Al Leydecker

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.

Integrated coastal reserve planning: making the land–sea connection

Land use, watershed processes, and coastal biodiversity are often intricately linked, yet land–sea interactions are usually ignored when selecting terrestrial and marine reserves with existing models. Such oversight increases the risk that reserves will fail to achieve their conservation objectives. The conceptual model underlying existing reserve selection models presumes each site is a closed ecological system, unaffected by inputs from elsewhere. As a short-term objective, we recommend extending land-conservation analyses to account for effects on marine biodiversity by considering linkages between ecosystems. This level of integration seems feasible and directly relevant to agencies and conservancies engaged in protecting coastal lands. We propose an approach that evaluates terrestrial sites based on whether they benefit or harm marine species or habitats. We then consider a hypothetical example involving estuarine nurseries. Whether this approach will produce more effective terrestrial reserves remai...

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.

Processes regulating the solute concentrations of snowmelt runoff in two subalpine catchments of the Sierra Nevada, California

Geochemical processes regulating solute concentrations in snowmelt runoff were investigated for the snowmelt periods of 1992 and 1993 in two subalpine catchments in Sequoia National Park, California. The catchments, 0.5 and 0.2 ha, have 10 and 25% soil coverage, respectively; the remaining area is exposed granodiorite bedrock. Mineral weathering was the major source of solutes in runoff. Calcium export in excess of stoichiometric plagioclase weathering was attributed to dry deposition and the weathering of mafic minerals and disseminated calcite. Cation exchange was important in the regulation of K and increased base cation export in response to rain-on-snow events and LiBr tracer. Sulfate fluxes were attributed to snowpack elution and SO4 desorption after the beginning of melt. The selective retention of Li during tracer experiments suggests that acid-neutralizing capacity is linked to soil stocks along convoluted flow paths and is not necessarily compromised in areas of sparse soil cover.

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.

Estimating evaporation in seasonally snow-covered catchments in the Sierra Nevada, California

Abstract We used Mortons CRAE model, a simple model based on the complementary or advection–aridity relationship, to estimate annual evaporation for six alpine and sub-alpine catchments in the central and southern Sierra Nevada for periods of four to six years. Ten-day averages of air temperature, dew-point and sunshine duration were used as input. Modeled winter evaporation compared well with mean-profile calculations at the meteorological stations, and average annual totals were similar to those calculated from water balances and the evaporative concentration of conservative ions, i.e. chloride and sulfate. Regression relationships between weather stations were used successfully to extend limited meteorological data. Using standard lapse rates to provide data for catchments without weather stations was less successful, but still useful. The average annual catchment evaporation estimated by the model was ∼36% of annual precipitation (the average water balance and evaporative concentration estimates were 31 and 38%, respectively), showing that annual evaporative loss in the Sierra Nevada is appreciable.

Episodic variations in nutrient concentrations in coastal Californian streams

The numerous streams from Californias coastal mountains transport nutrients and sediments into coasta1 estuaries and the near-shore oceanic environment. In central California, these streams traverse catchments with land covers and uses that include chaparral, grazed grasslands, orchards, industria! agricu1ture and suburban and urban development. Fluvial nutrient concentrations and fluxes vary as a function of the landscape characteristics and as a function of considerable rain fal! fluctuations. The strong seasonal pattern o f winter rainfall followed by many warm months without rain, typical o f Californias central coast, is shared with other regions around the world, generally termed Mediterranean climates. A pronounced transition from dry to wet conditions with intense rainfall and rapid runoff from mountainous catchments results in large changes in nutrient concentrations within and between storms. As part of a long-term investigation o f mobilization and fluvial transport of nutrients in catchments bordering the Santa Barbara Channel (ROBINSON et al. 2002), we intensively sampled nutrient concentrations and measured discharge at multiple sites in several streams. These data permit inferences about the relative contributions of nutrients from different portions o f the catchments as a function of storm size. We present analyses o f changes in nitrate, ammonium and phosphate concentrations in Mission Creek, Santa Barbara, California, during a series o f winter storms to examine the premise that the more developed portion o f the catchment i s the primary source o f nutrients to the stream.