INTEGRATING BIOLOGICAL AND DREDGING PROJECT DATA (cont.)

ERDC TN-DOER-E9

May 2000

thresholds. In contrast, projects generating persistent, high suspended-sediment concentrations

represent the most problematic set of circumstances. Whereas acute exposures at high concentra-

tions for short intervals of time might occur in association with many dredging project scenarios,

these would often be limited in spatial scale. Perhaps the more difficult assessment involves

exposures at low concentrations, but for extended durations, as might occur in restricted waterways.

Nonetheless, placing the dredging operation accurately within the exposure matrix is a necessary

first step in assessing probabilities of detrimental impacts.

This literature review brings to light the small number of species for which relevant data are available

concerning biological responses to suspended-sediments within both the range of concentration and

exposure durations associated with dredging projects. The prevalence of data come from bioassay-

type tests that measure end points (usually mortality) under conditions that do not reflect what

organisms are likely to encounter in the field. Most of the studies summarized in this review

measured dose-response relationships under laboratory conditions that simulated a worst-case

scenario for motile organisms subjected to continuous exposure. Motile organisms can generally

avoid unsuitable conditions in the field. The effects of intermittent exposures to elevated suspended-

sediment concentrations, which simulate conditions in many areas that are subject to tidal flushing,

or are typical of hopper dredging operations (in which dredging is discontinued during transit to

and from the placement site) are not addressed in the literature. Under most scenarios, fish and

other motile organisms encounter localized suspended-sediment plumes for exposure durations of

minutes to hours, unless the organism is attracted to the plume and follows its location. Fish eggs

and larvae are more sensitive to suspended-sediment impacts than older life history stages; however,

most of the available data for eggs and larvae pertain to freshwater conditions (Table 1). If a

probable dredging-induced dosage of ≤1,500 mg/L for ≤1 day is assumed for motile fishes (as

indicated in Figure 1), documented detrimental impacts observed for juveniles and adults were

limited to tests that used fuller's earth rather than natural sediments. Fuller's earth produces negative

responses at lower concentrations than natural sediment (Sherk et al. 1974). Adult fish responses

to suspended sediments for durations of less than 1 day at concentrations ≤1,500 mg/L, i.e.,

conditions relevant to most dredging project scenarios, have not been sufficiently studied to reach

definite conclusions.

For sessile organisms, exposure durations at a typical hydraulic cutterhead dredging site in the

absence of tidal flushing are not likely to exceed 5 days (Figure 2), and in many cases may be

substantially shorter. Although adult bivalve molluscs are silt-tolerant organisms (Sherk 1972),

they can be affected by high suspended-sediment concentrations. Hard clams (Pratt and Campbell

1956) and oysters (Kirby 1994) exposed to fine silty-clay sediments have exhibited reduced growth

and survival, respectively. Suspended-sediment concentrations required to elicit these responses

and mortality, however, are extremely high, i.e., beyond the upper limits of concentrations reported

for most estuarine systems under natural conditions and typical concentrations associated with

dredging operations. Sublethal effects, such as reduced pumping rates and growth, were evident

for adult bivalves at concentrations that occur under natural conditions, but may be of short-term

(hours to days) duration, for example, during a storm (Schubel 1971; Turner and Miller 1991). As

with estuarine fish, the egg and larval stages of shellfish are more sensitive to suspended-sediment

impacts than the adults. Estimates of suspended-sediment impacts to these pelagic, early life history

stages must consider the local hydrodynamics of the dredging site, which strongly influence the

likelihood of extended exposure to suspended-sediment plumes.