Stephen P. Opsahl

Stable isotopic signatures, tissue stoichiometry, and nutrient cycling (C and N) of native and invasive freshwater bivalves

Abstract Filter-feeding mussels historically comprised most of benthic biomass in many streams. They contribute to stream ecosystem functioning by linking the water column and benthic habitats. Both native and nonnative species coexist in many streams, but their ecological roles are not well quantified. The invasive bivalve, Corbicula fluminea, has the potential to alter profoundly organic matter dynamics and nutrient cycling in streams. We compared stable isotope ratios and tissue and biodeposit stoichiometry of the native freshwater mussel, Elliptio crassidens, and C. fluminea in a Coastal Plain stream (Ichawaynochaway Creek, a tributary to the lower Flint River, Georgia, USA) to assess their trophic niche space and potential effects on nutrient cycling. We hypothesized that C. fluminea would assimilate a larger range of materials than E. crassidens. To determine dietary overlap of C. fluminea and E. crassidens, we measured the elemental and stable isotopic compositions (δ13C and δ15N) of their tissue. Corbicula fluminea showed lower trophic fidelity than E. crassidens and was able to acquire and assimilate a wide range of resources, as illustrated by their wide range of δ13C values. Corbicula fluminea also might alter nutrient cycling in the benthic environment of streams because they retain less N than E. crassidens, as reflected by their higher tissue C∶N. In the laboratory, we measured C and N in biodeposits (feces and pseudofeces) from the 2 species. Corbicula fluminea released more N through their biodeposits relative to E. crassidens by mass, a result implying that C. fluminea might modify nutrient cycling in streams. Our results show important differences in the food resources assimilated and the nutrients deposited as feces and pseudofeces by these 2 bivalves. Furthermore, our results demonstrate how invasive species, such as C. fluminea, can alter aquatic environments through differences in species traits within a functional group.

Suspended material availability and filtration–biodeposition processes performed by a native and invasive bivalve species in streams

Unionid mussels are among the most threatened group of freshwater organisms globally. They are known for their ability to filter food particles from flowing and standing waters. However, invasive bivalve species, such as the Asian clam (Corbicula fluminea) in North America, have the potential to overlap in feeding and potentially out-compete the native species. Yet, the feeding preferences of unionid mussels and C. fluminea are incompletely understood. We hypothesized that Elliptio crassidens (native) and C. fluminea (invasive) would select for specific organic components present within seston. We examined changes in seston (dry mass and ash-free dry mass) resulting from bivalve feeding activity for three size classes of material that were isolated using gravimetric filtration. The treatments were also sub-sampled for flow cytometry (FC) which separated the suspended materials in the stream water into five categories: detritus, heterotrophic bacteria, picoautotrophs, nanoautotrophs, and heterotrophic nanoeukaryotes. Our results indicated that both species of bivalve showed preferences for organic and living materials. E. crassidens preferentially filtered nanoeukaryotes, whose decreases were associated with an increase in bacteria. In contrast, C. fluminea preferred smaller materials through selective filtration of picoautotrophs. In addition, both species increased the concentration of large materials toward the end of the experiment because of the suspension of their pseudofeces biodeposits. To our knowledge, this study is the first to examine grazing by bivalve species on natural stream particulate matter using FC. Our results suggest that native and non-native mussels have different functional roles, which has important implications for organic matter processing and food webs in streams.

Stream discharge and floodplain connections affect seston quality and stable isotopic signatures in a coastal plain stream

Abstract Connections of a stream to its floodplain are important ecological linkages that affect spatial and temporal dynamics of the basal resources available to primary consumers in streams. Suspended organic material and associated microorganisms (seston) vary in quality seasonally and interannually within streams because of changing inputs from riparian and floodplain sources. Researchers have investigated the quality of different size fractions of material, but these differences have not been assessed with respect to the hydrology and the geomorphic structure of streams. We investigated how quality, represented by the stoichiometric ratio C:N, and stable isotopic signature (δ13C and δ15N) of 3 seston size classes varied in Ichawaynochaway Creek, a 5th-order tributary of the lower Flint River in the Coastal Plain of southwestern Georgia, USA. Samples were collected throughout the basin during varying flow regimes to estimate the quality and source of materials available over different temporal and spatial scales. Our results indicate significant differences in quality and stable isotopic signature based on particle size, discharge, and geomorphic structure of the stream and floodplain (constrained vs unconstrained reaches). The constrained portions of this stream occur in the lower portions of the basin. During low flow conditions, seston had higher quality with less depleted δ13C and more enriched δ15N signatures in the constrained than in the unconstrained portions of the stream. However, during high flow conditions, higher quality seston entered the stream from the adjacent floodplain in all portions of the basin. Insights gained from our study indicate how terrestrial and aquatic linkages and the natural flow regime affect the dynamics of basal resources and their availability to primary consumers in streams.

Physicochemical habitat association of a native and a non-native crayfish in the lower Flint river, Georgia: implications for invasion success

Invasive species must cope with a suite of environmental conditions that are different from those in their native ranges. We examine how the physicochemical environment contributes to the invasion success of Orconectes palmeri, a non-native crayfish, in the lower Flint River basin, Georgia, USA. We examined the distribution of Procambarus spiculifer, a native crayfish, and O. palmeri within the lower Flint River basin, and examined associations between species relative abundance and physicochemical variables. Within the lower Flint River, O. palmeri was found almost exclusively at upstream sites, while P. spiculifer was more abundant downstream. We did not detect small O. palmeri downstream, suggesting little recruitment in this area. Within tributaries, only native crayfish were observed. O. palmeri was more abundant at sites with warmer water temperatures, and P. spiculifer was more abundant at sites with cooler water temperatures, which were maintained by ground water seepage. P. spiculifer abundance was also positively associated with abundance of coarse wood. Laboratory studies demonstrated that O. palmeri selected warmer temperatures than P. spiculifer, suggesting that warmer temperatures in the upstream area favor O. palmeri. Water temperatures may have increased in recent years due to human withdrawals from the aquifer and the installation of upstream dams, creating a thermal regime that P. spiculifer has not historically encountered. Our findings suggest that the maintenance of groundwater inputs and a forested riparian buffer is crucial to protect populations of P. spiculifer and to prevent further spread by O. palmeri, especially if climatic changes result in warmer waters.

Resource-Consumer Relationships and Baseline Stable Isotopic Signatures of Food Webs in Isolated Wetlands

We examined isolated wetland food webs using stable C and N isotopes to understand resource-consumer relationships and controls on baseline isotopic signatures. Marshes were usually more 13C-enriched than cypress savannas and cypress gum swamps. Analysis of coarse particulate organic matter (CPOM) fractions indicated that C3 plants contributed the majority of organic matter to isolated wetlands. Individual wetlands of the same type were sometimes significantly different in δ13C and/or δ15N, suggesting that differences in stable isotopic signatures were related to baseline isotopic values rather than feeding preferences. Bi-plots were normalized using baseline values and resulting resource-consumer relationships were similar among the three wetland types. Food webs appeared to be supported by basal resources including periphyton and conditioned particulate organic matter, although indirect evidence suggested phytoplankton as an important basal resource. Differences in baseline isotopic signatures may have been partly a function of vascular plant decomposition based on significant positive relationships between the median δ13C of individual wetland food webs, the δ13C of dissolved inorganic carbon (DIC), dissolved organic carbon (DOC) and CPOM pools, and a strong positive relationship between δ13C-DIC and δ13C-DOC. Mixing model calculations showed that other mechanisms were also important for establishing baseline isotopic signatures.

Source, variability, and transformation of nitrate in a regional karst aquifer: Edwards aquifer, central Texas

Many karst regions are undergoing rapid population growth and expansion of urban land accompanied by increases in wastewater generation and changing patterns of nitrate (NO3(-)) loading to surface and groundwater. We investigate variability and sources of NO3(-) in a regional karst aquifer system, the Edwards aquifer of central Texas. Samples from streams recharging the aquifer, groundwater wells, and springs were collected during 2008-12 from the Barton Springs and San Antonio segments of the Edwards aquifer and analyzed for nitrogen (N) species concentrations and NO3(-) stable isotopes (δ(15)N and δ(18)O). These data were augmented by historical data collected from 1937 to 2007. NO3(-) concentrations and discharge data indicate that short-term variability (days to months) in groundwater NO3(-) concentrations in the Barton Springs segment is controlled by occurrence of individual storms and multi-annual wet-dry cycles, whereas the lack of short-term variability in groundwater in the San Antonio segment indicates the dominance of transport along regional flow paths. In both segments, longer-term increases (years to decades) in NO3(-) concentrations cannot be attributed to hydrologic conditions; rather, isotopic ratios and land-use change indicate that septic systems and land application of treated wastewater might be the source of increased loading of NO3(-). These results highlight the vulnerability of karst aquifers to NO3(-) contamination from urban wastewater. An analysis of N-species loading in recharge and discharge for the Barton Springs segment during 2008-10 indicates an overall mass balance in total N, but recharge contains higher concentrations of organic N and lower concentrations of NO3(-) than does discharge, consistent with nitrification of organic N within the aquifer and consumption of dissolved oxygen. This study demonstrates that subaqueous nitrification of organic N in the aquifer, as opposed to in soils, might be a previously unrecognized source of NO3(-) to karst groundwater or other oxic groundwater systems.

Tracking suspended particle transport via radium isotopes (226Ra and 228Ra) through the Apalachicola-Chattahoochee-Flint River system

Suspended particles in rivers can carry metals, nutrients, and pollutants downstream which can become bioactive in estuaries and coastal marine waters. In river systems with multiple sources of both suspended particles and contamination sources, it is important to assess the hydrologic conditions under which contaminated particles can be delivered to downstream ecosystems. The Apalachicola-Chattahoochee-Flint (ACF) River system in the southeastern United States represents an ideal system to study these hydrologic impacts on particle transport through a heavily-impacted river (the Chattahoochee River) and one much less impacted by anthropogenic activities (the Flint River). We demonstrate here the utility of natural radioisotopes as tracers of suspended particles through the ACF system, where particles contaminated with arsenic (As) and antimony (Sb) have been shown to be contributed from coal-fired power plants along the Chattahoochee River, and have elevated concentrations in the surficial sediments of the Apalachicola Bay Delta. Radium isotopes ((228)Ra and (226)Ra) on suspended particles should vary throughout the different geologic provinces of this river system, allowing differentiation of the relative contributions of the Chattahoochee and Flint Rivers to the suspended load delivered to Lake Seminole, the Apalachicola River, and ultimately to Apalachicola Bay. We also use various geochemical proxies ((40)K, organic carbon, and calcium) to assess the relative composition of suspended particles (lithogenic, organic, and carbonate fractions, respectively) under a range of hydrologic conditions. During low (base) flow conditions, the Flint River contributed 70% of the suspended particle load to both the Apalachicola River and the bay, whereas the Chattahoochee River became the dominant source during higher discharge, contributing 80% of the suspended load to the Apalachicola River and 62% of the particles entering the estuary. Neither of these hydrologic scenarios, which were moderately low flow regimes, appeared to transport particles contaminated with arsenic and antimony to Apalachicola Bay.

Data Release for Water-quality and chemical loading data from the Geronimo Creek and Plum Creek watersheds, south-central Texas, April 2015 March 2016

Located in south-central Texas, the Geronimo Creek and Plum Creek watersheds have long been characterized by elevated nitrate concentrations. From April 2015 through March 2016, an investigation was done by the U.S. Geological Survey, in cooperation with the Guadalupe-Blanco River Authority (GBRA) and the Texas State Soil and Water Conservation Board (TSSWCB), to assess nitrate (as nitrogen) concentrations and to document possible sources of elevated nitrate as nitrogen in these two watersheds. Water-quality samples were collected from 20 groundwater, spring, and stream sites across the two watersheds, along with precipitation samples and wastewater treatment plant (WWTP) effluent samples from the Plum Creek watershed. Stream, spring, and groundwater samples were collected from both watersheds during four synoptic sampling events to characterize spatial and temporal variations in water quality and chemical loadings. Water-quality and quantity data from the wastewater treatment plants and stream discharge data were also considered. Samples were analyzed for major ions, selected trace elements, nutrients, stable isotopes of water and nitrogen isotopes. Data collected for this project was used to calculate selected chemical loadings and used in interpretation of sources of nitrate in both watersheds.