Evaluation of the ecological status of a stream in middle Tennessee using periphyton characteristics

The upper reach of Big McAdoo Creek in the Lower Cumberland River Watershed in Middle Tennessee is listed as unimpaired by nutrient enrichment by the Tennessee Department of Environment and Conservation and the United States Environmental Pro ection Agency. The primary objective of this research was to determine if the reach sho uld remain listed as unimpaired. Water quality was evaluated by examining concentration of soluble reactive phosphorous, characteristics of periphyton, and the composition and structure of the diatom assembl age. An oligotrophic concentration of soluble reactive phosphorous of the water a nd mesotrophic concentrations of photoautotrophic periphyton indicate the stream site sampled was not impacted by nutrient enrichment. Oligotrophic to mesotrophic values for the Si ltation Index, Organic Pollution Index, and Pollution Tolerance Index for the diatom assemblage indicate t he assemblage was not negatively impacted by siltation, high concentrations of dissolved organi cs, or nutrient enrichment, respectively. The results indicate the upper reach of Big McAdoo Creek s hould have its water-quality status remain as unimpaired. Knowledge of the effects of nutrient concentration on the c omposition and structure of photoautotrophic periphyton is essential to understand the impac t of eutrophication on shallow lotic systems. Nutrient enrichment and sediments from nonpoint s urces are most responsible for the biological impairment of United States waters (Irvine & Mur phy 2009). Assessments of nutrient and sediment pollution are prerequisites to developing watershed m anagement plans to protect aquatic ecosystems (Smucker & Vis 2009). Biological monitoring is esse ntial to characterize and quantify the influences of water quality. Periodic water sampl ing for chemical analyses alone may not reveal the impact of nonpoint-source pollution because pollutants from n npoint sources often enter waterways in pulses. In addition, chemical analyses do not reveal the impacts of pollution on biological integrity (Taylor et al. 2007). Photoautotrophic periphyton are the most important prim ary producers in the majority of wadeable streams (Lambert & Steinman 1997). Nutrient enric hment of streams changes photoautotrophic periphyton characteristics and affects who le-stream ecological relationships. The composition, biomass, and physiological status of photoautotrophi c eriphyton are excellent indicators of water quality and are used universally to foll ow changes in aquatic environments (Eaton et al. 2005). High concentrations of chlorophyll a as a measurement of the biomass of photoautotrophic periphyton is a hallmark of eutrophication (Khan & Ansari 2005). Because chlorophyll-a concentration is influenced by many abiotic and biotic characters of a stream reach, measurements of chlorophylla concentration alone may not be adequate to demonstrate impair ment by nutrient enrichment (Kurle & Cardinale 2011). Lebkuecher et al.: Ecological status of a middle Tennessee stream 2 Diatoms are the focus of most studies characterizing impact s of eutrophication on periphyton composition because more autecological information exists fo r diatoms relative to other algae (Smucker & Vis 2013). Information from the composition of diat oms can be used to support proposed restoration and conservation policies because dia tom assemblages in impaired water often reflect the nature of impairment (Smucker & Vis 2009). Eva luation of diatom composition is a standardized protocol for monitoring changes of water qualit y in many European countries and states including Oklahoma, Montana, Kentucky, and Texas (Stevenson et al. 2008). Big McAdoo Creek is a major tributary of the Lower Cumber land River Watershed and joins the Cumberland River approximately 20 km south of Clarksville, Tennessee. The Lower Cumberland River Watershed is in the Western Pennyroyal Karst Level IV Ecoregion. The geologic base of the watershed is Mississippian-age limestone and includes some che rt, s ale, siltstone, sandstone, and dolomite. The soils are a thin loess mantle, highly erodible, and very fertile (Baskin et al. 1997). Forests are Western Mesophytic and consist largely of Quercus and Carya species (Baskin et al. 1997). The watershed encompasses approximately 2,338 square miles and has an estimated population of 155,000 people (TNCT 2015). Over 50% of the watershed is used to produce agriculture products including tobacco, corn, soybean, and l ivestock (TDEC 2012). The cumulative effects of erosion, agricultural runoff, livestock acces s to streams, and poorly functioning sewage systems result in poor quality water in the lower reaches of all of the major tributaries in the watershed. The upper reach of Big McAdoo Creek is listed as unimpaired by nutrient enrichment by the Tennessee Department of Environment and Conservation and t he United States Environmental Protection Agency, whereas the lower reach is listed as i mpaired (TDEC 2012). The primary objective of this research was to determine if the upper reach of Big McAdoo Creek should remain listed as unimpaired. We used multiple approaches to evaluat e the ecological status of Big McAdoo Creek, including determinations of soluble reactive phosphorous c ncentration in water samples, evaluations of periphyton biomass and physiological status, an d analyses of the structure of the diatom assemblage. Methods Periphyton and water were sampled at the lower end of the upp r reach of Big McAdoo Creek 100 m downstream of the creek crossing at Hwy 12, 30 km s outh of Clarksville, Tennessee, on 16 Sep 2015. Two transects from the waterlines of opposing banks , 10 m apart, were established at the sampling site. Transect widths and stream depths at 1/3 intervals of each transect were determined. Stream velocity was determined as the time re quired for a density-neutral object to travel 10 m downstream. Stream discharge was calculated as: Dis charge = Width Depth Velocity 0.9 (Robins & Crawford 1954). A water sample was collected mi stream, 5 cm below the surface, to determine the concentration of soluble reactive phosphorous using a Lachat QuickChem 8000 Flow Injection Analyzer (Lachat Instruments, Loveland, Colorad ). Six midstream plots in the 10-m reach were established wi th 0.25 m wire frames. The fractions of stones designated as very coarse gravel or la rger (considered stable substrate) and coarse gravel or smaller (considered unstable substrate) in each plot were recorded. Two cobbles nearest the plot center were removed. One cobble was used for determi nation of periphyton dry weight, determinations of pigment concentrations, and ash-free peri phyton dry weight. One cobble was used to evaluate diatom composition. One sample of unstable substr ate from each plot was removed with a core sampler for determinations of pigment concentrations associated with unstable substrate. Laboratory methods for measurements of ash-free periphyton dry weight and concentrations of periphyton chlorophyll (chl) a and pheophytin (pheo) a are described in Eaton et al. (2005). The surface area of cobble from which periphyton was removed was calculated by covering the upper surface with aluminum foil, weighing the foil, and extrapolati ng weight to surface area (Hauer & Lamberti 2006). The autotrophic index (AI) was calculated usi ng the equation of Crossy and LaPoint Lebkuecher et al.: Ecological status of a middle Tennessee stream 3 (1988): AI = [Ash-free periphyton dry weight (g /m)]/[chlorophyll a (g/m)]. Identification of diatoms and calculation of diatom indices followed the methods descr ib d in KDOW (2002) and Lebkuecher et al. (2015). Results and Discussion Stream site morphological characteristics were determ in d to provide detail of the abiotic characteristics of the sampling site (Table 1). Stable s ubstrate for periphyton growth (cobble and very coarse gravel) comprises approximately 1/3 of the benthic envi ronment which is typical for the heterogeneous stream bottoms of mid-order streams in Middle T ennessee (Lebkuecher et al. 2015). Table 1. Abiotic characteristics of the stream site ampled in Big McAdoo Creek.