Jason M. Taylor

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.

Multiscale Environmental Influences on Fish Assemblage Structure in Central Texas Streams

Abstract We investigated the influences of local and landscape-scale environmental variables on fish assemblage structure among 64 stream reaches in two large river basins in central Texas. The broad spatial extent of this study region provided an opportunity to examine fish assemblage–environment relationships at multiple scales across a range of stream types in landscapes exposed to varying degrees of anthropogenic alteration. We used nonmetric multidimensional scaling (NMS) combined with permutational analysis of variance, k-means cluster analysis, and indicator species analysis to evaluate broad-scale influences of ecoregional and large river basin boundaries on fish assemblage structure. We also estimated relationships between fish assemblage structure and environmental factors with NMS and rotational vector fitting across all ecoregions and within ecoregions. Ordinations of sites based on species composition grouped stream reaches together according to ecoregion, and k-means clustering identified th...

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.

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.

Ecoregional, catchment, and reach-scale environmental factors shape functional-trait structure of stream fish assemblages

Patterns of association between functional traits and environmental gradients can improve understanding of species assemblage structure from local to regional scales, and therefore may be useful for natural resource management. We measured functional traits related to trophic ecology, habitat use, and life-history strategies of fishes and examined their associations with environmental factors in the Brazos and Trinity River basins in Central Texas. We also examined the relationship between functional diversity of fish assemblages and indices of biotic integrity and habitat quality. Environmental characteristics at the local reach and catchment scales, including the extent of forested area in the watershed, amount of land developed for urban and agricultural uses, stream size, substrate characteristics, and availability of riffle and pool habitats, were significantly associated with functional trait composition of fish assemblages. Broad physiographic differences between ecoregions also had a large influence on taxonomic and functional assemblage structure. In general, the volume of functional trait space occupied by fish assemblages was greatest in streams with high habitat quality scores located within landscapes having less alteration from agriculture and urban development. Distributions of functional traits in fish assemblages might provide an additional basis for assessment of stream condition in relation to environmental impacts.

Coupling Fish Community Structure with Instream Flow and Habitat Connectivity between Two Hydrologically Extreme Years

Abstract Hydrologic variability and instream habitat connectivity play fundamental roles in structuring fish communities in lotic ecosystems. We collected fish assemblage and physical habitat data from 28 central Texas streams during the summers of 2006 (a drought year with minimal summer precipitation and low stream flow) and 2007 (an exceptionally wet year with periodic flooding in spring and sustained high flows throughout summer). We evaluated the correspondence between the magnitude of physical habitat and fish community composition change in stream reaches sampled in these two contrasting years using ordination, successional vector analysis, and indicator species analysis. In 2006, streams characterized by disconnected pools had different fish community structure and habitat characteristics than streams that had habitats connected by flowing water. The amount of interannual change in both fish community structure and habitat characteristics was greatest between streams that had disconnected pools in...

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.

Fish-mediated nutrient cycling and benthic microbial processes: can consumers influence stream nutrient cycling at multiple spatial scales?

Abstract.  Fish-mediated nutrient recycling influences nutrient dynamics in stream ecosystems, but its consequences for smaller-scale microbial processes in benthic habitats are not well understood. We quantified the effect of nutrient recycling by the grazing fish, Campostoma anomalum, on downstream periphyton in 12 flow-through stream mesocosms. We compared periphyton nutrient ratios and algal biomass (as chlorophyll a [chl a]) between tiles upstream and downstream of enclosures with and without fish to measure nutrient-cycling effects in streams with low (11) and high (177) surface-water dissolved N∶P molar ratios. No upstream–downstream changes in periphyton nutrient ratios were observed in low N∶P streams with or without fish. In high N∶P streams, periphyton C∶N decreased and C∶P and N∶P increased on tiles downstream of enclosures. In high N∶P streams, downstream changes in periphyton nutrient ratios were greater in streams with than without fish, and chl a significantly increased downstream of enclosures with fish. We linked nutrient-recycling effects to downstream microbial processes by comparing bacterial biomass production (BBP), photosynthesis (PS) rates, and the degree of coupling between the 2 processes on tiles downstream of enclosures. We estimated the degree of coupled production between algae and bacteria downstream of enclosures as the covariance between PS and BBP among replicates within each stream (COVPS–BBP). In high N∶P streams, areal BBP and PS rates and COVPS–BBP were higher downstream of enclosures with fish. Chl a and COVPS–BBP declined with increasing periphyton C∶N content, resulting in a positive relationship between COVPS–BBP and algal biomass across all treatments. Our results indicate that grazing fish alter stream ecosystem N and P dynamics through consumer-mediated recycling pathways, but downstream responses depend on background nutrient regimes. Fish-mediated changes in nutrient dynamics and algal biomass influence reliance of heterotrophs and autotrophs on nutrients recycled within periphyton communities to support benthic production.

Downstream fish assemblage response to river impoundment varies with degree of hydrologic alteration

Abstract River impoundments can fundamentally restructure downstream fish assemblages by altering flow regimes. However, the degree of alteration and associated ecological change may depend on pre-existing hydrologic regimes. We used long-term datasets to compare downstream hydrological and fish assemblage responses to impoundment in two catchments classified as having intermittent and perennial-flashy natural hydrologic regimes. We observed significant shifts in fish assemblage structure at both sites after stream impoundment. The historically intermittent stream shifted to a stable perennial flow regime. Changes in fish assemblage structure covaried with changes in five different components of the flow regime; most species that increased in abundance require fluvial habitats and likely benefited from increased flows during historically low flow seasons. Shifts in fish assemblage structure were also observed in the perennial stream, despite minimal flow alteration after impoundment; however, most species shifts were associated with lentic environments, and were more likely related to proximity of reservoirs in the drainage system rather than changes in stream flow. Findings from this study confirm that downstream fish assemblage response to river impoundment can be associated with high levels of hydrologic alteration, but other factors including expansion of lentic species into lotic environments also influence shifts in assemblage structure.

Habitat and nutrient enrichment affect decomposition of maize and willow oak detritus in Lower Mississippi River Basin bayous

Few investigators have studied leaf breakdown in bayous or oxbow lakes, dominant aquatic features in fertile floodplains that potentially influence storage and processing of detritus in agricultural landscapes. We compared decomposition rates of maize (Zea mays) and willow oak (Quercus phellos) from 3 bayous in the Lower Mississippi River Basin of northwestern Mississippi and in bayou mesocosms enriched to agriculturally relevant concentrations of N, P, and N+P. We hypothesized that maize would break down faster than willow oak and that bayou nutrient and habitat factors would influence decomposition for each species. Breakdown rates were significantly higher for maize (k = 0.0229–0.0386/d) than for willow oak (0.0026–0.0240/d). Decomposition was faster in lentic than lotic habitats for willow oak but not maize. In mesocosms, breakdown rates were higher for maize than willow oak, and decomposition of both species increased significantly in mesocosms enriched with N or N+P. Maize leaf C∶N and C∶P ratios were lower than willow oak, decreased with time, and did not differ between lotic and lentic habitats in the field study. Maize and willow oak leaf nutrient-ratio responses to enrichment in mesocosms varied with time. By the end of the experiment, C∶N ratios were lower in mesocosms enriched with N or N+P, and C∶P ratios were lower in mesocosms enriched with N+P for both litter species. Microbial respiration rates for maize were double that of willow oak, and enrichment effects varied with time, but N+P enrichment increased respiration for both species by the end of the experiment. Differences in breakdown between crop and native organic-matter sources combined with nutrient enrichment may affect water bodies in highly modified aquatic agroecosystems by increasing organic-matter breakdown rates and overall microbial respiration.