Marc Peipoch

Resistance and recovery of river biofilms receiving short pulses of Triclosan and Diuron

The effects of the herbicide Diuron (DIU) and the bactericide Triclosan (TCS) were assessed on laboratory-grown stream biofilms. Four week-old biofilms were exposed in mesocosms to 48-hours of short pulses of either DIU or TCS. The direct and indirect effects of each toxicant on the biofilms, and the subsequent recovery of the biofilms, were evaluated according to structural and functional biomarkers. These parameters were analyzed immediately before exposure, immediately after exposure, and 9 and 16days post-exposure. DIU caused an increase in diatom mortality (+79%), which persisted until the end of the experiment. TCS also affected diatom mortality (+41%), although the effect did not appear until 1week post-exposure. TCS caused an increase in bacterial mortality (+45%); however, this parameter returned to normal values 1week post-exposure. TCS compromised the cellular integrity of the green alga Spirogyra sp., whereas DIU did not. TCS also strongly inhibited phosphate uptake (-71%), which did not return to normal values until 2weeks post-exposure. DIU directly affected algae, but barely affected the heterotrophs, whereas TCS seriously impaired bacteria (direct effect) as well as autotrophs (indirect effect). However, the biofilms recovered their normal structure and function within only a few days to a few weeks. These findings demonstrate the capacity of biofilms to cope with periodic inputs of toxicants, but also the risks associated to repeated exposure or multi-contamination in aquatic ecosystems.

Influence of nitrate and ammonium availability on uptake kinetics of stream biofilms

Abstract. Human activity has significantly increased dissolved inorganic N (DIN) availability and has modified the relative proportion of NO3− and NH4+ species in many streams. Understanding the relationship between DIN concentration and DIN uptake is crucial to predicting how streams will respond to increased DIN loading. Nonetheless, this relationship remains unclear because of the complex interactions governing DIN uptake. We aimed to evaluate how biofilms from 2 streams differing in background DIN concentration would respond to increases in availability and changes in speciation (NO3− or NH4+) of DIN. We measured DIN uptake by biofilms in artificial flumes in each stream, using separate 15N-NO3− and 15N-NH4+ additions in a graded series of increasing DIN concentrations. The ambient uptake rate (U) was higher for NO3− than for NH4+ in both streams, but only U for NH4+ differed between streams. Uptake efficiency (UN-specific) at ambient conditions was higher in the low-N than in the high-N stream for both DIN species. A Michaelis–Menten model of uptake kinetics best fit the relationship between uptake and concentration in the case of NH4+ (for both streams) but not in the case of NO3− (neither stream). Moreover, saturation of NH4+ uptake occurred at lower rates (lower Umax) in the low-N than in the high-N stream, but affinity for NH4+ was higher (lower Ks) in the low-N stream. Together, these results indicate that the response capacity of biofilm communities to short-term increases of DIN concentration is determined primarily by the ambient DIN concentrations under which they develop. Our study also shows that DIN uptake by benthic biofilms varies with DIN availability and with DIN speciation, which often is modified by human activities.

Variability in δ15N natural abundance of basal resources in fluvial ecosystems: a meta-analysis

Abstract Variation in N stable isotope (&dgr;15N) signatures of basal resources can influence interpretation of trophic relationships in ecosystems, and significant variation in &dgr;15N signatures has been reported in streams and rivers. However, a comprehensive understanding of the main factors driving &dgr;15N variability is lacking, and this variability confounds the consumers trophic-level position during &dgr;15N analysis. We conducted a meta-analysis to examine the variability in &dgr;15N natural abundance of basal resources and dissolved inorganic N (DIN) in streams and rivers in relation to the environmental factors that may drive this variability. The meta-analysis was based on a literature review over the last 20 y (1989–2009) and contained signatures of &dgr;15N-DIN (&dgr;15N-NO3 and &dgr;15N-NH4) and &dgr;15N-basal resources (&dgr;15N-detrital compartments, &dgr;15N-biofilm, &dgr;15N-algae, and &dgr;15N-macrophytes) from >100 rivers or streams. Signatures of &dgr;15N-DIN varied widely (−8.4–19.4‰), and we found fewer values for &dgr;15N-NH4 than &dgr;15N-NO3, even though NH4+ is assimilated rapidly by basal resources. The range of &dgr;15N-basal resources was also broad (−4–16‰) within and among compartments. Human land use was the most significant factor explaining variability in &dgr;15N-DIN and &dgr;15N-basal resource signatures. We found significant differences between &dgr;15N signatures of photoautotrophic (i.e., autochthonous) and detrital (i.e., allochthonous) basal resources. Our results point out the difficulty in defining a baseline &dgr;15N signature of the food web, and provide a basis to explain confounding results in studies using &dgr;15N analysis to identify trophic linkages in fluvial food webs.

Nitrogen Stable Isotopes in Primary Uptake Compartments Across Streams Differing in Nutrient Availability

High variability in the natural abundance of nitrogen stable isotopes (δ(15)N) has been reported for primary uptake compartments (PUCs; e.g., epilithon, filamentous algae, bryophytes, macrophytes) in human-impacted aquatic ecosystems, but the origin of this variability is not yet well understood. We examined how δ(15)N of different PUC types relate to δ(15)N of dissolved inorganic nitrogen (DIN) species (nitrate and ammonium) and to the stream nutrient concentrations in which they grow. We selected 25 reaches located across the fluvial network of La Tordera catchment (NE Spain, 868.5 km(2)), encompassing a gradient of human pressures from headwaters to the river valley. δ(15)N-PUC variability was mostly explained by location within the fluvial network and was strongly related to the δ(15)N of DIN species, especially of ammonium. Models were stronger for PUCs growing within the stream channel and thus using streamwater as their main source of nutrients. Regression models including nutrient concentrations improved the prediction power for δ(15)N-PUCs, suggesting that nutrient concentrations and stoichiometry cannot be ignored in explaining the natural abundance of nitrogen isotopes in PUCs. These results provide insights into what controls variability in δ(15)N of PUCs within a stream network, with implications for the application of stables isotopes as an ecological tool.

Temporal Variability of Nitrogen Stable Isotopes in Primary Uptake Compartments in Four Streams Differing in Human Impacts

Understanding the variability of the natural abundance in nitrogen stable isotopes (expressed as δ(15)N) of primary uptake compartments (PUCs; e.g., epilithon or macrophytes) is important due to the multiple applications of stable isotopes in freshwater research and can give insights into environmental and anthropogenic factors controlling N dynamics in streams. While previous research has shown how δ(15)N of PUCs varies with δ(15)N of dissolved inorganic N (DIN) among streams, less is known about how δ(15)N of PUCs varies over time. Here, we examined monthly variation of δ(15)N of PUCs and of DIN species (nitrate and ammonium) over a year, and compared it among streams with contrasting human impacts and PUC types. Our results showed no evidence of isotopic seasonal patterns. Temporal variability in δ(15)N-PUCs increased with human impact, being the highest in the urban stream, probably influenced by the high variability of δ(15)N-DIN. Among compartments, in-stream PUCs characterized by fast turnover rates, such as filamentous algae, showed the highest temporal variability in δ(15)N values (from -3.6 to 23.2 ‰). Our study elucidates some of the environmental and biological controls of temporal variability of δ(15)N in streams, which should be taken into account when using stable isotopes as an ecological tool.

Small-scale heterogeneity of microbial N uptake in streams and its implications at the ecosystem level.

Large-scale factors associated with the environmental context of streams can explain a notable amount of variability in patterns of stream N cycling at the reach scale. However, when environmental factors fail to accurately predict stream responses at the reach level, focusing on emergent properties from small-scale heterogeneity in N cycling rates may help understand observed patterns in stream N cycling. To address how small-scale heterogeneity may contribute to shape patterns in whole-reach N uptake, we examined the drivers and variation in microbial N uptake at small spatial scales in two stream reaches with different environmental constraints (i.e., riparian canopy). Our experimental design was based on two ¹⁵N additions combined with a hierarchical sampling design from reach to microhabitat scales. Regardless of the degree of canopy cover, small-scale heterogeneity of microbial N uptake ranged by three orders of magnitude, and was characterized by a low abundance of highly active microhabitats (i.e., hot spots). The presence of those hot spots of N uptake resulted in a nonlinear spatial distribution of microbial N uptake rates within the streambed, especially in the case of epilithon assemblages. Small-scale heterogeneity in N uptake and turnover rates at the microhabitat scale was primarily driven by power relationships between N cycling rates and stream water velocity. Overall, fine benthic organic matter (FBOM) assemblages responded clearly to changes in the degree of canopy cover, overwhelming small-scale heterogeneity in its N uptake rates, and suggesting that FBOM contribution to whole-reach N uptake was principally imposed by environmental constraints from larger scales. In contrast, N uptake rates by epilithon showed no significant response to different environmental influences, but identical local drivers and spatial variation in each study reach. Therefore, contribution of epilithon assemblages to whole-reach N uptake was mainly associated with emerging properties from small-scale heterogeneity at lower spatial scales.

Contrasts among macrophyte riparian species in their use of stream water nitrate and ammonium: insights from 15N natural abundance

We examined the relevance of dissolved inorganic nitrogen (DIN) forms (nitrate and ammonium) in stream water as N sources for different macrophyte species. To do this, we investigated the variability and relationships between 15N natural abundance of DIN forms and of four different macrophyte species in five different streams influenced by inputs from wastewater treatment plants and over time within one of these streams. Results showed that 15N signatures were similar in species of submersed and amphibious macrophytes and in stream water DIN, whereas 15N signatures of the riparian species were not. 15N signatures of macrophytes were generally closer to 15N signatures of nitrate, regardless of the species considered. Our results showed significant relationships between 15N signatures of DIN and those of submersed Callitriche stagnalis and amphibious Veronica beccabunga and Apium nodiflorum, suggesting stream water DIN as a relevant N source for these two functional groups. Moreover, results from a mixing model suggested that stream water DIN taken up by the submersed and amphibious species was mostly in the form of nitrate. Together, these results suggest different contribution to in-stream N uptake among the spatially-segregated species of macrophytes. While submersed and amphibious species can contribute to in-stream N uptake by assimilation of DIN, macrophyte species located at stream channel edges do not seem to rely on stream water DIN as an N source. Ultimately, these results add a functional dimension to the current use of macrophytes for the restoration of stream channel morphology, indicating that they can also contribute to reduce excess DIN in streams.

Spatial Drivers of Ecosystem Structure and Function in a Floodplain Riverscape: Springbrook Nutrient Dynamics

On riverine flood plains, reorganization by fluvial processes creates and maintains a mosaic of aquatic and riparian landscape elements across a biophysical gradient of disturbance and succession. Across flood plains of gravel-bottom rivers, spring brooks emerge from points of groundwater discharge that may occur in distinct landscape positions. We investigated how ecosystem processes in spring brooks differ spatially across biophysical zones, reflecting how landscape position dictates severity of flood disturbance, allochthonous loading from riparian forests, and inputs from groundwater systems. Between July and October 2011, we quantified aspects of ecosystem structure and function among 6 spring brooks of the Nyack flood plain, Flathead River, Montana. Structural features varied predictably across near-channel (i.e., parafluvial) and late successional (i.e., orthofluvial) biophysical zones. Large wood standing stocks increased >40× (0.19–9.19 kg/m2), dominant particle size class differed by an order of magnitude (median particle size [D50] = 2–27), and measures of vertical hydraulic gradient (–0.06 to +0.12 cm/cm) reflected differences in landscape position. We found fine sediment accumulation, stronger groundwater inputs, and greater benthic and large wood standing stocks in orthofluvial than in parafluvial spring brooks. Algal biomass was negatively correlated with insolation and positively related to vertical hydraulic gradient. Results from microcosm experiments showed increasing N uptake across the gradient from parafluvial to orthofluvial spring brooks. Functional response to landscape-scale organization of springbrook structure underscores the need for a spatially explicit model of floodplain ecology.

Spatial Patterns in Biofilm Diversity across Hierarchical Levels of River-Floodplain Landscapes

River-floodplain systems are among the most diverse and productive ecosystems, but the effects of biophysical complexity at multiple scales on microbial biodiversity have not been studied. Here, we investigated how the hierarchical organization of river systems (i.e., region, floodplain, zone, habitats, and microhabitats) influences epilithic biofilm community assemblage patterns by characterizing microbial communities using 16S rRNA gene sequence data and analyzing bacterial species distribution across local and regional scales. Results indicate that regional and local environmental filters concurrently sort bacterial species, suggesting that spatial configuration of epilithic biofilms resembles patterns of larger organisms in floodplain ecosystems. Along the hierarchical organization of fluvial systems, floodplains constitute a vector of maximum environmental heterogeneity and consequently act as a major landscape filter for biofilm species. Thus, river basins and associated floodplains may simply reflect very large scale ‘patches’ within which environmental conditions select for community composition of epilithic biofilms.