Quantification of River Metabolism Along a Heavy Metal Contamination Gradient Through the Development and Application of a Smart Tracer SystemEPA Grant Number: FP917110
Title: Quantification of River Metabolism Along a Heavy Metal Contamination Gradient Through the Development and Application of a Smart Tracer System
Investigators: Stanaway, Daniel J
Institution: Boise State University
EPA Project Officer: Carleton, James N
Project Period: August 23, 2010 through August 22, 2013
Project Amount: $74,000
RFA: STAR Graduate Fellowships (2010) RFA Text | Recipients Lists
Research Category: Academic Fellowships , Fellowship - Ecosystem Services: Aquatic Systems Ecology
Contrary to current models of ecosystem response to chronic metal induced stress, microbial communities of the Clark Fork River in western Montana, the largest Superfund site in the United States, have high species diversity with repressed functional characteristics. Our study attempts to quantify the magnitude of metal contamination-induced limits on in situ heterotrophic microbial activity in metal tolerant communities via the application of a novel metabolically reactive hydrologic tracer.
Mining activity has contaminated numerous waterways with heavy metals. Vibrant microbial communities have evolved to thrive in these toxic environments. However, the presence of the pollutants appears to continually inhibit ecosystem services. We are employing a novel metabolically reactive hydrologic tracer that directly links ecosystem processes to flow hydraulics to interrogate how chronic contamination affects ecosystem function through quantifying in situ microbial community metabolism.
We hypothesize that chronic metal contamination produces a measureable metabolic cost to tolerant communities. To test this hypothesis, hyporheic sediments, supporting intact microbial communities, will be collected from sites along the Clark Fork River contamination gradient and pristine reference sites. Rates of microbial heterotrophic metabolism will be assessed in the presence and absence of an acute metal exposure to determine the metabolic cost of communities evolved for metal tolerance. Further, this approach allows us to assess metabolic costs in absolute and relative terms within and among communities evolved under different selective pressures (e.g, levels of metal contamination). We hypothesize that both community types will be negatively affected by the acute exposure, thereby indicating that even in tolerant communities, exposure to persistent pollutants exacts an energetic toll on community metabolism. To interrogate these communities, flow-through columns replicating hyporheic conditions will be packed with contaminated or clean sediment from six site pairs (contaminated vs. pristine). From each field site, three replicate columns will be treated with cadmium (Cd) (metal stressor), resazurin (metabolically reactive hydrologic tracer), and chloride (conservative hydrologic tracer); another set of three control columns will be treated with chloride and resazurin only. Of the metal cocktail that exists in the Clark Fork River, cadmium was selected as the experimental treatment because of its high toxicity and inability to abiotically reduce resazurin. Dissolved oxygen (DO) measurements will also be taken at the upstream and downstream end of the columns through non-intrusive fluorescence quenching to support the findings of the smart tracer. The Raz-Rru reactive advection dispersion equation (ADE), a modified version of the standard ADE equation, is used to determine the reaction rate of the biological reduction of resazurin to resorufin within a given hydrological setting. A Markov chain Monte Carlo approach will be developed to optimize line fitting to data for populating the ADE. The outcome of this approach will be a direct measure of the influence of metal stress on heterotrophic metabolism by microbial communities inhabiting the hyporheic zone of the chronically contaminated Clark Fork River.
The reduction rate of resazurin to resorufin provides direct quantification of the rate of microbial community respiration in the context of the overarching hydrological parameters such as velocity, dispersivity, and retardation. In aerobic heterotrophic microbial communities, the overall metabolic rate is a functional variable indicative of ecosystem health. In the absence of acute metal exposure, it is expected that the metabolic rate of communities from contaminated sites will be reduced relative to the corresponding pristine site, with the greatest inhibition observed in communities from the most heavily contaminated sites. Additionally, it is expected that in the presence of an acute metal exposure, resazurin reduction rate will be reduced in all communities relative to rates observed in the absence of an acute metal stress. The magnitude of inhibition of the metabolic rate reflects the continued metabolic cost of exposure to persistent metal pollutants even after long periods of selection for tolerant organisms. More specifically, communities from pristine and low contamination sites are expected to be more dramatically affected by the acute Cd exposure than communities associated with sediments from sites with higher in situ contamination levels. Secondarily, our study strives to further develop the Raz Rru Smart Tracer system as a viable tool for directly linking ecosystem processes with hydraulic parameters. It is expected that a strong correlation will exist between the reduction of resazurin to resorufin and dissolved oxygen consumption, thus corroborating the use of this system as a eco-hydrological tool.
Potential to Further Environmental/Human Health Protection:
The findings of this study will be important in supporting a new perspective of ecosystem response to chronic stress. Quantification of in-stream microbial function has a breadth of potential ecological and environmental regulatory applications. Development of an index relating changes in microbial respiration to metabolic function will allow for the determination of the consequences of anthropogenic contamination in terms of ecosystem services, such as hyporheic biogeochemical cycling, productivity, and nutrient retention. This relationship is of global importance in the context of carbon cycling, nutrient availability, and ecosystem integrity in systems impacted by the presence of persistent pollutants or other chronic stressors such as global climate change. This index, with appropriate tools, has the potential to advance the science governing environmental regulation and monitoring. Hyporheic biofilms can be more sensitive indicators of environmental stress than the ichthyological and macro-invertebrate based protocols currently employed because they form the base of aquatic food webs and have a high degree of exposure. Changes in hyporheic microbial assemblages have been detected at concentrations nearly an order of magnitude less than that at which responses in benthic macro-invertebrates can be measured. Therefore, a mechanism such as the Raz Rru Smart Tracer system that directly interrogates the in situ metabolic status of the microbial community has potential to improve water quality monitoring techniques and implementation of higher resolution environmental regulations based on impacts to quantifiable ecosystem services.