2003 Progress Report: The Influence of Climate-Induced Alterations in Dissolved Organic Matter on Metal Toxicity and UV Radiation in Rocky Mountain Streams

EPA Grant Number: R829640
Title: The Influence of Climate-Induced Alterations in Dissolved Organic Matter on Metal Toxicity and UV Radiation in Rocky Mountain Streams
Investigators: Clements, William , Baron, Jill S. , McKnight, Diane M. , Meyer, Joseph S.
Institution: Colorado State University , University of Colorado at Boulder , University of Wyoming
EPA Project Officer: Hiscock, Michael
Project Period: April 1, 2002 through April 1, 2005
Project Period Covered by this Report: April 1, 2003 through April 1, 2004
Project Amount: $896,212
RFA: Assessing the Consequences of Global Change for Aquatic Ecosystems: Climate, Land Use, and UV Radiation (2001) RFA Text |  Recipients Lists
Research Category: Global Climate Change , Ecological Indicators/Assessment/Restoration , Water , Ecosystems , Climate Change


The objective of this research project is to investigate the influence of climate-induced changes in hydrology and dissolved organic material (DOM) on responses of stream ecosystems to the combined stress of ultraviolet radiation (UVR) and heavy metals. We hypothesize that changes in climate and UVR will alter the quality and quantity of DOM in Rocky Mountain streams. Because DOM regulates light attenuation and metal bioavailability in these systems, we predict that exposure to UVR and metals will increase as a result of changes in DOM.

Progress Summary:

I. Biogeochemical Modeling

We used a modeling approach to examine the potential effects of climate change on stream hydrology and biogeochemistry. DayCent, an ecosystem biogeochemistry model, was coupled with PHREEQC, a soil and water chemical equilibrium model. The linked DayCent/PHREEQC model tracks soil cation exchange reactions and cycling of ammonium (NH4+), nitrate (NO3-), organic N, inorganic and organic P and S, base cations (Ca, Mg, K, Na), alkalinity, metals (Al, Fe, Mn), and pH in forest, grassland, cropland, and savanna systems. We tested the DayCent/PHREEQC model for alpine and subalpine catchments using 23 years of continuous records from the Loch Vale Watershed in Rocky Mountain National Park, Colorado.

Results . The model simulated discharge and dissolved organic carbon (DOC) well, although predictions of DOC concentrations were better for alpine than subalpine catchments. These results indicate that the DayCent/PHREEQC model can be used to predict changes in DOC and other chemical parameters under changing climate scenarios.

II. Field Monitoring

The objectives of our field monitoring program were to: (1) characterize the relationship between stream hydrology and DOM in metal-polluted and unpolluted streams; (2) measure the photodegradation and metal-binding affinity of DOM; (3) characterize the influence of source and composition of DOM on metal binding and light attenuation; and (4) measure spatial and seasonal changes in the relative contributions of fulvic acids to DOM.

We investigated the photolysis of DOM and Cu complexation and calculated how photobleaching of DOM would increase UV exposure to the streambed in natural waters. We measured the photodegradation and metal-binding affinity of fulvic acids and assessed the influence of source on fulvic acid metal binding and light attenuation. XAD-8-isolated fulvic acid samples were normalized to an absorption coefficient of 14.0 m-1 at 300 nm and irradiated for 4.5 hours in a 1,000 W solar simulator. The change in absorbance at 300 nm was monitored over the 4.5-hour period. Cu-fulvic acid titrations were performed on the XAD-8 isolates using a cupric ion-selective electrode before and after irradiation.

Results. Highest stream discharge and DOC concentrations were observed in late spring, corresponding to the peak in spring runoff. Across all 21 streams, there was a significant relationship between DOC concentration and stream discharge. These spatial and temporal patterns suggest that stream discharge is an important regulator of DOC concentrations.

Cu complexation with DOM did not differ between wetland versus riverine sources; however, photodegradation of DOM was greater in spring than summer, suggesting potential seasonal differences in toxicity and bioavailability of Cu. We found that photobleaching decreased DOM absorbance of UV by 25 ± 5 percent (mean ± standard deviation) in 24-hour exposures, the equivalent of approximately 3 days of insolation at our field sites.

Absorbance of fulvic acids isolated using an XAD-8 resin column decreased for all sites during the irradiation period, regardless of source. Changes in absorbance were inversely related to aromatic content, suggesting that fulvic acids with a higher content of plant material, which provide more aromatic groups, may buffer the effects of increased amounts of UVR more successfully. The metal-binding capacity of fulvic acids showed little variation between pristine and moderately contaminated sites. Snake River, the highly metal-contaminated site, however, had lower binding capacities than that of the other sampling sites. Decreased ligand concentrations in Snake River may be the result of coprecipitation of DOM with metal oxides or by DOM sorption onto the metal oxides covering the streambed (McKnight, et al., 1992). This process preferentially removes molecules from the fulvic acids, which are important to metal binding. These results indicate that aquatic fulvic acids in heavily metal-contaminated sites may be less effective at decreasing metal bioavailability.

III. Field and Microcosm Experiments To Assess Effects of UV-B and Heavy Metals

Field and microcosm experiments were conducted to assess the interactive effects of UV-B radiation and heavy metals on the structure and function of benthic communities. The specific objectives of these experiments were to: (1) determine how communities with different metal exposure histories respond to the combined effects of UV-B and heavy metals; (2) measure the direct effects of UV-B radiation on the structure and function of benthic communities in the field along a metal contamination; and (3) assess the influence of UV-B on metal bioavailability and toxicity.

To determine how exposure history influences community responses to the combined effects of UV-B and metals, two consecutive microcosm experiments were conducted in 2003. Benthic communities from West Tennessee Creek (reference) and the Arkansas River (metal- contaminated) were collected using artificial substrates and transferred to the Stream Research Laboratory located at Colorado State University in Fort Collins, Colorado. The 16 microcosms were assigned randomly to one of four treatments (control, metals only, UV-B only, and metals + UV-B) in a 2 x 2 factorial design (metals x UV-B exposure).

Field experiments were conducted during the summer of 2003 to investigate the effects and interaction of UV-B radiation and heavy metal pollution on the structure of benthic communities. Twelve sites were selected across a gradient of metals (1 to 377 µg/ l Zn) and dissolved organic matter (0.72 to 3.77 mg/L DOM) from the Central Rocky Mountains of Colorado. Replicate (n = 3) PVC frames (1 x 2 m) were placed in riffle areas of each stream. One side of each structure was covered with a filter that removed UV-B. Substrate-filled trays and ceramic tiles placed under each structure were colonized by macroinvertebrates and periphyton for 60 days.

Two microcosm studies were conducted during the summer of 2004. One of the studies was designed to complement a previous experiment in which we found that total organic carbon can increase Zn concentrations associated with periphyton and interact with Zn to reduce respiration of benthic communities. Recently, we have completed the field and laboratory components of this experiment, but sample processing and data analysis are ongoing. A second microcosm study was conducted to address the effects of UV-B on benthic communities of differing metal exposure history. In stream microcosms, benthic communities from the two locations were exposed to UV-B and compared to controls. Algae and macroinvertebrate communities were collected as well as gross primary production (GPP), chlorophyll- a, and ash-free dry mass. The expected completion time for processing these samples is June 2005.

Results . Responses of benthic communities to metals and UV-B varied between the reference and metal-polluted streams. No differences in mayfly drift among treatments were observed for assemblages collected from the metal-polluted stream (Arkansas River). In contrast, drift of mayflies from the reference stream (West Tennessee Creek) increased significantly under both UV and metals exposure. Significant UV effects were observed on net community metabolism, and the combined effects of UV plus metals were greater than either stressor alone. In addition, effects of metals on GPP were greater in the reference stream than in the metal-impacted stream. Results from this study indicate that exposure history plays an important role in determining the sensitivity of aquatic communities to UV-B or metals.

Preliminary results suggest that chlorophyll- a responded to the removal of UV-B at background and low-metals sites, whereas medium- and high-metals sites showed no response. Differences in specific algal taxa among sites may have contributed to the response of chlorophyll- a. Benthic macroinvertebrate and algae communities responded to the metals gradient, as expected. Most measures of macroinvertebrate community structure were reduced in the medium- and high-metals groups. Typically, benthic community measures at the background and low-metals streams were lower in the no-UV treatments, but this trend was reversed in the medium- and high-metals groups. These findings suggest that Rocky Mountain streams receiving metal pollution from historic mining may be at a greater risk of impairment resulting from the additional stress of UV-B radiation.

In June 2004, Clements organized a special session at the North American Benthological Society Annual Meeting in Vancouver, British Columbia, Canada entitled, “Photoecology of Benthic Habitats: Interactions Among Ultraviolet Radiation, Organisms, and the Environment ” (http://www.benthos.org/Meeting/nabs2004/specialsessions.htm Exit ). This special session brought together researchers from three of the five projects funded in 2002 by the Science To Achieve Results program: “Assessing the Consequences of Global Climate Change for Aquatic Ecosystems: Climate, Land Use, and UV Radiation.”

Future Activities:

We will parameterize DayCent-CHEM for the Upper Arkansas watershed, a metal-polluted stream in central Colorado, and running the model under different climate and vegetation scenarios. We will determine if climate change-influenced changes in DOC affect the solubility of Cu, Zn, and Cd.

We will model photochemistry of individual surface waters, which will incorporate the dynamics of stream flow and geochemistry specific to each monitoring site to predict quantitative effects of increasing levels of UVR. Tools such as parallel factor analysis (PARAFAC) and spectral slope will be used to investigate the nature of the UVR-induced chemical changes of the fulvic acid isolates. These tools may help elucidate which chemical moieties are altered or lost during UVR, and thus are responsible for the observed changes in metal-binding capacity. Correlations between the metal-binding data and the PARAFAC data will be investigated to determine if PARAFAC can provide predictive capabilities for UV-induced changes of fulvic acid metal-binding affinities.

Currently, benthic samples collected from natural substrate during the 2003 field experiment are being processed. The expected completion time is December 2004. Results from the 2003 microcosm experiment are being analyzed for publication, with a plan to submit a manuscript for publication in Ecological Application in January of 2005. Macroinvertebrate and periphyton samples are being processed from the 2004 microcosm experiments. Physicochemical data collected during the field experiments, including levels of DOM and heavy metals, are being analyzed. We anticipate submitting this research for publication in the fall of 2005 to Environmental Toxicology and Chemistry . Additionally, we expect to publish a methods paper outlining the techniques used to quantify UV-B exposure to the streambed by May 2005.

Journal Articles on this Report : 3 Displayed | Download in RIS Format

Other project views: All 20 publications 5 publications in selected types All 5 journal articles
Type Citation Project Document Sources
Journal Article Kashian DR, Prusha BA, Clements WH. Influence of total organic carbon and UV-B radiation on zinc toxicity and bioaccumulation in aquatic communities. Environmental Science & Technology 2004;38(23):6371-6376. R829640 (2003)
R829640 (Final)
R829515 (2004)
R829515 (2005)
R829515 (Final)
  • Abstract from PubMed
  • Full-text: ACS-Full Text HTML
  • Abstract: ACS-Abstract
  • Other: ACS-Full Text PDF
  • Journal Article McKnight DM, Duren SM. Biogeochemical processes controlling midday ferrous iron maxima in stream waters affected by acid rock drainage. Applied Geochemistry 2004;19(7):1075-1084. R829640 (2002)
    R829640 (2003)
  • Full-text: Science Direct Full Text
  • Abstract: Science Direct Abstract
  • Other: Science Direct PDF
  • Journal Article Prusha BA, Clements WH. Landscape attributes, dissolved organic C, and metal bioaccumulation in aquatic macroinvertebrates (Arkansas River Basin, Colorado). Journal of the North American Benthological Society 2004;23(2):327-339. R829640 (2002)
    R829640 (2003)
    R829640 (Final)
    R829515 (2003)
    R829515 (2004)
    R829515 (Final)
  • Abstract: Freshwater Science-Abstract
  • Supplemental Keywords:

    watersheds, ecological effects, bioavailability, toxicity, toxics, heavy metals, ecosystem indicators, aquatic, macroinvertebrates, benthic, ecology, hydrology, geology, limnology, modeling, monitoring, field experiments, microcosms, Rocky Mountains, Colorado, CO, EPA Region 8,, RFA, Scientific Discipline, Air, Geographic Area, Water, Hydrology, Water & Watershed, climate change, State, Atmospheric Sciences, Ecological Risk Assessment, Watersheds, EPA Region, water resources, metal toxicity, dissolved organic matter, wetlands, environmental monitoring, global change, regional hydrologic vulnerability, aquatic food web, hydrologic models, climate models, Rocky Mountains, Rocky Mountain Streams, UV radiation, vulnerability assessment, aquatic ecosystems, watershed sustainablility, land and water resources, aquatic ecology, climate variability, climatic models, Global Climate Change, stream ecosystem, Colorado (CO), land use

    Relevant Websites:

    http://www.cnr.colostate.edu/~willc/index.htm Exit

    http://www.nrel.colostate.edu/projects/lvws/pages/homepage.htm Exit

    http://instaar.colorado.edu/people/bios/mcknight.html Exit

    http://uwadmnweb.uwyo.edu/Zoology/faculty/meyer/meyer.htm Exit

    http://www.benthos.org/Meeting/nabs2004/specialsessions.htm Exit

    Progress and Final Reports:

    Original Abstract
  • 2002 Progress Report
  • Final Report