The Biogeography of the Coastal Wetlands of the Puget Trough: Deltaic Form, Environment, and Marsh Community Structure

Abstract

Seventeen deltas in the Puget Trough lowlands of the Pacific Northwest were classified on the basis of the morphology and physical environments of their river basin-delta-receiving basin systems. The classification algorithm identified three environmental categories; two of the deltas remained unclassified. The intertidal marshes of each delta foreshore were censused, plant communities were identified by means of the COMPCLUS program, and the community profiles of each delta were used as a basis for the vegetational classification. Three vegetation units were identified. The concordance in the membership of the environmental and vegetational categories suggests that there is a simple relationship between the two. A canonical correlation analysis of nine environmental variables and five plant community variables supported this conclusion; a single domi- nant linkage between the two sets of variables was identified. This linkage is interpreted as a salinity gradient. The loadings of the Salicornia-Dis- tichlis and Scirpus communities on the canonical variable, and their response patterns with respect to an index variable (Discharge Effective- ness Ratio, which measures the relative magnitude of fluvial and maritime influences at the delta front) constitute the evidence for this interpretation. In contrast to the fairly uniform coastal marshes to north and south, the deltaic marshes of the Puget Trough are composed of a mixture of regional types. Inter-delta variation can be accounted for by the differences in flow volumes and discharge regimes of the contributing rivers, and the exposure of the delta fronts.