Michelle A. Evans-White

Rapid expansion of natural gas development poses a threat to surface waters

Extraction of natural gas from hard-to-reach reservoirs has expanded around the world and poses multiple environmental threats to surface waters. Improved drilling and extraction technology used to access low permeability natural gas requires millions of liters of water and a suite of chemicals that may be toxic to aquatic biota. There is growing concern among the scientific community and the general public that rapid and extensive natural gas development in the US could lead to degradation of natural resources. Gas wells are often close to surface waters that could be impacted by elevated sediment runoff from pipelines and roads, alteration of streamflow as a result of water extraction, and contamination from introduced chemicals or the resulting wastewater. However, the data required to fully understand these potential threats are currently lacking. Scientists therefore need to study the changes in ecosystem structure and function caused by natural gas extraction and to use such data to inform sound environmental policy.

Thresholds in macroinvertebrate biodiversity and stoichiometry across water-quality gradients in Central Plains (USA) streams

Abstract N and P often limit primary and secondary production in ecosystems, but they also can cause eutrophication and negatively influence sensitive species above a certain level or threshold point. Aquatic biodiversity can have negative threshold relationships with water-quality variables at large scales, but the specific mechanism(s) driving these threshold relationships are not well established. We hypothesized that resource quality (i.e., C:P) might partly drive primary consumer (grazer and detritivore) richness thresholds by altering competitive interactions among species with differing resource demands, but might have less influence on predator richness. We estimated total N (TN), total P (TP), and turbidity thresholds for macroinvertebrate richness across trophic levels and feeding groups in Central Plains (USA) streams. We also determined if mean taxon body C:P of groups with diversity losses were negatively related to TP, a pattern that would suggest that eutrophic communities were dominated by a few species with high dietary P demands. Primary consumers were more sensitive to TN and TP (threshold mean  =  1.0 mg N/L and 0.06 mg P/L) than secondary consumers (threshold mean  =  0.09 mg P/L), a result supporting the resource quality hypothesis. Turbidity reduced richness regardless of feeding mode (threshold mean  =  4.7 NTU), a result suggesting that turbidity and nutrient thresholds were driven by different factors. The TP-richness threshold could be driven partially by changes in food quality because the mean body C:P of shredding and collector-gathering taxa declined as TP increased (threshold mean  =  0.07 and 0.75 mg P/L, respectively). Mean scraper C:P was not related to TP, a result indicating other factors might be responsible for the scraper richness threshold. Our results suggest that changes in resource quality could contribute to large-scale losses in biodiversity in nutrient-enriched lotic ecosystems. Within shredder and collector-gatherer macroinvertebrate feeding groups, P-rich food might allow faster growing taxa with high body P demands to out-compete slower growing taxa adapted to lower quality food resources. This pattern suggests that biotic integrity is directly linked to nutrients in streams and that toxicity, low dissolved O2, and increased turbidity might not be the only mechanisms leading to reductions in diversity as nutrient concentrations increase.

A review of stream nutrient criteria development in the United States.

Elevated nutrients and sediments are the main factors contributing to the poor biological condition measured in over 40% of US waters, highlighting the need for criteria that can aid management efforts to protect or restore the quality of US waters. A large amount of literature on nutrient criteria has been generated since the USEPA called for their development in 1998. Our objective was to examine this peer-reviewed literature to evaluate two main approaches for criteria development in lotic ecosystems: percentile rank and bivariate predictive statistical analyses. The 25th percentile approach has been examined broadly across USEPA-aggregate nutrient ecoregions, and we found that USEPA-suggested criteria for these aggregate ecoregions were often more conservative than criteria estimated using more current regionally focused data based on our compiled data set. Furthermore, 25th percentile estimates were often less than 75th percentile estimates based on reference sites, suggesting that 75th percentile estimates were not more conservative than 25th percentile estimates. Predictive approaches have focused on establishing linear and nonlinear relationships between water quality and algae, macroinvertebrate, and fish communities; attributing causation; and determining whether threshold points exist that can aid in nutrient criteria development. Most of the predictive approaches have occurred at the state or watershed level and may not be directly comparable to USEPA aggregate ecoregions. However, percentile method estimates often fell within the confidence interval of biological threshold criteria estimates, suggesting overlap and some consensus between the two main approaches.

Stream Vulnerability to Widespread and Emergent Stressors: A Focus on Unconventional Oil and Gas.

Multiple stressors threaten stream physical and biological quality, including elevated nutrients and other contaminants, riparian and in-stream habitat degradation and altered natural flow regime. Unconventional oil and gas (UOG) development is one emerging stressor that spans the U.S. UOG development could alter stream sedimentation, riparian extent and composition, in-stream flow, and water quality. We developed indices to describe the watershed sensitivity and exposure to natural and anthropogenic disturbances and computed a vulnerability index from these two scores across stream catchments in six productive shale plays. We predicted that catchment vulnerability scores would vary across plays due to climatic, geologic and anthropogenic differences. Across-shale averages supported this prediction revealing differences in catchment sensitivity, exposure, and vulnerability scores that resulted from different natural and anthropogenic environmental conditions. For example, semi-arid Western shale play catchments (Mowry, Hilliard, and Bakken) tended to be more sensitive to stressors due to low annual average precipitation and extensive grassland. Catchments in the Barnett and Marcellus-Utica were naturally sensitive from more erosive soils and steeper catchment slopes, but these catchments also experienced areas with greater UOG densities and urbanization. Our analysis suggested Fayetteville and Barnett catchments were vulnerable due to existing anthropogenic exposure. However, all shale plays had catchments that spanned a wide vulnerability gradient. Our results identify vulnerable catchments that can help prioritize stream protection and monitoring efforts. Resource managers can also use these findings to guide local development activities to help reduce possible environmental effects.

Growth and stoichiometry of a common aquatic detritivore respond to changes in resource stoichiometry

Consumer growth determines the quantity of nutrients transferred through food webs. The extent to which leaf composition and consumer physiology interact to constrain consumer production is not well understood. For example, detritivore growth, and thus material transfer, could change with detrital elemental composition. Detrital type and associated microbial biofilms can mediate the amount and rate of detritus consumed and used towards growth. Detritivore body stoichiometry or the threshold elemental ratio, the food ratio resulting in optimal growth, may predict taxon-specific growth response to stoichiometrically-altered detritus. Empirical measures of detritivore growth responses across a range of detrital stoichiometry are rare. We fed a common detritivore, Tipula abdominalis, maple or oak leaves that spanned a gradient of carbon:phosphorus (C:P) to examine how leaf identity and C:P interact to influence growth, consumption, assimilation efficiencies, and post-assimilatory processes. Tipula abdominalis growth and consumption varied with leaf type and stoichiometry. Individuals fed oak grew faster and ate more compared to individuals fed maple. Individuals fed maple grew faster and ate more as leaf C:P decreased. All individuals lost most of the material they assimilated through respiration and excretion regardless of leaf type or leaf stoichiometry. Consumption and growth rates of T. abdominalis increased with maple nutrient enrichment, but not oak, indicating leaf-specific nutrient enrichment affected leaf palatability. Slightly non-homeostatic T. abdominalis C:P was maintained by varied consumption, carbon assimilation, and P excretion. Our study underlines the importance of how detritivore consumption and post-assimilatory processing could influence whole-stream material storage and nutrient cycling in detrital-based ecosystems.

A stream insect detritivore violates common assumptions of threshold elemental ratio bioenergetics models

Ecologists increasingly use threshold elemental ratios (TERs) to explain and predict organism responses to altered resource C∶P or C∶N. TER calculations are grounded in diet-dependent growth, but growth data are limited for most taxa. Thus, TERs are derived instead from bioenergetics models that rely on simplifying assumptions, such as fixed organism C∶P and no P excretion at peak growth. We examined stoichiometric regulation of the stream insect detritivore Pycnopsyche lepida to assess bioenergetics model assumptions and compared bioenergetics TERC∶P estimates to those based on growth. We fed P. lepida maple and oak leaf diets along a dietary C∶P gradient (molar C∶P range = 950–4180) and measured consumption, growth, stoichiometric homeostasis (H), and elemental assimilation and growth efficiencies over a 5-wk period in the laboratory. Pycnopsyche lepida responses to varying resource C∶P depended on litter identity and were strongest among oak diets, on which growth peaked at diet C∶P = 1620. Pycnopsyche lepida fed oak litter exhibited flexible body C∶P during growth and in response to altered diet C∶P (non-strict homeostasis; H = 4.74), low P use efficiencies, and P excretion at peak growth. These trends violated common bioenergetics model assumptions and caused deviation of estimated TERC∶P from C∶P = 1620. Bioenergetics TERC∶P further varied among P. lepida of differing growth status on varying diet C∶P (overall TERC∶P range = 1030–9540). Our study identifies novel effects of nutrient enrichment and litter identity on detritivore stoichiometric regulation and supports growth-based approaches for future TER calculations.

Effects of grazing minnows and crayfish on stream ecosystem structure and function

Abstract We compared the effects of 2 common grazers, southern redbelly dace (Phoxinus erythrogaster) and crayfish (Orconectes spp.), on ecosystem structure and function in experimental streams with pool and riffle habitats. Our goal was to identify potentially overlapping roles of these grazers in these systems. Measures of ecosystem structure included algal filament length, particulate organic matter (POM), densities of invertebrate taxa, and algal biomass. Ecosystem function was measured as gross primary productivity (GPP). Biomass-dependent effects of crayfish and dace on ecosystem properties were compared in autumn 2005 when mean water temperature was 12.9°C (range 7.6–27.9°C). Increasing crayfish biomass did not influence ecosystem properties, but increasing dace biomass negatively affected algal filament length and chironomid abundance and positively affected chydorid abundance. Effects of moderately high biomasses of dace and crayfish were compared in spring 2006 when mean water temperature was 21.4°C (range 17.5–29.9°C). Algal filament lengths were generally low relative to values obtained in autumn 2005 in both dace and crayfish treatments. In addition, algal filament length was shorter and chironomid density was lower in crayfish than in dace streams. The contrasting effects of dace and crayfish across sampling days, seasons, and habitats led us to hypothesize that physiological and behavioral traits of these species might limit the redundancy of their effects on ecosystems across broad spatial and temporal scales.

Leaf-litter stoichiometry is affected by streamwater phosphorus concentrations and litter type

Abstract.  The stoichiometric ratios of organisms and their food resources can influence C and nutrient dynamics in aquatic ecosystems. Several investigators have quantified linkages between nutrient enrichment and consumer stoichiometry for stream detritivores, but very few have systematically quantified the effect of P enrichment on leaf-litter stoichiometry. Here, we examine the potential stoichiometric changes of 2 species of leaf litter subjected to varying levels of P enrichment in laboratory microcosms and mixed species across a natural P gradient of streams in the Ozark Highlands Region, Arkansas, USA. Leaf-litter %P content increased and C∶P ratios decreased with increasing levels of P enrichment and with increasing lability of the leaf species. In the laboratory study, C∶P of maple and oak leaves in the control treatment was ∼2500, whereas this ratio decreased to 500 and 1000 in the high-P treatments, respectively. Total P (TP) was inversely related to leaf-litter C∶P along the natural P gradient of streams in the Ozarks. Our results add to the growing body of information on the potential bottom-up effects of anthropogenic nutrient loading in streams and the influence of water-column nutrients and leaf quality on this response.

Stream macroinvertebrate communities across a gradient of natural gas development in the Fayetteville Shale

Oil and gas extraction in shale plays expanded rapidly in the U.S. and is projected to expand globally in the coming decades. Arkansas has doubled the number of gas wells in the state since 2005 mostly by extracting gas from the Fayetteville Shale with activity concentrated in mixed pasture-deciduous forests. Concentrated well pads in close proximity to streams could have adverse effects on stream water quality and biota if sedimentation associated with developing infrastructure or contamination from fracturing fluid and waste occurs. Cumulative effects of gas activity and local habitat conditions on macroinvertebrate communities were investigated across a gradient of gas well activity (0.2-3.6 wells per km(2)) in ten stream catchments in spring 2010 and 2011. In 2010, macroinvertebrate density was positively related to well pad inverse flowpath distance from streams (r=0.84, p<0.001). Relatively tolerant mayflies Baetis and Caenis (r=0.64, p=0.04), filtering hydropsychid caddisflies (r=0.73, p=0.01), and chironomid midge densities (r=0.79, p=0.008) also increased in streams where more well pads were closer to stream channels. Macroinvertebrate trophic structure reflected environmental conditions with greater sediment and primary production in streams with more gas activity close to streams. However, stream water turbidity (r=0.69, p=0.02) and chlorophyll a (r=0.89, p<0.001) were the only in-stream variables correlated with gas well activities. In 2011, a year with record spring flooding, a different pattern emerged where mayfly density (p=0.74, p=0.01) and mayfly, stonefly, and caddisfly richness (r=0.78, p=0.008) increased in streams with greater well density and less silt cover. Hydrology and well pad placement in a catchment may interact to result in different relationships between biota and catchment activity between the two sample years. Our data show evidence of different macroinvertebrate communities expressed in catchments with different levels of gas activity that reinforce the need for more quantitative analyses of cumulative freshwater-effects from oil and gas development.

Stream primary producers relate positively to watershed natural gas measures in north-central Arkansas streams.

Construction of unconventional natural gas (UNG) infrastructure (e.g., well pads, pipelines) is an increasingly common anthropogenic stressor that increases potential sediment erosion. Increased sediment inputs into nearby streams may decrease autotrophic processes through burial and scour, or sediment bound nutrients could have a positive effect through alleviating potential nutrient limitations. Ten streams with varying catchment UNG well densities (0-3.6 wells/km(2)) were sampled during winter and spring of 2010 and 2011 to examine relationships between landscape scale disturbances associated with UNG activity and stream periphyton [chlorophyll a (Chl a)] and gross primary production (GPP). Local scale variables including light availability and water column physicochemical variables were measured for each study site. Correlation analyses examined the relationships of autotrophic processes and local scale variables with the landscape scale variables percent pasture land use and UNG metrics (well density and well pad inverse flow path length). Both GPP and Chl a were primarily positively associated with the UNG activity metrics during most sample periods; however, neither landscape variables nor response variables correlated well with local scale factors. These positive correlations do not confirm causation, but they do suggest that it is possible that UNG development can alleviate one or more limiting factors on autotrophic production within these streams. A secondary manipulative study was used to examine the link between nutrient limitation and algal growth across a gradient of streams impacted by natural gas activity. Nitrogen limitation was common among minimally impacted stream reaches and was alleviated in streams with high UNG activity. These data provide evidence that UNG may stimulate the primary production of Fayetteville shale streams via alleviation of N-limitation. Restricting UNG activities from the riparian zone along with better enforcement of best management practices should help reduce these possible impacts of UNG activities on stream autotrophic processes.