THE ROLE OF ANTHROPOGENICALLY INDUCED SEABED DISTURBANCES IN GULF COAST ESTUARIES: A CASE STUDY OF THE IMPACT OF SHRIMP TRAWLING ON GALVESTON BAY SEDIMENTS

The seabed of industrialized estuaries are susceptible to a variety of anthropogenic impacts which result in seabed disturbances and sediment erosion. In the Gulf Coast of the United States, one of the most significant anthropogenic processes which affect the seabed is shrimp trawling. To address significance of shrimp trawling on the seabed and contrast this with natural episodic sediment resuspension events (e.g., extratropical and tropical storms), a series of disturbance and monitoring experiments have been conducted at a site (2.5 m depth) in Galveston Bay, Texas. Preliminary data suggests that the resuspended loads of up to 160 mg/cm2 are produced in the immediate path of a shrimp trawl, an order of magnitude higher than pre-trawl loads and comparable to that caused by wave resuspension in a 20-25 knot wind event. Based on sediment profiles of radioisotopes, pore water dissolved oxygen profiles, shear strength, and porosity, trawl disturbance can reach up to several cm’s into the substrate in the path of the trawl doors and “tickler” chain. Critical shear stress of the sediment surface is increased and benthic flux chamber deployments reveal that nutrient fluxes (e.g., ammonia) are also significantly enhanced, post-trawl. Seasonal wind events, in contrast, lack the plowing aspect of trawling, and only mobilize (e.g., resuspend) the upper few mm’s of the seabed, however, these events impact the seabed across much of the bay simultaneously.