Timothy J. Hoellein

Microplastic is an Abundant and Distinct Microbial Habitat in an Urban River

Recent research has documented microplastic particles (< 5 mm in diameter) in ocean habitats worldwide and in the Laurentian Great Lakes. Microplastic interacts with biota, including microorganisms, in these habitats, raising concerns about its ecological effects. Rivers may transport microplastic to marine habitats and the Great Lakes, but data on microplastic in rivers is limited. In a highly urbanized river in Chicago, Illinois, USA, we measured concentrations of microplastic that met or exceeded those measured in oceans and the Great Lakes, and we demonstrated that wastewater treatment plant effluent was a point source of microplastic. Results from high-throughput sequencing showed that bacterial assemblages colonizing microplastic within the river were less diverse and were significantly different in taxonomic composition compared to those from the water column and suspended organic matter. Several taxa that include plastic decomposing organisms and pathogens were more abundant on microplastic. These results demonstrate that microplastic in rivers are a distinct microbial habitat and may be a novel vector for the downstream transport of unique bacterial assemblages. In addition, this study suggests that urban rivers are an overlooked and potentially significant component of the global microplastic life cycle.

Anthropogenic Litter in Urban Freshwater Ecosystems: Distribution and Microbial Interactions

Accumulation of anthropogenic litter (i.e. garbage; AL) and its ecosystem effects in marine environments are well documented. Rivers receive AL from terrestrial habitats and represent a major source of AL to marine environments, but AL is rarely studied within freshwater ecosystems. Our objectives were to 1) quantify AL density in urban freshwaters, 2) compare AL abundance among freshwater, terrestrial, and marine ecosystems, and 3) characterize the activity and composition of AL biofilms in freshwater habitats. We quantified AL from the Chicago River and Chicagos Lake Michigan shoreline, and found that AL abundance in Chicago freshwater ecosystems was comparable to previously reported data for marine and terrestrial ecosystems, although AL density and composition differed among habitats. To assess microbial interactions with AL, we incubated AL and natural substrates in 3 freshwater ecosystems, quantified biofilm metabolism as gross primary production (GPP) and community respiration (CR), and characterized biofilm bacterial community composition via high-throughput sequencing of 16S rRNA genes. The main driver of biofilm community composition was incubation location (e.g., river vs pond), but there were some significant differences in biofilm composition and metabolism among substrates. For example, biofilms on organic substrates (cardboard and leaves) had lower GPP than hard substrates (glass, plastic, aluminum and tiles). In addition, bacterial communities on organic substrates were distinct in composition from those on hard substrates, with higher relative abundances of bacteria associated with cellulose decomposition. Finally, we used our results to develop a conceptual diagram designed to unite the study of AL in terrestrial and freshwater environments with the well-established field of marine debris research. We suggest this broad perspective will be useful for future studies which synthesize AL sources, ecosystem effects, and fate across multiple ecosystem types, and will benefit management and reduction of global AL accumulations.

Temporal variation in substratum-specific rates of N uptake and metabolism and their contribution at the stream-reach scale

Abstract Whole-stream rates of nutrient uptake and metabolism vary with season in forested headwater streams, but seasonal variation in rates at the substratum scale, which collectively generate whole-stream patterns, have not previously been quantified. We measured gross primary production (GPP), community respiration (CR), and NO3− uptake rates (NO3− U) on 7 substrata using in situ microcosms during spring, summer, and autumn in 3 headwater streams in northern Michigan. We quantified the areal reach coverage of each substratum, scaled NO3− U and metabolism to the stream reach (100 m), and compared scaled NO3− U to whole-stream NO3− U (measured using short-term nutrient additions). Patterns of GPP, CR, and NO3− U were different among substratum types and seasons. Substratum-specific NO3− U was positively related to GPP on epilithic biofilms and to CR on epixylic biofilms, bryophytes, and coarse and fine benthic organic matter (FBOM). During summer, large wood and FBOM accounted for 86% of reach-scale NO3− U, whereas epilithon contributed 28% and 49% of reach-scale NO3− U in spring and autumn, respectively. This result was unexpected for these forested closed-canopy streams. When substratum-specific rates were scaled up, NO3− U was not different from whole-stream NO3− U calculated from short-term enrichment. For large wood, organic matter, and epilithon, high molar ratios of C:N uptake suggested that NO3− uptake did not satisfy overall N demand. Streambed substratum composition influenced seasonal variability of reach-scale nutrient uptake by its influence on metabolism and changed the relative contribution of several substrata to whole-stream rates through time.

Macroinvertebrate secondary production in 3 forested streams of the upper Midwest, USA

Abstract Macroinvertebrate secondary production was estimated for 2 reaches in each of 3 adjacent forested headwater streams. We had 3 objectives: 1) to compare macroinvertebrate secondary production and community structure both within and among streams to examine the spatial extent of variability, 2) to explore important habitat variables related to secondary production, and 3) to compare our secondary production values to values from other headwater streams in deciduous forests. Principal components analysis separated study streams on the basis of small differences in substrate composition, organic-matter standing crops, and instream wood, but geology, riparian tree species composition, and fine benthic organic-matter standing crops were similar among streams. Secondary production varied among streams (range ∼1.2 to 3.3 g ash-free dry mass m−2 y−1) and was low compared to estimates from other streams draining deciduous forest. Macroinvertebrate communities had relatively higher production of scrapers, predators, and collector-filterers and lower production of shredders and collector-gatherers as compared to other perennial eastern deciduous headwater streams. We expected differences in secondary production among streams to be related to leaf-litter standing crops; however, differences among streams were positively related to % cover of gravel and cobble and chlorophyll a concentrations on gravel (R2 = 0.87, p = 0.01). Total secondary production was negatively related to the number of large pieces of wood, leaf-litter standing crop, and % cover of sand. A similar, positive relationship between % cover of gravel and cobble and chlorophyll a concentrations was found for primary consumers (R2 = 0.89, p = 0.03), collector-filterers (R2 = 0.76, p = 0.02), and scrapers (R2 = 0.67, p = 0.04). Low amounts and patchy distributions of coarse benthic organic matter, leaf litter, and large wood probably resulted in more variable secondary production of predators and shredders within these streams than among all streams. Inorganic substrate composition, primary production, and water temperature probably were key factors regulating secondary production of the other functional feeding groups and total secondary production among streams.

Sediment, water column, and open‐channel denitrification in rivers measured using membrane‐inlet mass spectrometry

Riverine biogeochemical processes are understudied relative to headwaters, and reach-scale processes in rivers reflect both the water column and sediment. Denitrification in streams is difficult to measure, and is often assumed to occur only in sediment, but the water column is potentially important in rivers. Dissolved nitrogen (N) gas flux (as dinitrogen (N2)) and open-channel N2 exchange methods avoid many of the artificial conditions and expenses of common denitrification methods like acetylene block and 15N-tracer techniques. We used membrane-inlet mass spectrometry and microcosm incubations to quantify net N2 and oxygen flux from the sediment and water column of five Midwestern rivers spanning a land use gradient. Sediment and water column denitrification ranged from below detection to 1.8 mg N m−2 h−1 and from below detection to 4.9 mg N m−2 h−1, respectively. Water column activity was variable across rivers, accounting for 0–85% of combined microcosm denitrification and 39–85% of combined microcosm respiration. Finally, we estimated reach-scale denitrification at one Midwestern river using a diel, open-channel N2 exchange approach based on reach-scale metabolism methods, providing an integrative estimate of riverine denitrification. Reach-scale denitrification was 8.8 mg N m−2 h−1 (95% credible interval: 7.8–9.7 mg N m−2 h−1), higher than combined sediment and water column microcosm estimates from the same river (4.3 mg N m−2 h−1) and other estimates of reach-scale denitrification from streams. Our denitrification estimates, which span habitats and spatial scales, suggest that rivers can remove N via denitrification at equivalent or higher rates than headwater streams.

The invasive Asian clam (Corbicula fluminea) increases sediment denitrification and ammonium flux in 2 streams in the midwestern USA

The invasive Asian clam, Corbicula fluminea, can enhance conditions needed for denitrification, but the effects of C. fluminea on denitrification in streams have not been measured. In summer 2012, we incubated sand- and gravel-filled boxes in the North Branch of the Chicago River, Illinois (high nutrient), and Eagle Creek, Michigan, USA (low nutrient). One side of the box contained no clams (control), and the other contained C. fluminea. After 6 wk in situ, we measured inorganic N and dissolved-gas fluxes using a continuous-flow approach. At both sites, live C. fluminea significantly increased NH4+ and N2 flux out of sediments and O2 consumption (i.e., respiration) relative to control sediment. Clams did not affect NO3− flux. Clams probably increased NH4+ flux via direct NH4+ excretion or mineralization of clam wastes and increased N2 flux through either increased coupled nitrification–denitrification or enhanced exchange of nutrients between water column and sediment via bioturbation (i.e., burrowing). We used benthic clam density in each stream to scale up effects of live C. fluminea on in situ N fluxes. In the low-nutrient stream, live clams increased NH4+ flux more than N2 production. However, in the high-nutrient stream, live clams enhanced N2 production more than NH4+ flux. The influence of dense assemblages of burrowing bivalves on denitrification may be an overlooked and potentially significant component of lotic N uptake in low- and high-N streams.

Effect of Eastern Oysters (Crassostrea virginica) and Seasonality on Nitrite Reductase Gene Abundance (nirS, nirK, nrfA) in an Urban Estuary

The influence of oysters on nitrogen (N) cycling has received increased research attention. Previous work focused on fluxes of N solutes and gases, but the effects on microbes responsible for N transformations are unknown. In May 2010, we deployed eastern oysters (Crassostrea virginica) in mesh cages above sand-filled boxes at four sites across a nutrient gradient in Jamaica Bay, New York City. In fall and winter, we used quantitative PCR to measure abundance of 16S rRNA and nitrite reductase genes for denitrification (nirS and nirK) and dissimilatory nitrate reduction to ammonium (nrfA) in sediment. We measured water column nutrients and chlorophyll a, sediment C:N and organic matter (OM), exchangeable ammonium (NH4+), ammonification, nitrification, and denitrification potential (DNP). Oysters did not affect gene abundance in fall, when we predicted that their influence would be strongest, or in winter. However, gene abundance was significantly different among sites and seasons. Factors which explained 16S rRNA, nirS, and nirK gene abundance included sediment OM, water column N, and chlorophyll a, similar to previous research. Abundance of nrfA was lower than that of nir genes and positively related to sediment C:N, suggesting OM lability may drive the balance between nir and nrfA. Finally, nirS and nirK abundance was unrelated to DNP, which is consistent with variable results from the literature. More studies that combine molecular techniques with N transformation rates in the context of oyster reefs are needed. Results will advance models which predict the ecosystem effects of reef conservation and restoration under variable environmental conditions.

Environmental drivers of leaf breakdown in an urban watershed

Leaf-litter breakdown is an important ecosystem process in urban streams, but urbanization may have complicated effects on breakdown rates. Low abundance of macroinvertebrate shredders may slow breakdown, but rates may increase if high nutrient concentrations stimulate microbial decomposers or if flooding enhances leaf fragmentation. We measured the relative importance of multiple environmental drivers on breakdown of eastern cottonwood (Populus deltoides) leaves at 5 sites in the urbanized North Branch of the Chicago River watershed. Few specialized macroinvertebrate shredders were present, but generalist macroinvertebrates, including isopods (Asellus aquaticus) and amphipods (Gammarus sp.) were abundant at all sites. We tested macroinvertebrate effects on breakdown rate in large- and small-mesh bags. We measured discharge, nutrient concentrations, benthic macroinvertebrate community composition, and subwatershed land use at each site. Leaf breakdown differed significantly among sites and between mesh types. Discharge and isopod abundance were positively related to breakdown rates, whereas nutrient concentrations were unrelated to breakdown rates. Abundances of isopods and amphipods were significantly higher in litter bags than in benthic samples. We conducted follow-up experiments in artificial streams to measure the separate effects of water velocity and isopods on leaf breakdown based on conditions from field sites. Increasing water velocity from 0.02 (control) to 0.07 m/s (high velocity) increased leaf breakdown by 33%, and adding isopods (density = 1034 individuals/m2) increased leaf breakdown by 40%. Measuring environmental controls on leaf breakdown throughout urban watersheds is critical to the use of breakdown rates as an assessment tool for urban stream ecosystems. Our study provides input data for models of stream ecosystem function at urban sites and informs management approaches for urban streams at the watershed scale.

Longitudinal patterns of microplastic concentration and bacterial assemblages in surface and benthic habitats of an urban river

Rivers are a major source of microplastic particles (<5 mm) to oceans, but empirical measurements of microplastic movement in freshwater ecosystems are rare. The hard, buoyant surface of microplastic is a novel habitat that selects for unique microbial assemblages in rivers, especially downstream of wastewater treatment plant (WWTP) point sources. We measured microplastic in surface water and benthic habitats 50 m upstream and 50, 305, 1115, and 1900 m downstream of the effluent outfall from a large WWTP in an urban river. We used high-throughput sequencing to measure bacterial assemblages on microplastic from surface and benthic habitats and compared them to bacterial assemblages from seston, water, and sediment. Concentrations of total microplastic and microplastic types (fragment, pellet) in surface water did not change with distance downstream of the WWTP. Thus, microplastic transport showed no net deposition or resuspension. Microplastic concentrations were much higher in the benthic zone than surface water. Benthic deposition appears to be a plastic sink over longer time scales, but subsequent studies are needed to resolve microplastic transport dynamics by particle type, size, and habitat. Composition of microplastic-attached bacterial assemblages differed from that of assemblages in water, seston, and sediment and supports domestic wastewater as a point source of microplastic (e.g., gastrointestinal taxa). Shifts in microplastic assemblages with distance from the WWTP suggest succession toward a ‘stream-like’ bacterial assemblage. Future studies are required to quantify the metabolic capacity of microplastic-associated bacteria. Estimates of transport distance, microplastic storage, and microbial interactions are critical to include lotic ecosystems in accountings of global plastic budgets.

Citizen science datasets reveal drivers of spatial and temporal variation for anthropogenic litter on Great Lakes beaches

Accumulation of anthropogenic litter (AL) on marine beaches and its ecological effects have been a major focus of research. Recent studies suggest AL is also abundant in freshwater environments, but much less research has been conducted in freshwaters relative to oceans. The Adopt-a-BeachTM (AAB) program, administered by the Alliance for the Great Lakes, organizes volunteers to act as citizen scientists by collecting and maintaining data on AL abundance on Great Lakes beaches. Initial assessments of the AAB records quantified sources and abundance of AL on Lake Michigan beaches, and showed that plastic AL was >75% of AL on beaches across all five Great Lakes. However, AAB records have not yet been used to examine patterns of AL density and composition among beaches of all different substrate types (e.g., parks, rocky, sandy), across land-use categories (e.g., rural, suburban, urban), or among seasons (i.e., spring, summer, and fall). We found that most AL on beaches are consumer goods that most likely originate from beach visitors and nearby urban environments, rather than activities such as shipping, fishing, or illegal dumping. We also demonstrated that urban beaches and those with sand rather than rocks had higher AL density relative to other sites. Finally, we found that AL abundance is lowest during the summer, between the US holidays of Memorial Day (last Monday in May) and Labor Day (first Monday in September) at the urban beaches, while other beaches showed no seasonality. This research is a model for utilizing datasets collected by volunteers involved in citizen science programs, and will contribute to AL management by offering priorities for AL types and locations to maximize AL reduction.