Jeffrey A. Back

Does nutrient enrichment decouple algal–bacterial production in periphyton?

Abstract Coupled production between algae and bacteria in stream epilithon was assessed along a nutrient-enrichment gradient in 8 Texas streams with open canopies. Photosynthesis (PS) and bacterial biomass production (BBP) were measured simultaneously using a dual-label radioassay (14C-HCO3– uptake and 3H-L-leucine incorporation into protein) on multiple samples within a stream reach. PS and BBP were measured after light (1200–1500 μmol m−2 s−1) and dark incubations. The degree of coupled production between algae and bacteria within a stream was estimated as the covariation (i.e., correlation or covariance) between PS and BBP derived from unshaded replicates in each stream. Streamwater nutrients ranged from 0.18 to 8.1 mg/L total N and 0.009 to 2.0 mg/L total P. Epilithon N and P content (as % dry mass) and C:N:P ratios varied widely among streams and were positively correlated with streamwater nutrient concentrations. Mean BBP measured in light incubations (BBPL) was greater than mean BBP measured in dark incubations (BBPD), and the difference between the 2 means (BBPL – BBPD) was positively correlated with mean PS among streams (R2 = 0.53). Covariance between PS and BBPL within streams (COVPS–BBP) decreased as epilithon nutrient content increased. COVPS–BBP was positively correlated with both epilithon C:N (R2 = 0.78) and C:P (R2 = 0.77) among streams. These results suggest that algal and bacterial production are decoupled by nutrient enrichment, and that algae might rely more heavily on bacterial-regenerated nutrients than on streamwater nutrients to support production in nutrient-poor streams.

Influence of drought and total phosphorus on diel pH in wadeable streams: implications for ecological risk assessment of ionizable contaminants.

Climatological influences on site-specific ecohydrology are particularly germane in semiarid regions where instream flows are strongly influenced by effluent discharges. Because many traditional and emerging aquatic contaminants, such as pharmaceuticals, are ionizable, we examined diel surface water pH patterns (i.e., change in pH over a 24-h period) at 23 wadeable streams in central Texas, USA, representing a gradient of nutrient enrichment during consecutive summers of 2006 and 2007. The years of our study were characterized by decidedly different instream flows, which likely affected production:respiration dynamics and led to distinctions in diel pH patterns between 2006 and 2007. Site-specific ambient water quality criteria for NH(3) and the aquatic toxicity of the model weak base pharmaceutical sertraline were predicted using continuous water quality monitoring data from the sites. Drought conditions of 2006 significantly increased (p<0.05) diel pH changes compared to high instream flows of 2007,and the magnitude of diel pH variability was most pronounced at nutrient-enriched sites in 2006. Differences in diel pH change patterns between 2006 and 2007 affected predictions of the environmental fate and effects for model weak base pharmaceuticals and NH(3). Overall, site-specific diel pH was more variable at some sites than the difference in mean surface water pH between the 2 summers. Diel pH variability affected regulatory criteria, because 20% of the study sites in 2006 experienced greater than 5-fold differences in National Ambient Water Quality Criteria for NH(3) over 24-h periods. Our study emphasizes the potential uncertainty that diel pH variability may introduce in site-specific assessments and provides recommendations for environmental assessment of ionizable contaminants.

Sex and size matter: ontogenetic patterns of nutrient content of aquatic insects

Abstract.  C, N, and P content were measured across the ontogeny of lotic aquatic insects representing a diversity of life-history characteristics. The relationship between individual mass and nutrient content was used to show ontogenetic patterns of nutrient content by species. Species analyzed for C and N content exhibited a quasihomeostatic pattern across ontogeny. Percent C and %N varied among taxa irrespective of ontogeny, with %C ranging from 47.4 to 56.2% and %N ranging from 9.6 to 11.6%. P content also varied by species but declined nonlinearly across ontogeny and was best represented by a power function. Percent P varied from >7% in 1st-instar Tabanus larvae to only 0.34% in adult male Ambrysus circumcinctus. Females had more P per unit mass than males in 6 of the 10 species that could be sexed. In the leptophlebiid mayflies, %P increased in mature female nymphs relative to the penultimate developmental class, whereas %P content of males continued to decline to eclosion. Maximum terminal mass by species was the main factor driving the magnitude of change in %P through their ontogeny. Small-bodied, rapidly growing species exhibited the sharpest decline in P content. Nonhomeostatic patterns in %P across ontogeny and between sexes has important implications for population- and community-level dynamics and ecosystem processes. First, small-bodied, high-%P taxa have faster growth rates than larger individuals, which supports one of the predictions of the growth-rate hypothesis (GRH). Second, elemental imbalance between consumers and their food changes across ontogeny, and therefore, nutrient recycling rate by a species changes with population age structure. Last, community structure may reflect nutrient availability in food such that enriched environments are more likely to be dominated by taxa with high growth rates and, thus, relatively high P demand.

Influence of nitrogen and phosphorus concentrations and ratios on Lemna gibba growth responses to triclosan in laboratory and stream mesocosm experiments

The effects of co-occurring nutrient and contaminant stressors are very likely to interact in aquatic systems, particularly at the level of primary producers. Site-specific nitrogen (N) and phosphorus (P) concentrations are often much lower and differ in relative availability than those used in nutrient-saturated laboratory assays for aquatic plants, which can introduce uncertainty in prospective ecological hazard and risk assessments. Because triclosan, an antimicrobial agent included in personal care products, potentially presents high relative risk among antimicrobial agents to aquatic plants and algae, we performed laboratory experiments with the model aquatic macrophyte Lemna gibba across a gradient of environmentally relevant N:P levels with and without triclosan co-exposure. Frond numbers (7 d) were significantly higher in N:P treatments of 16 and 23 but were lower in N:P of 937 and 2,500 treatments relative to standardized control media (N:P=3). When triclosan co-exposure occurred at high nutrient concentrations, frond number median effective concentration values at N:P 0.75, 3, and 16 were more than twofold lower than triclosan median effective concentration values in low nutrient media N:P ratios. However, a triclosan median effective concentration for frond number was twofold lower at N:P of 2,500 than at other N:P ratios in low concentration media. Influences of P enrichment on triclosan toxicity to L. gibba were further explored during a 14-d outdoor experimental stream mesocosm study. Effects of 2.6 and 20.8 microg L(-1) triclosan on L. gibba growth rates were more pronounced with increasing P treatment levels, which was generally consistent with our laboratory observations. Findings from these laboratory and field studies indicate that site-specific nutrient concentrations and ratios should be considered during assessments of primary producer responses to chemical stressors.

Allochthonous inputs from grass-dominated wetlands support juvenile salmonids in headwater streams: evidence from stable isotopes of carbon, hydrogen, and nitrogen

Abstract.  We used dual-isotope mixing models (&dgr;13C/&dgr;15N and &dgr;2H/&dgr;15N) in a Bayesian framework to partition allochthonous and autochthonous energy sources for salmonids in 2 headwater streams in the Kenai Lowlands, Alaska (USA). Our 1st objective was to estimate the production base for juvenile coho salmon (Oncorhynchus kisutch) and Dolly Varden (Salvelinus malma). We hypothesized that consumers would be reliant on both autochthonous (filamentous algae and periphyton) and allochthonous sources, but that autochthonous sources would dominate because of the open canopy and lower-quality litter inputs provided by the riparian wetland vegetation, primarily bluejoint grass (Calamagrostis canadensis). Our 2nd objective was to evaluate the utility of stable H isotopes for tracing energy pathways in a northern-latitude ecosystem. We hypothesized that &dgr;2H-based models would provide more precise estimates of source partitioning than &dgr;13C-based models because of greater source separation. Allochthonous source contributions consistently exceeded autochthonous sources for all fish species and size classes at both study sites. However, diet shifted during ontogeny, and larger Dolly Varden relied more on autochthonous sources than did smaller individuals of both species. Last, we found good correspondence and similar levels of precision between the &dgr;13C- and &dgr;2H-based models despite greater source separation by &dgr;2H. Our results highlight the importance of allochthonous sources in headwater streams, and we suggest that litter inputs from grasses may be an under-appreciated subsidy to salmon production. Stable H isotopes can be an effective foodweb tracer in northern-latitude streams, but source partitioning results were not sufficiently different from stable C isotope models for us to recommend unequivocally using them to replace or enhance &dgr;13C in similar studies.

Breakdown rates, nutrient concentrations, and macroinvertebrate colonization of bluejoint grass litter in headwater streams of the Kenai Peninsula, Alaska

Abstract Grass litter can be a dominant detritus type in many streams. However, use of this C source in stream food webs often is viewed as insignificant because of its relatively slow breakdown rates and low nutritional quality. We deployed leaf packs containing senesced bluejoint grass (Calamagrostis canadensis) across a natural nutrient gradient of 6 salmon-rearing headwater streams on the lower Kenai Peninsula, Alaska. We hypothesized that litter-colonizing microbes would use dissolved stream nutrients and enhance breakdown rates, litter nutrient concentrations, and densities of macroinvertebrates across streams. Leaf-pack mass and nutrient concentrations were measured on the material at 0 (predeployment), 2, 4, 6, and 8 wk in all streams. Breakdown rates were calculated from the mass measurements with an exponential decay model. Macroinvertebrate composition and abundance were measured at 8 wk in all streams and every 2 wk in 1 stream. Breakdown rates of bluejoint litter were relatively low (20–30% mass loss over 2 mo), but similar to rates found in previous studies of senesced grass litter. Weighted regressions showed that bluejoint breakdown rates in the 6 streams were significantly (p < 0.05) related to dissolved stream nutrient concentrations (r2  =  0.94 and 0.67 for dissolved inorganic N and PO4-P, respectively), litter nutrient concentration (r2  =  0.72 and 0.96 for leaf % N and % P, respectively), total macroinvertebrates/g (r2  =  0.73), and nonmetric multidimensional scaling-axis-1 scores of macroinvertebrate community structure (r2  =  0.80). Litter nutrients changed after just 2 wk and were increasingly and significantly related to stream nutrient concentrations over time. NMS ordination showed that succession of macroinvertebrates on leaf packs from one stream followed a distinct direction over time, and a large shift in macroinvertebrate community structure occurred between weeks 6 and 8, a result potentially indicating a consumer response to microbial conditioning. The abundance and diversity of macroinvertebrate taxa using bluejoint litter provide evidence that it is an important habitat and energy pathway for consumers in headwater streams of the Kenai Peninsula. In addition, climate change has the potential to change terrestrial vegetation assemblages, which drive differences in stream nutrient concentrations in this region. If N-fixing shrubs become more abundant in the future, litter-decomposition rates will be positively affected by increases in both stream and leaf-litter nutrient concentrations.

Catchment topography and wetland geomorphology drive macroinvertebrate community structure and juvenile salmonid distributions in south-central Alaska headwater streams

Abstract.  Conservation and management of headwater streams amid rapid global change require an understanding of the spatial and environmental factors that drive species distributions and associated ecosystem processes. We used a hierarchical analytical framework to model effects of catchment-scale topography and wetland geomorphic classes on stream physical habitat, chemistry, and macroinvertebrate and fish communities in 30 headwater streams across the Kenai Lowlands, southcentral Alaska, USA. We identified 135 macroinvertebrate taxa, 122 of which were aquatic insects, of which 79 were dipterans. We collected only 6 species of fish, but juvenile coho salmon and Dolly Varden were collected in 17 and 25 of the 30 streams and reached densities >500 and 1300/km, respectively. Flow-weighted slope, an indicator of water residence time and gradient, was the best catchment-scale correlate of macroinvertebrate and fish community structure, and its effect was mediated by wetland geomorphic classes and numerous water chemistry, substrate composition, and channel geomorphology variables measured at the reach scale. Many macroinvertebrate taxa showed high fidelity to different levels of the topographic gradient, resulting in high &bgr; diversity but relatively similar levels of &agr; diversity across the gradient. Juvenile salmonids were segregated among streams by both species and age classes. Coho salmon fry and parr (<10 cm total length [TL]) had significant unimodal distributions that peaked in streams with intermediate slopes and gravel substrate, whereas presmolts (≥10 cm) were found only in lowest-sloping streams with mostly peat substrate and deep, slow channels. Large Dolly Varden (≥8 cm) were found across the entire gradient but were most abundant in high-sloping catchments, whereas small Dolly Varden (<8 cm) followed a similar distribution but were absent from the lowest-gradient sites with low flow velocity, dissolved O2, and gravel substrate. Predictive modeling indicated that all of the 547 km of headwater streams in the study area might serve as potential habitat for ≥1 species and age class of salmonids. Our study should assist in development of catchment management tools for identifying and prioritizing conservation efforts in the region and may serve as a framework for other studies concerning biodiversity and focal species conservation in headwater streams.

Ontogenic differences in mayfly stoichiometry influence growth rates in response to phosphorus enrichment

We contrasted the carbon, nitrogen, and phosphorus (C: N: P) stoichiometry of Caenis spp. (Ephemeroptera:Caenidae) nymphs from 2 stream reaches differing in P enrichment. We also estimated growth rates of nymphs reared on algae of different P content across four development classes in a laboratory experiment. C: N ratios of field-collected nymphs exhibited variable responses across development classes between sites whereas C: P and N: P ratios showed a clear unimodal response, increasing from classes II through IV but then declining sharply in class V (nymphs nearing eclosion) at both sites. C: P was lower at the highly enriched site for all but the last development class. Growth rates increased in response to P enrichment at the earliest development class, but this growth response diminished in later development classes resulting in a significant interaction between P treatments and development classes. Trends in field data imply that later stages of development have higher P requirements than earlier classes and nutrient enrichment may affect sequestration of P by nymphs. Laboratory data suggest that early development classes are more P limited but in light of field results, nymphs may shift P allocation from somatic growth to reproductive development as organisms mature.

Effects of pulsed atrazine exposures on autotrophic community structure, biomass, and production in field-based stream mesocosms

The authors performed a multiple-pulsed atrazine experiment to measure responses of autotrophic endpoints in outdoor stream mesocosms. The experiment was designed to synthetically simulate worst-case atrazine chemographs from streams in agricultural catchments to achieve 60-d mean concentrations of 0 μg/L (control), 10 μg/L, 20 μg/L, and 30 μg/L. The authors dosed triplicate streams with pulses of 0 μg/L, 50 μg/L, 100 μg/L, and 150 μg/L atrazine for 4 d, followed by 7 d without dosing. This 11-d cycle occurred 3 times, followed by a recovery (untreated) period from day 34 to day 60. Mean ± standard error 60-d atrazine concentrations were 0.07 ± 0.03 μg/L, 10.7 ± 0.05 μg/L, 20.9 ± 0.24 μg/L, and 31.0 ± 0.17 μg/L for the control, 10-μg/L, 20-μg/L, and 30-μg/L treatments, respectively. Multivariate analyses revealed that periphyton and phytoplankton community structure did not differ among treatments on any day of the experiment, including during the atrazine pulses. Control periphyton biomass in riffles was higher immediately following the peak of the first atrazine pulse and remained slightly higher than some of the atrazine treatments on most days through the peak of the last pulse. However, periphyton biomass was not different among treatments at the end of the present study. Phytoplankton biomass was not affected by atrazine. Metaphyton biomass in pools was higher in the controls near the midpoint of the present study and remained higher on most days for the remainder of the study. Ceratophyllum demersum, a submersed macrophyte, biomass was higher in controls than in 20-μg/L and 30-μg/L treatments before pulse 3 but was not different subsequent to pulse 3 through the end of the present study. Maximum daily dissolved oxygen (DO, percentage of saturation) declined during each pulse in approximate proportion to magnitude of dose but rapidly converged among treatments after the third pulse. However, DO increased in controls relative to all atrazine treatments during the last 17 d of the experiment, likely a result of metaphyton cover in the pools. Finally, atrazine significantly limited uptake of PO4(3-) and uptake and/or denitrification of NO3(-) but only during pulses; percentage of dose removed from the water column was >85% for P and >95% for N after pulse 3 through the end of the present study. Collectively, only DO and metaphyton biomass differed at the end of the present study and only slightly. Some other endpoints were affected but only during pulses, if at all. The high levels of primary production and accumulation of algal biomass in all streams suggest that effects of pulses of atrazine at the concentrations used in the present study appear transient and likely do not represent ecologically significant adverse outcomes to periphyton, phytoplankton, and aquatic macrophytes, particularly in agricultural streams subjected to high nutrient loads.

Grazing minnows increase benthic autotrophy and enhance the response of periphyton elemental composition to experimental phosphorus additions

Abstract.  Excessive nutrient inputs and grazers can influence biomass and elemental composition of primary producers in freshwater ecosystems. How interactions between nutrient enrichment and grazing fish alter benthic habitats through effects on periphyton autotrophy, biomass, and elemental composition has been studied rarely. We compared the effects of grazing by central stonerollers (Campostoma anomalum) on autotrophic and total periphyton biomass, sediment mass, and C, N, and P stoichiometry of periphyton in 12 flow-through stream mesocosms randomly assigned to 1 of 3 different PO4-P concentrations (control: 8 µg/L, low: 20 µg/L, high: 100 µg/L). Fish grazing suppressed periphyton ash-free dry mass (AFDM) and sediment accumulation, regardless of P treatment. However, grazing also increased the proportion of algal biomass in the periphyton, evidenced by a reduction in benthic C:chlorophyll a on grazed substrates. The response of periphyton stoichiometry to experimental P enrichment was stronger on grazed substrates because central stonerollers maintained a higher proportion of algae in the periphyton matrix. Grazing enhanced the response of P standing stocks to enrichment, reduced C∶P and C∶N in high-P streams, and increased N∶P in control and low-P streams. Shifts from detritus- and sediment-bound nutrients to algal resources probably increase the palatability of benthic food resources and nutrient availability for other grazing organisms. Grazing fish may play a stronger role in benthic processes, such as nutrient cycling, than is currently recognized. Our results suggest that fish drive periphyton toward autotrophy, enhance sequestration of excess nutrients in periphyton and, thus, may relax stoichiometric constraints on fast growing organisms in stream communities.