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Use of Stable Isotopes of Carbon and Nitrogen to Identify Sources of Organic Matter to Bed Sediments of the Tualatin River, Oregon
| Content Provider | Semantic Scholar |
|---|---|
| Author | Bonn, Bernadine A. Rounds, Stewart A. |
| Copyright Year | 2014 |
| Abstract | The potential sources of organic matter to bed sediment of the Tualatin River in northwestern Oregon were investigated by comparing the isotopic fractionation of carbon and nitrogen and the carbon/nitrogen ratios of potential sources and bed sediments. Samples of bed sediment, suspended sediment, and seston, as well as potential source materials, such as soil, plant litter, duckweed, and wastewater treatment facility effluent particulate were collected in 1998–2000. Based on the isotopic data, terrestrial plants and soils were determined to be the most likely sources of organic material to Tualatin River bed sediments. The δ13C fractionation matched well, and although the δ15N and carbon/ nitrogen ratio of fresh plant litter did not match those of bed sediments, the changes expected with decomposition would result in a good match. The fact that the isotopic composition of decomposed terrestrial plant material closely resembled that of soils and bed sediments supports this conclusion. Phytoplankton probably was not a major source of organic matter to bed sediments. Compared to the values for bed sediments, the δ13C values and carbon/nitrogen ratios of phytoplankton were too low and the δ15N values were too high. Decomposition would only exacerbate these differences. Although phytoplankton cannot be considered a major source of organic material to bed sediment, a few bed sediment samples in the lower reach of the river showed a small influence from phytoplankton as evidenced by lower δ13C values than in other bed sediment samples. Isotopic data and carbon/nitrogen ratios for bed sediments generally were similar throughout the basin, supporting the idea of a widespread source such as terrestrial material. The δ15N was slightly lower in tributaries and in the upper reaches of the river. Higher rates of sediment oxygen demand have been measured in the tributaries in previous studies and coupled with the isotopic data may indicate the presence of more labile organic matter in these areas. Results from this study indicate that strategies to improve oxygen conditions in the Tualatin River are likely to be more successful if they target sources of soil, leaf litter, and other terrestrially derived organic materials to the river rather than the instream growth of algae. Introduction Dissolved oxygen is essential to the aquatic health of many rivers and lakes, and the concentration of dissolved oxygen often is used as a water-quality standard to protect fish and aquatic life. The dissolved oxygen (DO) concentration in a waterbody is affected by many processes, such as exchange with the atmosphere (reaeration), photosynthesis by algae or aquatic plants, ammonia nitrification, respiration by algae and bacteria, and the bacterially mediated decomposition of organic matter suspended in the water column or in surficial sediments. The oxygen consumed through the decomposition of organic matter in surficial sediments is called sediment oxygen demand (SOD) and can be one of the most important loss processes for DO, particularly in shallow streams. Low DO conditions periodically occur in the lower reaches of the Tualatin River and its tributaries in northwestern Oregon (fig. 1), especially during low-flow periods when the water is warm but algal photosynthesis is minimal. During such periods, SOD can account for a large fraction of the total DO consumed. The U.S. Geological Survey (USGS) investigated the effects of SOD in the Tualatin River, measured a median SOD rate of 2.3 grams of oxygen per square meter of sediment per day (g/m2/d) (Rounds and Doyle, 1997), and determined that SOD and water-column oxygen demand are the largest overall sinks for DO in the Tualatin River (Rounds and others, 1999). In response to water-quality problems in the Tualatin River, Oregon's Department of Environmental Quality in 1988 adopted a set of Total Maximum Daily Load (TMDL) regulations in an effort to restore the aesthetic qualities of the river and protect the river against low DO concentrations and high pH levels. The TMDLs required substantial ammonia and phosphorus reductions, which were designed to increase DO concentrations and limit the size of algal blooms during the summer months. At that time, it was assumed that algal-derived biomass settling to the river bottom was an important source of decomposable organic matter, and that limiting the size of algal blooms would not only eliminate high pH levels but also decrease the SOD and increase DO concentrations. That assumption was challenged by later measurements that determined that SOD rates generally were Use of Stable Isotopes of Carbon and Nitrogen to Identify Sources of Organic Matter to Bed Sediments of the Tualatin River, Oregon By Bernadine A. Bonn and Stewart A. Rounds 2 Use of Stable Isotopes to Identify Sources of Organic Matter to Bed Sediments of the Tualatin River, Oregon not elevated in those reaches of the river that had the largest algae populations, except at one site in the lower river where a slightly higher rate might be ascribed to enhanced deposition of algal biomass (Rounds and Doyle, 1997). Furthermore, water-quality modeling by USGS showed that even if ammonia sources were fully controlled, the Tualatin River still could be subject to periodic low-DO conditions because of DO consumption by SOD (Rounds and others, 1999). Revision of the Tualatin River TMDLs in 2001 recognized the importance of SOD and called for significant decreases (20 percent or more) in the SOD rate of the river and its tributaries through the control of settleable organic materials (Oregon Department of Environmental Quality, 2001). In order to manage and potentially reduce the effects of SOD, it is important to determine the sources of the organic matter delivered to stream sediments. Potential sources of such organic matter include not only algae, but aquatic plants, soils, terrestrial plant material, and possibly the particulate in effluent from wastewater treatment facilities (WWTFs). Many of these materials, however, may be distinguished from one another through measurements of the content and characteristics of the carbon and nitrogen present in that organic matter. Measurements of stable isotopes of carbon and nitrogen have been used for many years to investigate the nature and sources of organic matter in freshwater systems (Middelburg and Nieuwenhuize, 1998; Finlay and Kendall, 2007). Figure 1. Tualatin River basin in northwestern Oregon. tac10-0486_fig01 RM 60 RM 70 RM 30 RM 40 RM 50 RM 10 RM 20 RM 0 WIAMETTE RVER IVER CUMBIA Henry Hagg Lake Gales reek Creek M cK ay Cre ek Fnno Creek TUALATIN RIVER Dairy Cr Scggins Lake Oswego Tualatin Sherwood Portland Tigard Beaverton Farmington Hillsboro Cornelius Forest Grove TULATIN M OUTAINS CHHALEM MONTAINS CO AS T R AN GE CO AS T R AN GE COLUMBIA WASHINGTON OREGON TILLAMOOK YAMHILL CLACKAMAS MULTNOMAH DURHAM ROCK CREEK Bverton Cr Ro ck WASHINGTON OREGON Study Area 5 5 205 205 84 26 217 0 10 10 20 MILES 0 20 KILOMETERS EXPLANATION Base modified from U.S. Geological Survey and Metro digital data sets (1:24,000) Projection: Oregon Lambert, North American Datum 1983 Designated Urban Growth Boundary (2001) Basin boundary Wastewater Treatment Facility 122°30' 122°45' 123° 123°15' 123°30' 45° 45' 45° 37' 30" 45° 30' 45° 22' 30" Introduction 3 Stable Isotopes An element is defined by the number of protons that it contains; for example, all carbon atoms contain six protons and all nitrogen atoms contain seven protons. Atoms of the same element, however, may contain different numbers of neutrons, which cause them to have different masses. Forms of the same element that have different masses are called isotopes; stable isotopes are forms that are not radioactive. Although isotopes vary in the number of neutrons that they contain, their chemical properties essentially are identical. Many elements of biological interest [carbon (C), nitrogen (N), oxygen (O), sulfur (S)] have different isotopic forms. The relative abundances of isotopes can be measured with great precision using a mass spectrometer, and such abundances have been used to study biogeochemical cycles and ecosystem dynamics. The use of stable isotope abundances is based on the fact that different isotopes of an element participate in the same chemical reactions, but at slightly different rates due to their different masses. Lighter isotopes typically react at a slightly faster rate, which causes the relative isotopic abundance in the reactants and products to differ. This process is called isotopic fractionation. For example, the growth of algae tends to create algal biomass that contains less of the heavier carbon-13 isotope and more of the lighter carbon-12 isotope. By examining the isotopic composition of potential source materials, it might be possible to identify the sources of organic matter to river bed sediments. Description of Study Area The Tualatin River basin is a 712 mi2 watershed in northwestern Oregon. Encompassing most of Washington County and parts of Multnomah, Clackamas, and several other counties, the basin includes the western part of the Portland metropolitan area and was home to approximately 450,000 people at the time of this study (U.S. Census Bureau, 2000). The river begins in the forested Coast Range mountains to the west, meanders through a valley bottom farmed for a wide variety of agricultural products, and skirts the southern boundary of the urban area before joining the Willamette River upstream (south) of Portland (fig. 1). Five major tributaries enter the Tualatin River, each of which has distinct characteristics. Scoggins Creek has most of its drainage in the Coast Range and contains the basin's only reservoir, Henry Hagg Lake, which stores water for irrigation, municipal use, and flow augmentation during summer. Gales Creek also has a significant headwater area in the Coast Range and primarily is forested (70 percent). Dairy Creek is located in the norther |
| File Format | PDF HTM / HTML |
| Alternate Webpage(s) | https://pubs.usgs.gov/sir/2010/5154/pdf/sir20105154.pdf |
| Alternate Webpage(s) | https://doi.org/10.3133/sir20105154 |
| Language | English |
| Access Restriction | Open |
| Content Type | Text |
| Resource Type | Article |
