Sediment Microbial Activity Research Articles

High rates of microbial carbon turnover in sediments in the deepest oceanic trench on Earth

Microbes regulate the decomposition of organic matter in marine sediments. Measurements at the deepest oceanic site on Earth reveal high rates of microbial activity, potentially fuelled by the deposition of organic matter. Microbes control the decomposition of organic matter inmarine sediments. Decomposition, in turn, contributes to oceanic nutrient regeneration and influences the preservation of organic carbon1. Generally, rates of benthic decomposition decline with increasing water depth, although given the vast extent of the abyss, deep-sea sediments are quantitatively important for the global carbon cycle2,3. However, the deepest regions of the ocean have remained virtually unexplored4. Here, we present observations of microbial activity in sediments at Challenger Deep in the Mariana Trench in the central west Pacific, which at almost 11,000 m depth represents the deepest oceanic site on Earth. We used an autonomous micro-profiling system to assess benthic oxygen consumption rates. We show that although the presence of macrofauna is restricted at Challenger Deep, rates of biological consumption of oxygen are high, exceeding rates at a nearby 6,000-m-deep site by a factor of two. Consistently, analyses of sediments collected from the two sites reveal higher concentrations of microbial cells at Challenger Deep. Furthermore, analyses of sediment 210Pb profiles reveal relatively high sediment deposition in the trench. We conclude that the elevated deposition of organic matter at Challenger Deep maintains intensified microbial activity at the extreme pressures that characterize this environment.

Importance of allochthonous matter for profundal macrozoobenthic communities in a deep oligotrophic lake

AbstractThis study evaluates the nutritional importance of allochthonous matter supply for profundal macrozoobenthic communities. It determines chironomid and oligochaete abundances and carbon sources at two sampling sites with different allochthonous contributions in the deep oligotrophic waters of Lake Constance. Site AL, characterized by sedimenting organic matter (POMsed) with a more allochthonous origin was dominated by oligochaetes, whereas site AU characterized by POMsed of predominantly autochthonous origin, was dominated by chironomids. Total macroinvertebrate abundances were 10–20 times higher at site AL compared to site AU. At site AL microbial biomass per gram dry sediment (indicated by ATP content and bacterial abundances) was lower, whereas microbial activity (incorporation of leucine, respiration of glucose) was higher than at site AU. A higher metabolic activity at site AL was also indicated by steeper oxygen gradients in sediment microprofiles. Mean δ13C stable isotope signatures of oligochaetes (δ13C = −33.24 ± 1.27‰) and, to an even greater extent, of the dominant chironomid Micropsectra sp. (δ13C = −40.08 ± 1.65‰) at site AL were depleted compared to site AU for oligochaetes (δ13C = −30.19 ± 0.60‰) and chironomids (δ13C = −30.40 ± 0.75‰). At site AL, mean δ13C values of chironomids and oligochaetes were 11‰ and 4‰ lower, respectively, than mean POMsed δ13C. At site AU, benthic macroinvertebrates and POMsed were of comparable magnitude. The lower invertebrate δ13C values at site AL suggest an increased contribution of methane oxidizing bacteria (MOB) to the diet of the benthic organisms. A mixing model based on stable carbon isotopes suggests that at site AL 21 to 44% of chironomid diet and 9 to 19% of oligochaete diet stems from MOB consumption, whereas calculated MOB consumption was zero at site AU. Mean δ15N values of chironomids and oligochaetes were higher at both sites compared to POMsed, but at site AL (oligochaetes = 7.07 ± 0.61‰, “Micropsectra sp.” = 8.62 ± 0.30‰) the difference was less pronounced than at site AU (oligochaetes = 13.71 ± 0.30‰, “Micropsectra sp.” = 14.00 ± 0.95‰). These differences are considered to reflect mainly the degree of biotic processing of POMsed before consumption by invertebrates. Given that autochthonous POMsed, i.e. chlorophyll sedimentation, was comparable at both sites, but differences were observed with respect to total POMsed sedimentation rates, δ13C and δ15N isotope signatures of POMsed, and microbial activities in the sediments, we suggest that there is a strong contribution of allochthonous POMsed supply to the nutrition of macrobenthic communities at site AL. This suggests that allochthonous carbon supply is at least partly bioavailable and stimulates microbial growth and metabolic activities in lake sediments, which in turn provides additional nutritional sources for benthic organisms. Furthermore, the example of Lake Constance shows that, even in a well‐oxygenated oligotrophic lake, at least part of the allochthonous POMsed is made available to higher trophic levels via methane production and MOB biomass.

Effects of Non-Indigenous Oysters on Microbial Diversity and Ecosystem Functioning

Invasive ecosystem engineers can physically and chemically alter the receiving environment, thereby affecting biodiversity and ecosystem functioning. The Pacific oyster, Crassostrea gigas, invasive throughout much of the world, can establish dense populations monopolising shorelines and possibly altering ecosystem processes including decomposition and nutrient cycling. The effects of increasing cover of invasive C. gigas on ecosystem processes and associated microbial assemblages in mud-flats were tested experimentally in the field. Pore-water nutrients (NH4 + and total oxidised nitrogen), sediment chlorophyll content, microbial activity, total carbon and nitrogen, and community respiration (CO2 and CH4) were measured to assess changes in ecosystem functioning. Assemblages of bacteria and functionally important microbes, including methanogens, methylotrophs and ammonia-oxidisers were assessed in the oxic and anoxic layers of sediment using terminal restriction length polymorphism of the bacterial 16S rRNA, mxaF, amoA and archaeal mcrA genes respectively. At higher covers (40 and 80%) of oysters there was significantly greater microbial activity, increased chlorophyll content, CO2 (13 fold greater) and CH4 (6 fold greater) emission from the sediment compared to mud-flats without C. gigas. At 10% cover, C. gigas increased the concentration of total oxidised nitrogen and altered the assemblage structure of ammonia-oxidisers and methanogens. Concentrations of pore-water NH4 + were increased by C. gigas regardless of cover. Invasive species can alter ecosystem functioning not only directly, but also indirectly, by affecting microbial communities vital for the maintenance of ecosystem processes, but the nature and magnitude of these effects can be non-linear, depending on invader abundance.

Internal wave-induced redox shifts affect biogeochemistry and microbial activity in sediments: a simulation experiment

Internal waves (seiches) are well-studied physical processes in stratified lakes, but their effects on sediment porewater chemistry and microbiology are still largely unexplored. Due to pycnocline oscillations, sediments are exposed to recurrent changes between epilimnetic and hypolimnetic water. This results in strong differences of environmental conditions, which should be reflected in the responses of redox-sensitive biogeochemical processes at both, the sediment–water interface and deeper sediment layers. We tested in a series of mesocosm experiments the influence of seiche-induced redox changes on porewater chemistry and bacterial activity in the sediments under well controlled conditions. Thereby, we excluded effects of changes in current and temperature regimes. For a period of 10 days, intact sediment cores from oligotrophic Lake Stechlin were incubated under constant (either oxic or anoxic) or alternating redox conditions. Solute concentrations were measured as porewater profiles in the sediment, while microbial activity was determined in the upper 0.5 cm of sediment. Oxic and alternating redox conditions resulted in similar ammonium, phosphate, and methane porewater concentrations, while concentrations of each analyte were considerably higher in anoxic cores. Microbial activity was clearly lower in the anoxic cores than in the oxic and the alternating cores. In conclusion, cores with intermittent anoxic phases of up to 24 hours do not differ in biogeochemistry and microbial activities from static oxic sediments. However, due to various physical processes seiches cause oxygen to penetrate deeper into sediment layers, which affects sediment redox gradients and increase microbial activity in seiche-influenced sediments.

The influence of authigenic clay formation on the mineralogy and stable isotopic record of lacustrine carbonates

The mineralogical, compositional and stable isotopic variability of lacustrine carbonates are frequently used as proxies for ancient paleoenvironmental change in continental settings, under the assumption that precipitated carbonates reflect conditions and chemistry of ancient lake waters. In some saline and alkaline lake systems, however, authigenic clay minerals, forming at or near the sediment water interface, are a major sedimentary component. Often these clays are rich in Mg, influencing the geochemical budget of lake waters, and are therefore expected to influence the properties of contemporaneous authigenic carbonate precipitates (which may also contain Mg). This paper documents evidence for a systematic feedback between clay mineral and carbonate authigenesis through multiple precessionally driven, m-scale sedimentary cycles in lacustrine oil-shale deposits of the Eocene Green River Formation from the Uinta Basin (NE Utah). In the studied section, authigenic, Mg-rich, trioctahedral smectite content varies cyclically between 9 and 39wt.%. The highest concentrations occur in oil-shales and calcareous mudstones deposited during high lake level intervals that favored sedimentary condensation, lengthening the time available for clay diagenesis and reducing dilution by other siliciclastic phases. An inverse relation between dolomite percentage of carbonate and trioctahedral smectite abundance suggests the Mg uptake during clay authigenesis provides a first order control on carbonate mineralogy that better explains carbonate mineralogical trends than the possible alternative controls of (1) variable Mg/Ca ratios in lake water and (2) degree of microbial activity in sediments. We also observe that cyclical change in carbonate mineralogy, believed to be induced by clay authigenesis, also causes isotopic covariation between δ13CPDB and δ18OPDB of bulk sediments because of differences in the equilibrium fractionation factors of dolomite and calcite (∼2‰ and ∼2.6%, respectively). This provides an alternative mechanism for the common pattern of isotopic covariation, which is typically attributed to the effect of simultaneous changes in water balance and biological activity on the carbon and oxygen isotopic composition of lake waters. These findings may help improve paleoenvironmental reconstructions based on lacustrine carbonate records by adding to the factors known to influence the mineralogical, compositional and stable isotopic signals recorded by lacustrine carbonates.

IODP Expedition 331: Strong and Expansive Subseafloor Hydrothermal Activities in the Okinawa Trough

Integrated Ocean Drilling Program (IODP) Expedition 331 drilled into the Iheya North hydrothermal system in the middle Okinawa Trough in order to investigate active subseafloor microbial ecosystems and their physical and chemical settings. We drilled five sites during Expedition 331 using special guide bases at three holes for reentry, casing, and capping, including installation of a steel mesh platform with valve controls for postcruise sampling of fluids. At Site C0016, drilling at the base of the North Big Chimney (NBC) mound yielded low recovery, but core included the first Kuroko-type black ore ever recovered from the modern subseafloor. The other four sites yielded interbedded hemipelagic and strongly pumiceous volcaniclastic sediment, along with volcanogenic breccias that are variably hydrothermally altered and mineralized. At most sites, analyses of interstitial water and headspace gas yielded complex patterns with depth and lateral distance of only a few meters. Documented processes included formation of brines and vapor-rich fluids by phase separation and segregation, uptake of Mg and Na by alteration minerals in exchange for Ca, leaching of K at high temperature and uptake at low temperature, anhydrite precipitation, potential microbial oxidation of organic matter and anaerobic oxidation of methane utilizing sulfate, and methanogenesis. Shipboard analyses have found evidence for microbial activity in sediments within the upper 10–30 m below seafloor (mbsf) where temperatures were relatively low, but little evidence in the deeper hydrothermally altered zones and hydrothermal fluid regime. doi:10.2204/iodp.sd.13.03.2011

IODP Expedition 331: Strong and Expansive Subseafloor Hydrothermal Activities in the Okinawa Trough

Integrated Ocean Drilling Program (IODP) Expedition 331 drilled into the Iheya North hydrothermal system in the middle Okinawa Trough in order to investigate active subseafloor microbial ecosystems and their physical and chemical settings. We drilled five sites during Expedition 331 using special guide bases at three holes for reentry, casing, and capping, including installation of a steel mesh platform with valve controls for postcruise sampling of fluids. At Site C0016, drilling at the base of the North Big Chimney (NBC) mound yielded low recovery, but core included the first Kuroko-type black ore ever recovered from the modern subseafloor. The other four sites yielded interbedded hemipelagic and strongly pumiceous volcaniclastic sediment, along with volcanogenic breccias that are variably hydrothermally altered and mineralized. At most sites, analyses of interstitial water and headspace gas yielded complex patterns with depth and lateral distance of only a few meters. Documented processes included formation of brines and vapor-rich fluids by phase separation and segregation, uptake of Mg and Na by alteration minerals in exchange for Ca, leaching of K at high temperature and uptake at low temperature, anhydrite precipitation, potential microbial oxidation of organic matter and anaerobic oxidation of methane utilizing sulfate, and methanogenesis. Shipboard analyses have found evidence for microbial activity in sediments within the upper 10–30 m below seafloor (mbsf) where temperatures were relatively low, but little evidence in the deeper hydrothermally altered zones and hydrothermal fluid regime. <br><br> doi:<a href="http://dx.doi.org/10.2204/iodp.sd.13.03.2011" target="_blank">10.2204/iodp.sd.13.03.2011</a>

Antibiotic resistant bacteria/genes dissemination in lacustrine sediments highly increased following cultural eutrophication of Lake Geneva (Switzerland)

This study investigates faecal indicator bacteria (FIB), multiple antibiotic resistant (MAR), and antibiotic resistance genes (ARGs), of sediment profiles from different parts of Lake Geneva (Switzerland) over the last decades. MARs consist to expose culturable Escherichia coli (EC) and Enterococcus (ENT) to mixed five antibiotics including Ampicillin, Tetracycline, Amoxicillin, Chloramphenicol and Erythromycin. Culture-independent is performed to assess the distribution of ARGs responsible for , β-lactams ( blaTEM; Amoxicillin/Ampicillin) , Streptomycin/Spectinomycin ( aadA) , Tetracycline ( tet) Chloramphenicol ( cmlA) and Vancomycin ( van). Bacterial cultures reveal that in the sediments deposited following eutrophication of Lake Geneva in the 1970s, the percentage of MARs to five antibiotics varied from 0.12% to 4.6% and 0.016% to 11.6% of total culturable EC and ENT, respectively. In these organic-rich bacteria-contaminated sediments, the blaTEM resistant of FIB varied from 22% to 48% and 16% to 37% for EC and ENT respectively, whereas the positive PCR assays responsible for tested ARGs were observed for EC, ENT, and total DNA from all samples. The aadA resistance gene was amplified for all the sediment samples, including those not influenced by WWTP effluent water. Our results demonstrate that bacteria MARs and ARGs highly increased in the sediments contaminated with WWTP effluent following the cultural eutrophication of Lake Geneva. Hence, the human-induced changing limnological conditions highly enhanced the sediment microbial activity, and therein the spreading of antibiotic resistant bacteria and genes in this aquatic environment used to supply drinking water in a highly populated area. Furthermore, the presence of the antibiotic resistance gene aadA in all the studied samples points out a regional dissemination of this emerging contaminant in freshwater sediments since at least the late nineteenth century.

Sediment microbial activities and physic-chemistry as progress indicators of salt marsh restoration processes

Physicochemical (organic matter content, relative humidity, humic acid content, carbon, nitrogen and phosphorous content) and microbiological activities (extracellular enzymatic activities, basal and substrate induced respiration and bioenergetical parameters) were evaluated in salt marsh degraded sediments, during a 1 year revegetation trial starting at the end of the growing season. For this revegetated with Halimione portulacoides and also in nearby sediments with savage H. portulacoides plants and bare sediments were monitored for comparison proposes. Sediments colonized by savage plants showed higher water content, organic matter, carbon, nitrogen and phosphorous contents, in contrast with the low values observed in bare and colonized by transplants sediments. Overlooking the microbial metabolic and respiratory parameters it was observed that the more efficient microbial communities were present in the sediments colonized by the savage plants. Also the extracellular enzymes showed differences. In the transplant colonized sediments and also in the bare ones, peroxidase showed high and similar activities. By the contrary urease showed lower in these last sediments. Only acid phosphatase showed a different trend being higher in the bare sediments and in the sediments colonized by savage plants. Applying a rhizosphere sediment microbial index (RSMI) it was assessed that the more important factors for these sediments evaluation were the Basal respiration, urease activity, metabolic coefficient, total phosphorous content and total carbon content. Using this index for evaluation it showed not only that the sediments colonized by savage plants presented microbial quality, but also it was shown that the bare sediments and the sediments colonized by the transplants after a 1 year period still have the same microbial characteristics and thereby similar microbial qualities. This allowed to conclude that over the duration of this experiment, the revegetated sediments were still far from reaching the quality levels of the sediments naturally colonized, indicating that the 1 year period of revegetation was not enough to a successful restoration of a degraded salt marsh. Although the restoration process was not successful, the elaboration of the RSMI proved to be useful with applications in future monitorization programs.

Environmental fate of pyrethroids in urban and suburban stream sediments and the appropriateness of Hyalella azteca model in determining ecological risk

According to several recent studies using standard acute Hyalella azteca sediment bioassays, increased pyrethroid use in urban and suburban regions in California has resulted in the accumulation of toxic concentrations of pyrethroids in sediments of area streams and estuaries. However, a critical review of the literature indicates that this is likely an overestimation of environmental risk. Hyalella azteca is consistently the most susceptible organism to both aqueous and sediment-associated pyrethroid exposures when compared to a suite of other aquatic taxa. In some cases, H. azteca LC50 values are less than the community HC10 values, suggesting that the amphipod is an overly conservative model for community- or ecosystem-level impacts of sediment-associated pyrethroids. Further, as a model for responses of field populations of H. azteca, the laboratory bioassays considerably overestimate exposure, because the amphipod is more appropriately characterized as an epibenthic organism, not a true sediment dweller; H. azteca preferentially inhabit aquatic macrophytes, periphyton mats, and leaf litter, which drastically reduces their exposure to contaminated sediments. Sediment-bound pyrethroids are transported via downstream washing of fine particulates resulting in longer range transport but also more efficient sequestration of the chemical. In addition, site-specific variables such as sediment organic carbon content, grain size, temperature, and microbial activity alter pyrethroid bioavailability, degradation, and toxicity on a microhabitat scale. The type and source of the carbon in particular, influences the pyrethroid sequestering ability of sediments. The resulting irregular distribution of pyrethroids in stream sediments suggests that sufficient nonimpacted habitat may exist as refugia for resident sediment-dwelling organisms for rapid recolonization to occur. Given these factors, we argue that the amphipod model provides, at best, a screening level assessment of pyrethroid impacts and can correctly identify those sediments not toxic to benthic organisms but cannot accurately predict where sediments will be toxic.

Distribution of nonprescription pharmaceuticals in central Indiana streams and effects on sediment microbial activity

Since the discovery of trace concentrations of pharmaceuticals in streams and treated drinking water around the world, a call has been made by both the scientific community and the general public to increase understanding of the potential effects these compounds may have on freshwater integrity. We measured abundance and distribution of pharmaceuticals in headwater streams across the Upper White River Watershed of central Indiana. Four nonprescription pharmaceuticals (1,7-dimethylxanthine, caffeine metabolite; acetaminophen; caffeine; cotinine, nicotine metabolite) were found at one or more sites with mean concentrations of 0.038, 0.109, 0.057 and 0.041 μg/l, respectively. Caffeine was measured at trace concentrations at all sites sampled. Higher pharmaceutical concentrations were associated with streams having >90% agricultural land use in the sub watershed, suggesting that nonpoint sources, such as septic tanks, may contribute to stream pharmaceutical contamination. To assess the influence of these pharmaceuticals on stream microbial activity, we measured changes in sediment respiration and nutrient uptake in response to pharmaceuticals using both in vitro and in situ techniques. For in vitro experiments, respiration rates were not significantly different from controls with pharmaceutical exposure. However, net NO(3) (-)-N uptake increased significantly with nicotine concentrations. Net NH(4)(+)-N uptake was reduced in response to caffeine and nicotine exposure. In situ experiments indicated nicotine exposure increased microbial respiration. Our data show pharmaceuticals are ubiquitous in headwater streams of central Indiana and likely influence stream microbial activity depending on the pharmaceutical compound and history of exposure.

Environmental effect of sediment dredging in lake:IV influences of dredging on microbial activity and functional diversity of microbial community in sediments and its significance

通过为期-年的疏浚模拟试验,在试验室培养疏浚与对照柱样研究了底泥疏浚对沉积物微生物活性和群落功能多样性的影响。研究结果表明,太湖梅梁湾研究区模拟疏浚表层30cm对沉积物物理、化学和微生物性质影响较大。疏浚沉积物微生物活性显著低于未疏浚沉积物的微生物活性,疏浚对沉积物微生物活性影响较大且在-年的试验周期内难以恢复。底泥疏浚对沉积物微生物功能多样性产生影响,疏浚后初期新生表层沉积物的微生物群落多样性指数显著低于未疏浚沉积物,底泥疏浚改变了沉积物中微生物群落组成,并会导致微生物群落功能多样性降低。底泥疏浚对微生物活性与微生物群落功能多样性产生影响,从而对沉积物中生源要素的循环产生影响。;A laboratory experiment for studying the effects of sediment dredging on microbial activity and functional diversity of mi-crobial community of sediments was carried out through one year. Using incubation of undredged(control) and dredged cores, wefound that dredging the upper 30cm layer of sediment remarkably affected physico-chemical and microbial characteristics of sedi-menu in the study area of Meiliang Bay, Lake Taihu. The microbial activity in dredged sediments was significantly lower(P<0.05)than that in undredged sediments. Sediment dredging affected considerably microbial activity in sediments, and the microbi-al activity in dredged sediments could not recovery in one year experiment. Sediment dredging also affected the functional diversityof microbial community in sediment, the diversity index of microbial community in early dredged sediments was significantly lower(P<0.05) than that in undredged sediments. Sediment dredging modified the community composition of sediment microbial eco-system and resulted in decreasing of diversity of microbial community. Sediment dredging affected microbial activity and functionaldiversity of microbial community, which consequently affected the cycling of biogenic elements in sediments.

Sediment microbial enzyme activity as an indicator of nutrient limitation in the great rivers of the Upper Mississippi River basin

We compared extracellular enzyme activity (EEA) of microbial assemblages in river sediments at 447 sites along the Upper Mississippi, Missouri, and Ohio Rivers with sediment and water chemistry, atmospheric deposition of nitrogen and sulfate, and catchment land uses. The sites represented five unique river reaches—impounded and unimpounded reaches of the Upper Mississippi River, the upper and lower reaches of the Missouri River, and the entire Ohio River. Land use and river chemistry varied significantly between rivers and reaches. There was more agriculture in the two Upper Mississippi River reaches, and this was reflected in higher nutrient concentrations at sites in these reaches. EEA was highest in the two Upper Mississippi River reaches, followed by the lower Missouri River reach. EEA was generally lowest in the upper Missouri River reach. Canonical correlation analysis revealed a strong correlation between EEA and the suite of water and sediment chemistry variables, and the percent of the catchment in anthropogenically dominated land uses, including agriculture and urban development. Nutrient ratios of the waters and sediments suggested carbon (C), nitrogen (N), or phosphorus (P) limitation at a large number of sites in each reach. C-limitation was most pronounced in the unimpounded Mississippi River and lower Missouri River reaches; N-limitation was prevalent in the two Missouri River reaches; and P-limitation dominated the Ohio River. Linking microbial enzyme activities to regional-scale anthropogenic stressors in these large river ecosystems suggests that microbial enzyme regulation of carbon and nutrient dynamics may be sensitive indicators of anthropogenic nutrient and carbon loading.

Mercury biomethylation assessment in the estuary of Bilbao (North of Spain)

The relationship between the microbial methylation of mercury and the microbial activities in sediments and water collected from the estuary of Bilbao (North of Spain) was studied in three different sampling points and in two different seasons. Three different cultures were prepared with a sediment slurry to distinguish between biotic and abiotic methylation pathways and the variations of the methylmercury concentration and the variations of the population of total number of bacteria (TDC), anaerobic heterotrophic bacteria (AHB), sulphate-reducing bacteria (SRB) and Desulfovibrio were measured. From this work, it can be concluded that the variation of MeHg concentrations is a result of the methylation/demethylation processes in the sediments, and that the abiotic processes have a negligible contribution to those processes. According to the statistical analysis of the results (partial least squares analysis) a significant statistical correlation was established between methylmercury and the SRB counts.

The functioning of the microbial community in bottom sediments of Lake Baikal, with special regard to hydrothermal and gas hydrate regions

The anaerobic microbial community in deep-water sediments of Lake Baikal is mainly methanogenic. Organic matter utilization as a result of methane production exceeds that resulting from sulfate reduction. The study of microbial activity in sediments of regions of hydrothermal venting and gas hydrates shows similarities with hydrothermal oceanic regions, and indicates an important role of methanogenic and methanotropic communities in the carbon cycle of these regions of Baikal.

Sediment microbial enzyme activity as an indicator of nutrient limitation in Great Lakes coastal wetlands

Summary1. We compared the extracellular enzyme activity (EEA) of sediment microbial assemblages with sediment and water chemistry, gradients in agricultural nutrient loading (derived from principal component analyses), atmospheric deposition and hydrological turnover time in coastal wetlands of the Laurentian Great Lakes.2. There were distinct increases in nutrient concentrations in the water and in atmospheric N deposition along the gradient from Lake Superior to Lake Ontario, but few differences between lakes in sediment carbon (C), nitrogen (N) or phosphorus (P). Wetland water and sediment chemistry were correlated with the agricultural stress gradient, hydrological turnover time and atmospheric deposition.3. The N : P ratio of wetland waters and sediments indicated that these coastal wetlands were N‐limited. Nutrient stoichiometry was correlated with the agricultural stress gradient, hydrological turnover time and atmospheric deposition.4. Extracellular enzyme activity was correlated with wetland sediment and water chemistry and stoichiometry, atmospheric N deposition, the agricultural stress gradient and the hydrological turnover time. The ratios of glycosidases to peptidases and phosphatases yielded estimates of nutrient limitation that agreed with those based solely on nutrient chemistry.5. This study, the first to link microbial enzyme activities to regional‐scale anthropogenic stressors, suggests that quantities and ratios of microbial enzymes are directly related to the concentrations and ratios of limiting nutrients, and may be sensitive indicators of nutrient dynamics in wetland ecosystems, but further work is needed to elucidate these relationships.

Rivers as groundwater-dependent ecosystems: a review of degrees of dependency, riverine processes and management implications

Many rivers are classified as groundwater-dependent ecosystems (GDEs), owing to the contribution of groundwater to their base flow. However, there has been little explicit recognition of the way groundwater influences riverine biota or processes, how degrees of ecological dependency may vary, and the management implications of this dependency. The permeable beds and banks of these GDEs where surface water and groundwater exchange are termed ‘hyporheic zones’. They are often inhabited by invertebrates, with varying reliance on groundwater, although the ecological roles of these invertebrates are little known. Upwelling hyporheic water can promote surface primary productivity, influence sediment microbial activity, and affect organic matter decomposition. In many intermittent streams, variable groundwater inputs alter the duration of flow or water permanence, and the duration and timing of these largely govern the biota and rates of many ecosystem processes (e.g. leaf decomposition). Not only is the physical presence of water important, thermal and chemical conditions arising from groundwater inputs also have direct and indirect effects on riverine biota and rates or types of in-stream processes. Differing degrees of dependency of rivers on groundwater mediate all these influences, and may change over time and in response to human activities. Alteration of groundwater inputs through extraction from riparian wells or changes in local water table have an impact on these GDEs, and some current management plans aim to restrict groundwater extraction from near permeable river channels. However, these are often ‘blanket’ restrictions and the mechanisms of GDE dependency or timing of groundwater requirements are poorly understood, hampering refinement of this management approach. More effective management of these GDEs into the future can result only from a better understanding of the mechanisms of the dependency, how these vary among river types and what in-stream changes might be predicted from alteration of groundwater inputs.

Temperature-driven decoupling of key phases of organic matter degradation in marine sediments.

The long-term burial of organic carbon in sediments results in the net accumulation of oxygen in the atmosphere, thereby mediating the redox state of the Earth's biosphere and atmosphere. Sediment microbial activity plays a major role in determining whether particulate organic carbon is recycled or buried. A diverse consortium of microorganisms that hydrolyze, ferment, and terminally oxidize organic compounds mediates anaerobic organic matter mineralization in anoxic sediments. Variable temperature regulation of the sequential processes, leading from the breakdown of complex particulate organic carbon to the production and subsequent consumption of labile, low-molecular weight, dissolved intermediates, could play a key role in controlling rates of overall organic carbon mineralization. We examined sediment organic carbon cycling in a sediment slurry and in flow through bioreactor experiments. The data show a variable temperature response of the microbial functional groups mediating organic matter mineralization in anoxic marine sediments, resulting in the temperature-driven decoupling of the production and consumption of organic intermediates. This temperature-driven decoupling leads to the accumulation of labile, low-molecular weight, dissolved organic carbon at low temperatures and low-molecular weight dissolved organic carbon limitation of terminal metabolism at higher temperatures.

Detailed observation and measurement of sewer sediment erosion under aerobic and anaerobic conditions

A greater understanding of the erosion behaviour of sewer sediments is necessary in order to reliably estimate the amount and nature of the sewer sediments released from deposits in sewers and transported either to waste water treatment plants or discharged into the environment. Research has indicated that microbial activity in sediment can influence the physical release of sediment from in-pipe deposits. This paper reports on a series of erosion tests in which sewer sediments from different sewer networks are kept under different environmental conditions and their resistance to erosion is examined. The erosion tests are carried out under aerobic and anaerobic conditions and two temperatures, one representing ambient sewer temperatures and a lower temperature that significantly suppresses bacterial activity.

Relative influence of bioturbation and predation on organic matter processing in river sediments: a microcosm experiment

Summary1. Our objective was to measure the effects of bioturbation and predation on the physical characteristics and biogeochemical processes in river sediments.2. We investigated the impacts of tubificid worms tested separately and together with an omnivore (Gammarus pulex), which does feed on tubificids, on sediment distribution, water flux, sediment organic carbon, biofilm biomass and microbial activities, and the concentrations of dissolved oxygen, dissolved organic carbon, PO, NO, NO and NH in slow filtration sand–gravel columns. We hypothesised that gammarids, which exploit the top 2–3 cm of the sediment, would modify the impact of worms at the sediment surface.3. In experiments both with and without gammarids, bioturbation by the tubificids modified both the distribution of surface particles in the sediment column and water flux. In addition, microbial aerobic (oxygen consumption) and anaerobic (denitrification and fermentative decomposition of organic matter) processes in the sediment were stimulated in the presence of tubificid worms. However, G. pulex did not affect either the density or bioturbation activity of the tubificid worms.4. Bioturbation by the benthos can be a major process in river habitats, contributing to the retention of organic matter in sediment dynamics. The presence of at least one predator had no effect on bioturbation in sediments. In such systems, physical heterogeneity may be sufficient for tubificids to escape from generalist predators, though more specialised ones might have more effect.