Effects of river beads on algal nutrient limitation following severe wildfire

Abstract

Severe wildfires often increase inputs of nitrogen (N) and phosphorus (P) to streams, which can alter nutrient limitation and primary production of benthic algae. Changes in nutrient limitation, however, vary over space and time due to physical and biological factors. In particular, the low gradient, wide valley-bottom segments of montane rivers (hereafter beads ) are zones of nutrient uptake and retention. Few studies, however, have evaluated the role of river beads in mediating algal nutrient limitation following wildfire in perennial streams. We hypothesized that river beads would act as nutrient sinks and increase downstream N limitation following severe wildfire. We also expected that N limitation would be stronger during low flow conditions in autumn, when stream N concentrations are lower relative to high flows in spring. To test effects of river beads across seasons on nutrient limitation, we deployed nutrient diffusing substrate treatments above and below beads in three perennial streams within the Cameron Peak Fire scar, which was the largest fire in Colorado state history. On average, the three focal catchments had 52% of their upstream area burned at moderate or high severity. Nutrient amendments included N and P in a 2 x 2 factorial design, which were deployed in high flows in spring and low flows in autumn. We quantified environmental factors at each location that were expected to mediate the strength of nutrient limitation, including macroinvertebrate grazers, canopy cover, water temperature, depth, stream nutrients, dissolved oxygen and flow velocity. We also characterized periphyton communities using 16S rRNA and 18S rRNA gene sequencing to determine if community structure varied across seasons or position relative to beads. N addition increased algal Chl a concentrations downstream but not upstream of beads during low flow and low stream nutrient conditions. In contrast, during high flow, high nutrient conditions, benthic algae were co-limited by N and P, and the magnitude of response was higher upstream of beads than downstream. Macroinvertebrate grazer density had a negative effect on the magnitude of Chl a responses to N-containing treatments, which exceeded the relative importance of the other six environmental variables. Periphyton sequencing indicated the relative abundance of eukaryotic algae was lowest above beads in spring and below beads in autumn. Taken together, these results confirm that river beads influenced periphyton nutrient limitation, with the spatial pattern of limitation changing across seasons. Because stream restoration efforts following wildfire often focus on enhancing the functioning of degraded river beads, these findings also provide baseline information about how management interventions may influence nutrient limitation and retention in burned catchments.