From fires to floodplains: interacting effects of unconfined channel segments and seasonal dynamics on algal nutrient limitation following wildfire

Abstract

Severe wildfire has the potential to alter ecosystem processes including nutrient cycling and primary production in streams. Nutrients such as nitrogen (N) can remain elevated in streams for decades following wildfire and can impact downstream water quality, aquatic habitat, and food webs. In this study, nutrient diffusing substrates were deployed in three burned watersheds within the Cameron Peak Fire scar in Colorado, USA during peak and base flows to examine seasonal nutrient limitation of benthic algae upstream and downstream of unconfined channel segments, which are generally zones of nutrient uptake and retention. We also quantified environmental factors expected to mediate the strength of nutrient limitation, including macroinvertebrate grazers, canopy cover, water temperature, depth, ambient nutrients, dissolved oxygen, and velocity. We hypothesized that unconfined segments would act as nutrient sinks during base flows resulting in stronger N limitation downstream. Our results showed that effects of geomorphic position on N limitation and algal responses varied across seasons, with reduced N limitation during high N concentration peak flows, and stronger N limitation downstream of unconfined channel segments during base flows. Of the seven environmental variables, macroinvertebrate grazer density was the strongest mediator of algal N responses. These results highlight the importance of geomorphic context, seasonality, and biological factors in mediating N limitation and are relevant to informing stream restoration efforts that aim to restore and enhance unconfined channel reaches following wildfire.