Linda C. Schaffner AIWA 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 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. LocalityHabitat typeRecovery Time Source James River, Virginia Freshwater mud+ 3 weeksDiaz 1994 Coos Bay, OregonDisturbed mud4 weeksMcCauley et al Mobile Bay, AlabamaChannel mud6 monthsClark et al Chesapeake BayMud-sand18 monthsPfitzenmeyer 1970 Dieppe, FranceSand -gravel> 2 yearsDesprez 1992 Dutch Coastal Waters Sand3 yearsDe Groot 1979, 1986 Tampa Bay, FloridaOyster shell> 4 yearsUSACE 1974 HawaiiCoral reef> 5 yearsMaragos 1979 Beaufort SeaSand-gravel12 yearsWright 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 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.” Control stations > 2 km

Experimental Design H M L N R 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 = 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) May Jul Sep Nov Jun Aug Oct Dec Feb Apr Cell B Cell C Monitoring began Fall 1987 Monitoring began Spring

“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)

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). TreatmentOverburden (cm)Recovery Time Near0No difference from reference Low< 5No difference from reference Mid5-15Minimal 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.