Sediment Organic Content Research Articles

Modeling hydrologic controls on sulfur processes in sulfate‐impacted wetland and stream sediments

AbstractRecent studies show sulfur redox processes in terrestrial settings are more important than previously considered, but much remains uncertain about how these processes respond to dynamic hydrologic conditions in natural field settings. We used field observations from a sulfate‐impacted wetland and stream in the mining region of Minnesota (USA) to calibrate a reactive transport model and evaluate sulfur and coupled geochemical processes under contrasting hydrogeochemical scenarios. Simulations of different hydrological conditions showed that flux and chemistry differences between surface water and deeper groundwater strongly control hyporheic zone geochemical profiles. However, model results for the stream channel versus wetlands indicate sediment organic carbon content to be the more important driver of sulfate reduction rates. A complex nonlinear relationship between sulfate reduction rates and geochemical conditions is apparent from the model's higher sensitivity to sulfate concentrations in settings with higher organic content. Across all scenarios, simulated e− balance results unexpectedly showed that sulfate reduction dominates iron reduction, which is contrary to the traditional thermodynamic ladder but corroborates recent experimental findings by Hansel et al. (2015) that “cryptic” sulfur cycling could drive sulfate reduction in preference over iron reduction. Following the thermodynamic ladder, our models shows that high surface water sulfate slows methanogenesis in shallow sediments, but field observations suggest that sulfate reduction may not entirely suppress methane. Overall, our results show that sulfate reduction may serve as a major component making up and influencing terrestrial redox processes, with dynamic hyporheic fluxes controlling sulfate concentrations and reaction rates, especially in high organic content settings.

Dissimilatory Nitrate Reduction to Ammonium in the Yellow River Estuary: Rates, Abundance, and Community Diversity

Dissimilatory nitrate reduction to ammonium (DNRA) is an important nitrate reduction process in estuarine sediments. This study reports the first investigation of DNRA in the Yellow River Estuary located in Eastern Shandong, China. Saltwater intrusion could affect the physicochemical characteristics and change the microbial community structure of sediments. In this study, the activity, abundance and community diversity of DNRA bacteria were investigated during saltwater intrusion. The slurry incubation experiments combined with isotope-tracing techniques and qPCR results showed that DNRA rates and nrfA (the functional gene of DNRA bacteria) gene abundance varied over wide ranges across different sites. DNRA rates had a positive and significant correlation with sediment organic content and extractable NH4+, while DNRA rates were weakly correlated with nrfA gene abundance. In comparison, the activities and abundance of DNRA bacteria did not change with a trend along salinity gradient. Pyrosequencing analysis of nrfA gene indicated that delta-proteobacteria was the most abundant at all sites, while epsilon-proteobacteria was hardly found. This study reveals that variability in the activities and community structure of DNRA bacteria is largely driven by changes in environmental factors and provides new insights into the characteristics of DNRA communities in estuarine ecosystems.

Secondary Production of Macrobenthic Communities in Seagrass (Zostera marina, Eelgrass) Beds and Bare Soft Sediments Across Differing Environmental Conditions in Atlantic Canada

In nearshore ecosystems, habitats with emergent structure are often assumed to have higher ecosystem functioning than habitats lacking structure. However, such habitat-specific differences may depend on the surrounding environment. In this study, I examine the robustness of habitat-specific differences in ecosystem functioning for seagrass (Zostera marina) and adjacent bare soft sediments across varying environmental conditions on the Atlantic Coast of Nova Scotia, Canada, using secondary production as a metric. I also examine relationships of community secondary production and faunal structure with measured environmental variables (water depth, temperature, exposure, sediment, and plant properties). Benthic secondary production (invertebrates ≥500 μm) was higher in seagrass compared to bare sediments only at exposed sites with sandy sediments low in organic content, deep and cool water, and high belowground plant biomass. A regression relating community secondary production to the environmental variables explained 56% of the variance, while a constrained ordination explained 16% of the community structure. Important environmental determinants of community production were shoot density, temperature, depth, exposure, sediment organic content, and belowground plant biomass. Community structure was influenced by these variables plus sediment sand content and canopy height. This study shows that habitat-specific differences in secondary production may not be consistent across varying environmental conditions. Furthermore, seagrass beds are not always associated with higher ecosystem functioning than adjacent bare sediment. Both the surrounding environmental conditions and the presence of habitat structure should be considered for optimal management of nearshore ecosystems.

Mediation of macronutrients and carbon by post-disturbance shelf sea sediment communities

Benthic communities play a major role in organic matter remineralisation and the mediation of many aspects of shelf sea biogeochemistry. Few studies have considered how changes in community structure associated with different levels of physical disturbance affect sediment macronutrients and carbon following the cessation of disturbance. Here, we investigate how faunal activity (sediment particle reworking and bioirrigation) in communities that have survived contrasting levels of bottom fishing affect sediment organic carbon content and macronutrient concentrations ([NH4–N], [NO2–N], [NO3–N], [PO4–P], [SiO4–Si]). We find that organic carbon content and [NO3–N] decline in cohesive sediment communities that have experienced an increased frequency of fishing, whilst [NH4–N], [NO2–N], [PO4–P] and [SiO4–Si] are not affected. [NH4–N] increases in non-cohesive sediments that have experienced a higher frequency of fishing. Further analyses reveal that the way communities are restructured by physical disturbance differs between sediment type and with fishing frequency, but that changes in community structure do little to affect bioturbation and associated levels of organic carbon and nutrient concentrations. Our results suggest that in the presence of physical disturbance, irrespective of sediment type, the mediation of macronutrient and carbon cycling increasingly reflects the decoupling of organism-sediment relations. Indeed, it is the traits of the species that reside at the sediment–water interface, or that occupy deeper parts of the sediment profile, that are disproportionately expressed post-disturbance, that are most important for sustaining biogeochemical functioning.

Nutrient and herbivore alterations cause uncoupled changes in producer diversity, biomass and ecosystem function, but not in overall multifunctionality

Altered nutrient cycles and consumer populations are among the top anthropogenic influences on ecosystems. However, studies on the simultaneous impacts of human-driven environmental alterations on ecosystem functions, and the overall change in system multifunctionality are scarce. We used estuarine tidal flats to study the effects of changes in herbivore density and nutrient availability on benthic microalgae (diversity, abundance and biomass) and ecosystem functions (N2-fixation, denitrification, extracellular polymeric substances -EPS- as a proxy for sediment cohesiveness, sediment water content as a proxy of water retention capacity and sediment organic matter). We found consistent strong impacts of modified herbivory and weak effects of increased nutrient availability on the abundance, biomass and diversity of benthic microalgae. However, the effects on specific ecosystem functions were disparate. Some functions were independently affected by nutrient addition (N2-fixation), modified herbivory (sediment organic matter and water content), or their interaction (denitrification), while others were not affected (EPS). Overall system multifunction remained invariant despite changes in specific functions. This study reveals that anthropogenic pressures can induce decoupled effects between community structure and specific ecosystem functions. Our results highlight the need to address several ecosystem functions simultaneously for better ecosystem characterization and management.

Sediment carbon and nutrient fluxes from cleared and intact temperate mangrove ecosystems and adjacent sandflats

The loss of mangrove ecosystems is associated with numerous impacts on coastal and estuarine function, including sediment carbon and nutrient cycling. In this study we compared in situ fluxes of carbon dioxide (CO2) from the sediment to the atmosphere, and fluxes of dissolved inorganic nutrients and oxygen across the sediment-water interface, in intact and cleared mangrove and sandflat ecosystems in a temperate estuary. Measurements were made 20 and 25months after mangrove clearance, in summer and winter, respectively.Sediment CO2 efflux was over two-fold higher from cleared than intact mangrove ecosystems at 20 and 25months after mangrove clearance. The higher CO2 efflux from the cleared site was explained by an increase in respiration of dead root material along with sediment disturbance following mangrove clearance. In contrast, sediment CO2 efflux from the sandflat site was negligible (≤9.13±1.18mmolm−2d−1), associated with lower sediment organic matter content. The fluxes of inorganic nutrients (NH4+, NOx and PO43−) from intact and cleared mangrove sediments were low (≤20.37±18.66μmolm−2h−1). The highest NH4+ fluxes were measured at the sandflat site (69.21±13.49μmolm−2h−1). Lower inorganic nutrient fluxes within the cleared and intact mangrove sites compared to the sandflat site were associated with lower abundance of larger burrowing macrofauna. Further, a higher fraction of organic matter, silt and clay content in mangrove sediments may have limited nutrient exchange.

Patterns of Benthic Fauna Distribution in Wells: The Role of Anthropogenic Impact and Geology

Core Ideas Geology does not seem to influence fauna composition in the studied wells. Water chemistry does not differ significantly in wells placed on limestone and flysch. Water pollution affects the abundance and diversity of wells fauna. Fauna distribution depends mainly on depth and age of the wells. Surface water fauna prevail; only three stygobiontic species were found. This study was undertaken to determine the factors that influence the composition of macroinvertebrate communities in wells completed into two different bedrocks, limestone and flysch. The chemical parameters of the water in wells of both kinds did not significantly differ. The anthropogenic impact on the quality of water was revealed in some wells in both areas, which resulted in relatively low fauna diversity and abundance in polluted wells. Canonical correspondence analyses suggested that depth, age, sediment organic matter content, water oxygenation, and conductivity best explained the variance in the distribution of benthic fauna taxa. Among the 19 taxa that were determined to the family level, oligochaetous Clitellata dominated. They were represented by 13 species and one genus. Among them, two stygobionts, Trichodrilus moravicus and Rhyacodrilus subterraneus, were determined. Moreover, only one crustacean stygobiont, Niphargus tatrensis, was found, and it occurred only in a single well. A low number and abundance of stygobionts and stygophiles compared with stygoxenes was observed. Our studies showed that bedrock substratum does not influence macroinvertebrate composition.

Pharmaceutical chemicals, steroids and xenoestrogens in water, sediments and fish from the tidal freshwater Potomac River (Virginia, USA)

ABSTRACTSelected pharmaceutical chemicals, steroids and xenoestrogens (PCSXs) consisting of 29 endocrine modulators, therapeutic drugs, pesticides, detergents, plastics, and active ingredients in household products were measured in water, riverbed sediments and fish collected in a tributary embayment of the Potomac River (Hunting Creek, Alexandria, VA, USA) in the vicinity of wastewater discharge. A total of 17 PCSXs were found in the Hunting Creek samples, with steroid hormones (e.g., progesterone and 17α-ethinylestradiol), triclosan, dextromethorphan and bisphenol A being the most prominent micropollutants detected.The geospatial distribution of the PCSXs in Hunting Creek indicated that the steroids correlated with wastewater treatment plant discharge in all matrices, but such an association is tentative in Hunting Creek given the complex nature of urban sources of PCSXs and hydrodynamics in an urban tidal river. The sediment PCSX concentrations correlated with sediment total organic carbon content at all sampling sites. For the most part, the PCSXs showed an enrichment in fish tissue relative to sediments when concentrations were normalized to lipids and sediment organic carbon contents, but the influence of endogenous steroids is also an important consideration for these chemicals.

A 25,000-year record of environmental change from Welsby Lagoon, North Stradbroke Island, in the Australian subtropics

There are few continuous Australian palaeoclimate records that extend beyond the Last Glacial Maximum (LGM), meaning that knowledge of regional climates before, during and after this period is limited. Understanding late-Pleistocene climates of the subtropics is important because of the fundamental role the region plays in the large-scale, global transfer of energy from low latitudes. Palaeoclimate studies of subtropical regions can help define the extent of warming/cooling during the large global climatic events which characterise the late-Pleistocene. Here we report the results from a multi-proxy analysis of a sediment record from Welsby Lagoon on North Stradbroke Island, in the eastern Australian subtropics, spanning the past ca. 25,000 years. Stable C and N isotope analysis and high resolution contiguous records of macrocharcoal deposition and sediment organic content are interpreted in conjunction with a previously published pollen record. Sediment organic content displayed a very strong correlation with total organic carbon (TOC) content as determined through elemental analysis and, given the peaty nature of the sediment, is interpreted as indicative of moisture balance. The proxies reflect wet subtropical climates in the lead up to the LGM which led to an expansion of the wetland. This was followed by a cool, dry and windy LGM (ca. 22.3–19.7'000 years before present; kyr BP), which was punctuated by a brief wet phase ca. 21.7–20.4 kyr BP. A salient feature of the deglacial period is a rapid increase in TOC around 15 kyr BP, coincident with the Antarctic Cold Reversal and Bølling-Allerød warm phase. Increased fire frequency is evident in the Holocene, which is characterised by otherwise stable climate and vegetation. This study supports the notion of variable climates during the LGM and finds an onset of deglacial warming in the Australian subtropics that predates the Holocene.

Holocene climate and environmental history of East Greenland inferred from lake sediments

Prediction of future Arctic climate and environmental changes, as well as associated ice-sheet behavior, requires placing present-day warming and reduced ice extent into a long-term context. Here we present a record of Holocene climate and glacier fluctuations inferred from the paleolimnology of small lakes near Istorvet ice cap in East Greenland. Calibrated radiocarbon dates of organic remains indicate deglaciation of the region before ~10,500 years BP, after which time the ice cap receded rapidly to a position similar to or less extensive than present, and lake sediments shifted from glacio-lacustrine clay to relatively organic-rich gyttja. The lack of glacio-lacustrine sediments throughout most of the record suggests that the ice cap was similar to or smaller than present throughout most of the Holocene. This restricted ice extent suggests that climate was similar to or warmer than present, in keeping with other records from Greenland that indicate a warm early and middle Holocene. Middle Holocene magnetic susceptibility oscillations, with a ~200-year frequency in one of the lakes, may relate to solar influence on local catchment processes. Following thousands of years of restricted extent, Istorvet ice cap advanced to within 365 m of its late Holocene limit at ~AD 1150. Variability in the timing of glacial and climate fluctuations, as well as of sediment organic content changes among East Greenland lacustrine records, may be a consequence of local factors, such as elevation, continentality, water depth, turbidity, and seabirds, and highlights the need for a detailed spatial array of datasets to address questions about Holocene climate change.

Effect of Coupling Process of Wetting-Drying Cycles and Seasonal Temperature Increasing on Sediment Nitrogen Minerization in the Water Level Fluctuating Zone

To reveal the effect of coupling process of wetting-drying and seasonal temperature on sediment nitrogen (N) minerization, surface sediment samples were collected from the water level fluctuating zone(WLFZ) of Pengxi River crossing two hydrological sections. The sediment samples were incubated under drying and submerging conditions at the controlled temperature. The result showed that NO3--N and sand% in the sediment of higher altitude of water level (170 m) were higher than those in low altitudes (150 and 160 m), whereas contents of TN, NH4+-N and clay% and silt% in low altitudes were much higher. Generally, Net N mineralization rate and cumulation were lower in higher altitude of water level during the drying period and submerging period. The ammonification rate decreased rapidly at the initial stage of incubation (0-7 d), and then had no obvious change, and no significant differences among altitudes was observed. The nitrification rate at low altitude decreased with incubation time, while it had only a little change at higher altitude; The nitrification contributed a higher fraction of net N mineralization than ammonification. Net N mineralization rate and its cumulation were significantly higher in the drying period than in the submerging period, while net N mineralization rate decreased with incubation time at all altitudes. Net N mineralization cumulation tended to rise first and then declined at all altitudes of the drying period, whereas it was continuously decreasing at the low water level altitude during the submerging period. Net N nitrogen mineralization rate of the drying period was positively correlate to both the sediment organic matter content and its C:N ratio, while it showed a negative correlation in the submerging period(P<0.001). Net N mineralization was sensitive to temperature increase (Q10>1) in the drying period, while it was insensitive during the submerging period of low altitude (Q10<1). Thus, the impact of temperature on Net N mineralization was relatively low in submerging period of winter and N was accumulated with low releasing rate. In contrast to winter, summer exhibited warmer and drying period, this two factors would lead to higher N mineralization rate and further induce the potential risk of eutrophication as N releasing into water body.

Inundation, Vegetation, and Sediment Effects on Litter Decomposition in Pacific Coast Tidal Marshes

The cycling and sequestration of carbon are important ecosystem functions of estuarine wetlands that may be affected by climate change. We conducted experiments across a latitudinal and climate gradient of tidal marshes in the northeast Pacific to evaluate the effects of climate- and vegetation-related factors on litter decomposition. We manipulated tidal exposure and litter type in experimental mesocosms at two sites and used variation across marsh landscapes at seven sites to test for relationships between decomposition and marsh elevation, soil temperature, vegetation composition, litter quality, and sediment organic content. A greater than tenfold increase in manipulated tidal inundation resulted in small increases in decomposition of roots and rhizomes of two species, but no significant change in decay rates of shoots of three other species. In contrast, across the latitudinal gradient, decomposition rates of Salicornia pacifica litter were greater in high marsh than in low marsh. Rates were not correlated with sediment temperature or organic content, but were associated with plant assemblage structure including above-ground cover, species composition, and species richness. Decomposition rates also varied by litter type; at two sites in the Pacific Northwest, the grasses Deschampsia cespitosa and Distichlis spicata decomposed more slowly than the forb S. pacifica. Our data suggest that elevation gradients and vegetation structure in tidal marshes both affect rates of litter decay, potentially leading to complex spatial patterns in sediment carbon dynamics. Climate change may thus have direct effects on rates of decomposition through increased inundation from sea-level rise and indirect effects through changing plant community composition.

Organophosphate Esters in Sediment of the Great Lakes.

This is the first study on organophosphate ester (OPEs) flame retardants and plasticizers in the sediment of the Great Lakes. Concentrations of 14 OPEs were measured in three sediment cores and 88 Ponar surface grabs collected from Lakes Ontario, Michigan, and Superior of North America. The sum of these OPEs (Σ14OPEs) in Ponar grabs averaged 2.2, 4.7, and 16.6 ng g-1 dw in Lakes Superior, Michigan, and Ontario, respectively. Multiple linear regression analyses demonstrated statistically significant associations between logarithm concentrations of Σ14OPEs as well as selected congeners in surface grab samples and sediment organic carbon content as well as a newly developed urban distance factor. Temporal trends observed in dated sediment cores from Lake Michigan demonstrated that the recent increase in depositional flux to sediment is dominated by chlorinated OPEs, particularly tris(2-chloroisopropyl) phosphate (TCPP), which has a doubling time of about 20 years. Downward diffusion within sediment may have caused vertical fractionation of OPEs over time. Two relatively hydrophilic OPEs including TCPP had much higher concentrations in sediment than estimated based on equilibria between water and sediment organic carbon. Approximately a quarter (17 tonnes) of the estimated total OPE burden (63 tonnes) in Lake Michigan resides in sediment, which may act as a secondary source releasing OPEs to the water column for years to come.

An adsorption and thermodynamic study of ofloxacin on marine sediments

Environmental contextOfloxacin, a widely used fluorinated antibiotic, is resistant to biodegradation and hence can accumulate in the environment. A systematic investigation of ofloxacin on marine sediments showed that sediment organic carbon and heterogeneous sites on sediments play important roles in adsorption processes. The results help our understanding of the environmental behaviour and fate of ofloxacin in marine systems. AbstractThe adsorption behaviour of ofloxacin (OFL) on marine sediments treated by different methods was investigated using batch experiments. Three factors (sediment organic carbon content, salinity and temperature) that may affect the adsorption behaviour of OFL were analysed. The equilibrium time for OFL adsorption on marine sediment in natural seawater was ~4–5h. The adsorption of OFL on all sediments with different treatments fitted the Freundlich model well. The adsorption parameter Kf value was in the order of Kf (H2O2 treatment)&lt;Kf (H2O treatment)&lt;Kf (HCl treatment) over the studied concentration range. The adsorption of OFL was influenced not only by the sediment organic carbon content but also by external factors such as salinity of media and temperature. The adsorption was favourably influenced by decreased salinity and temperature of seawater. The adsorption capacity of OFL on marine sediments decreased with an increase of temperature and salinity. The Kf values decreased from 33.73±1.66 to 22.54±1.12(Lkg−1)1/n when the temperature increased from 283 to 313K. The changes in standard Gibbs free energy (ΔG0) and enthalpy (ΔH0) were −6.62±0.34kJmol−1 and −7.58±0.38kJmol−1 respectively, indicating that the adsorption process of OFL was spontaneous and exothermic. The positive value of the entropy change ΔS0 (i.e. 3.38±0.17JK−1mol−1) suggests that the degree of freedom increased during the adsorption process.

Factors Influencing Brook Trout (Salvelinus frontinalis) Egg Survival and Development in Streams Influenced by Agriculture

Increased sedimentation in surface waters due to anthropogenic activities can lead to the deterioration of salmonid spawning habitat through infiltration of excessive fine sediment into redds. This study assessed the effects of sediment on salmonid reproductive habitat on Prince Edward Island, Canada by evaluating egg survival and fry length using brook trout egg in situ incubations. In-stream egg incubators were buried in the substrate of streams in three watersheds with between 10 and 69% agricultural land-use during 2012-2013 and 2013-2014. Accumulated incubator sediment load and associated particle size, incubator organic matter and stream parameters temperature, velocity, bed sediment and particle size were measured over the period of study. There was no significant difference in embryo survival among watersheds. Survival was not related to fine sediment; instead, it increased with higher average temperature that was in turn associated with the presence of groundwater upwelling in the stream. The size of the fry was also not significantly different between watersheds with varying agricultural activity. Fry length was most strongly related to sediment organic matter content. Brook trout are widespread in PEI rivers, including in many rivers that have high sediment loads due to intensive agricultural land use. This species may avoid the adverse effect of fine sediment through their natural adaptation of using groundwater upwelling for spawning.

Pore water physicochemical constraints on the endangered clubshell mussel (Pleurobema clava)

Freshwater mussels are in decline worldwide, but it remains challenging to link specific stressors to mussel declines. The clubshell mussel (Pleurobema clava) is a federally endangered species that spends most of its life completely buried beneath stream sediments. We tested the hypothesis that clubshell’s decline stems, in part, from low pore water dissolved oxygen (DO) concentrations and toxic ammonia (NH3) levels, resulting from sedimentation of interstitial pore spaces. We measured pore water DO, NH3, interstitial sedimentation rates, and sediment organic matter content in the Tippecanoe River (Indiana, USA) at sites that spanned a range of clubshell populations, including two sites devoid of clubshell. We found little evidence for pore water NH3stress, but pore water DO generally declined with clubshell population and dipped below stress thresholds more frequently at non-clubshell sites than at sites with clubshell. In addition, interstitial sedimentation rates generally increased as clubshell populations declined, suggesting that the low DO concentrations were the result of decreased pore water – surface water exchange. As a result, we conclude that maintaining or improving habitat for clubshell mussels will require the reduction of riverine sediment loading.

Impacts of Sediment Organic Matter Content and pH on Ecotoxicity of Coexposure of TiO2 Nanoparticles and Cadmium to Freshwater Snails Bellamya aeruginosa.

The environmental factors are expected to affect the ecotoxicity of heavy metals in the presence of engineered nanoparticles (NPs) in aquatic ecosystems. However, in sediment scenario, little is known regarding their impacts on the ecotoxicity of co-exposure of sediment-associated heavy metals and NPs. This study evaluated the impacts of different levels of organic matter (OM) (4.8-11.6%) and pH (6-9) on the ecotoxicological effects of co-exposure of sediment-associated titanium dioxide nanoparticles (TiO2-NPs) and cadmium (Cd) to a freshwater gastropod Bellamya aeruginosa. The burdens of Ti and Cd and biomarkers of DNA damage, Na+/K+-ATPase, lipid peroxidation (LPO), and protein carbonylation (PC) in the hepatopancreas were determined following 21 days of exposure. At background level of OM (4.8%) in sediments, TiO2-NPs significantly promoted Cd accumulation in low-Cd treatments (5mg/kg) but did not promote Cd accumulation in high-Cd treatments (25mg/kg). At the relatively higher OM levels (7.1 and 11.6%), TiO2-NPs significantly enhanced Cd accumulation and toxicity as evidenced by aggravated DNA damage, decreased Na+/K+-ATPase activities, and increased LPO and PC levels. Moreover, Cd burdens in both low-Cd and high-Cd treatment were positively correlated with corresponding Ti burdens, indicating TiO2-NPs partially acted as carrier of Cd. At all pH levels, in low-Cd treatments, TiO2-NPs did not affect Cd accumulation, LPO, and PC levels but significantly enhanced DNA damage and slightly facilitated the inhibition of Na+/K+-ATPase activities. In high-Cd treatments, only at pH 9, TiO2-NPs significantly enhanced Cd accumulation and toxicity. Our results implied that interaction between TiO2-NPs and OM or pH significantly affected the accumulation and toxicity of Cd in B. aeruginosa, but the underlying mechanisms need further investigation. Additionally, it should be noted that the potential ecological risk of co-exposure of NPs and coexisting pollutants might be closely species-specific and related to environmental media.

Nitrogen uptake and coupled nitrification–denitrification in riverine sediments with benthic microalgae and rooted macrophytes

Riverine ecosystems receive nitrogen loads from point and diffuse sources that are transferred downstream. Such loads may undergo poorly explored retention and dissipation processes, varying along gradients of nitrogen availability due to different interactions among primary producers and microbial communities. We measured carbon uptake and nitrogen fluxes in microcosms containing riverine sediments with benthic algae and submerged macrophytes (Vallisneria spiralis L.). Coupled nitrification–denitrification rates were determined via 15N–NH4 + injection in the pore water and quantification of the produced 29N2 and 30N2. Two sites with different N–NO3 − concentration and sediment organic content were investigated. We hypothesized that: (1) N–NO3 − availability promotes water column N uptake and attenuates primary producers-bacteria competition; (2) coupled nitrification–denitrification is stimulated by radial oxygen loss from roots; (3) macrophyte meadows favour both temporary nitrogen retention and permanent removal. Sediments with V. spiralis were mostly inorganic C and N sinks and always displayed higher coupled nitrification–denitrification rates compared to sediments with microphytobenthos. Highest rates, up to 100 µmol N m−2 h−1, were measured at the more eutrophic site and in the light. This is likely due to a shift from root to leaf uptake at the N–NO3 − rich site, attenuating plant-bacteria competition, and to increased radial oxygen loss in organic-rich sediments, stimulating nitrification. High rates of N uptake and loss in lotic sediments represent natural buffers preventing N transport downstream and stress the need to preserve aquatic vegetation and its ecosystem services.

Input, behaviour and distribution of multiple elements in abiotic matrices along a transect within the Okavango Delta, northern Botswana.

Wetlands fed by rivers can be a sink for elements depending on elemental concentrations, wetland hydrology, geochemistry, vegetation and climate. In the case of the Okavango Delta, northern Botswana, the outflow discharge is a small fraction (2-5%) of the inflow. This has strong potential consequences for the Delta, as it strongly affects element cycling and storage within the Delta. We estimated the inputs, behaviour and distribution of multiple elements along a longitudinal transect within the Okavango Delta, to show potential effects of retention mechanisms of different elements. High annual element input is rather attributed to discharge than to the concentration within the water, which is generally extremely low. We observed minimal enrichment of the elements within the water pathway along the transect from inflow to outlets, implying that element output is negligible. For most elements, we observed a high correlation between storage and sediment organic matter content. The organic matter content within the sediments was higher in the vegetated sediments than in non-vegetated sediments (factor∼10), and a similar trend was found for most elements. In conclusion, organic matter dominated in sediments from vegetated plots and thus plays an important role in retaining the elements within the sediments of the Delta. This finding has major implications for e.g. planning constructed wetlands for water purification or element retention especially in areas with high evapotranspiration.

Structural equation modelling reveals factors regulating surface sediment organic carbon content and CO2 efflux in a subtropical mangrove

Mangroves are blue carbon ecosystems that sequester significant carbon but release CO2, and to a lesser extent CH4, from the sediment through oxidation of organic carbon or from overlying water when flooded. Previous studies, e.g. Leopold et al. (2015), have investigated sediment organic carbon (SOC) content and CO2 flux separately, but could not provide a holistic perspective for both components of blue carbon. Based on field data from a mangrove in southeast Queensland, Australia, we used a structural equation model to elucidate (1) the biotic and abiotic drivers of surface SOC (10cm) and sediment CO2 flux; (2) the effect of SOC on sediment CO2 flux; and (3) the covariation among the environmental drivers assessed. Sediment water content, the percentage of fine-grained sediment (<63μm), surface sediment chlorophyll and light condition collectively drive sediment CO2 flux, explaining 41% of their variation. Sediment water content, the percentage of fine sediment, season, landform setting, mangrove species, sediment salinity and chlorophyll collectively drive surface SOC, explaining 93% of its variance. Sediment water content and the percentage of fine sediment have a negative impact on sediment CO2 flux but a positive effect on surface SOC content, while sediment chlorophyll is a positive driver of both. Surface SOC was significantly higher in Avicennia marina (2994±186gm−2, mean±SD) than in Rhizophora stylosa (2383±209gm−2). SOC was significantly higher in winter (2771±192gm−2) than in summer (2599±211gm−2). SOC significantly increased from creek-side (865±89gm−2) through mid (3298±137gm−2) to landward (3933±138gm−2) locations. Sediment salinity was a positive driver of SOC. Sediment CO2 flux without the influence of biogenic structures (crab burrows, aerial roots) averaged 15.4mmolm−2d−1 in A. marina stands under dark conditions, lower than the global average dark flux (61mmolm−2d−1) for mangroves.