linda c. schaffner aiwa conference november 18, 2010
TRANSCRIPT
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State of the Science: Effects of Dredged Material Disposal on Bottom Communities of Lower Chesapeake Bay
Linda C. SchaffnerAIWA Conference
November 18, 2010
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Introduction• Coastal disposal of dredged material
is an environmental concern worldwide and increasingly the focus of conservation and legislative pressures.
• Both removal and disposal have direct and indirect effects on bottom communities – e.g. smothering, changes in hydrology.
• The US Army Corps of Engineers continues to seek ways to minimize impacts of open water disposal operations.
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Synthesis over last 10-15 years has lead to the development of a management framework and highlighted areas in need of further research…
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Dredging as an ecological disturbance:
• Disturbance – results in mortality of individuals
• Both natural and anthropogenic disturbances are common in shallow coastal areas.
• Ecologists have long studied how communities react and rebound in the face of disturbances in order to learn about succession and community resilience.
• Responses to disturbance vary depending on disturbance type and other factors, e.g. is the habitat structured (oyster reef) or unstructured (soft-bottom)?
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Rates of recovery from dredged material disposal have been shown to vary by habitat type.
Locality Habitat type Recovery Time
Source
James River, Virginia Freshwater mud + 3 weeks Diaz 1994
Coos Bay, Oregon Disturbed mud 4 weeks McCauley et al. 1977
Mobile Bay, Alabama Channel mud 6 months Clark et al. 1990
Chesapeake Bay Mud-sand 18 months Pfitzenmeyer 1970
Dieppe, France Sand -gravel > 2 years Desprez 1992
Dutch Coastal Waters
Sand 3 years De Groot 1979, 1986
Tampa Bay, Florida Oyster shell > 4 years USACE 1974
Hawaii Coral reef > 5 years Maragos 1979
Beaufort Sea Sand-gravel 12 years Wright 1977
Modified from Newell et al. 1998
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Thin-Layer Placement of Dredged Material
• Early work, primarily in the laboratory, showed that some bottom-dwelling animals migrate upward through a sediment overburden.
• Thin-layer placement is the intentional spreading of hydraulically pumped dredged material over broad areas to achieve overburdens less than 12 inches thick.
• The objective of thin-layer placement is to minimize impacts on bottom-dwelling fauna and to speed community recovery, particularly in estuarine environments.
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Study Region – Lower Chesapeake Bay
• Wolf Trap Disposal Area is a designated open water disposal site for uncontaminated sediments dredged from shipping channels in lower Chesapeake Bay
• The Corps of Engineers designed a disposal plan for this study which allowed for assessment of effects of varying thicknesses of dredged material overburden on benthic community structure and recovery rates.
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B
C
1.8 km
5.55 km
10-12 m
• The Wolf Trap (alternate) disposal area is a situated within a natural bathymetric depression.
• Samples were collected following two disposal events at different locations called “cells.”
Controlstations
> 2 km
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Experimental Design
H M L N R 10 -
12 m
< 1 km
> 2 km
dredged sediment
Cells were mapped using a sediment profiling camera in order to determine the thickness of deposited sediment.
DM overburden (thickness) criteria: low = < 5 cm; mid = 5-15 cm; high = >15 cm (often much more than that); N = near, edge of disposal cell; R = reference (control) stations not affected by dredging operations
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Disposal History(cumulative percent cubic meters)
100
50
0May Jul Sep Nov
Jun Aug Oct Dec Feb Apr
Cell B
Cell C
Monitoring began Fall 1987
Monitoring began Spring 1989
100
50
0
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“High” dredged material overburden
(image is 6” wide)
Natural bottom(image is 6” wide)
Natural bottom(anemone is 3” across)
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Community analyses showed no or minimal effect (low, mid) or rapid recovery (high)
referencenearlowmid
F 87
F 87
W 88Sp 88
W 88
S 88
F 87
F 87
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Summary of results:
• Communities got back to “normal” fairly quickly.• There were minimal effects of low and medium levels of overburden (<
15 cm).• It took 1.5 years or less for the high overburden sites to converge with
reference sites. • In this region of Chesapeake Bay, thin-layer disposal of clean material
had minimal impacts on benthic communities in the long run. • Results for recovery rates at “high” overburden sites are consistent with
previous studies based on habitat type (unstructured, silts and sands.
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Rates of recovery from dredged material disposal by overburden in this study (Schaffner 2010).
Treatment Overburden (cm) Recovery Time
Near 0 No difference from reference
Low < 5 No difference from reference
Mid 5-15 Minimal difference from reference
High > 15 < 1.5 years from initiation of monitoring program
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One surprising finding
• Multi-year trends in species richness and variable recruitment of key species occurred regionally during the study, at both the reference sites and within the disposal cells.
• These trends may have been associated with climate variations, or other factors not measured during the study.
• As a result, conditions for evaluating any measure of community recovery shifted through time.
• Sampling reference sites was important for assessing recovery.
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Thank you!Any questions?