ASSESSMENT OF POTENTIAL IMPACTS OF BUCKET DREDGING PLUMES ON WALLEYE SPAWNING HABITAT IN MAUMEE BAY, OHIO

摘要

Annual dredging of a major navigation channel in Lake Erie's Maumee Bay occurs in close proximity to walleye (Sander vitreus) spawning and nursery habitat. Concerns raised by regulatory agencies over potential impacts focus largely upon sediment resuspension. Hypothetical impacts include smothering of demersal eggs by re-deposited sediment, altered egg incubation and hatching success due to increased turbidity effects on water temperature regimes, and clogging or abrasion of gill tissues caused by suspended particles. Monitoring of a clamshell bucket dredging operation was conducted to assess the risk factors posed by typical maintenance dredging in Maumee Bay. Efforts included deployment of optical backscatter sensors (OBS) for time series records of turbidity and acoustic Doppler current profiler (ADCP) surveys to detennine the spatial extent, concentration gradient structure, and temporal dynamics of resuspended sediment plumes. Estimates of total suspended solids (TSS) concentrations were derived from ADCP relative backscatter data. Water samples were collected for gravimetric analysis and used to calibrate the acoustic backscatter data. Results indicated a rapid settling of suspended sediments within a relatively short distance from the dredging source. TSS concentrations fell from 800 mg/L at the source to less than 300 mg/L -over a 25 m span. Maximum observed TSS concentrations decreased to 40 mg/L (15 mg/L above background) at a distance of 115 m from the source. Detectable plume signatures against background became indistinct at distances greater than 125 m, where TSS values did not exceed 5 to 10 mg/L above background. Plume signatures were not detected in surface waters beyond 60 m or in the lower water column beyond 200 m. Waters overlying adjacent shoals, which represented walleye habitat, were examined for elevated TSS attributable to the dredging operation. TSS concentration ranges observed on shoals closest to the dredging activity were not measurably different than on shoals outside the area influenced by plumes. TSS concentrations on the shoals remained generally within 25 mg/L background levels and were consistent with background concentrations for all depth strata within the navigation channel. Turbidities were also monitored in both the navigation channel and adjacent shoals. Turbidities within the plume generally did not exceed 400 nephelometric turbidity units (NTU) at 25 m and 300 NTU at 46 m from the source in the lower water column, but peaked at 500 to 700 NTU in short duration spikes when the dredge advanced to within 15 m of a moored OBS. In contrast, ambient turbidities in the navigation channel did not exceed 25 NTU. Background turbidities measured at 5 stations located on the adjacent shoals ranged from 5 to 15 NTU. At one of three stations located on the shoals immediately adjacent to the dredge, measurements exceeded background conditions twice during ten minute pulses that occurred approximately 3.5 hours apart. At two sensors located 157 and 186 m from the dredge, single occurrences were recorded in which ambient conditions were exceeded by 3 to 10 NTU. In summary, it is very unlikely that bucket dredging operations conducted under similar conditions in Maumee Bay pose a meaningful risk to walleye in terms of either physical disturbance of spawning habitat or exposure of eggs to problematic sedimentation. Prevailing water current velocities were relatively slow, with depth-averaged velocities of 0.17 m/sec in the channel and 0.21 m/sec over the shoals. In the absence of swifter current flows to drive far-field dispersion of plumes, the spatial extent of plumes at any point in time would be limited such that exposures of larvae in the water column to elevated doses of suspended sediments or other altered water quality parameters would be minimal.