Impact of wildfire ash on benthic periphyton in aridland streams

While wildfires are not historically common to the Sonoran Desert, wildfire disturbances are becoming increasingly frequent in aridland ecosystems due to human activity, invasive grass, and altered seasons. Sycamore Creek, an intermittent stream in central Arizona, has recently experienced two large fire events: the 2020 Bush Fire, which burned half of the stream catchment, and the 2024 Sand Stone Fire, which burned a smaller extent of the catchment. Wildfire ash is transported to the stream during rain storms and influences stream ecosystems by decreasing dissolved oxygen and increasing dissolved organic carbon, organic nitrogen, and inorganic nitrogen; these carbon and nitrogen inputs are important because they interact with stream biota in metabolic functioning and biogeochemical cycles. Benthic periphyton contains a mixture of autotrophic algae, heterotrophic invertebrates, bacteria, fungi, and detritus, in addition to inorganic components. Algae are vital to stream ecosystems because they are responsible for most primary production and play an important role in nitrogen and carbon cycling. The ash nitrogen input is expected to enrich algal biomass growth and pigment production because nitrogen has been shown to be limiting to primary production in this stream. However, the addition of dissolved organic carbon from the ash could benefit the heterotrophic elements of periphyton more than the autotrophs, increasing competition for nutrients against algae. Because previous experiments have not commonly focused on the algal component of stream response to fire events, this study aims to begin to fill in this knowledge gap as wildfires become increasingly prevalent in aridland watersheds. A field experiment of nutrient diffusing substrata found that ash exposure slightly stimulates growth of autotrophs more than heterotrophs in stream periphyton, particularly in comparison to environments with increased nutrient availability.