linda c. schaffner aiwa conference november 18, 2010
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State of the Science: Effects of Dredged Material Disposal on Bottom Communities of Lower Chesapeake Bay
Linda C. SchaffnerAIWA ConferenceNovember 18, 2010
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.
Synthesis over last 10-15 years has lead to the development of a management framework and highlighted areas in need of further research…
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)?
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
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.
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.
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
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
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
“High” dredged material overburden
(image is 6” wide)
Natural bottom(image is 6” wide)
Natural bottom(anemone is 3” across)
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
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.
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
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.
Thank you!Any questions?
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