Stream Biofilms Research Articles

Disentangling abiotic and biotic effects of treated wastewater on stream biofilm resistomes enables the discovery of a new planctomycete beta-lactamase

BackgroundEnvironmental reservoirs of antibiotic resistance pose a threat to human and animal health. Aquatic biofilms impacted by wastewater effluent (WW) are known environmental reservoirs for antibiotic resistance; however, the relative importance of biotic factors and abiotic factors from WW on the abundance of antibiotic resistance genes (ARGs) within aquatic biofilms remains unclear. Additionally, experimental evidence is limited within complex aquatic microbial communities as to whether genes bearing low sequence similarity to validated reference ARGs are functional as ARGs.ResultsTo disentangle the effects of abiotic and biotic factors on ARG abundances, natural biofilms were previously grown in flume systems with different proportions of stream water and either ultrafiltered or non-ultrafiltered WW. In this study, we conducted deep shotgun metagenomic sequencing of 75 biofilm, stream, and WW samples from these flume systems and compared the taxonomic and functional microbiome and resistome composition. Statistical analysis revealed an alignment of the resistome and microbiome composition and a significant association with experimental treatment. Several ARG classes exhibited an increase in normalized metagenomic abundances in biofilms grown with increasing percentages of non-ultrafiltered WW. In contrast, sulfonamide and extended-spectrum beta-lactamase ARGs showed greater abundances in biofilms grown in ultrafiltered WW compared to non-ultrafiltered WW. Overall, our results pointed toward the dominance of biotic factors over abiotic factors in determining ARG abundances in WW-impacted stream biofilms and suggested gene family-specific mechanisms for ARGs that exhibited divergent abundance patterns. To investigate one of these specific ARG families experimentally, we biochemically characterized a new beta-lactamase from the Planctomycetota (Phycisphaeraceae). This beta-lactamase displayed activity in the cleavage of cephalosporin analog despite sharing a low sequence identity with known ARGs.ConclusionsThis discovery of a functional planctomycete beta-lactamase ARG is noteworthy, not only because it was the first beta-lactamase to be biochemically characterized from this phylum, but also because it was not detected by standard homology-based ARG tools. In summary, this study conducted a metagenomic analysis of the relative importance of biotic and abiotic factors in the context of WW discharge and their impact on both known and new ARGs in aquatic biofilms.DkJa1T-YWptaXXeH_hsDtBVideo

Exploring freshwater stream bacterial communities as indicators of land use intensity

BackgroundStream ecosystems comprise complex interactions among biological communities and their physicochemical surroundings, contributing to their overall ecological health. Despite this, many monitoring programs ignore changes in the bacterial communities that are the base of food webs in streams, often focusing on stream physicochemical assessments or macroinvertebrate community diversity instead. We used 16S rRNA gene sequencing to assess bacterial community compositions within 600 New Zealand stream biofilm samples from 204 sites within a 6-week period (February–March 2010). Sites were either dominated by indigenous forests, exotic plantation forests, horticulture, or pastoral grasslands in the upstream catchment. We sought to predict each site’s catchment land use and environmental conditions based on the composition of the stream bacterial communities.ResultsRandom forest modelling allowed us to use bacterial community composition to predict upstream catchment land use with 65% accuracy; urban sites were correctly assigned 90% of the time. Despite the variation inherent when sampling across a ~ 1000-km distance, bacterial community data could correctly differentiate undisturbed sites, grouped by their dominant environmental properties, with 75% accuracy. The positive correlations between actual values and those predicted by the models built using the stream biofilm bacterial data ranged from weak (average log N concentration in the stream water, R2 = 0.02) to strong (annual mean air temperature, R2 = 0.69).ConclusionsFreshwater bacterial community data provide useful insights into land use impacts on stream ecosystems; they may be used as an additional measure to screen stream catchment attributes.

Substrate promiscuity of xenobiotic-transforming hydrolases from stream biofilms impacted by treated wastewater

Organic contaminants enter aquatic ecosystems from various sources, including wastewater treatment plant effluent. Freshwater biofilms play a major role in the removal of organic contaminants from receiving water bodies, but knowledge of the molecular mechanisms driving contaminant biotransformations in complex stream biofilm (periphyton) communities remains limited. Previously, we demonstrated that biofilms in experimental flume systems grown at higher ratios of treated wastewater (WW) to stream water displayed an increased biotransformation potential for a number of organic contaminants. We identified a positive correlation between WW percentage and biofilm biotransformation rates for the widely-used insect repellent, N,N-diethyl-meta-toluamide (DEET) and a number of other wastewater-borne contaminants with hydrolyzable moieties. Here, we conducted deep shotgun sequencing of flume biofilms and identified a positive correlation between WW percentage and metagenomic read abundances of DEET hydrolase (DH) homologs. To test the causality of this association, we constructed a targeted metagenomic library of DH homologs from flume biofilms. We screened our complete metagenomic library for activity with four different substrates, including DEET, and a subset thereof with 183 WW-related organic compounds. The majority of active hydrolases in the metagenomic library preferred aliphatic and aromatic ester substrates while, remarkably, only a single reference enzyme was capable of DEET hydrolysis. Of the 626 total enzyme-substrate combinations tested, approximately 5% were active enzyme-substrate pairs. Metagenomic DH family homologs revealed a broad substrate promiscuity spanning 22 different compounds when summed across all enzymes tested. We biochemically characterized the most promiscuous and active enzymes identified based on metagenomic analysis from uncultivated Rhodospirillaceae and Planctomycetaceae. In addition to characterizing new DH family enzymes, we exemplified a framework for linking metagenome-guided hypothesis generation with experimental validation. Overall, this study expands the scope of known enzymatic contaminant biotransformations for metagenomic hydrolases from WW-receiving stream biofilm communities.

Biotic and abiotic factors interplay in structuring the dynamics of microbial co-occurrence patterns in tropical mountainsides

Ecological interactions are important for maintaining biodiversity and ecosystem functions. Particularly in stream biofilms, little is known about the distributional patterns of different taxonomic groups and their potential interactions along elevational gradients. Here, we investigated the bacterial and fungal community structures of stream biofilms across elevational gradients on Mount Kilimanjaro, and explored patterns of their distribution, diversity, community structures, and taxa co-occurrence. We found that fungal and bacterial richness were more convergent at higher elevations, while their community structures became significantly more divergent. Inferred network complexity and stability significantly decreased with increasing elevation for fungi, while an opposite trend was observed for bacteria. Further quantitative analyses showed that network structures of bacteria and fungi were more divergent as elevation increased. This pattern was strongly associated with shifts in abiotic factors, such as mean annual temperatures, water PO43--P, and stream width. By constructing bipartite networks, we showed the fungal-bacterial network to be less redundant, more clustering, and unstable with increasing elevation. Abiotic factors (e.g., temperatures and stream width) and microbial community properties (i.e., structure and composition) significantly explained the dynamic changes in fungal-bacterial network properties. Taken together, this study provides evidence for the interplay of biotic and abiotic factors structuring potential microbial interactions in stream biofilms along a mountainside elevational gradient.

The role of phages for microdiverse bacterial communities in proglacial stream biofilms

Viruses modulate the diversity and activity of microbial communities. However, little is known about their role for the structure of stream bacterial biofilm communities. Here, we present insights into the diversity and composition of viral communities in various streams draining three proglacial floodplains in Switzerland. Proglacial streams are characterized by extreme environmental conditions, including near-freezing temperatures and ultra-oligotrophy. These conditions select for few but well-adapted bacterial clades, which dominate biofilm communities and occupy niches via microdiversification. We used metagenomic sequencing to reveal a diverse biofilm viral assemblage in these streams. Across the different floodplains and streams, viral community composition was tightly coupled to that of the bacterial hosts, which was underscored by generally high host specificity. Combining predictions of phage-host interactions with auxiliary metabolic genes (AMGs), we identify specific AMGs shared by phages infecting microdiverse clade members. Our work provides a step towards a better understanding of the complex interactions among bacteria and phages in stream biofilm communities in general and streams influenced by glacier meltwaters and characterized by microdiversity in particular.

Cross-domain interactions confer stability to benthic biofilms in proglacial streams

Cross-domain interactions are an integral part of the success of biofilms in natural environments but remain poorly understood. Here, we describe cross-domain interactions in stream biofilms draining proglacial floodplains in the Swiss Alps. These streams, as a consequence of the retreat of glaciers, are characterised by multiple environmental gradients and perturbations (e.g., changes in channel geomorphology, discharge) that depend on the time since deglaciation. We evaluate co-occurrence of bacteria and eukaryotic communities along streams and show that key community members have disproportionate effects on the stability of community networks. The topology of the networks, here quantified as the arrangement of the constituent nodes formed by specific taxa, was independent of stream type and their apparent environmental stability. However, network stability against fragmentation was higher in the streams draining proglacial terrain that was more recently deglaciated. We find that bacteria, eukaryotic photoautotrophs, and fungi are central to the stability of these networks, which fragment upon the removal of both pro- and eukaryotic taxa. Key taxa are not always abundant, suggesting an underlying functional component to their contributions. Thus, we show that there is a key role played by individual taxa in determining microbial community stability of glacier-fed streams.

Climate and Environmental Variables Drive Stream Biofilm Bacterial and Fungal Diversity on Tropical Mountainsides.

High mountain freshwater systems are particularly sensitive to the impacts of global warming and relevant environmental changes. Microorganisms contribute substantially to biogeochemical processes, yet their distribution patterns and driving mechanism in alpine streams remain understudied. Here, we examined the bacterial and fungal community compositions in stream biofilm along the elevational gradient of 745-1874 m on Mt. Kilimanjaro and explored their alpha and beta diversity patterns and the underlying environmental drivers. We found that the species richness and evenness monotonically increased towards higher elevations for bacteria, while were non-significant for fungi. However, both bacterial and fungal communities showed consistent elevational distance-decay relationships, i.e., the dissimilarity of assemblage composition increased with greater elevational differences. Bacterial alpha diversity patterns were mainly affected by chemical variables such as total nitrogen and phosphorus, while fungi were affectedby physical variables such as riparian shading and stream width. Notably, climatic variables such as mean annual temperature strongly affected the elevational succession of bacterial and fungal community compositions. Our study is the first exploration of microbial biodiversity and their underlying driving mechanisms for stream ecosystems in tropical alpine regions. Our findings provide insights on the response patterns of tropical aquatic microbial community composition and diversity under climate change.

Temporal patterns of stream biofilm in a mountain catchment: one-year monthly samplings across streams of the Orobic Alps (Northern Italy)

Periphyton is the dominant primary producer in mountain streams and sustains the higher trophic levels. While certain periphyton groups, particularly diatoms, have received extensive study, the comprehensive characterization of the entire community has been largely neglected. This study aims to investigate the temporal pattern of biofilm in mountain streams characterized by different water thermal regimes. A one-year quantitative campaign, involving monthly samplings, was conducted in five subalpine streams in Northern Italy’s Orobic Alps to collect epilithic biofilm from a wide surface area. The total biomass was quantified and the periphyton was analyzed both for composition (diatoms, green algae, cyanobacteria, and red algae) and for pigments. Disturbance, water temperature, physico-chemical conditions, nutrients, substrate diversity, and light availability were assessed concurrently with biofilm samplings. Results show sharp biofilm variations over months. In all sites, the disturbance was the primary factor reducing biomass and pigment content. Annually, all sites experienced similar turnover in periphyton composition mainly associated with light and water temperature. Overall, the study indicates that frequent quantitative investigations of biofilm help understand intra-annual variations and identify key drivers. Such information is useful to understand the ecosystem processes and the food web dynamics.

Unexpected functional diversity of stream biofilms within and across proglacial floodplains despite close spatial proximity

AbstractHigh‐mountain streams are particularly vulnerable to climate change because they intimately interface with the cryosphere. As glaciers shrink and snowpack diminishes, proglacial streams will eventually shift from being glacier‐fed to streams fed by groundwater, snowmelt and precipitation. This shift will affect both the flow regime and physico‐chemical characteristics of streams, possibly also the structure and function of their benthic microbiome. Here, we applied genome‐resolved metagenomics to benthic biofilms from glacier‐fed streams and their groundwater‐fed tributaries within three proglacial floodplains in the Swiss Alps. Despite the close spatial proximity between both stream types, their microbiome structure differed consistently for both prokaryotes and eukaryotes. The glacier‐fed stream microbiome was taxonomically and functionally less diverse than its groundwater‐fed counterpart, and had smaller genomes, but with conservation of the central metabolic functions primarily related to nitrogen and sulfur cycling. Consequently, much higher functional variability was associated with the microbiome of tributary streams, mainly due to abundant genes involved in the transport and degradation of organic matter. Our findings suggest that glacier shrinkage will cause shifts in the functioning of benthic microbiomes of proglacial floodplains with yet unknown consequences for downstream biogeochemistry.

Nitrogen retention in stream biofilms – A potential contribution to the self-cleaning capacity

Surface waters are under increasing pressure due to human activities, such as nutrient emissions from wastewater treatment plants (WWTPs). Using the retention of nitrogen (N) released from WWTPs as a proxy, we assessed the contribution of biofilms grown on inorganic and organic substrates to the self-cleaning capacity of second-order streams within the biosphere reserve Vosges du Nord/Palatinate forest (France/Germany). The uptake of N from anthropogenic sources, which is enriched with the heavy isotope 15N, into biofilms was assessed up- and downstream of WWTPs after five weeks of substrate deployment. Biofilms at downstream sites showed a significant positive linear relationship between δ15N and the relative contribution of wastewater to the streams' discharge. Furthermore, δ15N substantially increased in areas affected by WWTP effluent (∼8.5‰ and ∼7‰ for inorganic and organic substrate-associated biofilms, respectively) and afterwards declined with increasing distance to the WWTP effluent, approaching levels of upstream sections. The present study highlights that biofilms contribute to nutrient retention and likely the self-cleaning capacity of streams. This function seems, however, to be limited by the fact that biofilms are restricted in their capacity to process excessive N loads with large differences between individual reaches (e.g., δ15N: −3.25 to 12.81‰), influenced by surrounding conditions (e.g., land use) and modulated through climatic factors and thus impacted by climate change. Consequently, the impact of WWTPs located close to the source of a stream are dampened by the biofilms’ capacity to retain N only to a minor share and suggest substantial N loads being transported downstream.

Trace metal–macronutrient colimitation of algal biofilms in streams with differing ambient inorganic nutrients

Trace metal–macronutrient colimitation of algal biofilms in streams with differing ambient inorganic nutrients

Dissipation of pesticides by stream biofilms is influenced by hydrological histories.

To evaluate the effects of hydrological variability on pesticide dissipation capacity by stream biofilms, we conducted a microcosm study. We exposed biofilms to short and frequent droughts (daily frequency), long and less frequent droughts (weekly frequency) and permanently immersed controls, prior to test their capacities to dissipate a cocktail of pesticides composed of tebuconazole, terbuthylazine, imidacloprid, glyphosate and its metabolite aminomethylphosphonic acid. A range of structural and functional descriptors of biofilms (algal and bacterial biomass, extracellular polymeric matrix (EPS) concentration, microbial respiration, phosphorus uptake and community-level physiological profiles) were measured to assess drought effects. In addition, various parameters were measured to characterise the dynamics of pesticide dissipation by biofilms in the different hydrological treatments (% dissipation, peak asymmetry, bioconcentration factor, among others). Results showed higher pesticide dissipation rates in biofilms exposed to short and frequent droughts, despite of their lower biomass and EPS concentration, compared to biofilms in immersed controls or exposed to long and less frequent droughts. High accumulation of hydrophobic pesticides (tebuconazole and terbuthylazine) was measured in biofilms despite the short exposure time (few minutes) in our open-flow microcosm approach. This research demonstrated the stream biofilms capacity to adsorb hydrophobic pesticides even in stressed drought environments.

Spatial and temporal patterns of stream nutrient limitation in an Arctic catchment

Arctic stream biofilm responses to ongoing climate-related changes in physical and chemical conditions have major implications for stream food webs and biogeochemical cycles. Yet, such effects have rarely been studied outside summer months or at sub-catchment scales in the Arctic. We used deployments of nutrient diffusing substrates (NDS) to assess the spatial (20 deployments) and seasonal patterns (10 deployments) and physical and chemical drivers of nutrient limitation within an Arctic stream catchment. Results show that nutrient limitation of autotrophic processes was common during summer, but that light inhibited biomass accrual under the ice in winter. Alongside single N, P and C responses, co-limitation dominated the overall pattern of limitation over time and across the catchment. However, the primary limiting nutrient to autotrophs changed from N to P in parts of the catchment with higher N concentrations. As Arctic studies are often conducted at individual sites during summer, these may miss shifts in the drivers of stream productivity that arise from variable nutrient, temperature, and light regimes. Our results caution against focusing on one single most important limiting nutrient, as we found that this can shift seasonally and over small spatial scales in this Arctic catchment.

Cross-domain interactions induce community stability to benthic biofilms in proglacial streams

Cross-domain interactions induce community stability to benthic biofilms in proglacial streams

Effects of combining flow intermittency and exposure to emerging contaminants on the composition and metabolic response of streambed biofilm bacterial communities

Freshwater ecosystems are characterised by the co-occurrence of stressors that simultaneously affect the biota. Among these, flow intermittency and chemical pollution severely impair the diversity and functioning of streambed bacterial communities. Using an artificial streams mesocosm facility, this study examined how desiccation and pollution caused by emerging contaminants affect the composition of stream biofilm bacterial communities, their metabolic profiles, and interactions with their environment. Through an integrative analysis of the composition of biofilm communities, characterization of their metabolome and composition of the dissolved organic matter, we found strong genotype-to-phenotype interconnections. The strongest correlation was found between the composition and metabolism of the bacterial community, both of which were influenced by incubation time and desiccation. Unexpectedly, no effect of the emerging contaminants was observed, which was due to the low concentration of the emerging contaminants and the dominant impact of desiccation. However, biofilm bacterial communities modified the chemical composition of their environment under the effect of pollution. Considering the tentatively identified classes of metabolites, we hypothesised that the biofilm response to desiccation was mainly intracellular while the response to chemical pollution was extracellular. The present study demonstrates that metabolite and dissolved organic matter profiling may be effectively integrated with compositional analysis of stream biofilm communities to yield a more complete picture of changes in response to stressors.

Effects of global change on the ability of stream biofilm to dissipate the herbicide glyphosate

The herbicide glyphosate is contaminating a large number of freshwater ecosystems worldwide and its fate and effects remains uncertain in light of the effects of global change. The present study examines how variations in water temperature and light availability relative to global change affect the ability of stream biofilms to degrade the herbicide glyphosate. Biofilms were exposed in microcosms to two levels of water temperature simulating global warming (Ambient = 19–22 °C and Warm = 21–24 °C) and three levels of light representative of riparian habitat destruction due to land use change (Dark = 0, Intermediate = 600, High = 1200 μmol photons m−2 s−1). Biofilms were acclimated to six different experimental treatments, namely i) ambient temperature without light (AMB_D), ii) ambient temperature and intermediate light (AMB_IL), iii) ambient temperature and high light (AMB_HL), iv) warm temperature without light (WARM_D), v) warm temperature and intermediate light (WARM_IL) and vi) warm temperature and high light (WARM_HL). The ability of biofilms to degrade 50 μg L−1 of glyphosate was tested. Results showed that water temperature increase, but not light availability increase, significantly increased aminomethyl phosphonic acid (AMPA) production by biofilms. However, the combined increase of temperature and light generated the shortest time to dissipate half of the glyphosate supplied and/or half of the maximum AMPA produced (6.4 and 5.4 days, respectively) by biofilms. Despite light had a major effect in modulating biofilm structural and functional descriptors, the response of certain descriptors (i. e. chlorophyll-a concentration, bacterial density and diversity, nutrient content and PHO activity) to light availability increase depended on water temperature. Specifically, the biofilms in the WARM_HL treatment displayed the highest Glucosidase: Peptidase and Glucosidase: Phosphatase enzyme activity ratios and the lowest biomass C: N molar ratios compared to the other treatments. According to these results, warmer temperatures and high light availability could have been exacerbating the decomposition of organic C compounds in biofilms, including the use of glyphosate as a C source for microbial heterotrophs. This study shows that ecoenzymatic stoichiometry and xenobiotic biodegradation approaches can be combined to better understand the functioning of biofilms in pesticide-polluted streams.

Homogeneous Environmental Selection Structures the Bacterial Communities of Benthic Biofilms in Proglacial Floodplain Streams.

In proglacial floodplains, glacier recession promotes biogeochemical and ecological gradients across relatively small spatial scales. The resulting environmental heterogeneity induces remarkable microbial biodiversity among proglacial stream biofilms. Yet the relative importance of environmental constraints in forming biofilm communities remains largely unknown. Extreme environmental conditions in proglacial streams may lead to the homogenizing selection of biofilm-forming microorganisms. However, environmental differences between proglacial streams may impose different selective forces, resulting in nested, spatially structured assembly processes. Here, we investigated bacterial community assembly processes by unraveling ecologically successful phylogenetic clades in two stream types (glacier-fed mainstems and non-glacier-fed tributaries) draining three proglacial floodplains in the Swiss Alps. Clades with low phylogenetic turnover rates were present in all stream types, including Gammaproteobacteria and Alphaproteobacteria, while the other clades were specific to one stream type. These clades constituted up to 34.8% and 31.1% of the community diversity and up to 61.3% and 50.9% of the relative abundances in mainstems and tributaries, respectively, highlighting their importance and success in these communities. Furthermore, the proportion of bacteria under homogeneous selection was inversely related to the abundance of photoautotrophs, and these clades may therefore decrease in abundance with the future "greening" of proglacial habitats. Finally, we found little effect of physical distance from the glacier on clades under selection in glacier-fed streams, probably due to the high hydrological connectivity of our study reaches. Overall, these findings shed new light on the mechanisms of microbial biofilm assembly in proglacial streams and help us to predict their future in a rapidly changing environment. IMPORTANCE Streams draining proglacial floodplains harbor benthic biofilms comprised of diverse microbial communities. These high-mountain ecosystems are rapidly changing with climate warming, and it is therefore critical to better understand the mechanisms underlying the assembly of their microbial communities. We found that homogeneous selection dominates the structuring of bacterial communities in benthic biofilms in both glacier-fed mainstems and nonglacier tributary streams within three proglacial floodplains in the Swiss Alps. However, differences between glacier-fed and tributary ecosystems may impose differential selective forces. Here, we uncovered nested, spatially structured assembly processes for proglacial floodplain communities. Our analyses additionally provided insights into linkages between aquatic photoautotrophs and the bacterial taxa under homogeneous selection, potentially by providing a labile source of carbon in these otherwise carbon-deprived systems. In the future, we expect a shift in the bacterial communities under homogeneous selection in glacier-fed streams as primary production becomes more important and streams become "greener".

Resource-dependent foraging behaviour of grazers enhances effects of nutrient enrichment on algal biomass

Both the quantity and nutritional quality of food resources can strongly influence the foraging movements of herbivores, which in turn determine the strength of top-down control on primary producer biomass. Nutrient enrichment can alter the biomass and nutritional quality of primary producers, but the consequences for the foraging of herbivores and hence for top-down control are still poorly understood. In this study, we combined a two-factorial experiment (two nutrient levels × grazing by the freshwater gastropod Ancylus fluviatilis) with video analyses tracking grazers’ movements to investigate nutrient enrichment effects on spatial ranges of grazing activity and algal biomass removal. Natural stream biofilms were grown in phosphorus-enriched (P+) and phosphorus-poor flumes (P−) for two weeks before A. fluviatilis were added to the flumes and allowed to graze on biofilm for an additional 2 weeks. Total periphyton biomass was enhanced by P+ and reduced by grazer presence. However, the total grazer effect depended on the nutrient level: at the end of the experiment, on average 95% of algal cover were removed by grazing in the P− flumes versus 26% in the P+ flumes. Fast movements of A. fluviatilis were detected significantly more often in the P− treatment, whereas grazers were detected resting more often in the P+ treatment. Our results demonstrate that nutrient enrichment can increase primary producer biomass both directly and indirectly by limiting the foraging ranges of herbivores. The resulting feedback loop between reduced grazing activity and increased plant biomass might in turn exacerbate eutrophication effects on habitat structure.

Short‐term effects of a large dam decommissioning on biofilm structure and functioning

Aging dams and the rising efforts to restore stream ecosystems are increasing the number of dam decommissioning programs. Although dam decommissioning aims at improving in‐stream habitat, biodiversity, and ecosystem functioning in the long term, it might also cause ecological impacts in the short term due to the mobilization of the sediment accumulated in the reservoir. Benthic biofilm in particular can be impaired by episodes of high turbidity and scouring. We conducted a multiple before‐after/control‐impact experiment to assess the effects of the drawdown of a large dam (42 m tall), a first step to its decommissioning, on biofilm structure (biomass and chlorophyll‐a) and functioning (metabolism, nutrient uptake, and organic matter breakdown). Our results show that the reservoir drawdown reduced the autotrophic biofilm biomass (chlorophyll‐a) downstream from the dam, which in turn lowered metabolism. However, nitrogen and phosphorus uptake by the biofilm was not affected. Organic matter breakdown was slower below the dam than in nearby undammed reaches before and during drawdown. All drawdown effects quickly disappeared and reaches downstream from the dam approached values found in nearby undammed reaches. Thus, our results indicate that the effects of reservoir drawdown on stream biofilms exist but may be small and disappear rapidly.

Resource limitation of autotrophs and heterotrophs in boreal forest headwater streams

Autotrophic and heterotrophic microbes in stream biofilms dominate biogeochemical cycling and rely on nutrient and energy resources for growth and productivity. In the boreal forest, variation in these resources can originate from permafrost distribution and controls competition for nutrients between stream autotrophs and heterotrophs. We investigated which resources control nutrient uptake and metabolism in headwater stream biofilms of subarctic Alaska, USA, and how resource availability affects competition for inorganic nutrients. We hypothesized that the competitive outcome between autotrophs and heterotrophs for inorganic nutrients would be dependent on availability of organic C, or inorganic nutrients (N and P). To test our hypotheses, we measured resource limitation at the patch and reach scales along a permafrost gradient in interior Alaska. At the patch scale, nutrient diffusing substrata revealed that, secondary to light, N and P were colimiting to autotrophic growth, whereas C was primarily limiting to heterotrophic respiration. In the presence of labile C, heterotrophs exhibited a larger response to nutrient enrichment and outcompeted autotrophs for inorganic nutrients. At the reach scale, light availability had the largest influence on nutrient uptake, but inorganic nutrients were also important. The positive response to increased nutrient and C availability at the patch scale suggests that the predicted increase in exports into fluvial networks with permafrost degradation will alter biofilm structure and function. Ultimately, biofilm communities will shift to more heterotroph-dominated patches if heterotrophs outcompete autotrophs for inorganic nutrients. As permafrost thaws and nutrients and organic C mobilize into streams, nutrient uptake dynamics and competition within biofilms will be altered, affecting nutrient use and export.