Oyster reefs have declined globally. Interest in their restoration has motivated research into oyster-mediated ecosystem services including effects on biodiversity, filtration, and nitrogen (N) ...cycling. Recent evidence suggests oysters may promote denitrification, or anaerobic respiration of nitrate (NO
3
−
) into di-nitrogen gas, via benthic deposition of carbon (C) and N-rich biodeposits. However, the mechanisms whereby biodeposits promote N transformations prerequisite to denitrification (e.g., mineralization and nitrification) are unclear. Previous research has also not measured oysters' influence on N cycling in urbanized areas. 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 (New York, USA). Oysters were arranged at four densities: 0, 40, 85, and 150 oysters/m
2
. For 17 months we measured water-column nutrients and chlorophyll
a
, every two weeks to monthly. Every two months we measured sediment ash-free dry mass (AFDM), exchangeable ammonium (NH
4
+
), ammonification, nitrification, denitrification potential (DNP), and NO
3
−
and C limitation of DNP. Oysters increased sediment AFDM at three of four sites, with the greatest increase at high density. Oysters did not affect any N pools or transformations. However, variation among sites and dates illustrated environmental drivers of C and N biogeochemistry in this urban estuary. Overall, nitrification was positively related to net ammonification, water column NH
4
+
, and sediment NH
4
+
, but was not correlated with DNP. Denitrification was consistently and strongly NO
3
−
limited, while C was not limiting or secondarily limiting. Therefore, the oyster-mediated increase in AFDM did not affect DNP because C was not its primary driver. Also, because DNP was unrelated to nitrification, it is unlikely that biodeposit N was converted to NO
3
−
for use as a denitrification substrate. Predicting times or sites where denitrification is driven by the C and N species originating from oyster biodeposits remains a challenge under eutrophic conditions. Towards this goal, we synthesized our conclusions with literature predictions in a conceptual model for pathways whereby oysters might influence C and N dynamics differently in oligotrophic relative to eutrophic ecosystems.
Microplastic is a contaminant of concern in freshwater ecosystems worldwide. Microplastic particles within aquatic habitats are colonized by dense microbial biofilms, and previous studies have shown ...that microplastic microbiomes are distinct in taxonomic composition from bacterial assemblages in the surrounding environment. However, questions remain about the degree to which microplastic selects for specific bacterial taxa across diverse aquatic habitats. We used laboratory microcosms inoculated with water from 3 rivers in northern Illinois watersheds with distinct land-use types to test the hypothesis that microbiomes present on microplastic would be similar even when the microplastic was incubated with microbial assemblages from different source waters. When microplastic from a commercial soap and ceramic tiles were incubated with water from each of the 3 rivers for ~1 mo, the bacterial assemblages that colonized the microplastic were remarkably consistent in taxonomic composition, whereas the planktonic and tile bacterial assemblages originating from the 3 rivers were distinct. The number of bacterial operational taxonomic units found within the microplastic microbiomes was consistent across source water treatments and ~3× lower than in the surrounding water and tiles. Some of the bacterial taxa that were over-represented in the microplastic microbiomes in our experiment can metabolize plastic or plastic-associated compounds. If microplastic microbiomes are metabolizing plastic polymers, this process could have significant implications for the long-term fate of microplastic in freshwater habitats.
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.
In aquatic ecosystems, plastic litter is a substrate for biofilms. Biofilms on plastic and natural surfaces share similar composition and activity, with some differences due to factors such as ...porosity. In freshwaters, most studies have examined biofilms on benthic substrates, while little research has compared the activity and composition of biofilms on buoyant plastic and natural surfaces. Additionally, the influence of substrate size and successional stage on biofilm composition has not been commonly assessed. We incubated three plastics of distinct textures that are buoyant in rivers, low‐density polyethylene (rigid; 1.7 mm thick), low‐density polyethylene film (flexible; 0.0254 mm thick), and foamed polystyrene (brittle; 6.5 mm thick), as well as wood substrates (untreated oak veneer; 0.6 mm thick) in the Chicago River. Each material was incubated at three sizes (1, 7.5, and 15 cm2). Substrates were incubated at 2–10 cm depths and removed weekly for 6 weeks. On each substrate we measured chlorophyll concentration, biofilm biomass, respiration, and flux of nitrogen gas. We sequenced 16S and 23S rRNA genes at Weeks 1, 3, and 6 to capture biofilm community composition across successional stages. Chlorophyll, biomass, and N2 flux were similar across substrates, but respiration was greater on wood than plastics. Bacterial and algal richness and diversity were highest on foam and wood compared to polyethylene substrates. Bacterial biofilm community composition was distinct between wood and plastic substrates, while the algal community was distinct on wood and foam, which were different from each other and polyethylene substrates. These results indicate that polymer properties influence biofilm alpha and beta diversity, which may affect transport and distribution of plastic pollution and associated microbes, as well as biogeochemical processes in urban rivers. This study provides valuable insights into the effects of substrate on biofilm characteristics, and the ecological impacts of plastic pollution on urban rivers.
Practitioner Points
Plastic physical and chemical properties act as forces of selection for biofilm.
Biofilm activity was similar among three different types of plastic.
Community composition between plastic and wood was different.
Biofilm activity among plastic and wood substrates was similar, except for higher respiration on wood. The unique physical and chemical properties of wood and plastic substrates acted as forces of selection that led to distinct bacterial communities.
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.
Key Points
We adapted dissolved N2 methods and a novel modeling approach to estimate denitrification in rivers
The water column accounted for 0–85% and 39–85% of combined denitrification and respiration
Rivers can remove nitrogen via denitrification at equivalent or higher rates than headwaters
Anthropogenic disturbances may be increasing jellyfish populations globally. Epibenthic jellyfish are ideal organisms for studying this phenomenon due to their sessile lifestyle, broad geographic ...distribution, and prevalence in near-shore coastal environments. There are few studies, however, that have documented epibenthic jellyfish abundance and measured their impact on ecological processes in tropical ecosystems. In this study, the density and size of the upside-down jellyfish (Cassiopea spp.) were measured in Codrington Lagoon, Barbuda. A sediment core incubation study, with and without Cassiopea, also was performed to determine their impact on benthic oxygen and nutrient fluxes. Densities of Cassiopea were 24–168 m−2, among the highest reported values in the literature. Under illuminated conditions, Cassiopea increased oxygen production >300% compared to sediment alone, and they changed sediments from net heterotrophy to net autotrophy. Cassiopea increased benthic ammonium uptake, but reduced nitrate uptake, suggesting they can significantly alter nitrogen cycling. Future studies should quantify the abundance of Cassiopea and measure their impacts on ecosystem processes, in order to further determine how anthropogenic-related changes may be altering the function of tropical coastal ecosystems.
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•High densities (24-168 m−2) of Cassiopea sp. found in Barbuda.•Cassiopea sp. increase benthic oxygen production.•Ammonium uptake is increased and nitrate uptake is reduced by Cassiopea sp.
Anthropogenic particles (AP), which include microplastics and other synthetic, semisynthetic, and anthropogenically modified materials, are pollutants of concern in aquatic ecosystems worldwide. ...Rivers are important conduits and retention sites for AP, and time series data on the movement of these particles in lotic ecosystems are needed to assess the role of rivers in the global AP cycle. Much research assessing AP pollution extrapolates stream loads based on single time point measurements, but lotic ecosystems are highly variable over time (e.g., seasonality and storm events). The accuracy of models describing AP dynamics in rivers is constrained by the limited studies that examine how frequent changes in discharge drive particle retention and transport. This study addressed this knowledge gap by using automated, high‐resolution sampling to track AP concentrations and fluxes during multiple storm events in an urban river (Milwaukee River) and comparing these measurements to commonly monitored water quality metrics. AP concentrations and fluxes varied significantly across four storm events, highlighting the temporal variability of AP dynamics. When data from the sampling periods were pooled, there were increases in particle concentration and flux during the early phases of the storms, suggesting that floods may flush AP into the river and/or resuspend particles from the benthic zone. AP flux was closely linked to river discharge, suggesting large loads of AP are delivered downstream during storms. Unexpectedly, AP concentrations were not correlated with other simultaneously measured water quality metrics, including total suspended solids, fecal coliforms, chloride, nitrate, and sulfate, indicating that these metrics cannot be used to estimate AP. These data will contribute to more accurate models of particle dynamics in rivers and global plastic export to oceans.
Practitioner Points
Anthropogenic particle (AP) concentrations and fluxes in an urban river varied across four storm events.
AP concentrations and fluxes were the highest during the early phases of the storms.
Storms increased AP transport downstream compared with baseflow.
AP concentrations did not correlate with other water quality metrics during storms.
This study used automated samplers to collect anthropogenic particles (AP) from the Milwaukee River during four storm events. AP concentrations and fluxes varied with storm phase, and storms increased AP transport downstream compared with baseflow. AP concentrations did not correlate with other water quality metrics during storms.
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.
Seagrass meadows are important sites of nitrogen (N) transformations in estuaries, however, the role of N loading in driving relative rates of N fixation and denitrification in seagrass habitats is ...unclear. The current study quantified N fluxes in eelgrass meadows (Zostera marina (L.)) and nearby unvegetated sand in trials representing in situ and N enriched conditions. Net N2 fluxes were low or negative under in situ conditions in both eelgrass and sand. Under N enriched conditions, denitrification was higher than N-fixation, and denitrification in eelgrass was significantly higher than sand. Denitrification of water column NO3− was more significant than coupled nitrification-denitrification in the eelgrass. Denitrification was likely supported by greater organic carbon and N within the eelgrass sediment compared to sand. Eelgrass meadows in Shinnecock Bay may facilitate the ecosystem service of N removal and retention during short-term nutrient pulses that can originate from groundwater discharge and stormwater runoff.
•Eelgrass meadows can be hotspots for nitrogen cycling.•Mechanisms that favor nitrogen fixation or denitrification are unclear.•N2 fluxes were low or negative in eelgrass meadows with low water column nitrogen.•Denitrification was high in eelgrass habitat under nitrogen enriched conditions.•Denitrification was supported by water column nitrate and sediment carbon.
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