The overarching conceptual hypothesis of our decadal research plan is that altered climate, changing snow regime and changes in atmospheric composition are driving biogeochemical and trophic changes in high elevation ecosystems. Given the continuing climatic trend toward warming temperatures, driven principally by warmer summers, and uncertainty of how this trend will influence deposition of snow and rain at high elevations in the Sierra Nevada, we have continued our long-term studies of atmospheric deposition (Fig. 2). Snow water equivalence has varied by more than a factor of five, ranging from 590 mm to 3177 mm, and averaged 1391±700 mm (±standard deviation). Rainfall amounts varied by over a factor of 20, ranging from 20 mm to 430 mm and averaging 180±98 mm. High interannual variability in precipitation has strong influence on nitrate concentrations and sources, lake acid neutralizing capacity and on lake metabolism.
Atmospheric particulate matter (PM) sampling was recently conducted at the Lower Kaweah monitoring station (10 km west of Emerald Lake, 1905 m asl), which had access to power required to operate the Stacked Filter Unit and Micro-Orifice Uniform Deposit Impactor(Vicars et al. 2010; Vicars and Sickman 2011; Vicars 2009). Total P (TP), inorganic P (IP), and Al, Fe, Ca, Mg and V were determined. Aerosol concentrations were elevated, primarily due to transport from offsite and emissions from local and regional wildfires. The dry depositional flux of TP ranged between 7 and 118 μg m-2 d-1 (mean of 40 ± 27 μg m-2 d-1). Relative rates of dry deposition of P and N are consistent with increasing N limitation of phytoplankton. PM concentrations were highest during the dry season, averaging 8.8 ± 3.7 and 11.1 ± 7.5 μg m-3 for the coarse and fine fractions, while winter months had PM concentrations < 1 μg m-3. Fe/Al and Fe/Ca (Fig. 3) ratios suggest a mixture of dust from regional agriculture and long-range transport of dust from Asia.
We identified major soil P pools and categorized them into biologically or geochemically controlled (Fig. 4). On average, 14% of the total P in A horizons is labile or considered plant available, 62% is mostly bound to Al and Fe, and 24% is considered refractory. For B horizons, 9% of the total P is labile, 61% is bound to Al and Fe, and 30% is refractory. Biologically controlled soil P pools represent 62% of the total-P in A horizons and 53% in B horizons. Phosphorus appears to not be in short supply in soils. Microbial biomass P concentrations are highest during winter when soils are snow covered and concentrations decline over time into the autumn. This pattern indicates the importance of microbial P-immobilization in subnivean soils and highlights a potential mechanism for soil P retention that may be sensitive to climate change.
We conducted field incubations of lake sediments to determine rates of P release to the water column and used N2 to decrease the concentration of O2 in water above sediment cores to determine rates of P release under anoxia. There was a net consumption of water column P by sediments under oxic conditions; under N2-induced anoxia we measured zero or slightly positive net flux of P from the sediments. Together with observations of hypolimnetic P concentrations, these experiments suggest that sediments cannot explain long-term increases in P availability to phytoplankton.
A sequential P fractionation was used to measure the content of P in Emerald Lake sediments, and Fe:Al ratios were used to assess the potential for P desorption. As an aspect of interpreting P cycling in Emerald Lake, we are characterizing P fractions and Fe:Al ratios in surficial sediments from 50 lakes throughout the Sierra Nevada and short sediment cores collected from five lakes including Emerald.
Though our time series had indicated a trend toward nitrogen limitation of phytoplankton (Fig. 5), and we hypothesized the cause to be increased P inputs (Sickman et al. 2003a), recent results indicate periods with P limitation as well as N or co-limitation (Fig. 5; Heard 2013). Hence, we conducted experimental bioassays in five Sierra Nevada lakes, including Emerald, using a gradient of nitrogen concentrations and a submersible fluorometer to frequently determine chlorophyll responses. Using the experimental data and logistic growth modeling, we have been able to identify nutrient criteria for N and P concentrations in lakes that can be applied to synoptic survey results to estimate the proportion of Sierra Nevada lakes that have been altered by atmospheric deposition of nutrients. These periodic surveys allow us to place our measurements at Emerald Lake and in the Tokopah basin in the broader context of other Sierran lakes. We have conducted occasional sampling of multiple lakes throughout the region (e.g., Melack et al. 1985, Sickman et al. 2003a). The most recent surveys (2006-2008) included 150 lakes in 14 catchments spanning the latitudinal extent of the Sierra Nevada (Sadro et al. 2012) and 50 lakes in National Parks and Forest Service wilderness areas of the central Sierra Nevada (Bennett et al., in review) to support development of diatom-inferences models.
The ratio between daily whole-lake gross primary production and community respiration characterizes the balance between autotrophy and heterotrophy and provides an index of whole-lake metabolism. In recent years we have added direct measurements of ecosystem metabolism to our regular sampling regime. We have accumulated a three-year base-line of metabolic rate measurements, including net metabolic balance, while also characterizing the impact of a large rain storm (Sadro et al. 2011a,b,c; Sadro and Melack 2012; Fig. 6). Ecosystem metabolism has important implications for energy flow, biogeochemical cycling and food web dynamics, and it is a valuable metric of environmental change which is likely to be sensitive to interannual variability of snowmelt and lake temperature.
