Dissolved Oxygen Thresholds For Fish and Invertebrate Assemblages In Plains Streams

This study investigated the dissolved oxygen (DO) concentrations at which fish and invertebrates assemblage in Plains streams in southwestern Louisiana began to demonstrate biological threholds. The study area included 35 sites located on streams divided between the South Central Plains (14), Western Gulf Coastal Plain (17), and Mississippi Alluvial Plain (4) ecoregions. Piecewise regressions indicated that average threshold values for taxa richness, diversity, and abundance metrics were 2.6 and 2.3 mg/L, for the invertebrate and fish assemblages, respectively. These thresholds are approximately twice the DO concentration that some native fish species are capable of tolerating and are comparable to existing DO criteria for some coastal streams in Louisiana and Texas. Sites with DO minima less than 2.5 mg/L were favored by extremely tolerant taxa, which had respiratory adaptations that gave them a competitive advantage.

 In general, DO minima were negatively related to the amount of agriculture in the buffer area; however, DO at sites with both low and high amounts of agriculture (including three least-disturbed sites) declined to less than 2.5 mg/L. Thus, although DO sometimes declined below a concentration that was identified as an approximate biological threshold, naturally occurring allochthonous material (rather than anthropogenic activity) was sometimes responsible.