Heather A. Bechtold

Pharmaceuticals suppress algal growth and microbial respiration and alter bacterial communities in stream biofilms

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.

PLANT AND MICROBE CONTRIBUTION TO COMMUNITY RESILIENCE IN A DIRECTIONALLY CHANGING ENVIRONMENT

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.

The consequence of species loss on ecosystem nitrogen cycling depends on community compensation

Repercussions of species loss on ecosystem processes depend on the effects of the lost species as well as the compensatory responses of the remaining species in the community. We experimentally removed two co-dominant plant species and added a 15N tracer in alpine tundra to compare how species’ functional differences influence community structure and N cycling. For both of the species, production compensated for the biomass removed by the second year. However, the responses of the remaining species depended on which species was removed. These differences in compensation influenced how species loss impacted ecosystem processes. After the removal of one of the co-dominant species, Acomastylis rossii, there were few changes in the relative abundance of the remaining species, and differences in functioning could be predicted based on effects associated with the removed species. In contrast, the removal of the other co-dominant, Deschampsia caespitosa, was associated with subsequent changes in community structure (species relative abundances and diversity) and impacts on ecosystem properties (microbial biomass N, dissolved organic N, and N uptake of subordinate species). Variation in compensation may contribute to the resulting effects on ecosystem functioning, with the potential to buffer or accelerate the effects of species loss.

Effects of temperature and concentration on nutrient release rates from nutrient diffusing substrates

Abstract Nutrient diffusing substrates (NDS) are an important tool for evaluating periphyton nutrient limitation. The rate at which nutrients are released from NDS depends on both the initial nutrient concentration and the length of time that NDS are in place. Whether temperature also affects nutrient release rates from NDS is unclear. However, this information is important because temperature effects on release rates could confound experimental results for NDS-based experiments testing rates of accumulation of periphyton biomass when stream water temperature is variable. We measured N and P release rates from NDS vials with 3 initial concentrations (0.05, 0.1, and 0.5 mol/L) of nutrients at 3 temperatures (4, 15, and 21°C) for 21 d. Release rates of both nutrients were greater for vials with higher nutrient concentrations and for vials at warmer temperatures. For all concentrations, release rates decreased log linearly with time, a result that might have important implications for patterns of colonization and subsequent interspecific interactions within the periphyton community. In our opinion, temperature-caused differences in release rates are not biologically important because the differences were much smaller (3%) than expected changes in periphyton maximum growth rates over similar temperature ranges (∼300%). Our results suggest that seasonal and site-related differences in temperature will not significantly affect nutrient release rates within the range of temperatures we tested, but researchers should consider nutrient concentration carefully when planning studies using NDS.

The antihistamine cimetidine alters invertebrate growth and population dynamics in artificial streams

Abstract.  Many pharmaceutical compounds have been detected in surface waters, but their effects on stream ecosystem function are not currently understood. Concentrations of cimetidine, a widely used antihistamine, have increased in streams and rivers. Invertebrates may be affected by exposure to cimetidine because they use histamines to regulate olfactory and stomatogastric function. Primary producers, such as algal biofilms, also may be affected by cimetidine, which may in turn, alter metabolism and ultimately invertebrate population dynamics. We conducted a long-term (83 d) experiment in artificial streams to measure the chronic effects of cimetidine on benthic biofilm function and stream invertebrate growth and population dynamics. We exposed 2 common invertebrate species, Gammarus fasciatus and Psephenus herricki, and biofilm to concentrations of cimetidine similar to what is found in USA surface waters (0.07–70.0 µg/L). We found no consistent effect of cimetidine on biofilm chlorophyll a or function (gross primary production, respiration). Growth and final biomass of reproducing G. fasciatus was reduced across all cimetidine treatments compared to the control. In addition, no individuals of the smallest size class occurred at lower concentrations of cimetidine suggesting that cimetidine may either more strongly affect invertebrates of smaller size classes or may suppress adult reproduction. We also found that higher concentrations of cimetidine significantly reduced survivorship of P. herricki. Low concentrations of cimetidine appear to have no effect on primary producers, but our observations indicate there are indirect negative effects on invertebrate growth and population dynamics.

A practical method for measuring integrated solar radiation reaching streambeds using photodegrading dyes

Abstract.  Incoming solar radiation is an important driver of aquatic ecosystem processes, such as gross primary production and photodegradation of organic matter. Rates of incoming solar radiation can be estimated in several ways, but their utility is limited for collecting a large number of replicate samples needed to quantify variation in light availability within and among streams. We evaluated the utility of 2 photodegrading organic dyes (rhodamine WT [RWT] and fluorescein) for measuring light exposure, especially at the level of the stream bed. We attached vials with known concentrations of the RWT or fluorescein to the stream bed and used regressions of concentration vs accumulated light to estimate photodegradation rates. Initial concentrations of RWT (20–100 µg/L) did not affect rate of photodegradation, but RWT decay rates were 93% slower in the dark than in the light. We also tested fluorescein, which degrades faster than RWT when exposed to light and is stable when kept in the dark. On average, RWT degraded at a slower rate (3.5 µg L−1 d−1) than fluorescein (40.8 µg L−1 d−1) when exposed to similar levels of light accumulation. Water temperature did not affect the decay rate of RWT at 10 or 20°C, but RWT did not decay significantly at 30°C, a result suggesting that high temperatures might affect decay rates differently than lower temperatures. Water temperature did not affect the decay rate of fluorescein. The strength of this method is that it enables researchers to integrate light measurements into a single value. Researchers can deploy multiple arrays within a reach to develop a relative measure of incoming light that has the potential to cover large spatial and temporal scales.

High resolution measurement of light in terrestrial ecosystems using photodegrading dyes.

Incoming solar radiation is the main determinant of terrestrial ecosystem processes, such as primary production, litter decomposition, or soil mineralization rates. Light in terrestrial ecosystems is spatially and temporally heterogeneous due to the interaction among sunlight angle, cloud cover and tree-canopy structure. To integrate this variability and to know light distribution over time and space, a high number of measurements are needed, but tools to do this are usually expensive and limited. An easy-to-use and inexpensive method that can be used to measure light over time and space is needed. We used two photodegrading fluorescent organic dyes, rhodamine WT (RWT) and fluorescein, for the quantification of light. We measured dye photodegradation as the decrease in fluorescence across an irradiance gradient from full sunlight to deep shade. Then, we correlated it to accumulated light measured with PAR quantum sensors and obtained a model for this behavior. Rhodamine WT and fluorescein photodegradation followed an exponential decay curve with respect to accumulated light. Rhodamine WT degraded slower than fluorescein and remained unaltered after exposure to temperature changes. Under controlled conditions, fluorescence of both dyes decreased when temperatures increased, but returned to its initial values after cooling to the pre-heating temperature, indicating no degradation. RWT and fluorescein can be used to measure light under a varying range of light conditions in terrestrial ecosystems. This method is particularly useful to integrate solar radiation over time and to measure light simultaneously at different locations, and might be a better alternative to the expensive and time consuming traditional light measurement methods. The accuracy, low price and ease of this method make it a powerful tool for intensive sampling of large areas and for developing high resolution maps of light in an ecosystem.

Frontiers in Ecosystem Science

In this final chapter, a few examples of areas that seem poised for rapid progress, and that show the vitality of the field, are offered. Some of these areas are urbanization, global change, restoration ecology, and the integration of humans into ecosystems.