Pharmaceutical and personal care products are ubiquitous in surface waters but their effects on aquatic biofilms and associated ecosystem properties are not well understood. We measured in situ responses of stream biofilms to six common pharmaceutical compounds (caffeine, cimetidine, ciprofloxacin, diphenhydramine, metformin, ranitidine, and a mixture of each) by deploying pharmaceutical-diffusing substrates in streams in Indiana, Maryland, and New York. Results were consistent across seasons and geographic locations. On average, algal biomass was suppressed by 22%, 4%, 22%, and 18% relative to controls by caffeine, ciprofloxacin, diphenhydramine, and the mixed treatment, respectively. Biofilm respiration was significantly suppressed by caffeine (53%), cimetidine (51%), ciprofloxacin (91%), diphenhydramine (63%), and the mixed treatment (40%). In autumn in New York, photosynthesis was also significantly suppressed by diphenhydramine (99%) and the mixed treatment (88%). Pyrosequencing of 16S rRNA genes was used to examine the effects of caffeine and diphenhydramine on biofilm bacterial community composition at the three sites. Relative to the controls, diphenhydramine exposure significantly altered bacterial community composition and resulted in significant relative increases in Pseudomonas sp. and decreases in Flavobacterium sp. in all three streams. These ubiquitous pharmaceuticals, alone or in combination, influenced stream biofilms, which could have consequences for higher trophic levels and important ecosystem processes.

To understand the role biota play in resilience or vulnerability to environmental change, we investigated soil, plant, and microbial responses to a widespread environmental change, increased nitrogen (N). Our aim was to test the plant–soil threshold hypothesis: that changed biotic structure influences resilience to accumulated changes in N. For six years, we removed one of two codominant species, Geum rossii and Deschampsia caespitosa, in moist‐meadow alpine tundra in Colorado, USA. We also manipulated nutrient availability by adding carbon (C) or N, separately and in combination with the species removals. Consistent with our hypothesis, Geum was associated with soil feedbacks that slowed rates of N cycling and Deschampsia with feedbacks that increased rates of N cycling. After a four‐year initial resilience period, Geum dramatically declined (by almost 70%) due to increasing N availability. In contrast, Deschampsia abundance did not respond to changes in N supply; it only responded to the removal of Geum. Forbs and graminoids responded more positively to Deschampsia removal than to Geum removal, indicating stronger competitive effects by Deschampsia. The changed biotic interactions appear to have community‐level consequences: after six years of Geum (but not Deschampsia) removal, evenness of the community declined by over 35%. Increased N affected the soil–microbial feedbacks, particularly in association with Geum. Microbial biomass N declined at higher N, as did the activities of two C‐acquiring and one N‐acquiring extracellular microbial enzymes. In the presence of Geum, N fertilization slowed the activity of phenol oxidase, a tannin‐degrading enzyme, suggesting that microbes shift from degrading Geum‐derived compounds. In the absence of Geum, acid phosphatase activity increased, suggesting increased phosphorus limitation in association with Deschampsia. With continued N deposition forecast for this system, these results suggest that initial resilience of Geum to increased N will be overwhelmed through elimination of microbial feedbacks. Once Geum declines, the loss will indirectly facilitate Deschampsia via competitive release. Because Deschampsia exerts strong competitive effects on subordinate species, increased Deschampsia abundance may be accompanied by a community‐wide drop in diversity. We conclude that plant–soil feedbacks through the microbial community can influence vulnerability to exogenous changes in N and contribute to threshold dynamics.

Abstract. Urban streams are exposed to multiple different stressors on a regular basis, with increased hydrological flashiness representing a common urban stream stressor. Stream metabolism, the coupled ecosystem functions of gross primary production (GPP) and ecosystem respiration (ER), controls numerous other ecosystem functions and integrates multiple processes occurring within streams. We examined the effect of one large (Superstorm Sandy) and multiple small and moderately sized flood events in Baltimore, Maryland, to quantify the response and recovery of urban stream GPP and ER before and after floods of different magnitudes. We also compared GPP and ER before and after Superstorm Sandy to literature values. We found that both GPP and ER decreased dramatically immediately following floods of varying magnitudes, but on average GPP was more reduced than ER (80% and 66% average reduction in GPP and ER, respectively). Both GPP and ER recovered rapidly following floods within 4-18 d, and recovery intervals did not differ significantly between GPP and ER. During the two-week recovery following Superstorm Sandy, two urban streams exhibited a range of metabolic activity equivalent to~15% of the entire range of GPP and ER reported in a recent meta-analysis of stream metabolism. Urban streams exhibit a substantial proportion of the natural variation in metabolism found across stream ecosystems over relatively short time scales. Not only does urbanization cause increased hydrological flashiness, it appears that metabolic activity in urban streams may be less resistant, but also more resilient to floods than in other streams draining undeveloped watersheds, which have been more studied. Our results show that antecedent conditions must be accounted for when drawing conclusions about stream metabolism measurements, and the rapid recovery and resilience of urban streams should be considered in watershed management and stream restoration strategies targeting ecosystem functions and services.

We examined the effect of agricultural land use on whole-stream nutrient availability, biofilm standing crop, and biofilm nutrient limitation and uptake in 21 stream locations in southeastern Idaho. Higher stream water concentrations of dissolved organic carbon (DOC), but not nitrate (NO 3 -N) or phosphate (PO 4 -P), were associated with % agriculture in the watershed. Streambed chlorophyll a (Chl a) and ash free dry weight (AF dry wt) also increased with agricultural land use. Nutrient diffusing substrate (NDS) bioassays, which determine biofilm nutrient limitation, showed that Chl a was NO 3 -N limited and suppressed by labile DOC, whereas AF dry wt did not respond to either NO 3 -N or labile DOC additions. Together, the different responses of Chl a and AF dry wt suggest potential competition between biofilm autotrophs and heterotrophs for nutrients or other resources. Nutrient uptake, determined with short-term releases of NO 3 -N, PO 4 -P, and DOC (as glucose) showed that NO 3 -N uptake velocity (V f ) increased with agricultural land use. Despite the N-limited status of biofilms indicated by the NDS results, uptake of NO 3 -N could not be consistently detected at all locations. The differences in response of biofilm to organic carbon enrichment suggests a difference in DOC quality, with labile DOC added with NDS compared to perhaps less-labile DOC found in-stream. Linking the interactive responses of biofilm communities to altered nutrient availability is an important step toward understanding whole ecosystem responses to land-use change.