Emma J. Rosi-Marshall

A review of allochthonous organic matter dynamics and metabolism in streams

Abstract The role of allochthonous organic matter in lotic ecosystems has been an important research topic among aquatic ecologists since the seminal work by Lindeman was published in 1942. Since 1986, studies on organic matter budgets, ecosystem metabolism, and decomposition published in J-NABS have made significant contributions to the overall understanding of organic matter dynamics in streams. In this review, we summarize the utility of organic matter budgets, cover the major advances in research on ecosystem metabolism, and describe the intrinsic and extrinsic factors influencing organic matter decomposition. We also discuss future directions and current applications of research and highlight the need for additional studies on the role of land use and climate change, as well as continued use of organic matter processing as a functional metric in biomonitoring studies. We emphasize the need for continued data synthesis into comprehensive organic matter budgets. Such comparative studies can elucidate important drivers of organic matter dynamics and can assist in the understanding of large continental/global changes that might be occurring now and in the near future. In general, continued emphasis on synthesizing information into a larger framework for streams and rivers will improve our overall understanding of the importance of organic matter in lotic ecosystems.

Toxins in transgenic crop byproducts may affect headwater stream ecosystems

Corn (Zea mays L.) that has been genetically engineered to produce the Cry1Ab protein (Bt corn) is resistant to lepidopteran pests. Bt corn is widely planted in the midwestern United States, often adjacent to headwater streams. We show that corn byproducts, such as pollen and detritus, enter headwater streams and are subject to storage, consumption, and transport to downstream water bodies. Laboratory feeding trials showed that consumption of Bt corn byproducts reduced growth and increased mortality of nontarget stream insects. Stream insects are important prey for aquatic and riparian predators, and widespread planting of Bt crops has unexpected ecosystem-scale consequences.

ARE RIVERS JUST BIG STREAMS? A PULSE METHOD TO QUANTIFY NITROGEN DEMAND IN A LARGE RIVER

Given recent focus on large rivers as conduits for excess nutrients to coastal zones, their role in processing and retaining nutrients has been overlooked and understudied. Empirical measurements of nutrient uptake in large rivers are lacking, despite a substantial body of knowledge on nutrient transport and removal in smaller streams. Researchers interested in nutrient transport by rivers (discharge >10000 L/s) are left to extrapolate riverine nutrient demand using a modeling framework or a mass balance approach. To begin to fill this knowledge gap, we present data using a pulse method to measure inorganic nitrogen. (N) transport and removal in the Upper Snake River, Wyoming, USA (seventh order, discharge 12000 L/s). We found that the Upper Snake had surprisingly high biotic demand relative to smaller streams in the same river network for both ammonium (NH4+) and nitrate (NO3-). Placed in the context of a meta-analysis of previously published nutrient uptake studies, these data suggest that large rivers may have similar biotic demand for N as smaller tributaries. We also found that demand for different forms of inorganic N (NH4+ vs. NO3-) scaled differently with stream size. Data from rivers like the Upper Snake and larger are essential for effective water quality management at the scale of river networks. Empirical measurements of solute dynamics in large rivers are needed to understand the role of whole river networks (as opposed to stream reaches) in patterns of nutrient export at regional and continental scales.

Ecosystem ecology meets adaptive management: food web response to a controlled flood on the Colorado River, Glen Canyon

Large dams have been constructed on rivers to meet human demands for water, electricity, navigation, and recreation. As a consequence, flow and temperature regimes have been altered, strongly affecting river food webs and ecosystem processes. Experimental high-flow dam releases, i.e., controlled floods, have been implemented on the Colorado River, U.S.A., in an effort to reestablish pulsed flood events, redistribute sediments, improve conditions for native fishes, and increase understanding of how dam operations affect physical and biological processes. We quantified secondary production and organic matter flows in the food web below Glen Canyon dam for two years prior and one year after an experimental controlled flood in March 2008. Invertebrate biomass and secondary production declined significantly following the flood (total biomass, 55% decline; total production, 56% decline), with most of the decline driven by reductions in two nonnative invertebrate taxa, Potamopyrgus antipodarum and Gammarus lacustris. Diatoms dominated the trophic basis of invertebrate production before and after the controlled flood, and the largest organic matter flows were from diatoms to the three most productive invertebrate taxa (P. antipodarum, G. lacustris, and Tubificida). In contrast to invertebrates, production of rainbow trout (Oncorhynchus mykiss) increased substantially (194%) following the flood, despite the large decline in total secondary production of the invertebrate assemblage. This counterintuitive result is reconciled by a post-flood increase in production and drift concentrations of select invertebrate prey (i.e., Chironomidae and Simuliidae) that supported a large proportion of trout production but had relatively low secondary production. In addition, interaction strengths, measured as species impact values, were strongest between rainbow trout and these two taxa before and after the flood, demonstrating that the dominant consumer-resource interactions were not necessarily congruent with the dominant organic matter flows. Our study illustrates the value of detailed food web analysis for elucidating pathways by which dam management may alter production and strengths of species interactions in river food webs. We suggest that controlled floods may increase production of nonnative rainbow trout, and this information can be used to help guide future dam management decisions.

Wastewater Treatment Effluent Reduces the Abundance and Diversity of Benthic Bacterial Communities in Urban and Suburban Rivers

ABSTRACT In highly urbanized areas, wastewater treatment plant (WWTP) effluent can represent a significant component of freshwater ecosystems. As it is impossible for the composition of WWTP effluent to match the composition of the receiving system, the potential exists for effluent to significantly impact the chemical and biological characteristics of the receiving ecosystem. We assessed the impacts of WWTP effluent on the size, activity, and composition of benthic microbial communities by comparing two distinct field sites in the Chicago metropolitan region: a highly urbanized river receiving effluent from a large WWTP and a suburban river receiving effluent from a much smaller WWTP. At sites upstream of effluent input, the urban and suburban rivers differed significantly in chemical characteristics and in the composition of their sediment bacterial communities. Although effluent resulted in significant increases in inorganic nutrients in both rivers, surprisingly, it also resulted in significant decreases in the population size and diversity of sediment bacterial communities. Tag pyrosequencing of bacterial 16S rRNA genes revealed significant effects of effluent on sediment bacterial community composition in both rivers, including decreases in abundances of Deltaproteobacteria, Desulfococcus, Dechloromonas, and Chloroflexi sequences and increases in abundances of Nitrospirae and Sphingobacteriales sequences. The overall effect of the WWTP inputs was that the two rivers, which were distinct in chemical and biological properties upstream of the WWTPs, were almost indistinguishable downstream. These results suggest that WWTP effluent has the potential to reduce the natural variability that exists among river ecosystems and indicate that WWTP effluent may contribute to biotic homogenization.

Pharmaceutical Compounds and Ecosystem Function: An Emerging Research Challenge for Aquatic Ecologists

The number of anthropogenic compounds that occur in aquatic ecosystems today is in the thousands, many at trace concentrations. One group of compounds that has captured the interest of both the scientific community and the general public is pharmaceutical and personal care products (PPCPs), for example, hormones, chemotherapy drugs, antihistamines, stimulants, antimicrobials and various cosmetic additives. Toxicology of some PPCPs is currently understood, but their effect on ecological structure and function of aquatic ecosystems is largely unknown. We review sources and fates of these compounds in aquatic ecosystems and discuss how methods developed to study aquatic ecosystem ecology can contribute to our understanding of the influence of PPCPs on aquatic ecosystems. We argue that aquatic ecology has a well-developed tool kit for measuring the transformation, fate, and transport of solutes using assays and experiments and that these methods could be employed to investigate how PPCPs impact ecological function. We discuss the details of these approaches and conclude that application of existing ecological methods to the study of this issue could substantially improve our understanding of the effect of these compounds in aquatic ecosystems.

Food‐web dynamics in a large river discontinuum

Nearly all ecosystems have been altered by human activities, and most communities are now composed of interacting species that have not co-evolved. These changes may modify species interactions, energy and material flows, and food-web stability. Although structural changes to ecosystems have been widely reported, few studies have linked such changes to dynamic food-web attributes and patterns of energy flow. Moreover, there have been few tests of food-web stability theory in highly disturbed and intensely managed freshwater ecosystems. Such synthetic approaches are needed for predicting the future trajectory of ecosystems, including how they may respond to natural or anthropogenic perturbations. We constructed flow food webs at six locations along a 386-km segment of the Colorado River in Grand Canyon (Arizona, USA) for three years. We characterized food-web structure and production, trophic basis of production, energy efficiencies, and interaction-strength distributions across a spatial gradient of pertu...

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.

Triclosan exposure increases triclosan resistance and influences taxonomic composition of benthic bacterial communities.

Triclosan (TCS) is a broad-spectrum antimicrobial compound that is incorporated into numerous consumer products. TCS has been detected in aquatic ecosystems across the U.S., raising concern about its potential ecological effects. We conducted a field survey and an artificial stream experiment to assess effects of TCS on benthic bacterial communities. Field sampling indicated that TCS concentrations in stream sediments increased with degree of urbanization. There was significant correlation between sediment TCS concentration and the proportion of cultivable benthic bacteria that were resistant to TCS, demonstrating that the levels of TCS present in these streams was affecting the native communities. An artificial stream experiment confirmed that TCS exposure could trigger increases in TCS resistance within cultivable benthic bacteria, and pyrosequencing analysis indicated that TCS resulted in decreased benthic bacterial diversity and shifts in bacterial community composition. One notable change was a 6-fold increase in the relative abundance of cyanobacterial sequences and a dramatic die-off of algae within the artificial streams. Selection of cyanobacteria over algae could have significant implications for higher trophic levels within streams. Finally, there were no observed effects of TCS on bacterial abundance or respiration rates, suggesting that bacterial density and function were highly resilient to TCS exposure.

Occurrence of maize detritus and a transgenic insecticidal protein (Cry1Ab) within the stream network of an agricultural landscape

Widespread planting of maize throughout the agricultural Midwest may result in detritus entering adjacent stream ecosystems, and 63% of the 2009 US maize crop was genetically modified to express insecticidal Cry proteins derived from Bacillus thuringiensis. Six months after harvest, we conducted a synoptic survey of 217 stream sites in Indiana to determine the extent of maize detritus and presence of Cry1Ab protein in the stream network. We found that 86% of stream sites contained maize leaves, cobs, husks, and/or stalks in the active stream channel. We also detected Cry1Ab protein in stream-channel maize at 13% of sites and in the water column at 23% of sites. We found that 82% of stream sites were adjacent to maize fields, and Geographical Information Systems analyses indicated that 100% of sites containing Cry1Ab-positive detritus in the active stream channel had maize planted within 500 m during the previous crop year. Maize detritus likely enters streams throughout the Corn Belt; using US Department of Agriculture land cover data, we estimate that 91% of the 256,446 km of streams/rivers in Iowa, Illinois, and Indiana are located within 500 m of a maize field. Maize detritus is common in low-gradient stream channels in northwestern Indiana, and Cry1Ab proteins persist in maize leaves and can be measured in the water column even 6 mo after harvest. Hence, maize detritus, and associated Cry1Ab proteins, are widely distributed and persistent in the headwater streams of a Corn Belt landscape.