The effects of shrimp trawling on sediment oxygen consumption and the fluxes of trace metals and nutrients from estuarine sediments

Abstract

The effects of shrimp trawling on sediment oxygen consumption and the sediment–water exchange fluxes of nutrients (ammonium, phosphate and silicate) and trace metals (Mn, Ni, Cu, Cd and Pb) were determined for two separate experiments conducted during July 1998 and May 1999. Oxygen microelectrode measurements showed that during the 1998 experiment, the surface sediments were oxic down to depths of approximately 15 mm suggesting that the study area had not been recently trawled. However, after passage of the trawl gear oxygen was present to depths of only 5 mm. This suggests resuspension of the upper 1 cm of sediment and resulted in ammonium and manganese fluxes increasing by a factor of 2–3, which was attributed to a similar mechanism, i.e. a strengthening of the diffusive gradient. The measured fluxes of oxygen, phosphate and the trace metals Ni, Cu and Pb were not affected by trawling, while the flux of Cd was affected. During the 1999 shrimp trawl experiment, pre- and post-trawl oxygen penetration was limited to the upper 1 mm of sediment suggesting the area had been recently trawled. The diffusive fluxes of oxygen were in agreement with directly measured oxygen fluxes, both prior to and after trawling; further indication that trawling had little to no effect on the exchange of oxygen across the sediment–water interface during this time. Macrofaunal abundance was significantly reduced during 1999 and consistent with sediment oxygen consumption rates, which were also lower during this time. This resulted in the fluxes of oxygen, which were enhanced by biological processes during 1998, becoming diffusion controlled during 1999. This resulted in pre- and post-trawl fluxes of oxygen, ammonium, silicate, Mn, Ni, Cu and Pb, when considering the measurement error, which did not differ significantly. Cd again seemed affected by shrimp trawling activities. An increase or decrease in a trace metal's turnover time (the time in days required to replenish the water column concentration) can have important implications towards the overall health of an estuary. In Galveston Bay, trace metal turnover times averaged 0.4±0.5, 52±32, 27±15, 22±23 and 5.3±3.0 d for Mn, Ni, Cu, Cd and Pb, respectively. These relatively short turnover times indicate that the sediments can be an important source of trace metals to the overlying water column, especially during low-flow periods of the Trinity River, which can sometimes last more than 1 yr. This, in turn, could possibly lead to the non-conservative mixing behaviors, for both nutrients and trace metals, previously reported for the Trinity Bay region of Galveston Bay, as conservative mixing is observed in the lower estuary. Our study shows that the effects of shrimp trawling are largely dependent on the prevailing sediment redox conditions due to the shallow penetration depth of both oxygen and the trawl gear. Therefore, it is unlikely that trawling activities adversely affect the overall health of Galveston Bay. However, repeated trawling with removal of the upper oxic sediment layers could trend surface sediments towards anoxia and ultimately lead to changes in benthic–pelagic coupling.