Biogeochemical processes at the sediment–water interface, Bombah Broadwater, Myall Lakes

Myall Lakes has experienced algal blooms in recent years which threaten water quality. Biomarkers, benthic fluxes measured with chambers, and pore water metabolites were used to identify the nature and reactivity of organic matter (OM) in the sediments of Bombah Broadwater (BB), and the processes controlling sediment-nutrient release into the overlying waters. The OM in the sediments was principally from algal sources although terrestrial OM was found near the Myall River. Terrestrial faecal matter was identified in muddy sediments and was probably sourced via runoff from farm lands. The reactive OM which released nutrients into the overlying waters was from diatoms, dinoflagellates and probably cyanobacteria. Microcystis filaments were observed in surface sediments. OM degradation rates varied between 5.3 and 47.1 mmol m^?2 day^?1 (64–565 mg m^?2 day^?1), were highest in the muddy sediments and sulphate reduction rates accounted for 20–40% of the OM degraded. Diatoms, being heavy sink rapidly, and are an important vector to transport catchment N and P to sites of denitrification and P-trapping in the sediments. Denitrification rates (mean ～4 mmol N m^?2 day^?1), up to 7 mmol N m^?2 day^?1 (105 mg N m^?2 day^?1) were measured, and denitrification efficiencies were highest (mean = 86 ± 4%) in the sandy sediments (～20% of the area of BB), but lower in the muddy sediments (mean = 63 ± 15%). These differences probably result from higher OM loads and anaerobic respiration in muddy sediments. Most DIP (>70%) from OM degradation was not released into overlying waters but remained trapped in surface sediments. Biophysical (advective) processes were responsible for the measured metabolite (O₂, CO₂, DSi, DIN and DIP) fluxes across the sediment–water interface.     

Carbon dioxide and methane emissions from interfluvial wetlands in the upper Negro River basin, Brazil

Extensive interfluvial wetlands occur in the upper Negro River basin (Brazil) and contain a mosaic of vegetation dominated by emergent grasses and sedges with patches of shrubs and palms. To characterize the release of carbon dioxide and methane from these habitats, diffusive and ebullitive emissions and transport through plant aerenchyma were measured monthly during 2005 in permanently and seasonally flooded areas. CO₂ emissions averaged 2193 mg C m^?2 day^?1. Methane was consumed in unflooded environments and emitted in flooded environments with average values of ?4.8 and 60 mg C m^?2 day^?1, respectively. Bubbles were emitted primarily during falling water periods when hydrostatic pressure at the sediment?Cwater interface declined. CO₂ and CH₄ emissions increased when dissolved O₂ decreased and vegetation was more abundant. Total area and seasonally varying flooded areas for two wetlands, located north and south of the Negro River, were determined through analysis of synthetic aperture radar and optical remotely sensed data. The combined areas of these two wetlands (3000 km²) emitted 1147 Gg C year^?1 as CO₂ and 31 Gg C year^?1 as CH₄. If these rates are extrapolated to the area occupied by hydromorphic soils in the upper Negro basin, 63 Tg C year^?1 of CO₂ and 1.7 Tg C year^?1 as CH₄ are estimated as the regional evasion to the atmosphere.     

Productivity and Temperature as Drivers of Seasonal and Spatial Variations of Dissolved Methane in the Southern Bight of the North Sea

Dissolved CH₄ concentrations in the Belgian coastal zone (North Sea) ranged between 670 nmol l^?1 nearshore and 4 nmol l^?1 offshore. Spatial variations of CH₄ were related to sediment organic matter (OM) content and gassy sediments. In nearshore stations with fine sand or muddy sediments, the CH₄ seasonal cycle followed water temperature, suggesting methanogenesis control by temperature in these OM-rich sediments. In offshore stations with permeable sediments, the CH₄ seasonal cycle showed a yearly peak following the chlorophyll-a spring peak, suggesting that in these OM-poor sediments, methanogenesis depended on freshly produced OM delivery. This does not exclude the possibility that some CH₄ might originate from dimethylsulfide (DMS) or dimethylsulfoniopropionate (DMSP) or methylphosphonate transformations in the most offshore stations. Yet, the average seasonal CH₄ cycle was unrelated to those of DMS(P), very abundant during the Phaeocystis bloom. The annual average CH₄ emission was 126 mmol m^?2 y^?1 in the most nearshore stations (~4 km from the coast) and 28 mmol m^?2 y^?1 in the most offshore stations (~23 km from the coast), 1260–280 times higher than the open ocean average value (0.1 mmol m^?2 y^?1). The strong control of CH₄ by sediment OM content and by temperature suggests that marine coastal CH₄ emissions, in particular in shallow areas, should respond to future eutrophication and warming of climate. This is supported by the comparison of CH₄ concentrations at five stations obtained in March 1990 and 2016, showing a decreasing trend consistent with alleviation of eutrophication in the area.     

Contribution of transparent exopolymer particles to carbon sinking flux in an oligotrophic reservoir

Transparent exopolymer particles (TEP) compose an important pool of particulate organic matter (POM) in aquatic systems. However, no studies of TEP contribution to C export to sediment exist for freshwaters. We quantify the contribution of TEP to C sinking fluxes in an oligotrophic reservoir (Quéntar, Southern Spain) by monitoring TEP in the water column and TEP, particulate organic carbon (POC) and dry weight in sedimentation traps. TEP sinking fluxes ranged from 0.73 to 183.23 mg C m^?2 day^?1 and from 0.51 to 177.04 mg C m^?2 day^?1 at the surface and at the bottom layer, respectively. These values represent that, over an annual basis, 5.59 Ton TEP-C (over 61.32 Ton POC) are exported, on an average, from the water column to the sediment of Quentar reservoir. TEP concentrations (average = 48.0 μg XG eq l^?1) were lower than the scarce data reported for freshwaters. No significant relationships between TEP and Chl a concentrations or BA were observed. Average value for daily sedimentation flux (6.63 g Dry Weight m^?2 day^?1) in the study reservoir was higher than that documented for low productive natural aquatic ecosystems as a consequence of the high amount of allochthonous material input characterizing reservoirs. TEP contributed to C export to sediment with a value that range from 0.02 to 31%. Our results show that even in man-made systems, which are predominantly controlled by allochthonous inputs, TEP may be relevant for explaining POM settling fluxes.     

The relative contributions of biological and abiotic processes to carbon dynamics in subarctic sea ice

Knowledge on the relative effects of biological activity and precipitation/dissolution of calcium carbonate (CaCO₃) in influencing the air-ice CO₂ exchange in sea-ice-covered season is currently lacking. Furthermore, the spatial and temporal occurrence of CaCO₃ and other biogeochemical parameters in sea ice are still not well described. Here we investigated autotrophic and heterotrophic activity as well as the precipitation/dissolution of CaCO₃ in subarctic sea ice in South West Greenland. Integrated over the entire ice season (71 days), the sea ice was net autotrophic with a net carbon fixation of 56 mg C m^?2, derived from a sea-ice-related gross primary production of 153 mg C m^?2 and a bacterial carbon demand of 97 mg C m^?2. Primary production contributed only marginally to the TCO₂ depletion of the sea ice (7–25 %), which was mainly controlled by physical export by brine drainage and CaCO₃ precipitation. The net biological production could only explain 4 % of this sea-ice-driven CO₂ uptake. Abiotic processes contributed to an air-sea CO₂ uptake of 1.5 mmol m^?2 sea ice day^?1, and dissolution of CaCO₃ increased the air-sea CO₂ uptake by 36 % compared to a theoretical estimate of melting CaCO₃-free sea ice. There was a considerable spatial and temporal variability of CaCO₃ and the other biogeochemical parameters measured (dissolved organic and inorganic nutrients).     

Gaseous and fluvial carbon export from an Amazon forest watershed

The transfer of carbon (C) from Amazon forests to aquatic ecosystems as CO₂ supersaturated in groundwater that outgases to the atmosphere after it reaches small streams has been postulated to be an important component of terrestrial ecosystem C budgets. We measured C losses as soil respiration and methane (CH₄) flux, direct CO₂ and CH₄ fluxes from the stream surface and fluvial export of dissolved inorganic C (DIC), dissolved organic C (DOC), and particulate C over an annual hydrologic cycle from a 1,319-ha forested Amazon perennial first-order headwater watershed at Tanguro Ranch in the southern Amazon state of Mato Grosso. Stream pCO₂ concentrations ranged from 6,491 to 14,976 ??atm and directly-measured stream CO₂ outgassing flux was 5,994 ± 677 g C m^?2 y^?1 of stream surface. Stream pCH₄ concentrations ranged from 291 to 438 ??atm and measured stream CH₄ outgassing flux was 987 ± 221 g C m^?2 y^?1. Despite high flux rates from the stream surface, the small area of stream itself (970 m², or 0.007% of watershed area) led to small directly-measured annual fluxes of CO₂ (0.44 ± 0.05 g C m² y^?1) and CH₄ (0.07 ± 0.02 g C m² y^?1) per unit watershed land area. Measured fluvial export of DIC (0.78 ± 0.04 g C m^?2 y^?1), DOC (0.16 ± 0.03 g C m^?2 y^?1) and coarse plus fine particulate C (0.001 ± 0.001 g C m^?2 y^?1) per unit watershed land area were also small. However, stream discharge accounted for only 12% of the modeled annual watershed water output because deep groundwater flows dominated total runoff from the watershed. When C in this bypassing groundwater was included, total watershed export was 10.83 g C m^?2 y^?1 as CO₂ outgassing, 11.29 g C m^?2 y^?1 as fluvial DIC and 0.64 g C m^?2 y^?1 as fluvial DOC. Outgassing fluxes were somewhat lower than the 40?C50 g C m^?2 y^?1 reported from other Amazon watersheds and may result in part from lower annual rainfall at Tanguro. Total stream-associated gaseous C losses were two orders of magnitude less than soil respiration (696 ± 147 g C m^?2 y^?1), but total losses of C transported by water comprised up to about 20% of the ± 150 g C m^?2 (±1.5 Mg C ha^?1) that is exchanged annually across Amazon tropical forest canopies.     

When Water Vanishes: Magnitude and Regulation of Carbon Dioxide Emissions from Dry Temporary Streams

Most fluvial networks worldwide include watercourses that recurrently cease to flow and run dry. The spatial and temporal extent of the dry phase of these temporary watercourses is increasing as a result of global change. Yet, current estimates of carbon emissions from fluvial networks do not consider temporary watercourses when they are dry. We characterized the magnitude and variability of carbon emissions from dry watercourses by measuring the carbon dioxide (CO₂) flux from 10 dry streambeds of a fluvial network during the dry period and comparing it to the CO₂ flux from the same streambeds during the flowing period and to the CO₂ flux from their adjacent upland soils. We also looked for potential drivers regulating the CO₂ emissions by examining the main physical and chemical properties of dry streambed sediments and adjacent upland soils. The CO₂ efflux from dry streambeds (mean ± SD = 781.4 ± 390.2 mmol m^?2 day^?1) doubled the CO₂ efflux from flowing streambeds (305.6 ± 206.1 mmol m^?2 day^?1) and was comparable to the CO₂ efflux from upland soils (896.1 ± 263.2 mmol m^?2 day^?1). However, dry streambed sediments and upland soils were physicochemically distinct and differed in the variables regulating their CO₂ efflux. Overall, our results indicate that dry streambeds constitute a unique and biogeochemically active habitat that can emit significant amounts of CO₂ to the atmosphere. Thus, omitting CO₂ emissions from temporary streams when they are dry may overlook the role of a key component of the carbon balance of fluvial networks.     

CO2 dynamics along Danish lowland streams: water–air gradients, piston velocities and evasion rates

We measured CO₂ concentration and determined evasion rate and piston velocity across the water–air interface in flow-through chambers at eight stations along two 20 km long streams in agricultural landscapes in Zealand, Denmark. Both streams were 9–18-fold supersaturated in CO₂ with daily means of 240 and 340 μM in January–March and 130 and 180 μM in June–August. Annual CO₂ medians were 212 μM in six other streams and 460 μM in four groundwater wells, while seven lakes were weakly supersaturated (29 μM). Air concentrations immediately above stream surfaces were close to mean atmospheric conditions except during calm summer nights. Piston velocity from 0.4 to 21.6 cm h^?1 was closely related to current velocity permitting calculation of evasion rates for entire streams. CO₂ evasion rates were highest in midstream reaches (170–1,200 mmol m^?2 day^?1) where CO₂-rich soil water entered fast stream flow, while rates were tenfold lower (25–100 mmol m^?2 day^?1) in slow-flowing lower reaches. CO₂ evasion mainly derived from the input of CO₂ in soil water. The variability of CO₂ evasion along the two lowland streams covered much of the range in sub-Arctic and temperate streams reported previously. In budgets for the two stream catchments, loss of carbon from soils via the hydrological cycle was substantial (3.2–5.7 mmol m^?2 day^?1) and dominated by CO₂ consumed to form HCO₃ ^? by mineral dissolution (69–76%) and export of organic carbon (15–23%) relative to dissolved CO₂ export (7–9%).     

The carbon dioxide evasion cycle of an intermittent first-order stream: contrasting water–air and soil–air exchange

Ephemeral streams and wetlands are characterized by complex cycles of submersion and emersion, which influence the greenhouse gas flux rates. In this study we quantify the spatiotemporal variability in CO₂ and CH₄ concentrations and fluxes of an intermittent first-order stream over three consecutive wet and dry cycles spanning 56 days, to assess how hydrologic phase transitions influence greenhouse gas evasion. Water column excess CO₂ ranged from ?11 to 1600 μM, and excess CH₄ from 1 to 15 μM. After accounting for temporal changes in the ratio of wet versus dry streambed hydraulic radius, total CO₂–C fluxes ranged from 12 to 156 mmol m^?2 day^?1, with an integrated daily mean of 61 ± 25 mmol m^?2 day^?1. Soil–air evasion rates were approximately equal to those of water–air evasion. Rainfall increased background water–air CO₂–C fluxes by up to 780% due to an increase in gas transfer velocity in the otherwise still waters. CH₄–C fluxes increased 19-fold over the duration of the initial, longer wet-cycle from 0.1 to 1.9 mmol m^?2 day^?1. Temporal shifts in water depth and site-specific ephemerality were key drivers of carbon dynamics in the upper Jamison Creek watercourse. Based on these findings, we hypothesise that the cyclic periodicity of fluxes of biogenic gases from frequently intermittent streams (wet and dry cycles ranging from days to weeks) and seasonally ephemeral watercourses (dry for months at a time) are likely to differ, and therefore these differences should be considered when integrating transient systems into regional carbon budgets and models of global change.     

Carbon Sequestration in a Large Hydroelectric Reservoir: An Integrative Seismic Approach

Artificial reservoirs likely accumulate more carbon than natural lakes due to their unusually high sedimentation rates. Nevertheless, the actual magnitude of carbon accumulating in reservoirs is poorly known due to a lack of whole-system studies of carbon burial. We determined the organic carbon (OC) burial rate and the total OC stock in the sediments of a tropical hydroelectric reservoir by combining a seismic survey with sediment core sampling. Our data suggest that no sediment accumulation occurs along the margins of the reservoir and that irregular bottom morphology leads to irregular sediment deposition. Such heterogeneous sedimentation resulted in high spatial variation in OC burial—from 0 to 209 g C m^?2 y^?1. Based on a regression between sediment accumulation and OC burial rates (R ² = 0.94), and on the mean reservoir sediment accumulation rate (0.51 cm y^?1, from the seismic survey), the whole-reservoir OC burial rate was estimated at 42.2 g C m^?2 y^?1. This rate was equivalent to 70% of the reported carbon emissions from the reservoir surface to the atmosphere and corresponded to a total sediment OC accumulation of 0.62 Tg C since the reservoir was created. The approach we propose here allows an inexpensive and integrative assessment of OC burial in reservoirs by taking into account the high degree of spatial variability and based on a single assessment. Because burial can be assessed shortly after the survey, the approach combining a seismic survey and coring could, if applied on a larger scale, contribute to a more complete estimate of carbon stocks in freshwater systems in a relatively short period of time.     

Carbon flows in eutrophic Lake Rotsee: a <Superscript>13</Superscript>C-labelling experiment

The microbial segment of food webs plays a crucial role in lacustrine food-web functioning and carbon transfer, thereby influencing carbon storage and CO₂ emission and uptake in freshwater environments. Variability in microbial carbon processing (autotrophic and heterotrophic production and respiration based on glucose) with depth was investigated in eutrophic, methane-rich Lake Rotsee, Switzerland. In June 2011, ¹³C-labelling experiments were carried out at six depth intervals in the water column under ambient light as well as dark conditions to evaluate the relative importance of (chemo)autotrophic, mixotrophic and heterotrophic production. Label incorporation rates of phospholipid-derived fatty acid (PLFA) biomarkers allowed us to differentiate between microbial producers and calculate group-specific production. We conclude that at 6 m, net primary production (NPP) rates were highest, dominated by algal photoautotrophic production. At 10 m —the base of the oxycline— a distinct low-light community was able to fix inorganic carbon, while in the hypolimnion, heterotrophic production prevailed. At 2 m depth, high label incorporation into POC could only be traced to nonspecific PLFA, which prevented definite identification, but suggests cyanobacteria as dominating organisms. There was also depth zonation in extracellular carbon release and heterotrophic bacterial growth on recently fixed carbon. Large differences were observed between concentrations and label incorporation of POC and biomarkers, with large pools of inactive biomass settling in the hypolimnion, suggesting late-/post-bloom conditions. Net primary production (115 mmol C m^?2 d^?1) reached highest values in the epilimnion and was higher than glucose-based production (3.3 mmol C m^?2 d^?1, highest rates in the hypolimnion) and respiration (5.9 mmol C m^?2 d^?1, highest rates in the epilimnion). Hence, eutrophic Lake Rotsee was net autotrophic during our experiments, potentially storing large amounts of carbon.     

Drivers of Sediment Accumulation and Nutrient Burial in Coastal Stormwater Detention Ponds,South Carolina,USA

Stormwater detention ponds are widely utilized as control structures to manage runoff during storm events. These ponds also represent biogeochemical hotspots, where carbon (C) and nutrients can be processed and buried in sediments. This study quantified C and nutrient [nitrogen (N) and phosphorus (P)] sources and burial rates in 14 stormwater detention ponds representative of typical residential development in coastal South Carolina. Bulk sediment accumulation was directly correlated with catchment impervious surface coverage (R² = 0.90) with sediment accumulation rates ranging from 0.06 to 0.50 cm y^?1. These rates of sediment accumulation and consequent pond volume loss were lower than anticipated based on maintenance guidelines provided by the State. N-alkanes were used as biomarkers of sediment source; the derived terrestrial aquatic ratio (TAR_HC) index was strongly correlated with sediment accumulation rate (R² = 0.71) which, in conjunction with high C/N ratios (16–33), suggests that terrestrial biomass drives this sediment accumulation, with relatively minimal contributions from algal derived material. This is counter to expectations that were based on the high algal productivity generally observed in stormwater ponds and previous studies of natural lakes. Sediment C and nutrient concentrations were consistent among ponds, such that differences in burial rates were a simple function of bulk sediment accumulation rate. These burial rates (C: 8.7–161 g m^?2 y^?1, N: 0.65–6.4 g m^?2 y^?1, P: 0.238–4.13 g m^?2 y^?1) were similar to those observed in natural lake systems, but lower than those observed in reservoirs or impoundments. Though individual ponds were small in area (930–41,000 m²), they are regionally abundant and, when mean burial rates are extrapolated to the regional scale (≈ 21,000 ponds), ultimately sequester 2.0 × 10⁹ g C y^?1, 9.5 × 10⁷ g N y^?1, and 3.7 × 10⁷ g P y^?1 in the coastal region of South Carolina alone. Stormwater ponds represent a relatively new but increasingly significant feature of the coastal landscape and, thus, are a key component in understanding how urbanization alters the transport and transformations of C and nutrients between terrestrial uplands and downstream receiving waters.     

Primary production of aquatic macrophytes and their epiphytes in two shallow lakes (Peipsi and Võrtsjärv) in Estonia

In shallow lakes with large littoral zones, epiphytes and submerged macrophytes can make an important contribution to the total annual primary production. We investigated the primary production (PP) of phytoplankton, submerged macrophytes, and their epiphytes, from June to August 2005, in two large shallow lakes. The production of pelagic and littoral phytoplankton and of the dominant submerged macrophytes in the littoral zone (Potamogeton perfoliatus in Lake Peipsi and P. perfoliatus and Myriopyllum spicatum in Lake Võrtsjärv) and of their epiphytes was measured using a modified ¹⁴C method. The total PP of the submerged macrophyte area was similar in both lakes: 12.4 g C m^?2 day^?1 in Peipsi and 12.0 g C m^?2 day^?1 in Võrtsjärv. In Peipsi, 84.2% of this production was accounted for by macrophytes, while the shares of phytoplankton and epiphytes were low (15.6 and 0.16%, respectively). In Võrtsjärv, macrophytes contributed 58%, phytoplankton 41.9% and epiphytes 0.1% of the PP in the submerged macrophyte area. Epiphyte production in both lakes was very low in comparison with that of phytoplankton and macrophytes: 0.01, 5.04, and 6.97 g C m^?2 day^?1, respectively, in Võrtsjärv, and 0.02, 1.93, and 10.5 g C m^?2 day^?1, respectively, in Peipsi. The PP of the littoral area contributed 10% of the total summer PP of Lake Peipsi sensu stricto and 35.5% of the total summer PP of Lake Võrtsjärv.     

Methane fluxes from a patterned fen of the northeastern part of the La Grande river watershed, James Bay, Canada

In northeastern Canada, at the ecotonal limit of the forest tundra and lichen woodland, a rise of the regional water table in the peatland systems was registered since Little Ice Age resulting in increasing pool compartment at the expense of terrestrial surfaces. We hypothesized that, with a mean water table closer to peat surface and higher pool density, these ecosystems would be great CH₄ emitters. In summers 2009 and 2010, methane fluxes were measured in a patterned fen located in the northeastern portion of the La Grande river watershed to determine the contribution of the different microforms (lawns, hollows, hummocks, string, pools) to the annual CH₄ budget. Mean seasonal CH₄ fluxes from terrestrial microforms ranged between 12.9 and 49.4 mg m^?2 day^?1 in 2009 and 15.4 and 47.3 mg m^?2 day^?1 in 2010. Pool fluxes (which do not include ebullition fluxes) ranged between 102.6 and 197.6 mg CH₄ m^?2 day^?1 in 2009 and 76.5 and 188.1 mg CH₄ m^?2 day^?1 in 2010. Highest fluxes were measured in microforms with water table closer to peat surface but no significant relationship was observed between water table depth and CH₄ fluxes. Spatially weighted CH₄ budget demonstrates that, during the growing season, the studied peatland emitted 66 ± 31 in 2009 and 55 ± 26 mg CH₄ m^?2 day^?1 in 2010, 79 % of which is accounted by pool fluxes. In a context where climate projections predict greater precipitations in northeastern Canada, these results indicate that this type of peatlands could contribute to modify the methane balance in the atmosphere.     

CNP budgets of a coral-dominated fringing reef at La Réunion,France: coupling of oceanic phosphate and groundwater nitrate

Productivity, nutrient input, nutrient uptake, and release rates were determined for a coral-dominated reef flat at La Réunion, France, to assess the influence of groundwater nitrogen on carbon and nutrient budgets. Water samples were collected offshore in the ocean, at the reef crest and back reef for nutrients, picoplankton, pH, and total alkalinity. Volume transport of ocean water across the reef flat was measured using both current meters and drogues. Groundwater advected onto the reef flat and mixed with incoming ocean water. Metabolic rates for the reef community were determined to be: gross primary production = 1,000 mmol C m⁻² d⁻¹, community respiration = 960 mmol C m⁻² d⁻¹, and community calcification = 210 mmol C m⁻² d⁻¹. Across the reef flat, silicate behaved conservatively, there was net uptake of phosphate (0.06 mmol P m⁻² d⁻¹) and net release of nitrate, ammonia, dissolved and particulate organic nitrogen (total 7.0 mmol N m⁻² d⁻¹). Groundwater nitrate contributed 37% of the increase in nitrate plus ammonia. The first-order mass transfer coefficient of phosphate was 3.3 m d⁻¹, and for nitrate plus ammonia, 5.9 m d⁻¹. Gross N and P uptake from estimates of mass transfer and uptake of particles were 0.37 mmol P m⁻² d⁻¹ and 7.2 mmol N m⁻² d⁻¹, respectively giving an N:P uptake ratio of 20:1. Thus, the elevation of nitrogen across the reef flat maintains a high N:P flux, enhancing algal growth downstream of the transect. We conclude that net community production (40 mmol C m⁻² d⁻¹) was sustained by net uptake of phosphate from the ocean and net uptake of new nitrogen from groundwater.

     

Microbial sulfate-reducing activities in anoxic sediment from Marine Lake Grevelingen: screening of electron donors and acceptors

Sulfate-reducing bacteria in marine sediments mainly utilize sulfate as a terminal electron acceptor with different organic compounds as electron donors. This study investigated microbial sulfate-reducing activity of coastal sediment from Marine Lake Grevelingen (MLG), the Netherlands using different electron donors and electron acceptors. All four electron donors (ethanol, lactate, acetate and methane) showed sulfate-reducing activity with sulfate as electron acceptor, suggesting the presence of an active sulfate-reducing bacterial population in the sediment, even at dissolved sulfide concentrations exceeding 12 mM. Ethanol showed the highest sulfate reduction rate of 55 µmol g _VSS ^?1 day^?1 compared to lactate (32 µmol g _VSS ^?1 day^?1), acetate (26 µmol g _VSS ^?1 day^?1) and methane (4.7 µmol g _VSS ^?1 day^?1). Sulfide production using thiosulfate and elemental sulfur as electron acceptors and methane as the electron donor was observed, however, mainly by disproportionation rather than by anaerobic oxidation of methane coupled to sulfate reduction. This study showed that the MLG sediment is capable of performing sulfate reduction by using diverse electron donors, including the gaseous and cheap electron donor methane.     

Underwater sinkhole sediments sequester Lake Huron’s carbon

Lake Huron’s submerged sinkhole habitats are impacted by high-conductivity groundwater that allows photosynthetic cyanobacterial mats to form over thick, carbon-rich sediments. To better understand nutrient cycling in these habitats, we measured the stable isotopic content of carbon and nitrogen in organic and inorganic carbon pools in Middle Island sinkhole, a ~23 m deep feature influenced by both groundwater and overlying lake water. Two distinct sources of dissolved CO₂ (DIC) were available to primary producers. Lake water DIC (δ ¹³C = ?0.1 ‰) differed by +5.9 ‰ from groundwater DIC (δ ¹³C = ?6.0 ‰). Organic carbon fixed by primary producers reflected the two DIC sources. Phytoplankton utilizing lake water DIC were more enriched in ¹³C (δ ¹³C = ?22.2 to ?23.2 ‰) than mat cyanobacteria utilizing groundwater DIC (δ ¹³C = ?26.3 to ?30.0 ‰). Sinkhole sediments displayed an isotopic signature (δ ¹³C = ?23.1 ‰) more similar to sedimenting phytoplankton than the cyanobacterial mat. Corroborated by sediment C/N ratios, these data suggest that the carbon deposited in sinkhole sediments originates primarily from planktonic rather than benthic sources. ²¹⁰Pb/¹³⁷Cs radiodating suggests rapid sediment accumulation and sub-bottom imaging indicated a massive deposit of organic carbon beneath the sediment surface. We conclude that submerged sinkholes may therefore act as nutrient sinks within the larger lake ecosystem.     

Influence of weather variables on methane and carbon dioxide flux from a shallow pond

Freshwaters are important sources of the greenhouse gases methane (CH₄) and carbon dioxide (CO₂) to the atmosphere. Knowledge about temporal variability in these fluxes is very limited, yet critical for proper study design and evaluating flux data. Further, to understand the reasons for the variability and allow predictive modeling, the temporal variability has to be related to relevant environmental variables. Here we analyzed the effect of weather variables on CH₄ and CO₂ flux from a small shallow pond during a period of 4 months. Mean CH₄ flux and surface water CH₄ concentration were 8.0 [3.3–15.1] ± 3.1 mmol m^?2 day^?1 (mean [range] ± 1 SD) and 1.3 [0.3–3.5] ± 0.9 µM respectively. Mean CO₂ flux was 1.1 [?9.8 to 16.0] ± 6.9 mmol m^?2 day^?1. Substantial diel changes in CO₂ flux and surface water CH₄ concentration were observed during detailed measurements over a 24 h cycle. Thus diel patterns need to be accounted for in future measurements. Significant positive correlations of CH₄ emissions with temperature were found and could include both direct temperature effects as well as indirect effects (e.g. related to the growth season and macrophyte primary productivity providing organic substrates). CO₂ flux on the other hand was negatively correlated to temperature and solar radiation, presumably because CO₂ consumption by plants was higher relative to CO₂ production by respiration during warm sunny days. Interestingly, CH₄ fluxes were comparable to ponds with similar morphometry and macrophyte abundance in the tropics. We therefore hypothesize that CH₄ and CO₂ summer emissions from ponds could be more related to the morphometry and dominating primary producers rather than latitude per se. Data indicate that CH₄ emissions, given the system characteristic frameworks, is positively affected by increased temperatures or prolonged growth seasons.     

Response of long-, medium- and short-term processes of the carbon budget to overgrazing-induced crusts in the Tibetan Plateau

The Kobresia pastures of the Tibetan Plateau represent the world’s largest alpine grassland ecosystem. These pastures remained stable during the last millennia of nomadic animal husbandry. However, strongly increased herds’ density has promoted overgrazing, with unclear consequences for vegetation and soils, particularly for cycles of carbon (C), nutrients and water. Vegetation-free patches of dead root-mat covered by blue-green algae and crustose lichens (crusts) are common in overgrazed Kobresia pastures, but their effect on C turnover processes is completely unknown. We tested the hypothesis that the crusts strongly affect the C cycle by examining: (i) the long-term C stock measured as soil organic matter content; (ii) medium-term C stock as dead roots; (iii) recent C fluxes analyzed as living roots and CO₂ efflux; and (iv) fast decomposition of root exudates. Up to 7.5 times less aboveground and 1.9 times less belowground living biomass were found in crust patches, reflecting a much smaller C input to soil as compared with the non-crust Kobresia patches. A lower C input initially changed the long-term C stock under crusts in the upper root-mat horizon. Linear regression between living roots and CO₂ efflux showed that roots contributed 23% to total CO₂ under non-crust areas (mean July–August 5.4 g C m^?2 day^?1) and 18% under crusts (5.1 g C m^?2 day^?1). To identify differences in the fast turnover processes in soil, we added ¹³C labeled glucose, glycine and acetic acid, representing the three main groups of root exudates. The decomposition rates of glucose (0.7 day^?1), glycine (1.5 day^?1) and acetic acid (1.2 day^?1) did not differ under crusts and non-crusts. More ¹³C, however, remained in soil under crusts, reflecting less complete decomposition of exudates and less root uptake. This shows that the crust patches decrease the rates of medium-term C turnover in response to the much lower C input. Very high ¹³C amounts recovered in plants from non-crust areas as well as the two times lower uptake by roots under crusts indicate that very dense roots are efficient competitors with microorganisms for soluble organics. In conclusion, the altered C cycle in the overgrazing-induced crustose lichens and blue-green algae crusts is connected with strongly decreased C input and reduced medium-term C turnover.     

Upper limits of photosynthetic productivity and problems of scaling

Some 1,370 W m^?2 of light energy reaches the outer atmosphere of earth and on average only 240 W m^?2 reaches the earth’s surface. Only a fraction of this is used to fix CO₂ through photosynthesis, and efficiencies ranging from 0.1?8% for total irradiance have been reported. The theoretical maximum quantum efficiency of carbon fixation is 0.125 mol C (mol quanta)^?1 which relates to a maximum productivity of about 12 g C m^?2 day^?1 or 29.8 g(dw) m^?2 day^?1. This could increase to a maximum of 200 g(dw) m^?2 day^?1 in intermittent light of high frequencies, which is on average eight times higher than the average measured under field conditions where rates approaching 25 g(dw) m^?2 day^?1 are considered high. Several possibilities exist for achieving higher yields and photosynthetic efficiencies, such as limiting the antennae sizes and pulsing light at frequencies equivalent to electron turnover in the electron transport chains of photosynthesis. Scaling from laboratory experimental conditions to large commercial photobioreactors is a major stumbling block and may be the single most important factor responsible for the overall low reported areal production rates.