Development of Puget Sound Benthic Indicators

A Washington State Department of Ecology report establishing benthic indicators for Puget Sound. Benthic macrofauna are known to be good indicators of the status of marine environments, and benthic indices are often used as an assessment tool.

Executive Summary

The Puget Sound Ecosystem Monitoring Program (PSEMP), formerly known as the Puget Sound Ambient Monitoring Program and the Puget Sound Assessment and Monitoring Program (PSAMP), has collected benthic macrofaunal samples since 1989, but has yet to make full use of that data because of challenges in interpreting what constitutes abnormal deviation from an expected biological assemblage. In other regions of the country, such interpretation is facilitated by use of benthic indices that remove much of the subjectivity and provide a simple means for communicating complex information to managers. The goal of this project is to develop, calibrate, and validate several benthic indices for Puget Sound data and determine whether they perform well enough to justify using them in assessments of Puget Sound benthic condition.

Data from five Sound-wide benthic surveys were combined and used to (1) assess the number and distribution of benthic macrofaunal assemblages in Puget Sound, (2) calibrate five benthic indices that were originally developed and applied elsewhere for use in Puget Sound, (3) create a validation data set to evaluate performance of the calibrated Puget Sound benthic indices, based on best professional judgment (BPJ) of expert benthic ecologists, and (4) evaluate the performance of the calibrated Puget Sound benthic indices. The data used for the assessment included benthic macrofaunal species abundance data and habitat data from 1,023 site events from 1989 to 2008, which were segregated into a 983 sample calibration data set, and a 40 sample validation data set. The calibration data were used for benthic index development, while the validation data, which were independent of the calibration data, were used to validate and evaluate the calibrated benthic indices.

The Puget Sound Benthic Assemblage

A single benthic macrofaunal assemblage was identified in Puget Sound. As a result, development of a single set of habitat-related benthic indices should suffice for assessing benthic condition in Puget Sound. The benthic macrofaunal assemblage was identified by hierarchical cluster analysis of macrobenthic species abundance data from 601 sites selected from the 1,023 available site events. These sites were not affected by poor sediment chemistry or associated with areas of known toxicity. Groups of samples with similar species composition were identified by the cluster analysis and tested for differentiation based on five habitat variables: sediment grain size, depth, salinity, latitude, and longitude. Habitat-related sample groups were identified by sequentially examining splits in the cluster analysis dendrograms, to assess whether each split reflected habitat differentiation. Although five habitat-related sample groups were identified with significant Mann-Whitney-Wilcoxon differences in habitat variables across dendrogram splits, five lines of evidence indicated that the sample groupings are sub-assemblages of a single naturally occurring Puget Sound benthic assemblage rather than five distinct assemblages. The main reason was that few abundant species occurred exclusively in single groupings; instead, many abundant species occurred in large percentages of the samples in many of the groupings.

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