Wildfire variably impacted stream water quality and gross primary production in adjacent, similarly burned, montane watersheds

Wildfires increase the fluxes of water, ash, sediment, nutrients, and carbon into streams and rivers. However, the unpredictable timing and location of wildfires have resulted in a lack of continuous water quality records prior to and following wildfires. This limits our understanding of how post-fire increases of material transport impact water quality and gross primary production (GPP). We explored water quality and GPP responses in two adjacent montane second-order streams in northern New Mexico that were similarly impacted by the Las Conchas wildfire (2011). We conducted a before-after paired trend analysis using a subset of measurements from each year from a period with the most complete water quality data to facilitate between-year comparison. The two streams exhibited similar temporal trends in pre- and post-fire water quality during this late-summer period for 3 years prior to, and 6 years following the wildfire. An immediate increase in specific conductance – SC (2x increase) and turbidity (10x increase) were observed, with a return to near pre-fire levels over 6 years. However, pre- and post-fire trends in GPP differed between streams. To investigate this variability, a mixed effect model was used to assess which environmental parameters were drivers of GPP pre- and post-fire. SC and turbidity had varying effects on GPP pre-fire and negative effects post-fire. The stream gradient of the first-order tributaries impacted by fire, which differed between streams, may have also contributed to the differential metabolic response. We found the impacts of wildfires on GPP were variable and emphasize the need for additional research to better understand how wildfire impacts ecosystem processes that regulate energy and material fluxes in lotic systems.