Stressor Identification in an Agricultural Watershed: Little Floyd River, Iowa

摘要

In Iowa, the use of the stressor identification (SI) protocol was prompted by stream impairments of the fish and benthic macroinvertebrate communities. The Little Floyd River was included on the 1998 303(d) list of impaired waterbodies based on a 1990 stream-use assessment. At that time, no fish were observed and only pollution-tolerant benthic macroinvertebrate species were present. Fish have since recolonized the stream. The Little Floyd River is a third-order, warm-water stream located in the Northwest Iowa Loess Prairies Ecoregion. Land use in the watershed is dominated by row-crop agriculture and livestock production. Candidate causes for this biological impairment included flow alteration, substrate alteration, turbidity, altered basal food source, low dissolved oxygen concentrations, high temperature, and high ammonia concentrations. The Iowa Department of Natural Resources (IDNR) characterized the impairment of the stream using chemical and biological samples, as well as observations of the physical habitat collected at four sites over the course of four years. The evidence was reanalyzed by comparing data among the four sites using a less impaired comparator site within the Little Floyd to determine the co-occurrence of stressors with benthic macroinvertebrates and fish. Other types of evidence used in the case were complete causal pathway, stressor response from other field studies, and stressor-response studies from laboratory data and other studies. IDNR identified the primary probable causes of biological impairment as deposited sediment and low dissolved oxygen. Based on the original SI completed by the IDNR, total maximum daily load (TMDL) for sediment and dissolved oxygen were submitted to and approved by the U.S. Environmental Protection Agency in 2005.