Harbor dredging and benthic infauna : a case study Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/fn107205v

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  • This study of the immediate, localized effects of a small dredging operation on the benthic invertebrate community in the shipping channel of Coos Bay, Oregon, was designed: 1) to measure the extent of the physical removal of benthic macro- and meiofauna by hopper dredging; 2) to record the subsequent benthic effects of mid-channel spoiling; and 3) to monitor the rate and pattern of biological readjustment in the affected areas. Replicate Shipek grab samples before and after dredge operations were restricted spatially and temporally to delineate rapid and localized biological responses. Faunal abundance, taxa composition, species diversity and qualitative sediment characteristics were studied. Immediate declines in faunal abundance at the dredge and spoil sites were temporary and re-adjusted to pre-dredging abundance levels within 28 days. Temporary increases in diversity reflected changes in the relative abundance of taxa arising from siltation and burial of organisms. Following re-adjustment, populations increased in all areas except the dredge channel. Localized population declines are hypothesized to result from the unsuitability of newly exposed sediment for settlement of pelagic larvae. Although most taxa were adversely affected by dredging activity, Capitella ovincola was relatively unaffected and increased dramatically within the dredge channel within 18 days after dredging. Qualitative sediment characteristics were generally the same before and after dredge operations except for localized increases in wood debris at both dredge and spoil sites. The effects of dredging activity are thought to be dependent on: 1) the size and duration of the dredge operation, 2) pre-dredging history and frequency of dredging, 3) the type of benthic community, 4) depth of water and sediment type, 5) draft and size of the dredging vessel and 6) shipping and related harbor activities. The direct, benthic effects of this dredging operation were short term. The temporary nature of these changes was linked to the small scope of the dredging operation and the adaptability of the benthic community. It is hypothesized that periodic disruption of the sediment surface by small scale maintenance operations may have less effect on the benthic community than the daily presence of heavy shipping and industrial and domestic pollution.
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