Use of Stable Nitrogen Isotopes and Permeable Membrane Devices to Study what Factors Influence Freshwater Mollusk Survival in the Conasauaga River

Abstract

Recent biological inventory data shows severe declines in freshwater mussel abundance and biodiversity in the Conasauga River Basin in Northwest Georgia, USA. Based on assessments of habitat conditions, mussel populations should be sustainable. We conducted a study of sediment and water quality to evaluate the impact of anthropogenic contamination on mussel populations. Permeable membrane devices (PMD), polar organic chemical integrative samplers (POCIS), conventional water and sediment quality analyses, and stable nitrogen isotope ratio analyses (delta15N) of snails and sediments were used to assess sediment and water quality at target sites throughout the basin. Ambient concentrations of organic contaminants in water were well below any aquatic life criteria; concentrations of some nutrients were detected above aquatic life criteria levels. Most mussel species in the river are endangered or threatened; therefore, snails were collected for delta15N analyses. Mean delta15N values for snails collected at forested upper watershed sites (national forest areas) were significantly lower than delta15N values from snails in agricultural areas. Delta15N values for raw cow manure and manure-treated soil were similar to delta15N values for snails collected in agricultural areas. Dissolved nitrate from water samples had elevated delta15N values similar to the upper range of delta15N values for snails in agricultural areas. Data, particularly stable nitrogen isotope data, indicates that a land use change from national forest land to agriculture alters nitrogen sources to the basin and snails. Implications of nutrient release on freshwater molluscan reproduction, growth, and survival are discussed.

Long-Term Monitoring Data Provide Evidence of Declining Species Richness in a River Valued for Biodiversity Conservation

“…Mainstem water samples from the study reach show occurrence of particularly high levels of total nitrogen (e.g., .5 mg/l) and total phosphorus (e.g., .300 ug/L; Freeman et al 2007;Sharpe and Nichols 2007;Lasier et al 2016). Stable nitrogen isotope ratios of herbivorous consumers in the Conasauga mainstem downstream from the national forest also show evidence of nitrogen enrichment from agricultural sources (Sharpe and Nichols 2007;Baker 2012). Other studies have found changes in fish community structure associated with increased nutrients (Evans-White et al 2013;Taylor et al 2014), although mechanisms are unclear.…”

“…Several studies have reported elevated levels of agriculturally derived nutrients and contaminants in the Conasauga River study reach (Sharpe and Nichols 2007;Jacobs 2013;Lasier et al 2016). Jacobs (2013) additionally documents occurrence of bioavailable estrogen in water and sediments of tributaries that join the Conasauga mainstem either upstream or in our study reach, occurrence of intersex condition in approximately 13% of the fish collected in these tributaries, and lowered growth in two fish species exposed to sediment from tributaries with the highest estrogen levels.…”

“…Genetic analyses (George et al 2010) and survey data (Hagler et al 2011) have provided evidence of population decline in the endangered Conasauga Logperch. Native mollusks have disappeared from many historically occupied sites in the Conasauga River system (Evans 2001) and biologists have documented potentially harmful contaminants in the river mainstem and tributaries (Sharpe and Nichols 2007;Jacobs 2013;Lasier et al 2016). Conservation concerns in the Etowah River system have focused on potential effects of urban growth on imperiled fishes (Burkhead et al 1997;Wenger et al 2010), based in part on analyses showing negative correlations between present-day occurrences of native stream fishes and impervious land-cover associated with urban development (Wenger et al 2008).…”

Reassessing Enigmatic Mussel Declines in the United States

“…This is a compelling mechanism in some cases. For example, the Red River, Tennessee, the Little and Nolin rivers in Kentucky, and the Conasauga River in Georgia are in intensely agricultural regions and show elevated nitrogen loading, which creates conditions favorable for ammonia formation; pesticide contamination is also prevalent in these streams (Sharpe and Nichols 2007;Haag et al 2019).…”

The conservation implications of riparian land use on river turtles

“…The proximate effects of the loss of undisturbed land cover associated with agricultural practices on instream habitat or biota have not been evaluated for the creeks in our study. Runoff from agricultural land increases nitrogen loading, which can increase abundance of macrophytes and algae (Allan, 1995), while reducing the abundances of fishes (Jones et al ., 1999; Burcher et al ., 2008), mollusks (Poole & Downing, 2004; Sharpe & Nichols, 2007) and other macroinvertebrates (Wang et al ., 1997; Lammert & Allan, 1999). With these types of shifts in resources available within agriculturally dominated streams, we would expect to see increases in omnivores, like T. scripta and generalized herbivores such as P. concinna , and decreases in dietary specialists like G. barbouri and S. minor .…”