Showing papers by "Diane M. McKnight published in 2007"

Chemical characteristics of fulvic acids from Arctic surface waters: Microbial contributions and photochemical transformations

Abstract: [1] Dissolved organic matter (DOM) originating from the extensive Arctic tundra is an important source of organic material to the Arctic Ocean. Chemical characteristics of whole water dissolved organic matter (DOM) and the fulvic acid fraction of DOM were studied from nine surface waters in the Arctic region of Alaska to gain insight into the extent of microbial and photochemical transformation of this DOM. All the fulvic acids had a strong terrestrial/higher plant signature, with uniformly depleted δ13C values of −28‰, and low fluorescence indices around 1.3. Several of the measured chemical characteristics of the Arctic fulvic acids were related to water residence time, a measure of environmental exposure to sunlight and microbial activity. For example, fulvic acids from Arctic streams had higher aromatic contents, higher specific absorbance values, lower nitrogen content, lower amino acid–like fluorescence and were more depleted in δ15N relative to fulvic acids isolated from lake and coastal surface waters. The differences in the nitrogen signature between the lake and coastal fulvic acids compared to the stream fulvic acids indicated that microbial contributions to the fulvic acid pool increased with increasing water residence time. The photo-lability of the fulvic acids was positively correlated with water residence time, suggesting that the fulvic acids isolated from source waters with larger water residence times (i.e., lakes and coastal waters) have experienced greater photochemical degradation than the stream fulvic acids. In addition, many of the initial differences in fulvic acid chemical characteristics across the gradient of water residence times were consistent with changes observed in fulvic acid photolysis experiments. Taken together, results from this study suggest that photochemical processes predominantly control the chemical character of fulvic acids in Arctic surface waters. Our findings show that hydrologic transport in addition to biogeochemical alteration of the organic matter must be considered in order to predict the ultimate fate of Arctic DOM.

Biogeochemical stoichiometry of Antarctic Dry Valley ecosystems

Abstract: operate over 10–10 6 years. The simple organisms (microbial and metazoan) comprising dry valley foodwebs adhere to strict biochemical requirements in the composition of their biomass, and when activated by availability of liquid water, they influence the chemical composition of their environment according to these ratios. Nitrogen and phosphorus varied significantly in terrestrial and aquatic ecosystems occurring on landscape surfaces across a wide range of exposure ages, indicating strong influences of landscape development and geochemistry on nutrient availability. Biota control the elemental ratio of stream waters, while geochemical stoichiometry (e.g., weathering, atmospheric deposition) evidently limits the distribution of soil invertebrates. We present a conceptual model describing transformations across dry valley landscapes facilitated by exchanges of liquid water and biotic processing of dissolved nutrients. We conclude that contemporary ecosystem stoichiometry of Antarctic Dry Valley soils, glaciers, streams, and lakes results from a combination of extant biological processes superimposed on a legacy of landscape processes and previous climates.

Sources and fates of dissolved organic carbon in lakes as determined by whole-lake carbon isotope additions

Abstract: Four whole-lake inorganic 13C addition experiments were conducted in lakes of differing trophic status. Inorganic 13C addition enriched algal carbon in 13C and changed the $$\delta^{13}$$ C-DOC by +1.5‰ to +9.5‰, depending on the specific lake. This change in $$\delta^{13}$$ C-DOC represented a significant input of algal DOC that was not completely consumed by bacteria. We modeled the dynamics in $$\delta^{13}$$ C-DOC to estimate the fluxes of algal and terrestrial carbon to and from the DOC pool, and determine the composition of the standing stock. Two experiments in lightly stained, oligotrophic lakes indicated that algal production was the source of about 20% of the DOC pool. In the following year, the experiment was repeated in one of these lakes under conditions of nutrient enrichment, and in a third, more humic lake. Algal contributions to the DOC pool were 40% in the nutrient enriched lake and 5% in the more humic lake. Spectroscopic and elemental analyses corroborated the presence of increased algal DOC in the nutrient enriched lake. Natural abundance measurements of the $$\delta^{13}$$ C of DOC in 32 lakes also revealed the dual contributions of both terrestrial and algal carbon to DOC. From these results, we suggest an approach for inferring the contribution of algal and terrestrial DOC using easily measurable parameters.

Photooxidation of wetland and riverine dissolved organic matter: altered copper complexation and organic composition

Abstract: In natural waters, the uptake of transition metals such as copper (Cu) by aquatic biota depends on the activity of the free cupric ion ({Cu2+}) rather than on total Cu concentration. Thus, an important ecological function of dissolved organic matter (DOM) in aquatic ecosystems is Cu–DOM complexation, which greatly decreases the {Cu2+}. However, Cu bioavailability is greatly modified by source and environmental history of DOM because DOM affinity for Cu varies by orders of magnitude among DOM sources; moreover, DOM is photochemically unstable. During 72-h irradiation experiments at intensities approximating sunlight with DOM from a palustrine wetland and a third-order river, we investigated photooxidative effects on DOM complexation of Cu as well as spectral and chemical changes in DOM that might explain altered Cu complexation. Irradiation decreased Cu complexation by riverine DOM, but unexpectedly increased Cu complexation by wetland DOM, resulting in 150% greater {Cu2+} in riverine DOM at the same dissolved organic carbon concentrations. The specific ultraviolet absorption (SUVa) and humic substances tracked photochemical changes in the conditional stability constants of Cu–DOM complexes, suggesting that the aromaticity of DOM influences its affinity for Cu. Carbonyl concentration in 13C nuclear magnetic resonance spectra (13C-NMR) covaried directly with Cu binding-site densities in DOM. However, no aspect of Cu–DOM complexation consistently covaried with fluorophores (i.e., the fluorescence index) or low molecular weight organic acids. Our results suggest that global increases in UV radiation will affect Cu–DOM complexation and subsequent Cu toxicity depending on light regime as well as DOM source.

Chemical characterization of DOM in channels of a seasonal wetland

Abstract: Although wetlands are known to be important sources of dissolved organic matter (DOM) within watersheds, production of DOM within wetlands is not well understood. In the Okavango Delta, a large wetland located in Botswana, large amounts of DOM produced in the wetland are transported in the river network and to the subsurface. The purpose of this study was to gain insight into environmental processing of DOM in wetland surface waters by examining chemical characteristics of plant litter leachates and fulvic acids isolated from two surface water sites in the Panhandle (PHFA) and Seasonal Swamp (SSFA) of the Okavango Delta. Spectroscopic properties measured over the course of leaching experiments indicated a greater abundance of plant-derived DOM over time. Results of elemental and 13CNMR analyses showed that aromaticities and C:N ratios of PHFA and SSFA and a Cyperus papyrus leachate fulvic acid (CPLFA) were in the range typical for fulvic acids derived from vascular plants. Fluorescence analyses of fulvic acids using parallel factor analysis (PARAFAC) further indicated the importance of plant litter sources in surface water DOM. Environmental processing of DOM in downstream surface waters by bacterial and photodegradation was suggested by higher N and S content for SSFA compared to CPLFA and by differences in δ15N, δ34S, δ13C and fluorescence signatures among the 3 fulvic acid samples. These chemical characterization results suggest that a progressive enrichment of DOM by plant-derived material occurs with distance downstream and that this DOM undergoes some environmental processing within the surface water system.

Photochemical control of copper complexation by dissolved organic matter in Rocky Mountain streams, Colorado

Abstract: We investigated photochemical, seasonal, and source effects on copper (Cu) complexation by dissolved organic matter (DOM). Cu-DOM complexation regulates Cu toxicity by decreasing the activity of the cupric ion ({Cu2+}), the most bioavailable Cu species. However, DOM is photochemically unstable under solar insolation. We analyzed Cu-DOM complexation before and after photooxidation of DOM collected from six rivers during spring runoff and late summer (n = 12 DOM samples). After irradiation of DOM for 24 h in a solar simulator (~4 d of ambient insolation), we analyzed Cu-DOM complexation during potentiometric titrations of Cu into dissolved organic carbon concentrations of 5 mg L−1. In 10 DOM samples across the range of titrations (Cu, 7.8 x 10−8 to 8.7 x 10−6 mol L−1), photooxidation of DOM decreased Cu complexation, increasing {Cu2+} by an average of 156% ± 28% (mean ± SE). In one DOM sample, irradiation had no net effect on {Cu2+} (6% ± 12%), whereas in another Cu complexation was enhanced (30% ± 4%). Cu complexation that was indistinguishable before irradiation decreased significantly more during photooxidation of DOM in spring (185% ± 25%) than in summer (74% ± 14%). The specific ultraviolet absorption coefficient at 254 nm explained ~60% of the variation in conditional stability constants of Cu-DOM complexes regardless of DOM source, season, or extent of photooxidation. During a simulated contaminant event where 1.5 x 10−6 mol L−1 Cu was added to site waters, water chemistry reduced bioavailability in 6 of 12 cases to below the {Cu2+} expected to cause 50% mortality ({Cu2+}LC50) in larval fish. However, after 6 d of photooxidation, none of the site waters remained below {Cu2+}LC50.

Effects of Acid Rock Drainage on Stocked Rainbow Trout (Oncorhynchus mykiss): An In-Situ, Caged Fish Experiment

Abstract: In-situ caged rainbow trout (Oncorhynchus mykiss) studies reveal significant fish toxicity and fish stress in a river impacted by headwater acid rock drainage (ARD). Stocked trout survival and aqueous water chemistry were monitored for 10 days at 3 study sites in the Snake River watershed, Colorado, U.S.A. Trout mortality was positively correlated with concentrations of metals calculated to be approaching or exceeding conservative toxicity thresholds (Zn, Mn, Cu, Cd). Significant metal accumulation on the gills of fish stocked at ARD impacted study sites support an association between elevated metals and fish mortality. Observations of feeding behavior and significant differences in fish relative weights between study site and feeding treatment indicate feeding and metals-related fish stress. Together, these results demonstrate the utility of in-situ exposure studies for stream stakeholders in quantifying the relative role of aqueous contaminant exposures in limiting stocked fish survival.

Trends in discharge and flow season timing of the Onyx River, Wright Valley, Antarctica since 1969

Abstract: Flow records at the two stream gauges on the Onyx River represent the longest actively collected environmental records in the McMurdo Dry Valleys, Antarctica. The downstream gauge, near Lake Vanda, has been collecting data since 1969, and the upstream gauge, at Lower Wright Glacier (LWRT), has collected data since 1972. We analyzed these records to assess the long-term trends in annual discharge, flow season length, flow season start, and flow season end. Our results indicate overall decreasing trends in annual discharge (0.4x10 6 m 3 /decade at LWRT, 0.8 x10 6 m 3 /decade at Vanda), and increasing flow season lengths (by 7 d/decade at LWRT, and 2.7 d/decade at Vanda), influenced by earlier start and later end dates (5.2 and 0.8 d/decade, respectively at LWRT; 4.8, 1.4 d/decade, respectively at Vanda). This suggests that flow season climate patterns in the Dry Valleys are decreasing glacier melt intensity overall, but extending the period of meltwater generation.