The bacterial response to trawling and wind-induced sediment resuspension in the Pamlico River Estuary, North Carolina.

摘要

North Carolina's large, microtidal estuarine system has been exposed to a variety of anthropogenic impacts, including alteration of watershed drainage, nutrient loading, and bottom trawling. Research on the ecological impacts of trawling has focused on direct effects such as bycatch and benthic mortality. Accordingly, little is known of potential secondary impacts of trawling in this estuarine system. One potential effect, sediment resuspension by bottom trawling, may result in a direct alteration of bacterial abundance, activities, and assemblages. Such impacts could alter water column processes, benthic processes, benthic-pelagic coupling, and food web structure.; Bacteria are ubiquitous in aquatic ecosystems, and serve vital roles as decomposers, nutrient recyclers, and food sources for higher organisms. In addition, bacteria have rapid generation times, and therefore may be the first members of the food web to demonstrate a detectable response to trawling disturbance.; The study at hand attempts to characterize the response of water column bacterial communities to sediment resuspension by wind and trawling disturbance. Field and laboratory experiments were conducted from 2001 to 2004. Field experiments were performed in South Creek, North Carolina, a sub-tributary of the Pamlico River Estuary. Trawling was simulated in laboratory microcosms containing muddy-sand or silty-clay sediment types. Total suspended solids (TSS) served as a measure of sediment disturbance.; Bacterial abundances in the water column during trawling experiments ranged from 1.5 x 106 to 7.9 x 107 cells/ml. During trawling experiments, temporal variability in abundance was high and trawling effects were rarely significant. The portion of metabolically active cells demonstrated inconsistent effects in response to trawling disturbance. Occasionally the portion of cells metabolically active in the trawl sites would be significantly smaller than the corresponding non-trawled control site. Regression analysis of wind speed and bacterial abundance indicated a significant relationship during four of seven trawling experiments. When analyzed by directional origin, Northeast, South, and Southwest winds showed the strongest relationship. No relationships between wind speed and metabolic activity were detected.; Disturbance in laboratory microcosms resulted in immediate increases in the TSS concentration. Bacterial abundances were less variable (9.2 x 10 5 to 6.5 x 106) during microcosm experiments, and increased sharply following a simulated trawling disturbance. However, the percentage of active cells decreased following disturbance, and bacterial productivity (g C ml-1 hr-1) first declined, then rebounded to temporarily exceed pre-disturbance levels. These trends were apparent for both sediment types, yet most pronounced for the silty-clay sediment. Bacterial taxonomic richness and sulfate-reducer taxonomic richness analysis of total bacterial community DNA revealed no apparent changes in bacterial community structure following disturbance events in field and laboratory experiments. These data suggest that the water column of South Creek is well-mixed and sediment resuspension may have an inhibitory effect on water column bacteria. Resuspension of the upper few centimeters of the benthos did not greatly alter taxonomic richness in the water column. Our data suggest bottom trawling in the Pamlico River Estuary has little impact on the resident water column bacterial communities. Basin-wide processes such as wind and rain events, which affect sediment resuspension, runoff and nutrient loading, and stratification/destratification of the water column; are more likely to affect the bacterial community.