35–45 kHz and 1–10 kHz, respectively. The sub-bottom profiles shown were acquired using the system in chirp mode with a 1 ms long 2–10 kHz pulse. This system also received positions from the Seapath 330+.

The multibeam bathymetry data were cleaned and initially analyzed using the QPS QIMERA software (Version 2.1.0)², where grids were produced representing the seafloor bathymetry with resolutions of 0.5 × 0.5 m to 0.25 × 0.25 m. Multibeam backscatter was processed using QPS FMGT (Version 7.9.2) and the acoustic water column information with QPS FM Midwater (Version 7.9.0). The processed mapping data were subsequently imported to the Open Source Geographic Information System QGIS, version 3.10.0 A Coruña³, for further analyses and map making. The sub-bottom profiles were processed and displayed using software tools provided by the Geological Survey of Canada, courtesy of Bob Courtney.

The area was initially mapped with multibeam from 2 to 4 May 2018 (Figure 1), i.e., about 5.5 months before the trawling on 22 October the same year. The bathymetry data from October were adjusted by 2 cm to account for differences in seafloor bathymetry and/or sea level between the two dates (see Supplementary Section 2.1 for details). The corrected bathymetric surface produced after trawling (October) was subtracted from the surface before the trawling (May) for two 6 m-wide, 900 m-long corridors along each of the door tracks. This produced a detailed map, where positive values indicated depressions and negative values piles of sediment on the seabed. The volume of the furrows and piles produced by the trawling was also calculated from these corridors. Ten 20 m-long boxes along the track were identified as having the highest quality data and, of these, Boxes 2 and 7 were selected for detailed analyses of the dimensions of the trawl track features, since the small-scale topography in these two areas was most distinct, with between-survey differences close to 0 m outside the trawl marks.

The trawl track was re-mapped with multibeam on April 7, 2020, to investigate if the tracks still were visible in the seafloor or if they had been degraded by bottom processes, i.e., sedimentation, erosion, or bioturbation. This was followed by visual inspections by divers, as described below.

Four days after trawling (26 October 2018), a 25 m-long part of the track was inspected using a camera mounted on a BlueROV2 Remotely Operated Vehicle (ROV) equipped with an underwater positioning system linked to GPS (Figure 1). A second inspection was done on 7 April 2020 by divers in the same place as the 2019 ROV survey (Figure 1).

CTD casts were taken using a Seabird 911+ (Seabird Scientific) down to 0.5 m above the bottom (m.a.b.), continuously measuring depth, salinity, temperature, and turbidity (post-processing binned the data to every 0.25 m). A Rosette sampler fitted with twelve 5 L Niskin bottles was used to take water samples at various distances above the seabed, with a focus on the bottom water (0.5–10 m.a.b.).

At each water depth, water for dissolved methane analysis was immediately collected directly through silicone tubes from the CTD rosette bottles into 100 mL serum bottles and allowed to overflow twice. One hundred microliters of a 50% zinc chloride (ZnCl₂) solution was added to stop all biological activity and the bottles immediately sealed with butyl rubber septa and crimped, ensuring no air bubbles were trapped.

The rest of the water sample was mixed well and different subsamples taken. A known volume (c. 1 L) of water was filtered on a pre-weighed and pre-burnt 47 mm GF/F filter for quantification of suspended particulate matter (SPM) and particulate organic matter (POM). A further two samples were filtered in the same way for analysis of; (1) particulate carbon and nitrogen; and (2) particulate phosphorus. In between samples, funnels and filters were rinsed with 50 mL MilliQ water to remove any salt. All filters were frozen immediately at −20°C in foil. Duplicate 10 mL water samples were filtered through a disposable 0.47 μm filter into 2 vials for dissolved nutrient analysis [(NO₃^– + NO₂^–)-N, NH₄⁺-N, PO₄^3–-P, hereafter referred to as NO_x, NH₄, and PO₄] and frozen immediately at −20°C. Twelve millilitres of water was filtered through a 0.2 μm disposable filter into a vial for dissolved element analysis (Al, Fe, Mn, P, Ti) and 100 mL water was saved unfiltered in a factory clean plastic bottle for total element analysis. The vials and bottles were pre-filled with a volume of concentrated suprapure HNO₃ equivalent to 1% of the sample volume. Both were kept cool (8°C) and dark. MilliQ blanks were taken for all these analyses.

A multicorer (K.U.M. Umwelt und Meerestechnik Kiel), fitted with four acrylic core tubes [inner diameter (i.d.) 9 cm, height 60 cm] was used to take sediment cores. A total of 11 cores were taken 24 h after trawling at different locations within and outside the trawl track (Figure 1 and Supplementary Table 3), and sampled for a range of measurements (see below). Four further cores from the trawl door track and four from 100 m outside the trawl track (Figure 1 and Supplementary Table 3) were taken 2 days after trawling for O₂ microprofiling and for nutrient flux incubations. In these eight cores, the overlying water was removed and collected in one container per station for later use in the incubations. Each core was subsampled using a smaller Plexiglas core tube (i.d. 4.6 cm, length 30 cm). These minicores were capped top and bottom with rubber stoppers, ensuring no air bubbles in the overlying water, and immediately placed in a dark cool box for transport to the laboratory within a few hours of collection.

Porewater samples were extracted from three cores at 1–5 cm intervals (closer intervals near the sediment surface) with rhizons (Rhizosphere Research Products) (Seeberg-Elverfeldt et al., 2005). The rhizons had been pre-treated for 2 h in 2 M HCl, followed by two rinses with deionized water for 2 h and final storage in deionized water. The overlying water was drained and rhizons were connected to 10 mL disposable plastic syringes via 3-way luerlock stopcocks and inserted through tight-fitting, pre-drilled holes in the sediment tubes. The first mL of pore water was discarded from the syringe. Then two samples of c. 1 mL porewater were collected in separate Eppendorf tubes for analysis of dissolved elements and dissolved nutrients. No more than 2 mL were collected from each core to prevent cross-contamination of adjacent intervals (Seeberg-Elverfeldt et al., 2005) and samples were stored frozen.

Three cores were used for measurements of porewater dissolved methane. A sediment sample of 2.5 mL was taken with a 3 mL cut-off syringe through the side of taped, pre-drilled core tubes at 2 cm intervals. The sample was directly transferred to a 20 mL serum vial containing 5 mL 5 M NaCl, immediately closed with a thick septum and an aluminum crimp seal and shaken thoroughly to produce a slurry (Sawicka and Brüchert, 2017).

Four cores were sliced at 1 cm intervals down to 8 cm and thereafter 1 cm slices taken at 9–10 cm, 14–15 cm, 19–20 cm, and 24–25 cm deep. Each slice was frozen in a separate ziplock bag for analysis of sediment porosity, organic carbon content and grain size.

Sediment microprofiling for O₂ concentrations was conducted in two 4.6 cm i.d. cores using a Clark-type microelectrode (OX-50, Unisense), one core from the trawl track and one from outside the track. Three replicate microprofiles were carried out in each core. The microelectrode tip (50 μm) was inserted directly inside each sediment core, while an air stone was bubbling air to ensure sufficient water mixing during the measurements. The sensor was calibrated using a two-point calibration procedure in O₂ saturated bottom water and inside the sediment (zero reading). Oxygen penetration depth (OPD) was defined as the depth where O₂ concentration was consistently <1 μmol L^–1.

The eight 4.6 cm i.d. cores were used for a nutrient flux incubation experiment. All cores were fitted with individual magnetic stirrers in the overlying water and placed un-capped in a temperature-controlled incubator at in situ temperature (6°C) in the dark. The overlying water was carefully replaced with in situ bottom water, avoiding sediment resuspension, so that all cores were exposed to the same overlying water. After an 8 h stabilization period, a 15 h incubation was performed to estimate nutrient and oxygen fluxes across the sediment-water interface. Dissolved oxygen measurements and water samples for nutrients were taken immediately prior to capping the cores for the incubation and immediately after the stoppers were removed. Oxygen was measured directly using a microelectrode (OX-500, Unisense), calibrated with 0 and 100% O₂-saturated in situ bottom water (bubbled with N₂ or air, respectively). Water samples for nutrients were filtered on a GF/F filter and frozen prior to analysis of NO_x, NH₄ and PO₄. Net solute fluxes were calculated from the difference between final and initial concentrations according to Bonaglia et al. (2013). Positive values represent effluxes while negative values represent sediment uptake.

All statistics [analysis of variance (ANOVA), principle component analysis (PCA), multiple correlations] and contour plots were done with Statistica v. 13.3. Further details of the calculations are presented in the Supplementary Sections 2.2, 2.3, 3.2.1, 3.3.1, but a brief summary is given here.

The tracks created by the trawl doors comprised two regular rows of pits with sediment piles on the sides, rather than smooth consistent furrows (Figure 3). The piles of displaced sediment were particularly pronounced on the inner side of the furrow (Figures 3, 4C).

Using the areas with best quality multibeam data (Boxes 2 and 7, Figure 3b), the mean width of the furrows was 1.6 m and ranged from 6 to 12 cm deep. In the entire analyzed corridor shown in Figure 3b, the maximum furrow depth was 16 cm. The sediment piles on the inner sides of the trawl doors were higher than on the outer side of the track and were 1.4–1.9 m wide and up to 8 cm high in the whole corridor. These values should be seen as minimum measures of the average maximum dimensions as the measurements are made between the −2 cm contours.

In the area between the two trawl door tracks, shallow parallel depressions were seen in ROV images (Figure 4B). These were produced by the rollers on the groundgear moving over the seabed and had a depth of c. 2 cm, since they were only occasionally visible in the multibeam data which has a vertical resolution in the order of ±0.1% of the water depth, i.e., ±2 cm in 20 m water depth.

The furrows of the trawl door tracks were estimated from multibeam bathymetry to occupy volumes of 0.24–0.34 m³ m^–1 of track while the sediment piles range between 0.14 and 0.24 m³ m^–1 (Table 1). The relative contributions of the groundgear and trawl doors to the sediment disturbance were estimated; per m² disturbed, the doors have a larger impact (by a factor of c. 5–6). However, the total volume or mass disturbed for every m the trawl is dragged is potentially greater (factor of c. 2–3) for the groundgear, since the area affected comprises a relatively large proportion of the total track (92% of the total area and 70% of the total disturbed volume) (Supplementary Section 2.3). A total of about 1 m³ or 12 kg sediment was therefore disturbed per m of seabed trawled.

The trawl tracks may also be visible with sub-bottom profiling, since the locations of the trawl tracks in sub-bottom profile SBP 805 coincide with acoustically transparent spots (Supplementary Figure 2), which commonly appear where layered sediments are disturbed. Although several other similar spots are visible along the profile, it seems more than coincidental that these exactly match the trawl track locations. The vertical resolution of the sub-bottom profiles is on the order of 10–20 cm, so it is not impossible that the tracks, with their vertical topography of c. 20 cm, might be visible in this way.

The re-survey of the trawl tracks in April 2020, i.e., 18 months after they were formed, showed that the tracks were still preserved in the seafloor and clearly visible in multibeam images (Supplementary Figure 4). However, the backscatter signal was much fainter, and divers could not see the tracks.

During the week of the field experiment, the water column did not show strong stratification, but there were two water masses: an overlying stable layer (c. 7.5°C, salinity c. 7 psu) down to about 10–20 m water depth; and an underlying layer where temperature and salinity decreased steadily to c. 5.5°C and c. 7.3 psu at 0.5 m above the seafloor. There was a tendency for the surface layer to extend slightly deeper further to the SE. The depth of the overlying layer also increased during the week from c. 10 m to c. 20 m, particularly between 1 and 3 days after trawling.

Bottom water currents and speeds were spatially and temporally variable during the week. Current speeds were generally slow (<6 cm s^–1) and, in the area from the trawl track to where the suspended sediment was measured, the general direction of flow was between SW and SE, depending on the exact position, water depth and date (Figure 1 and Supplementary Figure 7).

Sediment profiles of porosity, organic carbon content, mean particle size and chlorophyll all suggested physical disturbance of the surface 3–5 cm in the trawl track (Supplementary Figure 5). Profiles of dissolved substances (e.g., Fe, Mn, nutrients, and dissolved methane) in the porewater also indicated disruption of the biogeochemistry of the surface sediment (Supplementary Figure 6). For several substances (e.g., chlorophyll, methane, porosity), these profiles confirm the removal or displacement of the surface sediment, and thus the apparent upward shift of these profiles compared to undisturbed sediment. The lack of a consistent type of disturbance across all cores is probably due to their exact positions relative to different features of the track.

The general appearance and dimensions we describe here, of deeper paired door tracks, c. 1.2–1.6 m wide and 6–12 cm (max. 16 cm) deep, to either side of less distinct parallel tracks caused by groundgear, are typical for otter trawl tracks on muddy seabeds (Krost et al., 1990; Tuck et al., 1998; Humborstad et al., 2004; Smith et al., 2007; Palanques et al., 2014). We clearly saw lateral displacement of sediment, particularly on the inside of the door furrow, as described by Gilkinson et al. (1998) and Ivanović et al. (2011). The broken pattern of the door track is also consistent with the “jumping otterboard” tracks described by Krost et al. (1990) and the “herring bone patterns” in Smith et al. (2007). Many of these studies used side scan sonar and video, the former allowing larger features to be seen over wide areas and the latter providing more detailed images of small-scale features. By using multibeam sonar we were able to combine these two aspects and provide track bathymetry data at a high horizontal and vertical resolution over >1 km of track. Only a few other studies have used multibeam sonar for assessing the physical impacts of bottom fishing gear; Malik and Mayer (2007) for scallop dredge tracks on coarse seabeds in the Gulf of Maine, and Depestele et al. (2016, 2018), Bruns et al. (2020), and Lüdmann et al. (2021) for beam trawls and otter trawls on soft seabeds in the North Sea. These studies have used multibeam data mainly to quantify the number of tracks in a given area and to estimate track dimensions, and sometimes depth penetration. In addition, we also used the multibeam data to quantify the total amount of sediment excavated and displaced; c. 1 m³ or 500 kg per m of trawl track.

In our study, the trawl track was clearly visible in multibeam bathymetry images 18 months later (Supplementary Figure 4). However, divers were unable to see any clear features, suggesting that the track may have been smoothed out and/or filled in by currents, sedimentation or bioturbation during this time. It appears, however, that the tracks just below the sediment surface are still well-preserved and detected by the multibeam since the acoustic signal penetrates the uppermost centimeters of the seafloor. These trawl-induced changes to surface seabed physical properties have the potential to affect the distribution and survival of benthic organisms that require particular sediment properties (e.g., water or organic carbon content) as well as altering biogeochemical gradients in the sediments. The persistence of the track is in agreement with other authors who have seen that, especially in areas with low background disturbance and on muddy seabeds, tracks may persist from months to years (Krost et al., 1990; Schwinghamer et al., 1998; Tuck et al., 1998; Smith et al., 2007; Palanques et al., 2014; Oberle et al., 2018; Bunke et al., 2019).

After the single passage of the trawl, a turbid plume (up to 6.9 mgDW L^–1) was detected in the water column for several days, up to about 10 m.a.b. and >1 km downstream from the trawl track. Turbidity values obtained during controlled field experiments vary widely, not only because of differences in gear types and towing speeds (O’Neill and Ivanović, 2016) or type of seabed (O’Neill and Summerbell, 2011; O’Neill and Ivanović, 2016) but also depending on how long after the trawling event, at what distance from the trawl track and height above the seabed the measurements were taken. Studies that have looked at immediate suspension have recorded turbidities of several hundred mg L^–1 close to the gear (e.g., Schoellhamer, 1996; Durrieu de Madron et al., 2005; Dellapenna et al., 2006; Dounas, 2006; Mengual et al., 2016), though these values decrease with time to become comparable to those seen in this study.

When sediment suspension has been calculated as a function of distance or area trawled, values of several hundred g per m² trawled are commonly reported (Dounas et al., 2005; Durrieu de Madron et al., 2005; Dellapenna et al., 2006; Bradshaw et al., 2012; Mengual et al., 2016), which agrees well with our estimate of 0.25 kg m^–2 (across the whole track). O’Neill and Summerbell (2011) showed that the trawl doors suspended more sediment per disturbed area (up to nearly 50 kg m^–2) than the groundgear (up to c. 10 kg m^–2). This can be compared to our values of 0.96 and 0.19 kg m^–2 for the doors and groundgear, respectively. The difference of an order of magnitude between the studies is probably due to our calculations being based on water turbidity 2 h after trawling, as opposed to seconds after (only 10–15% of the plume may remain in suspension after 1 h; Durrieu de Madron et al., 2005). However, the relative difference between the suspension caused by gear parts is the same in both studies (factor of about 5). Since the groundgear has a larger contact area with the sediment than the doors, it can contribute substantially to the total suspension across the whole swept width (Dounas, 2006), something we confirmed in this study (groundgear contributed about 70% of the total amount suspended).

The initial width of the sediment plume will be the same as the width of the trawl (i.e., distance between trawl doors) but will spread outwards and upwards due to hydrodynamic drag, energy and turbulence generated by the gear and by water currents. Estimates of the initial height of the plume are c. 3–4 m (a few minutes after trawling, Mengual et al., 2016), c. 5.5 m (30 min after trawling, Durrieu de Madron et al., 2005), 10 m (after c. 30 min, Linders et al., 2017), 15–18 m (after c. 10 min, Bradshaw et al., 2012), all of which are consistent with our results (c. 10–15 m after 2 h). Bradshaw et al. (2012) also identified a separate plume from each trawl door that reached higher into the water column (15–18 m) than the plume in the central area (3–4 m) that was probably produced by the groundgear.

Trawl-suspended matter comprises a range of particulate substances such as particulate C, N and P and phaeopigments (e.g., Dounas et al., 2005, 2007; Durrieu de Madron et al., 2005). Our study showed the suspension of both mineral grains (as indicated by particulate Al and Ti) as well as particulate nutrients (particulate N and P) and elements such as Fe and Mn in the sediment plume, and the concentrations of these were strongly correlated and changed with time and distance from the trawl track as the sediment plume moved. Oxidation (mineralization) of particulate organic matter can be enhanced if suspended into the water column, particularly if entrained from low oxygen sediment layers into oxygenated water (Blackburn, 1997; Wainright and Hopkinson, 1997; Sciberras et al., 2016); this may partly explain the decrease in O₂ in bottom water sometimes observed after trawling (Riemann and Hoffman, 1991), as well as changes in bottom water solutes.

Our results showed that otter trawling leads to major alterations of bottom water and sediment geochemical parameters. Concentrations of particle-reactive dissolved substances (Al, Fe, and Ti) in the bottom water decreased directly after trawling, presumably due to their rapid adsorption to the large amount of suspended matter in the sediment plume. Concentrations of other substances (NH₄, NO_x, Mn and to a lesser extent PO₄) increased in the bottom water 2 h after trawling, up to a few hundred meters downstream of the trawl track. Dissolved methane concentrations were around 3× background 20 h post-trawling. These changes could be due to: (a) upward mixing of porewater into the water as sediment is suspended (van de Velde et al., 2018) or as pore pressures in the sediment are altered around the moving gear (Gilkinson et al., 1998; Esmaeili and Ivanović, 2014); (b) desorption of substances from suspended particulate matter due to altered redox conditions in the plume (Tiano et al., 2019); (c) increased organic matter mineralization rates in sediments and/or suspended matter (Tiano et al., 2019) or; (d) strengthening or weakening of sediment-water diffusive gradients when sediment is removed from or deposited on the sediment surface (e.g., Warnken et al., 2003). From our calculations, we conclude that a direct release of porewater solutes from the top 2 cm into the overlying water could produce up to c. 100% immediate increase in bottom water concentrations. These estimates can be compared with Durrieu de Madron et al. (2005) who calculated that 1–10 cm sediment was needed to provide the measured concentrations of dissolved substances a few hundred meters behind the trawl. A modeling study by Blackburn (1997) estimated that sedimentary POM from c. 1 to 2.5 cm sediment must be suspended in order to make any major contribution to DON, NH₄ or NO₃ in the overlying water.

Even if only a few cm (or even mm) are disturbed, this layer is where most biological activity occurs; thus, even a shallow impact (e.g., from the groundgear or sweeps) may have a substantial impact on nutrient dynamics (Fanning et al., 1982; Dounas et al., 2005; Morys et al., 2021). We did not detect elevated concentrations of solutes in bottom water 20 h after trawling, but saw decreased fluxes of nutrients and oxygen in incubated cores taken from the trawl track 2 days after trawling, indicating that the sediments had not fully re-equilibrated. This could either be due to depletion of the pool of dissolved nutrients in the surface sediment and/or the slowing of biogeochemical processes due to the removal of the surface sediment, with its labile carbon, porewater solutes and biologically active components. The removal or disruption of the upper sediment layer is supported by sediment profiles of methane, PO₄, NH₄ and NO_x, which were all elevated in the top 5 cm compared to sediment profiles 100 m from the track (Supplementary Figure 6). In the undisturbed cores, these compounds only occur at high concentrations in the deeper parts of the sediment core. The high concentrations are due to the diagenetic release during anaerobic organic matter remineralization by manganese, iron, sulfate reduction and methanogenesis in the deeper anoxic parts of the sediment and their higher concentrations in the trawl track porewaters suggest physical mixing by the trawl or removal of the topmost sediment layer. Concentration profiles of chlorophyll, organic matter and porosity (Supplementary Figure 5) also confirm the removal or displacement of the surface sediment, and thus an apparent upward shift of these profiles compared to undisturbed sediment, in agreement with Morys et al. (2021) in a similar study in the area.

Changes in benthic O₂ microprofiles suggested that 2 days after trawling the impacted sediment was less reactive (greater oxygen penetration depth, OPD) than the control sediment. Similar trends in OPD were observed by Tiano et al. (2019) and in groundgear tracks by Dellapenna et al. (2006). However, the opposite trend was seen by Warnken et al. (2003), Dellapenna et al. (2006), and Ferguson et al. (2020) in trawl door tracks, the latter study emphasizing the variability in response in different parts of the track due to the different nature of the physical disturbance by different parts of the gear.

Changes in dissolved element fluxes due to physical disturbance are usually short-lived pulses (hours) that disappear as sediment and water chemistry re-equilibrate (Riemann and Hoffman, 1991; Blackburn, 1997; Duplisea et al., 2001; Dounas et al., 2005, 2007; Durrieu de Madron et al., 2005; Percival et al., 2005; Dounas, 2006), as we also observed in this study. Thus, the exact time at which measurements are made relative to the time of disturbance is critical, and effects may vary temporally and spatially within a single study (e.g., Riemann and Hoffman, 1991; Zacharia et al., 2006; Ferguson et al., 2020). Effects also depend on how well-oxygenated the sediments are and if disturbance disrupts the oxic/anoxic boundary (Duplisea et al., 2001; Trimmer et al., 2005; Ferguson et al., 2020). In the Baltic Sea, this boundary is commonly less than 2 cm deep in oxygenated waters (Bonaglia et al., 2013; Almroth-Rosell et al., 2015), so the potential for disruption by trawling is high.

Short term suspension effects will likely cause a transient impact on biogeochemical processes in the bottom water in the immediate vicinity of the trawl tracks, but more important is the spatial and temporal persistence of the sediment plume in relation to background turbidity. Field studies that have measured these aspects are difficult to compare as local sediment type and hydrographical conditions determine the spread and fate of suspended matter. However, Bradshaw et al. (2012) and Linders et al. (2017) showed that a plume of fine sediment was still clearly detectable several days after trawling, just as we found in this study. Those authors, as well as Schubel et al. (1979), Churchill (1989), Schoellhamer (1996), Palanques et al. (2001, 2014), and Mengual et al. (2016) concluded that trawling likely contributed substantially to overall suspension and turbidity in the bottom water in the frequently trawled areas where those studies were performed. The relative contribution can also be higher during seasons where fishing is most intensive and natural suspension and sedimentation processes are low (Churchill, 1989; Palanques et al., 2014; Mengual et al., 2016). When suspended sediment is transported away from trawled areas, it can cause large-scale export of sediment to other areas, for example off-shelf (Churchill, 1989; Palanques et al., 2001; Ferré et al., 2008; Oberle et al., 2016b, 2018; Paradis et al., 2018), thus potentially affecting areas that are not themselves trawled.

While some authors have argued that physical disturbance simply speeds up a release of sediment solutes that would anyway have occurred more slowly through diffusion or bioturbation (Sloth et al., 1996; Blackburn, 1997), others argue that alterations to sediment stability and structure, sediment redox conditions, and benthic communities (especially bioturbators) may lead to chronic longer-term changes in sediment biogeochemistry (e.g., Duplisea et al., 2001) and sediment-water fluxes. The frequency of disturbance is probably important; disturbance at a frequency greater than the timescale needed for re-equilibration of sediment biogeochemical gradients may result in these sediments always being in a transient state (Duplisea et al., 2001; van de Velde et al., 2018). Lastly, trawling may even have wider-reaching local or regional impacts. Dissolved effluxes from trawling have been estimated, for example, to contribute to annual N and Si budgets (Gulf of Maine; Pilskaln et al., 1998) or productivity (Gulf of Heraklion; Dounas et al., 2007), or to have chronically affected nutrient dynamics [North Sea; Percival et al., 2005 (in contrast to findings by Trimmer et al., 2005)]. Suspension of organic-rich particles is also known to increase organic matter remineralization and oxygen consumption (Riemann and Hoffman, 1991; van de Velde et al., 2018), with implications for the sediments’ carbon storage capacity (Legge et al., 2020; Sala et al., 2021). In the Baltic Sea, where commercial bottom trawling frequently occurs close to areas of low oxygen bottom water (van Denderen et al., 2020), further decreases in oxygen caused by trawling also have the potential to exacerbate existing conditions of oxygen stress.

A single trawling event with an otter trawl on a muddy seafloor created a distinct trawl track that was still detectable 18 months later. The trawling event displaced c. 500 t sediment per km of track, decreasing oxygen penetration depth and nutrient and oxygen fluxes across the sediment-water interface in the track for at least 48 h. Approximately 9.5 t sediment, including tens to hundreds of kg of associated particulate elements such as Al, Fe, P, and Mn, were suspended per km of track; the sediment plume in the near-bottom water was transported more than 1 km away over the following 3–4 days. A short term release (hours) of dissolved substances (e.g., N, P, Mn, methane) was also observed in the vicinity of the track, however dissolved concentrations of particle-reactive elements (e.g., Al, Fe, Ti) near the seafloor decreased immediately after trawling; we suggest that this is due to them rapidly adsorbing to the suspended particles in the sediment cloud.

In order to more fully clarify the mechanisms of the biogeochemical processes occurring in the days after trawling, we suggest future experimental field studies take more detailed measurements of the suspended particulate and dissolved phases that were not possible in the scope of this study. Fine resolution measurements of the suspended particulate matter, dissolved oxygen, pH, alkalinity and dissolved organic and inorganic carbon in the bottom water over space and time would contribute to a better understanding of organic matter mineralization and speciation of suspended elements. In addition, sediment profiling of oxygen and dissolved and particulate substances in different areas of the track (e.g., door furrows and adjacent sediment piles, groundgear tracks) and over a number days or weeks would improve the mechanistic understanding of effects on biogeochemical processes and their recovery from a trawling event.

Although this study focused on the short term effects of a single passage of a small trawl, it highlights the potential for larger and longer-term effects in areas where disturbance by commercial trawling is frequent. Not only may seabed biogeochemical processes not have sufficient time to re-equilibrate between trawling events, but trawl-induced turbidity in the bottom water may be semi-permanent in areas with high trawling intensity and affect local, or even regional, nutrient and element cycling. According to the Marine Strategy Framework Directive (EU, 2008), “seafloor integrity” (i.e., physical, chemical and biological characteristics, ecosystem processes, and spatial connectivity; Rice et al., 2012) shall be safeguarded in order to ensure “good environmental status,” i.e., a state where “the structure and functions of the ecosystems are safeguarded and benthic ecosystems, in particular, are not adversely affected” (EU, 2017). In addition, hydrographical conditions should not be altered in a way that adversely affect marine ecosystems. It is clear from our study that bottom trawling has the potential to affect physical, chemical and biological aspects of seafloor integrity, as well as water quality (i.e., hydrographical conditions) both within and outside trawled areas.