Determining Effects of Fire on Algal Growth and Nutrient Limitation in the Sacramento Mountains Forests

Wildfires have devastated large areas of forest in the US Southwest leaving lands barren and reshaping fragile ecosystems. The effects of these fires may extend to adjacent aquatic and riparian habitats and may reduce the ability of resident aquatic organisms to process organic matter and cycle nutrients. Depending on nutrient inputs, streams may exhibit either nitrogen (N) and phosphorus (P) limitation; this can affect primary productivity, consequently affecting food availability for macroinvertebrate communities. The objective of this study is to understand the effect of recent fires on algal biomass and nutrient use, and to examine the ability of stream communities to recover after a fire. To determine nutrient limitation of algal primary producers, we will use nutrient diffusing substrates (NDS) to measure algal response to known N, P, and N+P quantities over a three-week period. Field experiments will be conducted at sites that have either experienced a fire in the past 2 years, or have not been burned for over 10 years. Previous experiments completed during Summer 2011 at reference (unburned) sites showed that nutrient limitation is dependent on substrate type. Macrophyte substrate was associated with N limitation of algae, while algae at sites with rocky substrates were P limited. Sites dominated by sediment exhibited no nutrient limitation.  We hypothesize that changes in sediment deposition as a result of wildfires will change the dominant substrate type, and reduce the extent of nutrient limitation; whereas, sites burned 10 years ago will have algal nutrient limitation similar to reference conditions.