Impact of Bottom Trawling on Deep-Sea Sediment Properties along the Flanks of a Submarine Canyon

The offshore displacement of commercial bottom trawling has raised concerns about the impact of this destructive fishing practice on the deep seafloor, which is in general characterized by lower resilience than shallow water regions. This study focuses on the flanks of La Fonera (or Palamós) submarine canyon in the Northwestern Mediterranean, where an intensive bottom trawl fishery has been active during several decades in the 400–800 m depth range. To explore the degree of alteration of surface sediments (0–50 cm depth) caused by this industrial activity, fishing grounds and control (untrawled) sites were sampled along the canyon flanks with an interface multicorer. Sediment cores were analyzed to obtain vertical profiles of sediment grain-size, dry bulk density, organic carbon content and concentration of the radionuclide 210Pb. At control sites, surface sediments presented sedimentological characteristics typical of slope depositional systems, including a topmost unit of unconsolidated and bioturbated material overlying sediments progressively compacted with depth, with consistently high 210Pb inventories and exponential decaying profiles of 210Pb concentrations. Sediment accumulation rates at these untrawled sites ranged from 0.3 to 1.0 cm y−1. Sediment properties at most trawled sites departed from control sites and the sampled cores were characterized by denser sediments with lower 210Pb surface concentrations and inventories that indicate widespread erosion of recent sediments caused by trawling gears. Other alterations of the physical sediment properties, including thorough mixing or grain-size sorting, as well as organic carbon impoverishment, were also visible at trawled sites. This work contributes to the growing realization of the capacity of bottom trawling to alter the physical properties of surface sediments and affect the seafloor integrity over large spatial scales of the deep-sea.商业性底拖网捕捞向远海转移的趋势，引发了学界对这类破坏性捕捞活动对深海海底影响的广泛担忧；深海海底整体较浅水区生态恢复力更低。本研究聚焦西北地中海海域的拉福内拉（La Fonera，亦称帕拉莫斯）海底峡谷侧翼区域，该区域在400至800米水深范围内，已开展数十年的高强度底拖网捕捞作业。为探究该工业捕捞活动对表层沉积物（深度0至50厘米）的改造程度，研究团队借助界面多管采样器（interface multicorer），沿峡谷侧翼对捕捞作业区与对照（未拖网）样点进行了采样。研究人员对采集的沉积物岩芯开展分析，以获取沉积物粒度、干容重、有机碳含量以及放射性核素210Pb（radionuclide 210Pb）浓度的垂直分布剖面。在对照样点，表层沉积物呈现出斜坡沉积体系典型的沉积学特征：最顶层为未固结且受生物扰动的沉积物，其下沉积物随深度增加逐渐压实；同时该区域始终保持较高的210Pb活度蓄积量，且210Pb浓度呈现指数衰减的垂直分布特征。未拖网样点的沉积物沉积速率介于0.3至1.0厘米/年之间。多数拖网作业区的沉积物属性与对照样点存在显著差异：采集的岩芯显示沉积物更致密，且210Pb表层浓度与活度蓄积量更低，这表明拖网渔具已造成研究区域现代表层沉积物发生大范围侵蚀。拖网作业区还出现了沉积物物理属性的其他改变现象，包括沉积物被彻底混合、粒度分选发生变化，以及有机碳含量降低。本研究进一步佐证了学界日益达成的共识：底拖网捕捞能够改变深海表层沉积物的物理属性，并在大范围空间尺度上损害深海海底的生态完整性。