James O. Sickman

Ecological Effects of Nitrogen Deposition in the Western United States

Abstract In the western United States vast acreages of land are exposed to low levels of atmospheric nitrogen (N) deposition, with interspersed hotspots of elevated N deposition downwind of large, expanding metropolitan centers or large agricultural operations. Biological response studies in western North America demonstrate that some aquatic and terrestrial plant and microbial communities are significantly altered by N deposition. Greater plant productivity is counterbalanced by biotic community changes and deleterious effects on sensitive organisms (lichens and phytoplankton) that respond to low inputs of N (3 to 8 kilograms N per hectare per year). Streamwater nitrate concentrations are elevated in high-elevation catchments in Colorado and are unusually high in southern California and in some chaparral catchments in the southwestern Sierra Nevada. Chronic N deposition in the West is implicated in increased fire frequency in some areas and habitat alteration for threatened species. Between hotspots, N deposition is too low to cause noticeable effects or has not been studied.

Nitrogen Emissions, Deposition, and Monitoring in the Western United States

Abstract Nitrogen (N) deposition in the western United States ranges from 1 to 4 kilograms (kg) per hectare (ha) per year over much of the region to as high as 30 to 90 kg per ha per year downwind of major urban and agricultural areas. Primary N emissions sources are transportation, agriculture, and industry. Emissions of N as ammonia are about 50% as great as emissions of N as nitrogen oxides. An unknown amount of N deposition to the West Coast originates from Asia. Nitrogen deposition has increased in the West because of rapid increases in urbanization, population, distance driven, and large concentrated animal feeding operations. Studies of ecological effects suggest that emissions reductions are needed to protect sensitive ecosystem components. Deposition rates are unknown for most areas in the West, although reasonable estimates are available for sites in California, the Colorado Front Range, and central Arizona. National monitoring networks provide long-term wet deposition data and, more recently, estimated dry deposition data at remote sites. However, there is little information for many areas near emissions sources.

Mechanisms underlying export of N from high-elevation catchments during seasonal transitions

Mechanisms underlying catchment export of nitrogen (N) during seasonal transitions (i.e., winter to spring and summer to autumn) were investigated in high-elevation catchments of the Sierra Nevada using stable isotopes of nitrate and water, intensive monitoring of stream chemistry and detailed catchment N-budgets. We had four objectives: (1) determine the relative contribution of snowpack and soil nitrate to the spring nitrate pulse, (2) look for evidence of biotic control of N losses at the catchment scale, (3) examine dissolved organic nitrogen ( DON) export patterns to gain a better understanding of the biological and hydrological controls on DON loss, and (4) examine the relationship between soil physico-chemical conditions and N export. At the Emerald Lake watershed, nitrogen budgets and isotopic analyses of the spring nitrate pulse indicate that 50 to 70% of the total nitrate exported during snowmelt (ca. April to July) is derived from catchment soils and talus; the remainder is snowpack nitrate. The spring nitrate pulse occurred several weeks after the start of snowmelt and was different from export patterns of less biologically labile compounds such as silica and DON suggesting that: (1) nitrate is produced and released from soils only after intense flushing has occurred and (2) a microbial N-sink is operating in catchment soils during the early stages of snowmelt. DON concentrations varied less than 20–30% during snowmelt, indicating that soil processes tightly controlled DON losses.

Regional analysis of inorganic nitrogen yield and retention in high-elevation ecosystems of the Sierra Nevada and Rocky Mountains

Yields and retention of dissolved inorganic nitrogen (DIN: NO3− + NH4+) and nitrate concentrations in surface runoff are summarized for 28 high elevation watersheds in the Sierra Nevada of California and Rocky Mountains of Wyoming and Colorado. Catchments ranged in elevation from 2475 to 3603 m and from 15 to 1908 ha in area. Soil cover varied from 5% to nearly 97% of total catchment area. Runoff from these snow-dominated catchments ranged from 315 to 1265 mm per year. In the Sierra Nevada, annual volume-weighted mean (AVWM) nitrate concentrations ranged from 0.5 to 13 μM (overall average 5.4 μM), and peak concentrations measured during snowmelt ranged from 1.0 to 38 μM. Nitrate levels in the Rocky Mountain watersheds were about twice those in the Sierra Nevada; average AVWM NO3− was 9.4 μM and snowmelt peaks ranged from 15 to 50 μM. Mean DIN loading to Rocky Mountain watersheds, 3.6 kg ha−1 yr−1, was double the average measured for Sierra Nevada watersheds, 1.8 kg ha−1 yr−1. DIN yield in the Sierra Nevada, 0.69 kg ha−1 yr−1, was about 60% that measured in the Rocky Mountains, 1.1 kg ha−1 yr−1. Net inorganic N retention in Sierra Nevada catchments was 1.2 kg ha−1 yr−1 and represented about 55% of annual DIN loading. DIN retention in the Rocky Mountain catchments was greater in absolute terms, 2.5 kg ha−1 yr−1, and as a percentage of DIN loading, 72%.A correlation analysis using DIN yield, DIN retention and surface water nitrate concentrations as dependent variables and eight environmental features (catchment elevation, slope, aspect, roughness, area, runoff, soil cover and DIN loading) as independent variables was conducted. For the Sierra Nevada, elevation and soil cover had significant (p > 0.1) Pearson product moment correlations with catchment DIN yield, AVWM and peak snowmelt nitrate concentrations and DIN retention rates. Log-linear regression models using soil cover as the independent variable explained 82% of the variation in catchment DIN retention, 92% of the variability in AVWM nitrate and 85% of snowmelt peak NO3−. In the Rocky Mountains, soil cover was significantly (p < 0.05) correlated with DIN yield, AVWM NO3− and DIN retention expressed as a percentage of DIN loading (%DIN retention). Catchment mean slope and terrain roughness were positively correlated with steam nitrate concentrations and negatively related to %DIN retention. About 91% of the variation in DIN yield and 79% of the variability in AVWM NO3− were explained by log-linear models based on soil cover. A log-linear regression based on soil cover explained 90% of the variation of %DIN retention in the Rocky Mountains.

Nitrogen mass balances and abiotic controls on N retention and yield in high-elevation catchments of the Sierra Nevada, California, United States

Interannual variations in nitrogen mass balances for the Emerald Lake watershed (ELW) and six additional headwater basins of the Sierra Nevada of California are described and used to investigate the influence of physical (snow regime, runoff, and precipitation) and chemical (N loading) forcings on the observed variability in annual catchment yield and retention of N. At ELW, annual yield of N varied by a factor of 8 (0.4–3.2 kg ha−1 yr−1) and was a linear function of runoff (R2 = 0.89 and 0.74 for dissolved inorganic nitrogen and dissolved organic nitrogen, respectively). Nitrogen yield increased faster than increases in runoff; that is, ecosystem processes enhanced N losses during years with high runoff and retarded losses during dry years. The timing of snowmelt runoff had a large effect on catchment inorganic N dynamics: nitrate pulses were greater and DIN retention was lower in years with deep, late melting snowpacks. We hypothesize that in the Sierra Nevada, labile N pools in soils are increasingly stocked during years with high snowfall amounts. These findings and modeling studies in high-elevation watersheds suggest that if current trends toward warmer air temperatures and earlier snowmelt continue, N retention will increase in the Sierra Nevada.

Chapter 8 Methods for Measuring Atmospheric Nitrogen Deposition Inputs in Arid and Montane Ecosystems of Western North America

Abstract Measuring atmospheric deposition in arid and snow-dominated regions presents unique challenges. Throughfall, the flux of nutrients transported in solution to the forest floor, is generally the most practical method of estimating below-canopy deposition, particularly when monitoring multiple forest sites or over multiple years. However, more studies are needed to relate throughfall fluxes to total atmospheric deposition, particularly in seasonally dry regions. In seasonally snow-covered regions, the distribution of atmospheric deposition and subsequent nitrogen (N) fluxes are highly sensitive to the temporal and spatial dynamics of snow accumulation and melt. Recent developments in passive monitoring techniques for throughfall and measurement of gaseous pollutants greatly facilitate monitoring of atmospheric deposition and ambient pollutant concentrations over broader spatial scales than was previously possible. Here we focus primarily on N fluxes as N is both a limiting nutrient and a pollutant in many terrestrial ecosystems, and because sulfur (S) deposition is not a widespread problem in the West. Methods suggested for estimating spatially distributed atmospheric deposition in arid and snow-dominated systems include simulation modeling, inferential method, throughfall collection, branch rinsing, N accumulation in surface soils of arid zones, and snowpack sampling methods. Applying more than one approach is often necessary to capture the various atmospheric deposition pathways and the spatial and temporal variability of N deposition.

Episodic lake acidification in the Sierra Nevada, California

Seven high-altitude headwater catchments were studied from 1990 to 1994 to evaluate susceptibility to episodic acid neutralizing capacity (ANC) depression. Dilution (decreasing base cation concentrations) was the primary factor in ANC depression during snowmelt, accounting for 75 to 97% of the ANC reduction. In lakes where acidification (increasing anion concentrations) was noted, nitrate and sulfate were equally important during the first half of snowmelt, while sulfate dominated the latter half. A linear model, based on the relationship between minimum and fall-overturn ANC for the lakes in our study, estimated that none of the 114 lakes sampled during the 1985 EPA Western Lakes Survey had been episodically acidified (ANC < 0). Modifications of the model were used to predict that approximately 6 and 10% of Sierran lakes will become episodically acidified with increases in nitrate and sulfate deposition of 50 and 150%, respectively. No lakes will be chronically acidified with these depositional increases.

Sterols of the Syndinian Dinoflagellate Amoebophrya sp., a Parasite of the Dinoflagellate Alexandrium tamarense (Dinophyceae)

ABSTRACT. Several harmful photosynthetic dinoflagellates have been examined over past decades for unique chemical biomarker sterols. Little emphasis has been placed on important heterotrophic genera, such as Amoebophrya, an obligate, intracellular parasite of other, often harmful, dinoflagellates with the ability to control host populations naturally. Therefore, the sterol composition of Amoebophrya was examined throughout the course of an infective cycle within its host dinoflagellate, Alexandrium tamarense, with the primary intent of identifying potential sterol biomarkers. Amoebophrya possessed two primary C27 sterols, cholesterol and cholesta‐5,22Z‐dien‐3β‐ol (cis‐22‐dehydrocholesterol), which are not unique to this genus, but were found in high relative percentages that are uncommon to other genera of dinoflagellates. Because the host also possesses cholesterol as one of its major sterols, carbon‐stable isotope ratio characterization of cholesterol was performed in order to determine whether it was produced by Amoebophrya or derived intact from the host. Results indicated that cholesterol was not derived intact from the host. A comparison of the sterol profile of Amoebophrya to published sterol profiles of phylogenetic relatives revealed that its sterol profile most closely resembles that of the (proto)dinoflagellate Oxyrrhis marina rather than other extant genera.

20th Century Atmospheric Deposition and Acidification Trends in Lakes of the Sierra Nevada, California, USA

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.

Photosynthetic activity of phytoplankton in a high altitude lake (Emerald Lake, Sierra Nevada, California)

Photosynthetic activity by phytoplankton was measured during the ice-free seasons of 1984, 1985 and 1987 using the 14C radioassay in high altitude Emerald Lake (California). Relative quantum yield (αB) and light-saturated chlorophyll-specific carbon uptake (PmB) were calculated from the relationship of light and photosynthesis fitted to a hyperbolic tangent function. Temporal changes in PmB showed no regular pattern. Seasonal patterns of αB generally had peaks in the summer and autumn. Phytoplankton biomass (as measured by chlorophyll a) and light-saturated carbon uptake (Pm) had peaks in the summer and autumn which were associated with vertical mixing. Estimates of mean daily carbon production were similar among the three years: 57 mg C m−2 2 d−1 in 1984, 70 mg C m−2 2 d−1 in 1985 and 60 mg C m−2 d−1 in 1987. Primary productivity in Emerald Lake is low compared to other montane lakes of California and similar to high-altitude or high-latitude lakes in other regions.