Sodium as a subsidy in the spring: evidence for a phenology of sodium limitation.

Understanding the factors that mediate carbon (C) cycling is increasingly important as anthropogenic activities and climate change alter ecosystems. Decomposition rates mediate C cycling and are in part regulated by sodium (Na) where Na is limiting up to some threshold after which Na becomes stressful and reduces decomposition rates (i.e., the Sodium Subsidy-Stress hypothesis). An overlooked pathway by which decomposers encounter increased salts like NaCl is through plants, which often take up Na in proportion to soil concentrations. Here we tested the hypothesis that Na addition through litter (detritus) and water and their interaction would impact detrital processing and leachate chemistry. Laboratory riparian soil mesocosms received either artificial litter (100% cellulose sponges) soaked in 0.05% NaCl (NaClL) or just H2O (H2OL: control) and half of each litter treatment received weekly additions of 150 ml of either 0.05% NaCl water (NaClW) or just H2O (H2OW: control). After 8 weeks decomposition was higher in NaCl addition treatments (both NaClL and NaClW and their combo) than controls (H2OL + H2OW) but reflected a unimodal relationship where the saltiest treatment (NaClL + NaClW) was only marginally higher than controls indicating a subsidy-stress response. Previous studies in this system found that Na addition in either water or litter decreased decomposition. However, differences may reflect a phenology of Na demand where Na-limitation increases in the spring (this study). These results indicate that our understanding of how Na impacts detrital processes, C cycling, and aquatic-terrestrial linkages necessitates incorporation of temporal dynamics.

Interspecific homeostatic regulation and growth across aquatic invertebrate detritivores: a test of ecological stoichiometry theory.

Across resource quality gradients, primary consumers must regulate homeostasis and release of nutrients to optimize growth and fitness. Based primarily on internal body composition, the ecological stoichiometry theory (EST) offers a framework to generalize interspecific patterns of these responses, yet the predictions and underlying assumptions of EST remain poorly tested across many species. We used controlled laboratory feeding experiments to measure homeostasis, nutrient release, and growth across seven field-collected aquatic invertebrate detritivore taxa fed wide resource carbon:nitrogen (C:N) and carbon:phosphorus (C:P) gradients. We found that most invertebrates exhibited strict stoichiometric homeostasis (average 1/H = - 0.018 and 0.026 for C:N and C:P, respectively), supporting assumptions of EST. However, the stoichiometry of new tissue production during growth intervals (growth stoichiometry) deviated - 30 to + 54% and - 145 to + 74% from initial body C:N and C:P, respectively, and across species, growth stoichiometry was not correlated with initial body stoichiometry. Notably, smaller non- and hemimetabolous invertebrates exhibited low, decreasing growth C:N and C:P, whereas larger holometabolous invertebrates exhibited high, often increasing growth C:N and C:P. Despite predictions of EST, interspecific sensitivity of egestion stoichiometry and growth rates to the resource gradient were weakly related to internal body composition across species. While the sensitivity of these patterns differed across taxa, such differences carried a weak phylogenetic signal and were not well predicted by EST. Our findings suggest that traits beyond internal body composition, such as feeding behavior, selective assimilation, and ontogeny, are needed to generalize interspecific patterns in consumer growth and nutrient release across resource quality gradients.

Correction to: Are soils beneath coniferous tree stands more acidic than soils beneath deciduous tree stands?

Fig. 1. was amended to reduce the size of the map and improve formatting of the manuscript. The authors claim this amendment does not affect the information being conveyed.

Are soils beneath coniferous tree stands more acidic than soils beneath deciduous tree stands?

In 2008, the Mulberry River, a National Wild and Scenic River, was listed as impaired due to low pH (below pH 6.0). Over the last 50 years, the volume of conifers in the Ozark region has increased 115% since 1978 which may result in the acidification of nearby aquatic ecosystems. The objective of this study was to determine if differences exist in soil and litter chemical properties between deciduous and coniferous tree stands. Aboveground litter (n = 200) and soil (n = 400) at 0- to 5- and 5- to 15-cm depths were collected at paired deciduous and coniferous stands at 10 locations within the Mulberry River watershed and analyzed for a suite of chemical parameters. There were no differences (P > 0.05) in several measures of soil acidity between deciduous and coniferous stands. Litter collected from the coniferous stands was more acidic than deciduous litter (4.4 vs 4.7; P < 0.05). Cation exchange capacity, exchangeable Ca and Mg, and water-soluble P and Mg contents differed (P < 0.05) by stand and depth. Cation exchange capacity and exchangeable Ca and Mg were greatest in the 0- to 5-cm depth interval of the coniferous stands. Water-soluble P and Mg contents were greatest within the 0- to 5-cm depth interval which did not differ (P > 0.05) between stand but were greater than the 5- to 15-cm depth interval. Although limited to the top 15-cm of soil, the similarity in soil acidity between stands suggests that conifer growth may not be a substantial source of acidity to the Mulberry River.

Multiple riparian-stream connections are predicted to change in response to salinization.

Secondary freshwater salinization, a common anthropogenic alteration, has detrimental, lethal and sub-lethal effects on aquatic biota. Ions from secondary salinization can become toxic to terrestrial and aquatic organisms when exposed to salinized runoff that causes periodic high-concentration pulses. Gradual, low-level (less than 1000 ppm salinity) increases in salt concentrations are also commonly documented in regions with urbanization, agriculture, drilling and mining. Despite widespread low-level salt increases, little is known about the biological and ecological consequences in coupled riparian-stream systems. Recent research indicates lethal and even sub-lethal levels of ions can subsidize or stress microbial decomposer and macroinvertebrate detritivores that could lead to alterations of three riparian-stream pathways: (i) salinized runoff that changes microbial decomposer and macroinvertebrate detritivore and algae performance leading to changes in composition and processing of detrital pools; (ii) riparian plant salt uptake and altered litter chemistry, and litterfall for riparian and aquatic detritivores and their subsequent enrichment, stimulating decomposition rates and production of dissolved and fine organic matter; and (iii) salt consumption in salinized soils could increase riparian detritivore growth, decomposition and dissolved organic matter production. Subsidy-stress and reciprocal flows in coupled riparian-stream connections provide frameworks to identify the extent and magnitude of changes in detrital processing from salinization.This article is part of the theme issue 'Salt in freshwaters: causes, ecological consequences and future prospects'.

Quantity and quality limit detritivore growth: mechanisms revealed by ecological stoichiometry and co-limitation theory.

Resource quantity and quality are fundamental bottom-up constraints on consumers. Best understood in autotroph-based systems, co-occurrence of these constraints may be common but remains poorly studied in detrital-based systems. Here, we used a laboratory growth experiment to test limitation of the detritivorous caddisfly larvae Pycnopsyche lepida across a concurrent gradient of oak litter quantity (food supply) and quality (phosphorus : carbon [P:C ratios]). Growth increased simultaneously with quantity and quality, indicating co-limitation across the resource gradients. We merged approaches of ecological stoichiometry and co-limitation theory, showing how co-limitation reflected shifts in C and P acquisition throughout homeostatic regulation. Increased growth was best explained by elevated consumption rates and improved P assimilation, which both increased with elevated quantity and quality. Notably, C assimilation efficiencies remained unchanged and achieved maximum 18% at low quantity despite pronounced C limitation. Detrital C recalcitrance and substantive post-assimilatory C losses probably set a minimum quantity threshold to achieve positive C balance. Above this threshold, greater quality enhanced larval growth probably by improving P assimilation toward P-intensive growth. We suggest this interplay of C and P acquisition contributes to detritivore co-limitation, highlighting quantity and quality as potential simultaneous bottom-up controls in detrital-based ecosystems, including under anthropogenic change like nutrient enrichment.

Comparing the Ecological Stoichiometry in Green and Brown Food Webs - A Review and Meta-analysis of Freshwater Food Webs.

The framework of ecological stoichiometry was developed primarily within the context of "green" autotroph-based food webs. While stoichiometric principles also apply in "brown" detritus-based systems, these systems have been historically understudied and differ from green ones in several important aspects including carbon (C) quality and the nutrient [nitrogen (N) and phosphorus (P)] contents of food resources for consumers. In this paper, we review work over the last decade that has advanced the application of ecological stoichiometry from green to brown food webs, focusing on freshwater ecosystems. We first review three focal areas where green and brown food webs differ: (1) bottom-up controls by light and nutrient availability, (2) stoichiometric constraints on consumer growth and nutritional regulation, and (3) patterns in consumer-driven nutrient dynamics. Our review highlights the need for further study of how light and nutrient availability affect autotroph-heterotroph interactions on detritus and the subsequent effects on consumer feeding and growth. To complement this conceptual review, we formally quantified differences in stoichiometric principles between green and brown food webs using a meta-analysis across feeding studies of freshwater benthic invertebrates. From 257 datasets collated across 46 publications and several unpublished studies, we compared effect sizes (Pearson's r) of resource N:C and P:C on growth, consumption, excretion, and egestion between herbivorous and detritivorous consumers. The meta-analysis revealed that both herbivore and detritivore growth are limited by resource N:C and P:C contents, but effect sizes only among detritivores were significantly above zero. Consumption effect sizes were negative among herbivores but positive for detritivores in the case of both N:C and P:C, indicating distinct compensatory feeding responses across resource stoichiometry gradients. Herbivore P excretion rates responded significantly positively to resource P:C, whereas detritivore N and P excretion did not respond; detritivore N and P egestion responded positively to resource N:C and P:C, respectively. Our meta-analysis highlights resource N and P contents as broadly limiting in brown and green benthic food webs, but indicates contrasting mechanisms of limitation owing to differing consumer regulation. We suggest that green and brown food webs share fundamental stoichiometric principles, while identifying specific differences toward applying ecological stoichiometry across ecosystems.

Unconventional natural gas development did not result in detectable changes in water chemistry (within the South Fork Little Red River).

The Fayetteville Shale within north central Arkansas is an area of extensive unconventional natural gas (UNG) production. Recently, the Scott Henderson Gulf Mountain Wildlife Management Area (GMWMA) was leased from the state of Arkansas for NG exploration, raising concerns about potential impacts on water resources. From November 2010 through November 2014, we monitored four reaches of the South Fork Little Red River (SFLRR), within the GMWMA, establishing baseline physico-chemical characteristics prior to UNG development and assessing trends in parameters during and after UNG development. Water samples were collected monthly during baseflow conditions and analyzed for conductivity, turbidity, ions, total organic carbon (TOC), and metals. All parameters were flow-adjusted and evaluated for monotonic changes over time. The concentrations of all constituents measured in the SFLRR were generally low (e.g., nitrate ranged from <0.005 to 0.268 mg/l across all sites and sample periods), suggesting the SFLRR is of high water quality. Flow-adjusted conductivity measurements and sodium concentrations increased at site 1, while magnesium decreased across all four sites, TOC decreased at sites 1 and 3, and iron decreased at site 1 over the duration of the study. With the exception of conductivity and sodium, the physico-chemical parameters either decreased or did not change over the 4-year duration, indicating that UNG activities within the GMWMA have had minimal or no detectable impact on water quality within the SFLRR. Our study provides essential baseline information that can be used to evaluate water quality within the SFLRR in the future should UNG activity within the GMWMA expand.

A global database of nitrogen and phosphorus excretion rates of aquatic animals.

Animals can be important in modulating ecosystem-level nutrient cycling, although their importance varies greatly among species and ecosystems. Nutrient cycling rates of individual animals represent valuable data for testing the predictions of important frameworks such as the Metabolic Theory of Ecology (MTE) and ecological stoichiometry (ES). They also represent an important set of functional traits that may reflect both environmental and phylogenetic influences. Over the past two decades, studies of animal-mediated nutrient cycling have increased dramatically, especially in aquatic ecosystems. Here we present a global compilation of aquatic animal nutrient excretion rates. The dataset includes 10,534 observations from freshwater and marine animals of N and/or P excretion rates. These observations represent 491 species, including most aquatic phyla. Coverage varies greatly among phyla and other taxonomic levels. The dataset includes information on animal body size, ambient temperature, taxonomic affiliations, and animal body N:P. This data set was used to test predictions of MTE and ES, as described in Vanni and McIntyre (2016; Ecology DOI: 10.1002/ecy.1582).

Do biofilm communities respond to the chemical signatures of fracking? A test involving streams in North-central Arkansas.

BACKGROUND: Unconventional natural gas (UNG) extraction (fracking) is ongoing in 29 North American shale basins (20 states), with ~6000 wells found within the Fayetteville shale (north-central Arkansas). If the chemical signature of fracking is detectable in streams, it can be employed to bookmark potential impacts. We evaluated benthic biofilm community composition as a proxy for stream chemistry so as to segregate anthropogenic signatures in eight Arkansas River catchments. In doing so, we tested the hypothesis that fracking characteristics in study streams are statistically distinguishable from those produced by agriculture or urbanization. RESULTS: Four tributary catchments had UNG-wells significantly more dense and near to our sampling sites and were grouped as 'potentially-impacted catchment zones' (PICZ). Four others were characterized by significantly larger forested area with greater slope and elevation but reduced pasture, and were classified as 'minimally-impacted' (MICZ). Overall, 46 bacterial phyla/141 classes were identified, with 24 phyla (52%) and 54 classes (38%) across all samples. PICZ-sites were ecologically more variable than MICZ-sites, with significantly greater nutrient levels (total nitrogen, total phosphorous), and elevated Cyanobacteria as bioindicators that tracked these conditions. PICZ-sites also exhibited elevated conductance (a correlate of increased ion concentration) and depressed salt-intolerant Spartobacteria, suggesting the presence of brine as a fracking effect. Biofilm communities at PICZ-sites were significantly less variable than those at MICZ-sites. CONCLUSIONS: Study streams differed by Group according to morphology, land use, and water chemistry but not in biofilm community structure. Those at PICZ-sites covaried according to anthropogenic impact, and were qualitatively similar to communities found at sites disturbed by fracking. The hypothesis that fracking signatures in study streams are distinguishable from those produced by other anthropogenic effects was statistically rejected. Instead, alterations in biofilm community composition, as induced by fracking, may be less specific than initially predicted, and thus more easily confounded by agriculture and urbanization effects (among others). Study streams must be carefully categorized with regard to the magnitude and extent of anthropogenic impacts. They must also be segregated with statistical confidence (as herein) before fracking impacts are monitored.

Dietary and taxonomic controls on incorporation of microbial carbon and phosphorus by detritivorous caddisflies.

Heterotrophic microbes on detritus play critical roles in the nutrition of detritivorous animals, yet few studies have examined factors controlling the acquisition of microbial nutrients toward detritivore growth, which is termed "incorporation". Here, we assessed effects of detrital substrate identity (leaf type), background nutrients, and detritivore species identity on detritivore incorporation of microbial carbon (C) and phosphorus (P) in leaf litter diets. We fed oak and maple litter conditioned under two nutrient concentrations (50 or 500 µg P L(-1)) to the detritivorous caddisfly larvae Ironoquia spp., Lepidostoma spp., and Pycnopsyche lepida and used the radioisotopes 14C as glucose and 33P as phosphate to dually trace incorporation of microbial C and P by caddisflies. Incorporation efficiencies of microbial C (mean ± SE = 12.3 ± 1.3%) were one order of magnitude higher than gross growth efficiencies for bulk detrital C from recent studies (1.05 ± 0.08%). Litter type did not affect incorporation of microbial nutrients; however, caddisflies incorporated microbial P 11 % less efficiently when fed litter from the higher P concentration. Two lower body C:P species (Pycnopsyche and Ironoquia) exhibited 9.9 and 7.1% greater microbial C and 19.0 and 17.7% greater microbial P incorporation efficiencies, respectively, than the higher body C:P species (Lepidostoma). Our findings support ecological stoichiometry theory on post-ingestive regulation that animals fed lower C:P diets should reduce P incorporation efficiency due to excess diet P or alleviation of P-limited growth, and that lower C:P species must incorporate dietary C and P more efficiently to support fast growth of P-rich tissues.

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.

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.

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.

Responses of stream macroinvertebrates to Bt maize leaf detritus.

In the midwestern United States, maize detritus enters streams draining agricultural land. Genetically modified Bt maize is commonly planted along streams and can possibly affect benthic macroinvertebrates, specifically members of the order Trichoptera, which are closely related to target species of some Bt toxins and are important detritivores in streams. The significance of inputs of Bt maize to aquatic systems has only recently been recognized, and assessments of potential nontarget impacts on aquatic organisms are lacking. We conducted laboratory feeding trials and found that the leaf-shredding trichopteran, Lepidostoma liba, grew significantly slower when fed Bt maize compared to non-Bt maize, while other invertebrate taxa that we examined showed no negative effects. We also used field studies to assess the influence of Bt maize detritus on benthic macroinvertebrate abundance, diversity, biomass, and functional structure in situ in 12 streams adjacent to Bt maize or non-Bt maize fields. We found no significant differences in total abundance or biomass between Bt and non-Bt streams, and trichopterans comprised only a small percentage of invertebrate biomass at all sites (0-15%). Shannon diversity did not differ among Bt and non-Bt streams and was always low (H' range = 0.9-1.9). Highly tolerant taxa, such as oligochaetes and chironomids, were dominant in both Bt and non-Bt streams, and macroinvertebrate community composition was relatively constant across seasons. We used litterbags to examine macroinvertebrate colonization of Bt and non-Bt maize detritus and found no significant differences among litter or stream types. Our in situ findings did not support our laboratory results; this is likely because the streams we studied in this region are highly degraded and subject to multiple, persistent anthropogenic stressors (e.g., channelization, altered flow, nutrient and pesticide inputs). Invertebrate communities in these streams are a product of these degraded conditions, and thus the impact of a single stressor, such as Bt toxins, may not be readily discernable. Our results add to growing evidence that Bt toxins can have sublethal effects on nontarget aquatic taxa, but this evidence should be considered in the context of other anthropogenic impacts and alternative methods of pest control influencing streams draining agricultural regions.

Rapid decomposition of maize detritus in agricultural headwater streams.

Headwater streams draining agricultural landscapes receive maize leaves (Zea mays L.) via wind and surface runoff, yet the contribution of maize detritus to organic-matter processing in agricultural streams is largely unknown. We quantified decomposition and microbial respiration rates on conventional (non-Bt) and genetically engineered (Bt) maize in three low-order agricultural streams in northwestern Indiana, USA. We also examined how substrate quality and in-stream nutrient concentrations influenced microbial respiration on maize by comparing respiration on maize and red maple leaves (Acer rubrum) in three nutrient-rich agricultural streams and three low-nutrient forested streams. We found significantly higher rates of microbial respiration on maize vs. red maple leaves and higher rates in agricultural vs. forested streams. Thus both the elevated nutrient status of agricultural streams and the lability of maize detritus (e.g., low carbon-to-nitrogen ratio and low lignin content) result in a rapid incorporation of maize leaves into the aquatic microbial food web. We found that Bt maize had a faster decomposition rate than non-Bt maize, while microbial respiration rates did not differ between Bt and non-Bt maize. Decomposition rates were not negatively affected by genetic engineering, perhaps because the Bt toxin does not adversely affect the aquatic microbial assemblage involved in maize decomposition. Additionally, shredding caddisflies, which are known to have suppressed growth rates when fed Bt maize, were depauperate in these agricultural streams, and likely did not play a major role in maize decomposition. Overall, the conversion of native vegetation to row-crop agriculture appears to have altered the quantity, quality, and predictability of allochthonous carbon inputs to headwater streams, with unexplored effects on stream ecosystem structure and function.

Direct and indirect effects of a potential aquatic contaminant on grazer-algae interactions.

Contaminants have direct, harmful effects across multiple ecological scales, including the individual, the community, and the ecosystem levels. Less, however, is known about how indirect effects of contaminants on consumer physiology or behavior might alter community interactions or ecosystem processes. We examined whether a potential aquatic contaminant, an ionic liquid, can indirectly alter benthic algal biomass and primary production through direct effects on herbivorous snails. Ionic liquids are nonvolatile organic salts being considered as an environmentally friendly potential replacement for volatile organic compounds in industry. In two greenhouse experiments, we factorially crossed four concentrations of 1-N-butyl-3-methylimidazolium bromide (bmimBr; experiment 1: 0 or 10 mg/L; experiment 2: 0, 1, or 100 mg/L) with the presence or absence of the snail Physa acuta in aquatic mesocosms. Experimental results were weighted by their respective control (no bmimBr or P. acuta) and combined for statistical analysis. When both bmimBr and snails were present, chlorophyll a abundance and algal biovolume were higher than would be expected if both factors acted additively. In addition, snail growth rates, relative to those of controls, declined by 41 to 101% at 10 and 100 mg/L of bmimBr. Taken together, these two results suggest that snails were less efficient grazers in the presence of bmimBr, resulting in release of algae from the grazer control. Snails stimulated periphyton primary production in the absence, but not in the presence, of bmimBr, suggesting that bmimBr also can indirectly alter ecosystem function. These findings suggest that sublethal contaminant levels can negatively impact communities and ecosystem processes via complex interactions, and they provide baseline information regarding the potential effects of an emergent industrial chemical on aquatic systems.

Stoichiometry of consumer-driven nutrient recycling across nutrient regimes in streams.

Stoichiometric constraints within ecological interactions and their ecosystem consequences may depend on characteristics of the abiotic environment such as background nutrient levels. We assessed whether consumer identity, via differing body stoichiometry, could regulate periphyton stoichiometry across nutrient regimes in open systems. In 60 flow-through artificial streams, we factorially crossed dissolved inorganic nitrogen levels (elevated = 294 micog L(-1), ambient = 26 microg L(-1)) with dissolved inorganic phosphorus levels (DIP: elevated = 15 microg L(-1), ambient = 3 microg L(-1)) and consumer type [crayfish (body N : P = 18), snails (body N : P = 28) or a control]. At ambient DIP, periphyton in the crayfish treatment had a lower %P and a lower C : P than periphyton in the snail treatment suggesting that consumer identity, probably mediated by differing P-excretion, regulated periphyton P content. At high DIP, consumer identity no longer affected periphyton elemental composition. Therefore, the stoichiometry of consumer-driven nutrient recycling and consumer identity may be less important to ecosystem functioning in environments with elevated nutrient levels.

Acute and chronic toxicity of imidazolium-based ionic liquids on Daphnia magna.

Room-temperature ionic liquids (ILs) are considered to be green chemicals that may replace volatile organic solvents currently used by industry. However, IL effects on aquatic organisms and ecosystems are currently unknown. We studied the acute effects of imidazolium-based ILs on survival of the crustacean Daphnia magna and their chronic effects on number of first-brood neonates, total number of neonates, and average brood size. Lethal concentrations of imidazolium ILs with various anions (X-) ranged from a median lethal concentration (LC50) of 8.03 to 19.91 mg L(-1), whereas salts with a sodium cation (Na+ X-) were more than an order of magnitude higher (NaPF6 LC50, 9,344.81 mg L(-1); NaBF4 LC50, 4765.75 mg L(-1)). Thus, toxicity appeared to be related to the imidazolium cation and not to the various anions (e.g., Cl-, Br-, PF6-, and BF4-). The toxicity of imidazolium-based ILs is comparable to that of chemicals currently used in manufacturing and disinfection processes (e.g., ammonia and phenol), indicating that these green chemicals may be more harmful to aquatic organisms than current volatile organic solvents. We conducted 21-d chronic bioassays of individual D. magna exposed to nonlethal IL concentrations at constant food-resource levels. Daphnia magna produced significantly fewer total neonates, first-brood neonates, and average neonates when exposed to lower concentrations (0.3 mg L(-1)) of imidazolium-based ILs than in the presence of Na-based salts at higher concentrations (400 mg L(-1)). Such reductions in the reproductive output of Daphnia populations could cascade through natural freshwater ecosystems. The present study provides baseline information needed to assess the potential hazard that some ILs may pose should they be released into freshwater ecosystems.