H52C-01 INVITED
Estimation of coupled water and energy balance model parameters from equilibrium constraints on temperature and moisture
Salvucci [2001, WRR 37(5), 1357-1366] demonstrated that the conditionally averaged net moisture flux (dS/dt) on soil moisture storage (S) tends to zero due to equilibrating tendencies in the water balance, and that this property can be used to estimate the moisture dependence of net drainage and evaporation from precipitation measurements. Here we extend this idea to the coupled energy and water balance at the land surface. We show that, using conditional averaging and stationarity constraints, we can express a single objective function that measures the moisture and temperature dependent errors in land surface water and energy fluxes solely in terms of observed forcings (e.g. precipitation, radiation, wind speed) and surface states (moisture and temperature). It is through these conditional expectations – equivalent to the joint covariance of the states and forcing – that we can then estimate the model parameters. Here we illustrate through proof-of-concept examples and data from Ameriflux sites, that the combination of surface moisture and temperature data provides a robust, empirical basis for estimating evaporation models. Furthermore, because the method is derived only from stationarity and conservation statements (of energy and water), it is scale-free and thus derives effective land surface model parameters at the scale of the data applied in the estimation.
H52C-02 INVITED
Storms Induce Variable Changes in Phosphorus Release From a North Carolina Coastal Plain In-Stream Wetland
Wetlands are a valuable tool for water purification but vary in their effectiveness at sequestering phosphorus (P). A long water retention time and an extensive amount of sediment surface area in the wetland create optimum conditions for slow P exchange between the water column and storage pools (i.e., sediment pore water, sorbed to sediments, etc.). These P storage pools can be overwhelmed through hydrologic disturbances caused by intensive rainfall events and flooding associated with storms. Events such as hurricanes and tropical storms can differ in their rainfall delivery and residence time overland, thus causing variations in their hydrologic disturbances to wetlands. In some cases, storms can increase inflow into wetlands that exceeds its water storage capacity, thereby flushing P storage pools and accelerating P releases with outflow (Q). Between 1996 and 1999, this study examined 10 different storm event and wetland hydraulic characteristics (rainfall totals, residence time over mainland, wetland Q, etc.) on dissolved phosphorus (DP) exported from an animal waste impacted in-stream wetland and evaluated shifts in P storage pools (sediment pore water and sediment total phosphorus concentrations) that would account for potential DP releases. The 10 storms (6 hurricanes and 4 tropical storms) were found to differ greatly in their ability to accelerate P releases. For example, turbulence within the wetland caused by a single hurricane event such as by Bertha (in 1996) and Bonnie (in 1998) released less than 50 kg DP/mo, whereas, a single tropical storm (in 1997) promoted the release of 117 kg DP/mo. The higher DP mass load after the single tropical storm in 1997 was explained by severe disturbance to internal DP storage pools that increased outflow DP concentrations from 0.9 to 3.3 mg/L. Correspondingly, the range in outflow DP concentrations after the pair of single hurricane events was only between 0.2 to 0.3 mg/L. On a gross scale, massive rainfall totals by successive hurricanes in 1998 (Dennis, Floyd, and Irene) precipitated huge hydrologic disturbances within this wetland resulting in 1,119 kg of DP released. These three hurricanes in a short time period (2.5 mo) delivered a cumulative rainfall total of 606 mm that caused the in-stream wetland to be flushed several hundred-fold with equivalent volumes of water as inflow. Multiple flushing of the wetland by storms over 4 yrs also resulted in significant reductions in sediment pore water DP contents and in sediment total phosphorus concentrations. The large difference in DP mass loads released from this in-stream wetland was explained using regression analyses to evaluate mass loads versus Q differences between storm events. Examining 4 yrs of storm activity showed that some tropical storms can cause similar or higher DP releases than a single hurricane; still, multiple successive hurricanes in the same season delivering heavy rainfall totals significantly increased DP export.
H52C-03
Phosphorus Sorption Characteristics of the Bronx River Bed Sediments
Phosphorus (P) is a major nutrient for plant growth, and it is often the primary limiting nutrient controlling algal blooms in aquatic and semi-aquatic systems. The Bronx River of New York City, NY, USA constitutes freshwater and coastal water systems, and the water quality of the systems is considered lower than the standard levels. Algal blooms and subsequent oxygen (O2) depletion within the river have degraded water quality to some extent, in turn, endangered fishing, and limited recreational use. Soluble inorganic P enters solid or liquid phase through sorption/desorption processes, which determine the internal loading of P, one of the P availability indicators in the Bronx River. The objectives of this study were: to determine P sorption characteristics of the bed sediments in the Bronx River, and estimate the effects of physico-chemical properties of the sediments on P availability in the water column of the river. Bed sediments were collected from 15 sites along the Bronx River, from the origin in Davis Brook and Kensico Dam, Westchester County, through the Bronx to the Sound View Park estuary. Phosphorus sorption maxima Smax was significantly correlated with oxalate-extractable iron (Fe) and aluminum (Al), and acid-extractable magnesium (Mg) and calcium (Ca) along with total organic carbon (TOC). Similarly, the equilibrium P concentration (EPC0) was significantly correlated with Al and TOC. This study also showed that high percentage (>87%) of P sorbed by the bed sediments was determined as hysteretic P, as well as sediments from most of the sites had high initially adsorbed P, indicating potential large supply of P to the water column by the sediments could occur under changing hydro-climatic conditions as affected by global rise in temperature, e.g., changes in pH, ionic strength, redox conditions, etc., in turn, creating eutrophic conditions, and subsequent algal blooms.
H52C-04
Hydrochemical Response to Drought Conditions at an Alpine Watershed, Colorado Front Range
Extreme climate events play a key role in alpine hydrochemistry by altering source waters and flowpaths. Persistent drought conditions from 2000-2002 at Green Lakes Valley resulted in precipitation and streamflow about 75% of normal for the last 25 years. Surprisingly, both concentrations and fluxes of geochemical weathering products and nutrients increased during the drought at the higher elevation sites. Niwot Ridge LTER has continuously monitored streamflow, precipitation chemistry, and water quality for 25 years in Green Lakes Valley at 8 sites representing an elevation gradient extending from 3250 meters at the valley outlet to 4000 meters at the continental divide. Comparing continuous 5-year blocks of above-average precipitation (1993-1997) vs. below-average years (2000- 2004), both concentrations and fluxes were significantly higher during drought for base cations (p<0.05) throughout upper Green Lakes Valley. DAYCENT modeled predicted discharge correctly during the period of above-average precipitation but underpredicted discharge during drought conditions, suggesting an additional source of water. End Member Mixing Analyses (EMMA) conducted during 1996 constrains streamflow as a mixture of snowmelt, talus water, and groundwater with subsurface flowpaths contributing more than 50% of streamflow, even during snowmelt (Liu, 2004). However, EMMA results during drought years using chemical and isotopic compositions from surface water, talus springs, snowpits, snowmelt, soil water, and groundwater suggest an additional, unidentified source of streamflow. One possible end member is melting permafrost within the basin. We downscaled a qualitative, regional permafrost distribution model of the Colorado Front Range to investigate the potential role of melting permafrost on hydrochemical characteristics in Green Lakes Valley. Model results indicate that increasing mean annual air temperature by 1 degree Celsius results could melt 35% of permafrost in the watershed. Future monitoring and research efforts will examine the potential irreversible effects of extreme climate events and permafrost melt on alpine ecosystems.
H52C-05 INVITED
Impacts of Climate Change on the Resilience of a Eutrophic Lake
Phosphorus-limited lakes are believed to exhibit two meta-stable states: one characterized by low levels of phosphorus (P) and algae in the water column, and the other by high levels of P and turbid, eutrophic conditions. While brief spikes in P-loading from the watershed may cause the lake to shift from clear to turbid conditions, the turbid conditions may sustain themselves even after the loading drops by switching on recycling of P from the sediments. There are thus thresholds associated with lake P levels separating clear from turbid states, and associated with these states is a degree of resilience to environmental variability. In Wisconsin's seasonally eutrophic Lake Mendota, the long-term driver of lake quality is agricultural activity in the watershed, through manure and fertilizer application. This supply is mediated by the hydrological fluxes imposed by the prevailing climate. Lake Mendota's climate is expected to shift the balance of precipitation from snow towards rainfall, advance the onset of snowmelt and soil thaw, and lead to more variable summer precipitation, with concomitant effects on runoff and P loading. Using a distributed ecohydrological model of P fate and transport, we explore how scenarios of future climate change may affect the thresholds and resilience of Lake Mendota's trophic states to agricultural practices in the watershed.
H52C-06 INVITED
Controls over fungal communities and consequences for nutrient cycling
Soils harbor a high diversity of microbes-- as many as 100 species of fungi within a square meter. If different species target different components of litter, a more diverse community of fungi should lead to faster decomposition rates. We examined the hypotheses that variation in substrate use among fungal groups and variation in nitrogen availability are both important controls over the diversity of fungi in an Alaskan boreal forest. Nitrogen availability was considered because microbes are often N-limited, and because humans are altering N availability via anthropogenic N deposition and global warming. We used nucleotide analogs to link fungal groups with their role in decomposition in field samples. Leaf litter collected from the forest floor was supplemented with one of four N-containing compounds. Bromodeoxyuridine (BrdU, a thymidine analog) was also added. After 48 hours incubation, DNA was extracted. Most growing fungi should have assimilated the BrdU into new DNA. Their genetic identity was determined using oligonucleotide fingerprinting of rRNA genes (OFRG). OFRG is an rRNA gene profiling method that sorts genes into taxonomic groups with a high degree of resolution, and has a large capacity for sample processing. Fungal groups that proliferated following the addition of a given compound probably metabolized that compound. We found that fungal taxa varied in their responses to different substrates, indicating that they differed in substrate use. Specifically, community composition of fungi was significantly different among substrate treatments (P < 0.001). In addition, of the 15 dominant taxa, seven displayed significant preferences for one substrate over another. For instance, taxa within the Helotiales preferred glutamate (P = 0.001); Sporidiales, tannin-protein complexes (P = 0.014); Saccharomycetales, arginine (P = 0.042); and Polyporales, arginine and lignocellulose (P = 0.040). In a complementary experiment, we used BrdU labeling to characterize effects of N fertilization on fungal community composition. We observed that N fertilization decreased the richness of fungal taxa by 22%. Helotiales and Saccharomycetales tended to increase under N fertilization, whereas Polyporales did not change significantly. Together, these results indicate that shifts in the community composition of fungi under anthropogenic N deposition could lead to changes in nutrient dynamics.
H52C-07
Hydro-climatic Changes: Potential Non-linear Responses of Phosphorus Dynamic in Aquatic/Semi-aquatic Systems
Depending on resilience, threshold and lag times, hydro-climatic changes can cause nonlinear and/or irreversible changes in phosphorus (P) dynamic, and instigate P enrichment in aquatic/semi-aquatic systems. Thus, studying direct/indirect effects of expected global climate change on bioavailability of organic P in aquatic systems are in critical need, to help manage or increase the resilience of the ecosystem. The central hypothesis of this study is that P dynamic in aquatic, especially freshwater, ecosystem is likely to behave nonlinearly due to expected changes in sediment and water acidity, redox status, etc., because of potential hydro-climatic changes in the decades to come, thus, could face irreversible adverse changes. Devising possible biological and chemical treatments for the removal of P from eutrophic lakes, estuaries, etc, as well as helping in predicting the movement and fate of P under changing hydro-climatic conditions would be crucial to manage aquatic ecosystem in the near future. The critical question is not how much P is stored in any given aquatic/semi-aquatic system, but how the resilience and nonlinearity relate to the stability of stored P are affected due to the levels of environmental stressors, which are expected to fluctuate due to global change in the decades to come. Studies related to 31P Nuclear Magnetic Resonance Spectroscopy analysis, and multiple hydraulic retention cycles showed that, in general, frequent drying and reflooding of a semi-aquatic system such as wetland could significantly increase the bioavailability of P due to degradation of relatively less stable organic P, e.g., glycerophosphate and nucleoside monophosphate. Moreover, nutrients flux from sediments to the water column depended on the concentration gradients of the sediment-water interface and redox status. Shift in equilibrium P concentration of the water column as the water level rises, may cause release of adsorbed P from the sediments. Restoration of a eutrophic system may involve stepwise efforts including control of catchment nutrient inputs, internal nutrient loading, and biomanipulation, however, flooding, previously non-flooded areas, could export massive amount of P to nearby aquatic bodies, in turn, may cause collapse of the ecosystem.