Impacts of sediment resuspension on water quality in the Pamlico and Neuse River estuaries.

摘要

For at least the past several decades, North Carolina's Pamlico and Neuse River Estuaries have been subject to water quality problems related to increased eutrophication. Many research studies have been initiated in the past several years to determine the complex nutrient cycles in these estuarine systems. Most of this research, however, was concerned with the nutrient processes of the water column and the passive diffusion processes of the benthic sedimentary environment. Although diffusion of nutrients across the sediment/water interface may be important during quiescent periods of sediment deposition and short-term storage, wind events associated with storms throughout the year will resuspend newly deposited sediments resulting in the advective transport of sediment porewater, rich with nitrogen, phosphorus and carbon, into the water column. Sediment resuspension may increase water column nutrient concentrations, and therefore present estimates of nutrient and carbon inputs from the sediments may be too low. This study first evaluated benthic nutrient flux from resuspension by the performance of a preliminary laboratory study to examine the overall effectiveness of in situ sediment resuspension. Short-term sediment dynamics of natural resuspension events in these two estuaries were evaluated with the use of radioisotopes. Sediment cores at 9 sites in the estuaries have been collected at least bi-monthly. The short-term rate of sediment deposition was evaluated using Cs-137 and the naturally occurring radionuclides Be-land Th-234. Porewater nutrient inventories at all sites have also been determined. This technique allowed evaluation of the depth to which sediments have been disturbed and the advective flux of nutrients to the water column. Evaluating this advective flux of nutrients to the water column is crucial to understand estuarine nutrient cycling. Advective fluxes for ammonium and phosphate based on a resuspension event were approximately 6 times greater than the average diffusive flux. Meteorological, turbidity, and current data were collected indicating that these resuspension events occur when passing fronts developed wind speeds in excess of 4 m s−1. The regulatory framework for nutrient reduction strategies, policy implications of benthic nutrient sources, and the need for an appropriate indicator of benthic estuarine health are also addressed.