Northwest Fisheries Science Center

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Document Type: Contract Report
Center: NWFSC
Document ID: 6185
Title: Tongue point monitoring program, 1989-1992
Author/Editor: M. D. Siipola, Robert L. Emmett, Susan A. Hinton
Publication Year: 1993
Publisher: National Marine Fisheries Service and U.S. Army Corps of Engineers
Contracting Agency: U.S. Army Corps of Engineers. Portland, Oregon
Pages: 63 p. plus appendices (72 p.)
Date: 1993

This report presents data collected during 4 years of physical and environmental monitoring at the Tongue Point dredge site and the Ocean Dredged Material Disposal Site F (ODMDS-F), off the mouth of the Columbia River.  The ODMDS-F received fine grained material in 1989 from the Tongue Point, Oregon Navigation Improvements Project.  In summary, no significant contamination, toxicity, or bioaccumulation impacts due to dredging and disposal activities could be documented at either Tongue Point or ODMDS-F.

Dredged material deposited at ODMDS-F consisted of sediments finer than the ambient material, and formed a relatively stable and recognizable mound.  Bathymetry surveys of the dredged material mounds indicated its area remained relatively unchanged, while the maximum height decreased 30 months after disposal from 12 to 10 ft.  Coarser ambient material covered the finer-grained dredged material, further stabilizing the mound.

Dredged material disposal affected sediment characteristics at ODMDS-F, but this was not clearly reflected in the benthic invertebrate community structure.  Recolonization of the dredged material mound by benthic invertebrates was very rapid.  Six months (June 1990) after the cessation of dredged material disposal (December 1989) benthic invertebrate densities were higher at all stations except one when compared to the previous year.  Only Station D4 (the center of ODMDS F) showed a decline in benthic invertebrate density 6 months after dredged material disposal.  However, sediment characterization of the Station D4 sediments indicated it had reverted to ambient conditions by June 1990.

The presence of dredged material did not reduce benthic invertebrate densities, but altered benthic conununity structure compared to the surrounding stations.  In 1991, stations E3, C3, and 04, whose sediments consisted of dredged material, as shown by the large percentage of fine sediment, had different major benthic invertebrate taxa compared to stations unaffected by dredged material disposal.  However, stations E3 and C3 had the two highest benthic invertebrate densities of the 13 benthic invertebrate stations analyzed.  All benthic invertebrate stations showed changes in community structure from year to year.

Benthic invertebrate densities increased by an order of magnitude in the study area from 1989 to 1992.  This appeared to reflect long term annual variation and not a result of dredged material disposal.  The large annual variations of benthic invertebrate communities (changes in species composition and densities) off the mouth of the Columbia River over shadowed the ecological effect of dredged material disposal at ODMDS-F.  Long term benthic invertebrate data sets are needed to provide perspective on the ecology of nearshore benthic communities and the effect of dredged material disposal.

The benthic invertebrate conununity off the mouth of the Columbia River is very dynamic and was only slightly affected by dredged material disposal.  Although the dredged material deposited at ODMDS-F was unlike ambient material, recolonization of this material was rapid.  Recolonization species appeared to come from adjacent areas, an indication that many of these benthic species can tolerate a wide range of sediment characteristics.