Data_Sheet_1_Physical Disturbance by Bottom Trawling Suspends Particulate Matter and Alters Biogeochemical Processes on and Near the Seafloor.PDF

Bottom trawling is known to affect benthic faunal communities but its effects on sediment suspension and seabed biogeochemistry are less well described. In addition, few studies have been carried out in the Baltic Sea, despite decades of trawling in this unique brackish environment and the frequent occurrence of trawling in areas where hypoxia and low and variable salinity already act as ecosystem stressors. We measured the physical and biogeochemical impacts of an otter trawl on a muddy Baltic seabed. Multibeam bathymetry revealed a 36 m-wide trawl track, comprising parallel furrows and sediment piles caused by the trawl doors and shallower grooves from the groundgear, that displaced 1,000 m3 (500 t) sediment and suspended 9.5 t sediment per km of track. The trawl doors had less effect than the rest of the gear in terms of total sediment mass but per m2 the doors had 5× the displacement and 2× the suspension effect, due to their greater penetration and hydrodynamic drag. The suspended sediment spread >1 km away over the following 3–4 days, creating a 5–10 m thick layer of turbid bottom water. Turbidity reached 4.3 NTU (7 mgDW L–1), 550 m from the track, 20 h post-trawling. Particulate Al, Ti, Fe, P, and Mn were correlated with the spatio-temporal pattern of suspension. There was a pulse of dissolved N, P, and Mn to a height of 10 m above the seabed within a few hundred meters of the track, 2 h post-trawling. Dissolved methane concentrations were elevated in the water for at least 20 h. Sediment biogeochemistry in the door track was still perturbed after 48 h, with a decreased oxygen penetration depth and nutrient and oxygen fluxes across the sediment-water interface. These results clearly show the physical effects of bottom trawling, both on seabed topography (on the scale of km and years) and on sediment and particle suspension (on the scale of km and days-weeks). Alterations to biogeochemical processes suggest that, where bottom trawling is frequent, sediment biogeochemistry may not have time to recover between disturbance events and elevated turbidity may persist, even outside the trawled area.

底拖网捕捞（bottom trawling）已被证实会对底栖动物群落造成影响，但其对沉积物悬浮及海底生物地球化学过程的作用却鲜有详细报道。尽管该独特咸淡水环境已开展数十年拖网作业，且低氧、低盐度及盐度波动已作为生态系统胁迫因子的水域内拖网活动频发，但针对波罗的海（Baltic Sea）的相关研究仍为数不多。本研究针对波罗的海泥质海底，测定了翼网拖网（otter trawl）对其产生的物理与生物地球化学影响。多波束测深（multibeam bathymetry）结果显示，拖网轨迹（trawl track）宽度达36米，由拖网板（trawl doors）形成的平行犁沟与沉积物堆，以及触底装置（groundgear）形成的较浅凹槽共同构成；每千米拖痕可移置1000立方米（500吨）沉积物，并悬浮9.5吨沉积物。就总沉积物质量而言，拖网板的影响弱于其他拖网部件，但由于其穿透深度更大、水动力阻力更高，每平方米的沉积物移置量为其他部件的5倍，悬浮效应则为2倍。在后续3至4天内，悬浮沉积物扩散范围超过1千米，形成厚度5至10米的浑浊底层水层。拖网后20小时，距离轨迹550米处的浊度达到4.3 NTU（7 mgDW L–1）。颗粒态铝、钛、铁、磷及锰的分布与悬浮沉积物的时空分布模式显著相关。拖网后2小时，在轨迹数百米范围内的海底上方10米高度处，出现溶解态氮、磷及锰的浓度脉冲。水体中溶解态甲烷浓度至少在20小时内维持升高水平。拖网板轨迹区域的沉积物生物地球化学过程在48小时后仍受扰动，表现为氧渗透深度降低，以及沉积物-水界面处的营养盐与氧交换通量发生变化。上述结果清晰证实了底拖网捕捞对海底地形（千米尺度、年际尺度）以及沉积物与颗粒悬浮过程（千米尺度、数天至数周尺度）的物理影响。对生物地球化学过程的改变表明，在拖网作业频繁的区域，沉积物生物地球化学过程可能来不及在两次扰动事件间恢复，且即使在非拖网区域，升高的浊度也可能持续存在。