State of the Science: Effects of Dredged Material Disposal on Bottom Communities of Lower Chesapeake Bay

Linda C. SchaffnerAIWA Conference

November 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?