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PA Bulletin, Doc. No. 99-1676


Implementation of Revised Biological Metric

[29 Pa.B. 5149]

   On July 24, 1999 (29 Pa.B. 3960) the Department of Environmental Protection (Department) published a notice of availability and opportunity for public comment on a revision to the application of one of the biological metrics used in evaluating candidates for special water quality protection. The Department applies five biological metrics to develop a biological condition score for the candidate that is compared to that of a reference water body of known excellent quality.

   Generally, healthy aquatic systems are not dominated by any single type or group of organisms but, rather, are well balanced. 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). 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 water body. However, natural systems are varied and, when applied literally, this metric can yield misleading results in aquatic systems naturally dominated by one or two taxa or where the biological community is temporarily dominated by one or two taxabecause their reproductive cycle has yielded large numbers of individuals over a very brief interval. In order to reflect the science, the Department proposed to consider the Hilsenhoff Index number for the dominant 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. This will recognize that a sensitive biological community exists at the site even though it is dominated by a single taxon.

   All of the 18 comments received were in support of the change in implementation. They indicated that using the Hilsenhoff pollution tolerance index will provide more meaning to the percent dominant taxon score and assist in interpreting the metric. Based on these supportive comments, the Department will begin using this revised method for implementing the Percent Dominant Taxon metric upon publication of this notice in the Pennsylvania Bulletin.


[Pa.B. Doc. No. 99-1676. Filed for public inspection October 1, 1999, 9:00 a.m.]

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