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.

The New Zealand mud snail, Potamopyrgus antipodarum, is a widely distributed non-native species of management concern on four continents. In a southern California stream, P. antipodarum abundance, which ranged from ca. <10 to nearly 150,000 snails m−2, was related to discharge and temperature patterns. Laboratory experiments indicated that P. antipodarum (1) survivorship decreased from 13 to 27°C, but its growth rate was higher at 13 and 20°C than 27°C; (2) grazing rates were similar to those of native algivores in short-term trials; (3) grazing impact was greater than that of a native hydrobiid snail in longer-term trials; (4) ingested different diatom sizes than some other grazers; (5) reduced the abundances of medium-sized and large diatoms, and several filamentous cyanobacteria and chlorophytes, while increasing the relative abundances of tough filamentous chlorophytes (e.g., Cladophora); (6) impact on other grazing invertebrates was species specific, ranging from competition to facilitation; (7) reduced the survivorship of Anaxyrus boreas tadpoles; and (8) was consumed by non-native Procambarus clarkii and naiads of Aeshna and Argia. Ecological effects of introduced P.antipodarum are subtle, occurring primarily at transitory high densities, but flow regulation may enhance their effects by eliminating high flows that reduce their population sizes.

We compared the efficacy of stable carbon, hydrogen, and nitrogen isotope ratios in identifying the resources used by insect consumers in headwater streams of southern California. We also compared gut contents with consumer stable isotope ratios and mixing model estimates of resource contributions to predator diet. Stable hydrogen isotope ratios (as δ2H) of algivores were well separated from ratios for detritivores, whereas relationships between stable carbon (as δ13C) and nitrogen (as δ15N) ratios of consumers and their expected diets were weaker and more ambiguous. δ2H values of primary consumers more strongly reflected the proportions of their gut contents consisting of algae than δ13C values. The proportions of algivorous prey in predator gut contents increased with mixing model estimates of algivore contributions to predator diet using δ2H but not δ13C values. Our findings support the use of hydrogen isotope ratios in food web studies of streams in southern California and their potential use in assessing the effects of anthropogenic and natural disturbance on basal resource contributions to food webs that might not otherwise be identified using carbon isotope ratios.

We investigated multiple lines of evidence to determine if observed and paleo-reconstructed changes in acid neutralizing capacity (ANC) in Sierra Nevada lakes were the result of changes in 20th century atmospheric deposition. Spheroidal carbonaceous particles (SCPs) (indicator of anthropogenic atmospheric deposition) and biogenic silica and δ(13)C (productivity proxies) in lake sediments, nitrogen and sulfur emission inventories, climate variables, and long-term hydrochemistry records were compared to reconstructed ANC trends in Moat Lake. The initial decline in ANC at Moat Lake occurred between 1920 and 1930, when hydrogen ion deposition was approximately 74 eq ha(-1) yr(-1), and ANC recovered between 1970 and 2005. Reconstructed ANC in Moat Lake was negatively correlated with SCPs and sulfur dioxide emissions (p = 0.031 and p = 0.009). Reconstructed ANC patterns were not correlated with climate, productivity, or nitrogen oxide emissions. Late 20th century recovery of ANC at Moat Lake is supported by increasing ANC and decreasing sulfate in Emerald Lake between 1983 and 2011 (p < 0.0001). We conclude that ANC depletion at Moat and Emerald lakes was principally caused by acid deposition, and recovery in ANC after 1970 can be attributed to the United States Clean Air Act.

© 2014 John Wiley & Sons Ltd. We investigated the effects of a wildfire on stream physical, chemical and biological characteristics in a Mediterranean climate, comparing stream community structure and consumer resource use in burned versus unburned catchments in Santa Barbara County, CA, U.S.A. Canopy cover was lower and water temperature was higher in streams draining basins where the riparian vegetation burned than in streams in unburned basins or burned basins where riparian vegetation remained intact. Stream flow and suspended sediment concentrations during large post-fire storms and wet season nutrient levels were higher in burned than unburned catchments, with increased sedimentation after flood peaks. A year after fires, algal levels were highest in streams where riparian vegetation burned and lowest in streams in burned basins where the riparian canopy remained intact. In contrast, streams in burned basins had lower particulate organic matter, detritivore and predator levels than unburned basins, regardless of whether riparian vegetation burned. Where present, southern California steelhead trout (Oncorhynchus mykiss) were extirpated from burned basins. Algivore densities were high in streams with burned riparian vegetation for two post-fire years before declining to unburned stream levels. Shredder densities rebounded in streams in burned basins with intact riparian vegetation, but remained low for 4 years where riparian vegetation burned. Predatory invertebrate densities increased at sites where trout were eliminated by wildfire. Hydrogen stable isotope analysis indicated that the diets of most invertebrate taxa in streams with burned riparian vegetation a year after fires were comprised of a higher proportion of algal material than riparian detritus relative to invertebrates in streams with intact riparian vegetation. Wildfire impacts on stream food webs are determined, in part, by fire severity in the riparian zone. Streams with burned riparian canopies supported algal-based food webs and streams with intact riparian canopies sustained detrital-based food webs. Fire affected basal resources (nutrients, light, allochthonous inputs) with bottom-up effects on primary producers and consumers, but top-down effects were decoupled at the trophic link between invertebrate predators and primary consumers.