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PA Bulletin, Doc. No. 99-1189


Revised Biological Metric Availability

[29 Pa.B. 3960]

   The Department of Environmental Protection (Department) is making available for public comment a proposal to revise the way it interprets and applies one of the biological metrics used to evaluate the appropriateness of special water quality protection for candidate waterbodies throughout this Commonwealth.

   The Department has an ongoing process to evaluate waterbodies for possible inclusion in its Special Water Quality Protection Program. This process includes the application of five biological metrics to develop a biological condition score for the candidate waterbody. These metrics are designed to evaluate the structure and function of the invertebrate community and assess the overall health of the aquatic ecosystem. The composite score of the 5 metrics for a candidate waterbody is compared to that of a reference waterbody of known excellent quality and the percent difference is used, in conjunction with applicable regulatory criteria, to determine the appropriate Special Protection category (if any) for the candidate.

   One of the 5 metrics considers the dominance of a single taxon or group of aquatic invertebrates in the biological community of the waterbody in question. Generally, healthy aquatic systems are not dominated by any single type or group of organisms but, rather, are well balanced with numerous taxa present in relatively equal numbers. An invertebrate collection dominated by a single taxon is normally an indication that an outside stress has altered the system and created conditions that favor the proliferation of one group of invertebrates (usually pollution tolerant) over the more normal condition of multiple groups in balance with each other. The metric Percent Dominant Taxon detects this condition and, when compared to a healthy, balanced reference condition, results in a poor score for the candidate waterbody.

   This metric, in conjunction with the remaining 4 metrics, has been successfully used to evaluate many candidate waterbodies for special water quality protection. However, natural systems are varied, and it has come to the attention of Department staff that the metric Percent Dominant Taxon, when applied literally, can yield misleading results in aquatic systems naturally dominated by one or two taxa or in systems where the biological community is temporarily dominated by one or two taxa because the reproductive cycle for those taxa has yielded large numbers of individuals over a very brief interval of time. The following change in the procedure for scoring the metric Percent Dominant Taxon is proposed to take into account naturally occurring conditions or sample collections inadvertently timed to coincide with large hatches of one or two taxa of invertebrates.

   For invertebrate collections dominated by a single taxon, Department staff will consider the Hilsenhoff Index number for that group of invertebrates. This index (on a scale of 1--10) reflects the sensitivity of the organism to pollution. Low Hilsenhoff Index numbers are assigned to invertebrates that are sensitive to pollution. If the Hilsenhoff Index for the dominant taxon in a sample is 0, 1 or 2 (indicative of a pollution sensitive organism) the maximum score of 6 will be assigned for the Percent Dominant Taxon metric in that sample. This will recognize that a sensitive biological community exists at the site even though it is dominated by a single taxon.

   Persons wishing to comment on this proposed method of interpreting and scoring the invertebrate biological metric Percent Dominant Taxon may do so by submitting written comment to Edward R. Brezina, Chief, Division of Water Quality Assessment and Standards, Bureau of Watershed Conservation, P. O. Box 8555, Harrisburg, PA 17105-8555, e-mail

   Comments must be received by close of business August 24, 1999. Comments received by facsimile will not be accepted.


[Pa.B. Doc. No. 99-1189. Filed for public inspection July 23, 1999, 9:00 a.m.]

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