Assessing the ecological condition of a coastal plain watershed using a probabilistic survey design.

Using a probabilistic survey design, we assessed the ecological condition of the Florida (U.S.A.) portion of the Escambia River watershed using selected environmental and benthic macroinvertebrate data. Macroinvertebrates were sampled at 28 sites during July-August 1996, and 3414 individuals were identified. Taxonomic data were distilled into the 8 m of the Stream Condition Index (SCI), developed specifically for Florida waterways, and sites were classified on an ordinal scale as very good, good, poor, and very poor. The weighted cumulative distribution function of the SCI showed that 8% (+/- 7) of the stream and river miles in this landscape were in very good condition, while 25% (+/- 7), 51% (+/- 16), and 16% (+/- 15) were in good, poor, and very poor condition, respectively. The only environmental parameter significantly correlated with SCI was dissolved oxygen (DO), and two sites classified as very poor had oxygen levels around or below 2 mg L(-1). However, other sites exhibited similarly low SCI values without the attendant low DO, implying that factors determining site condition were complex and multivariate. The results of this survey corroborate Federal and state assessments demonstrating that many locations within this watershed exhibit significant degradation.