The Effects of a Prescribed Burn on Streambed Sediments, Macroinvertebrate Assemblages, and Water Quality in the Valle Toledo, Valles Caldera National Preserve, New Mexico

Jeanine K. McGann

A Professional Project Report Submitted in Partial Fulfillment of the Requirements for the Degree of Master of Water Resources, University of New Mexico

Abstract

Lotic ecosystems of the southwestern U.S. can be severely affected by wildfires causing alterations in water chemistry, sedimentation from runoff events, and changes in macroinvertebrate community structure. In these same systems, the use of prescribed burns as forest and/or grassland management tools also may cause changes. The severity of these changes in comparison to wildfires is of primary importance to the effectiveness of these management tools, as any degradation of aquatic habitats is of serious concern in a semi-arid region. This study evaluates the effects of a recent prescribed burn in the Valle Toledo section of the Valles Caldera National Preserve, November 2005. Factors affecting the outcome of the burn, including a marked lack of winter precipitation, are discussed and data are presented on changes in water chemistry, sedimentation, and macroinvertebrate assemblages, or the lack thereof, from pre-burn to post-burn and post-snowmelt conditions. Results of the study show no major overall degradation of the aquatic habitats in this area, although some localized changes did occur. Elevation of stream nitrate/nitrite concentrations were observed in the burn stream immediately post-burn, which did not decline until late spring. Carbon/nitrogen analysis of stream sediments revealed a localized effect of C:N ratio increase at snowmelt at two of the burn sampling sites. Macroinvertebrate assemblages were mostly unaltered immediately after the burn, though some decline in taxa richness and Jaccards similarity within the burn stream were observed at snowmelt. Continued monitoring will be needed to determine if these effects are mitigated and how quickly.