UC Santa Barbara

We studied the effects of wildfires in southern California on stream algal communities and their responses to nutrient enrichment. The intensity of scouring floods, concentrations of nutrients during ensuing rains (4-13× increases depending on nutrient), and postflood sediment deposition were greater at sites in burned than in unburned basins (UN sites). Postfire storms reduced algal biomass >10× more at sites in burned than in unburned basins. After the fire and subsequent floods, algal biomass, particularly of Cladophora, was up to 5 (chlorophyll a) to 20 (macroalgal mass) times higher at sites in burned basins with burned riparian vegetation (BRB sites) than at sites in unburned basins (UN sites), but algal abundance at sites in burned basins with intact riparian vegetation (BRI sites) was as low as 10 to 30% of that recorded at UN sites. Two to 3 mo after the wet season, nutrient and algal levels were similar among sites in UN, BRI, and BRB basins. We used summer nutrient diffusing substrata (NDS) experiments where N, P, N+P, or neither were added and found that algal density, chl a, and chl a/algal unit increased by ∼1.5 to 4.5× with N additions at sites in BRB and UN basins but did not change at sites in BRI basins. In contrast, algal biovolume increased 2 to 9× with N additions at all sites and with P addition at sites in BRI basins. General depressions in chl a by 40 to 50% with P addition were related to decreased chl a/algal unit. Diatoms increased with N and N+P additions at sites in UN and BRI basins and with P addition at sites in BRI basins but declined with P or N+P additions at sites in BRB basins, with cyanobacteria and green algae generally showing the opposite pattern. Algal responses to nutrient augmentation across sites were related to fire effects on riparian canopy cover, temperature, and ambient nutrient concentrations. Fire effects on algal communities and nutrient limitation appeared to be mediated through fire effects on flood disturbance and light regimes.