Publication Summary

Relationships between sediment-dwelling invertebrate communities, sediment quality, and levels of dissolved oxygen in Hood Canal waters were examined to evaluate the effects of low dissolved oxygen on the invertebrates. Hood Canal data collected from 1932 to 2005 was geographically mapped and examined for patterns and differences between locations and over time. Sediment texture was the most important factor controlling invertebrate community composition, followed by dissolved oxygen, organic carbon content of the sediments, and station depth. Steps taken to develop initial critical dissolved oxygen thresholds, and a summary of data gaps and associated recommendations for future work, are presented.

Sediment quality data and concentrations of water-column dissolved oxygen (DO) collected in Hood Canal from 1932 to 2005 were evaluated as part of the Hood Canal Dissolved Oxygen Program. The influence of these measures on the composition of sediment-dwelling invertebrate assemblages (benthos) was examined.

Sediment chemical contamination and toxicity was low, and confined to Port Gamble, Port Ludlow, and Dabob Bay. Coarse sands were found in northern Hood Canal and along shorelines. Fine-grained silts and clays were found in central and southern regions, at depth, and in shallow bays. Organic carbon concentrations increased in fine-grained sediments. DO concentrations decreased from north to south and from shallow to deep water. Minimum DO levels measured from 1932 through 2005 decreased over time, periodically falling below critical values at most southern stations and at an increasing number of central and northern stations.

Benthic assemblages were identified for three regions and nine sub-regions of Hood Canal. The number of individuals and species decreased and stress-tolerant species became dominant southward as sediment grain size and near-bottom DO decreased, and organic carbon content and depth increased. These factors, in this order, acting together may have influenced the composition of the benthos.

Obvious changes in assemblage structure occurred within DO ranges of >3 to 6 mg/L and < 1 mg/L. These two ranges may represent critical DO concentrations for Hood Canal benthos. Patterns of species succession over decreasing DO ranges were similar to responses by the benthos to stressors reported in fjords elsewhere.

Additional analyses indicated that there had been little change in northern Hood Canal benthos that could be attributed to declining oxygen levels. However, southern stations near the Great Bend have experienced changes consistent with declining DO concentrations since 1991.

Steps taken to develop initial critical DO values for the protection of the benthos, and a summary of data gaps and associated recommendations for future work on this topic, are presented.

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