Sediment Quality in Puget Sound Year 2 - Center for Coastal ...

and Conner, 1980; Gray, 1982: Becker et al., 1990; Ferraro et al., 1991 ; Llanso et al.,1998b).
Major differences in benthic communities can result from wide ranges in water depths, oxygen
concentrations at the sediment-water interface, the texture (grain size) and geochemical
composition of the sediment particles, porewater salinity as a function of proximity to a river or
stream, bottom water current velocity or physical disturbance as a result of natural scouring or
maritime traffic, and the effects of large predators. In addition, the composition of benthic
communities at any single location can be a function of seasonal or inter-annual changes in larval
recruitment, availability of food, proximity to adult brood stock, predation, and seasonal
differences in temperature, freshwater runoff, current velocity and physical disturbances.
In this survey of central Puget Sound, samples were collected in the deep waters of the central
basin, in protected waters of shallow embayments and coves, in scoured channels with strong
tidal currents, and in the lower reaches of a highly industrialized river. As a result of these major
differences in habitat, the abundance, diversity, and composition of benthic communities would
be expected to differ considerably from place to place.
Analyses of the benthic macroinfauna in the central Puget Sound survey indicated that the vast
majority of samples were populated by abundant and diverse infaunal assemblages. The numbers
of species and organisms varied considerably among sampling locations, indicative of the natural
degree of variability in abundance, community structure, and diversity among benthic samples in
Puget Sound. Calculated indices of evenness and dominance showed variability equal to that for
species counts and abundance.
With huge ranges in abundance, species composition, and diversity as a result of natural
environmental factors, it is difficult to discern the differences between degraded and un-degraded
(or "healthy") benthic assemblages. Some benthic assemblages may have relatively low species
richness and total abundance as a result of the effects of natural environmental factors. There
were a number of stations in which the benthos was very abundant and diverse despite the
presence of high chemical concentrations and high toxicity.
Both Long (1989) and Chapman (1 996) provided recommendations for graphical and tabular
presentations of data from the Sediment Quality Triad (i.e., measures of chemical contamination,
toxicity, and benthic community structure). The triad of measures was offered as an approach for
developing a weight-of-evidence to classify the relative quality of sediments (Long, 1989).
Chapman (1 996) later suggested that locations with chemical concentrations greater than effects-
based guidelines or criteria, and evidence of acute toxicity in laboratory tests (such as with the
amphipod survival bioassays), and alterations to resident infaunal communities constituted
"strong evidence of pollution-induced degradation". In contrast, he suggested that there was
"strong evidence against pollution-induced degradation" at sites lacking contamination, toxicity,
and benthic alterations. Several other permutations were described in which mixed or conflicting
results were obtained. In some cases, sediments could appear to be contaminated, but not toxic,
either with or without alterations to the benthos, or sediments were not contaminated with
measured substances, but, nevertheless, were toxic, either with or without benthic alterations.
Plausible explanations were offered for benthic "alterations" at non-contaminated and/or non-
toxic locations possibly attributable to natural factors, such as those identified above.
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