Trawling-induced resuspension and dispersal of muddy sediments and dissolved elements in the Gulf of Lion (NW Mediterranean)
Trawling-induced resuspension and dispersal of muddy sediments and dissolved elements in the Gulf of Lion (NW Mediterranean)
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DOI：
10.1016/j.csr.2005.08.002
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发表时间：
2005-12-01
影响因子：
2.3
通讯作者：
Bodiot, O
中科院分区：
文献类型：
--
作者：
de Madron, XD;Ferré, B;Bodiot, O
在狮子湾大陆架的三个地点进行了专门的拖网捕鱼实验，目的是评估不同类型的拖网在泥泞沉积物上引发的颗粒和溶解物质的再悬浮。不同的配置是：(i)底拖网，带地面绳线轴（Rockhopper）； （ii）底拖网，不带线轴（Medits）；（iii）在海床上方1至10米处拖拽的远洋拖网。利用拖曳式ADCP的声散射强度测量了重悬浮沉积物的羽状。在羽流内外采集了颗粒尺寸分布、光透射和水样的伴随剖面。通过对数据的分析，可以得出拖网产生的羽流的主要物理和化学特征；同样，还可以量化沉积物、颗粒（PN、POC）和溶解（营养物）元素的再悬浮通量。考虑颗粒物质的沉降速度和近底部湍流，对羽流的停留时间和扩散进行了监测和建模。结果表明，底拖网对沉积物的再悬浮作用显著，而近底和中水远洋拖网对沉积物的再悬浮作用无明显影响。海底拖网后方几百米处的沉积物云高3-6米，宽70-200米；它们是由水獭门和渔网产生的。烟羽中悬浮沉积物的平均浓度达到50 mg l（-1）。沿拖网路径的沉积物再悬浮通量从最粗的沉积物（粘土质淤泥）190克（-2）秒（-1）到最细的沉积物（粉质粘土）800克（-2）秒（-1）不等。虽然羽流段内溶解元素（营养物质）的重悬浮负荷表明孔隙水的释放，至少在沉积物的最初几厘米中存在，但颗粒物质负荷仅来自沉积物床厚度小于1毫米的重悬浮。这种差异表明，拖网铲出的沉积物中有很小一部分被有效地注入了水柱。对羽流沉降的监测表明，在其产生后的第一个小时内，沉积物负荷迅速衰减。一些泥沙（约为初始荷载的10-15%）仍处于悬浮状态；这可能是由于近底部的湍流阻止了细颗粒和聚集体的再沉积。羽流的横向扩散在很大程度上取决于水平流的可变性。(c) 2005 Elsevier Ltd版权所有。
A dedicated trawling experiment was performed at three sites on the Gulf of Lion continental shelf, with the aim of assessing the resuspension of particulate and dissolved matter triggered by different types of trawls on muddy sediments. The different configurations were: (i) bottom trawl, with bobbin for ground rope (Rockhopper): (ii) bottom trawl, without bobbin (Medits); and (iii) pelagic trawl, towed at 1 and 10m above the seabed.The plumes of resuspended sediment were measured using the acoustic backscattered intensity, from a towed ADCP. Concomitant profiles of particle size-distribution, light transmission and water samples were collected, outside and inside the plumes. The analysis of the data enabled derivation of the major physical and chemical characteristics of the plumes generated by the trawls; likewise, and to quantify the resuspension fluxes of sediment, particulate (PN, POC) and dissolved (nutrients) elements. The residence time and dispersal of the plumes were monitored and modelled, considering the settling velocity of the particulate matter and the near-bottom turbulence.The results indicate that the bottom trawls produce significant resuspension, whilst the near-bottom and mid-water pelagic trawls have no impact upon the sediment. The sediment clouds at several hundreds metres astern of the bottom trawls are 3-6m high and 70-200m, wide; they were generated both by the otter doors and the net. The average suspended sediment concentrations measured in the plumes reach 50 mg l(-1). Resuspension fluxes of sediment along the path of the trawls range from 190 g m(-2) s(-1), for the coarsest sediment (clayey silt) to 800 g m(-2) s(-1) for the finest sediment (silty clay). Whilst the resuspended loads of dissolved elements (nutrients) within the plume segment suggest a release of porewater, present at least in the first few centimetres of sediment, the particulate matter load only resulted from the resuspension of less than 1 mm thickness of the sediment bed. This discrepancy shows that a very small fraction of the sediment ploughed by the trawl is effectively injected into the water column.The monitoring of the settling of the plumes indicates a rapid decay of the sediment load, during the first hour after its generation. Some of the sediment (about 10-15% of the initial load) remains in suspension; this is due, probably, to the near-bottom turbulence that prevents the redeposition of the fine particles and aggregates. Lateral spreading of the plume is strongly dependent upon the variability of horizontal currents. (c) 2005 Elsevier Ltd. All rights reserved.