Minimum levels of sedimentation known to impact early life stages of shellfish

ERDC TN-DOER-E19

March 2005

transport, a relationship between transport and infiltration of bedload and fine sediment into

gravel and the result of embryo survival and gravel properties. The results of their modeling

effort indicated that further research was needed to clarify how sediment transport affects the

intergravel environment and in turn the potential for embryo survival. Their approach is not

directly transferable to estuarine environments but might provide some framework for

assessment of impacts of sediment transport in spawning areas.

Relatively high suspended sediment concentrations (>500 mg L-1) are known to have impacts on

early life stages of estuarine fish (Wilber and Clarke 2001, Berry et al. 2003). However, the

duration of exposure to suspended sediment from dredging or disposal must be related to the

type and residence time of eggs or larvae in an affected habitat. Morgan and Levings (1989)

demonstrated that after settlement, development of Pacific herring larvae is delayed at very high

levels of suspended sediment (10,000 mg L-1). But Boehlert and Morgan (1985) reported that

feeding rates of larval Pacific herring increased with increasing turbidity up to a point when

feeding was inhibited (2000 mg L-1). Longer-term effects of sediment deposition are highly

dependent on timing of egg attachment and larval settlement, which may be quite specific for a

given habitat.

Minimum levels of sedimentation known to impact early life stages of shellfish. Similar to

fish, the early life stages of shellfish can be affected by passage through high concentrations of

suspended sediment in the water column, but eventually shellfish must either attach to a hard

substratum or burrow into appropriate sediments. Bivalve larvae appear to tolerate relatively

high suspended sediment concentrations (up to 400-800 mg L-1 for oyster larvae and up to 2200

mg L-1 for quahog larvae for less than two days, Wilber and Clarke 2001). Oyster larvae require

a clean, hard substratum for attachment, but can tolerate thin layers of deposited sediments,

perhaps up to 1 mm.1 After attachment, oyster larvae can tolerate deposition of 2-3 mm, with 3-

5 mm and above likely to have some negative effects.1 Clam larvae are not likely to be affected

by sediment deposited before settlement (except for potential effects on "selection" of settlement

sites by larvae), but at the earliest stages, the newly settled larvae may not tolerate rapid

deposition of fine sediments. Deposition rate and thickness would have to exceed the burrowing

rate of the larval clams to have a negative impact. Suspended sediment and resuspended

sediment (for attached or burrowing post-larvae) can affect the feeding and growth of bivalves

(both larval and adult), and frequent or sustained exposure to high suspended sediment loads is

clearly detrimental to most species. Field or laboratory measurements should account for

stressors associated with high suspended load (including associated contaminants, ammonia, and

sulfides).

Minimum levels of sedimentation known to have an adverse impact on submerged aquatic

vegetation (SAV). Most assessments of loss of aquatic macrophytes have focused on impacts of

changes in underwater light from increased suspended sediment (e.g., Best et al. (2001, Dennison

et al. (1993). Accumulation of sediment may result in different responses among species of SAV

and, in turn, different effects on sediment entrainment around SAV (Fonseca and Fisher 1986).

Assessments along gradients of siltation in Southeast Asia have shown loss of species and

changes in species composition, supporting the potential for differential responses among species

Personal Communication. 2004. Roger Mann, Professor, Virginia Institute of Marine Science, Glouchester Point,

VA.