ALBERT

Description: Atlantis II Deep, a submarine basin of the Red Sea, is noteworthy because of its hydrothermally active brine pools. High-resolution temperature records from Poseidon Cruise during February 2011 revealed small steps thermal staircase in the lower transition zone from ≈2002 to 2008/2009 m depth at stations. Four vertically well-mixed convective layers, lower convective layer (LCL) and upper convective layers (UCL1–3), separated by high-temperature gradients at the interfaces were observed. The temperature of the layers UCL1–3 has dropped between 2008 and 2011. The top of UCL3 extends to about 2008/2009 m at stations and its average thickness has increased from 3.3 ± 0.5 m in 1992 to 7 m in 2011, whereas the thickness of layers UCL1–2 has decreased from 25.2 ± 0.3 m to 19.8 m and from 16.4 ± 0.5 m to 14.7 m, respectively, during this time. The upward buoyancy flux is 0.032 to 0.038 × 10−7 m2 s−3 which gives migration speed of UCL3 layer from 0.1 to 0.12 m year−1. With this speed, the thermal staircase ≈6 m thick will merge with UCL3 in 50 to 60 years increasing the thickness from 7 m to nearly 13 m.

Description: Sedimentwater fluxes are influenced by both hydrodynamics and sediment biogeochemical processes. However, fluxes at the sediment–water interface (SWI) are almost always analyzed from either a water- or sediment-side perspective. This study expands on previous work by comparing water-side (hydrodynamics and resulting diffusive boundary layer thickness, δDBL) and sediment-side (oxygen consumption and resulting sediment oxic zone) approaches for evaluating diffusive sediment oxygen uptake rate (JO2) and δDBL fro microprofiles. Dissolved oxygen microprofile and current velocity data were analyzed using five common methods to estimate JO2 and δDBL and to assess the robustness of the approaches. Comparable values for JO2 and δDBL were obtained (agreement within 20%), and turbulence-induced variations in these parameters were uniformly characterized with the five methods. JO2 estimates based on water-side data were consistently higher (+1.8 mmol m–2 d–1 or 25% on average) and δDBL estimates correspondingly lower (–0.4 mm or 35% on average) than those obtained using sediment-side data. This deviation may be attributed to definition of the sediment–water interface location, artifacts of the methods themselves, assumptions made on sediment properties, and/or variability in sediment oxygen-uptake processes. Our work emphasizes that sediment-side microprofile data may more accurately describe oxygen uptake at a particular location, whereas water-side data are representative of oxygen uptake over a broader sediment area. Regardless, our overall results show clearly that estimates of JO2 and δDBL are not strongly dependent on the method chosen for analysis.

Description: Methane (CH4) is one of the most important greenhouse gases (IPCC, 2005). Lakes and reservoirs have been identified as important, but overlooked, sources to the global CH4 budget. CH4 emission pathways include dissolved gas exchange at the water surface, bubble transport (ebullition), and degassing at the turbines of a hydropower dam or further downstream (Soumis, et al., 2005). Ebullition is an extremely effective pathway as bubbles mostly bypass oxidation at the sediment surface or in the water column and directly emit CH4. The stochastic nature of ebullition, however, makes it incredibly difficult to estimate; thus the aim of this study was to compare the traditional funnel method for measuring ebullition with a mass balance system analysis, atmospheric CH4 measurements, and hydroacoustic surveying. A yearlong CH4 survey was conducted at 2.5 km2 Lake Wohlen, a 90-yr-old run-of-river hydropower reservoir along the Aare River downstream of Bern, Switzerland. Dissolved CH4 ([CH4]d) profiles were measured monthly at the river inflow and at the dam. Sediment surface and water surface CH4 diffusion and CH4 oxidation in the water column were measured and/or calculated. Gas trap funnels measured ebullition near the seabed; drifting chambers captured total surface CH4 emissions. A bubble dissolution model was used to assess fractions of CH4 dissolving into the water and emitted to the atmosphere from bubbles. Complete method details in DelSontro, et al. (2010). Drifting chamber campaigns were accompanied by hydroacoustic surveys using an echosounder (Simrad EK60, 120 kHz). Eddy covariance measurements of atmospheric CH4 fluxes (EC/CH4) over the lake were made in conjunction with a cavity ringdown laser spectrometer (Los Gatos Research DLT-100). For details, see Eugster and Plüss (in press). It was discovered that [CH4]d increased by an order of magnitude along the reservoir and the [CH4]d accumulation was exponentially correlated with water temperature (T) (Figure 1a). The bubble dissolution model predicted that 70% of bubble-conveyed CH4 would reach the atmosphere, resulting in ~470 mg CH4 m-2 d-1 emitted to the atmosphere at T=17°C. Sediment and surface diffusions did not vary much with season and played a much lesser role in CH4 emissions than ebullition. Methane oxidation was negligible in this oxic reservoir with an average 2-day residence time. A system analysis was developed to better constrain the stochastic pattern of ebullition. Assuming no ebullition in winter (T〈10°C), sediment diffusion was estimated based on [CH4]d accumulation in water at a given flow rate. The [CH4]d accumulation and T regression was used to estimate [CH4]d from dissolving bubbles at various T regimes which, at T=17°C, agreed well with funnel measurements (140 and 220 mg CH4 m-2 d-1, respectively). Using the bubble dissolution model results, sediment ebullition and atmospheric emissions were calculated and agreed well with empirical results. Considering all CH4 dissolved into the water from rising bubbles will either degas at the turbines or further downstream, Lake Wohlen thus emits ~156 mg CH4 m-2 d-1 on average throughout the year (140 tons/yr; Figure 1b), the highest recorded for a temperate reservoir to date (Soumis, et al., 2005) and of which ~80% is from ebullition. Drifting chambers captured emissions (mean, 855 mg CH4 m-2 d-1) much higher than those estimated with the system analysis at 17°C, but chambers were deployed in a highly active ebullition area. The chamber emissions agreed, however, with the peak CH4 emissions measured by EC/CH4 in the same region and are comparable to emissions estimated via hydroacoustics. These findings further highlight the importance in a potentially warming climate of (1) temperature-correlated CH4 ebullition emissions from temperate water bodies, and (2) these promising techniques for quantifying them.

Description: Presently, oxygen minimum zones (OMZ) are incurring drastic changes from the combined impact of both rapidly declining O2 concentrations and increasing CO2 levels. The lower edge of OMZs, typically characterized by abundant benthic invertebrate communities and fish existing in already precarious conditions, are particularly susceptible to even the slightest O2 changes. Currently, there are very limited benthic O2 data from the upper- and lower-boundaries of OMZs. Using benthic landers and a sediment-water O2 micro-profiler, we resolved time series of O2 and temperature directly above the seafloor at the lower-boundary (depth range of 397 - 1015 m) of the Peruvian OMZ at a transect along 11°S. We observed an oscillating and persistent vertical movement of bottom water (BW) with displacement amplitudes exceeding 100 m. These vertical displacements have a ~ 12 hour period, and appear to be partially driven by tidal forcing. This regular BW vertical motion leads to distinct cyclic fluctuations of local O2 concentration and temperature. At ~ 1000 m water depth, O2 variability ranged from 23 to 44 µM. Cyclical benthic O2 fluctuations were observed with decreasing water depths until O2 concentrations 〈 3 µM were reached at ~ 400 m. The benthic environment immediately responds to the varying BW O2 levels. At ~ 1000 m, the diffusive oxygen uptake across the sediment water interface fluctuated between 0.5 to 1.4 mmol m-2 d-1, which is seemingly associated to variable O2 BW concentrations. Intermittent local benthic O2 levels affect the colonization and composition of both aerobic and anaerobic life at the lower boundary of the OMZ. The dominant communities have enormous consequences for the distribution and magnitude of benthic redox-sensitive biogeochemical processes. Similarly to investigations in other oxygen deficient environments, we found extremely high densities of epibenthic invertebrates that were sharply centered at ~ 650 m depth close to the oxic-anoxic interface. These organisms benefit substantially from the oscillating benthic oxygen levels. They may therefore inhabit regions deeper within the lower edge of the OMZ, thus exploiting increased availability of e.g. labile organic carbon, and perhaps taking a deep “breath” during high O2 conditions. These oxygen oscillations in association with intricate BW motions imply strong solute exchange between the benthos and the higher water column, which can have unforeseen consequences for the coupling of benthic and pelagic ecosystems in an increasingly oxygen deficient environment.

Description: Bubble transport of methane from shallow seep sites in the Black Sea west of the Crimea Peninsula between 70 and 112 m water depth has been studied by extrapolation of results gained through different hydroacoustic methods and direct sampling. Ship-based hydroacoustic echo sounders can locate bubble releasing seep sites very precisely and facilitate their correlation with geological or other features at the seafloor. Here, the backscatter strength of a multibeam system was integrated with single-beam data to estimate the amount of seeps/m2 for different backscatter intensities, resulting in 2709 vents in total. Direct flux measurements by submersible revealed methane fluxes from individual vents of 0.32–0.85 l/min or 14.5–37.8 mmol/min at ambient pressure and temperature conditions. A conservative estimate of 30 mmol/min per site was used to estimate the flux into the water to be 1219–1355 mmol/s. The flux to the atmosphere was calculated by applying a bubble dissolution model taking release depth, temperature, gas composition, and bubble size spectra into account. The flux into the atmosphere (3930–4533 mol/d) or into the mixed layer (6186–6899 mol/d) from the 21.8 km2 large study area is three times higher than independently measured fluxes of dissolved methane for the same area using geochemical methods (1030–2495 mol/d). The amount of methane dissolving in the mixed layer is 2256–2366 mol/d. This close match shows that the hydroacoustic approach for extrapolating the number of seeps/m2 and the applied bubble dissolution model are suitable to extrapolate methane fluxes over larger areas.

Description: In the majority of large river systems, flow is regulated and/or otherwise affected by operational and management activities, such as ship locking. The effect of lock operation on sediment-water oxygen fluxes was studied within a 12.9 km long impoundment at the Saar River (Germany) using eddy-correlation flux measurements. The continuous observations cover a time period of nearly 5 days and 39 individual locking events. Ship locking is associated with the generation of surges propagating back and forth through the impoundment which causes strong variations of near-bed current velocity and turbulence. These wave-induced flow variations cause variations in sediment-water oxygen fluxes. While the mean flux during time periods without lock operation was 0.5 6 0.1 g m�2 d�1, it increased by about a factor of 2 to 1.0 6 0.5 g m�2 d�1 within time periods with ship locking. Following the daily schedule of lock operations, fluxes are predominantly enhanced during daytime and follow a pronounced diurnal rhythm. The driving force for the increased flux is the enhancement of diffusive transport across the sediment-water interface by bottom-boundary layer turbulence and perhaps resuspension. Additional means by which the oxygen budget of the impoundment is affected by lock-induced flow variations are discussed.

Description: Methane (CH4) emissions from small rivers and streams, particularly via ebullition, are currently under-represented in the literature. Here, we quantify the methane effluxes and drivers in a small, Northern European river. Methane fluxes are comparable to those from tropical aquatic systems, with average emissions of 320 mg CH4 m-2 d-1. Two important drivers of methane flux variations were identified in the studied system: 1) temperature-driven sediment methane ebullition and 2) flow-dependent contribution suspected to be hydraulic exchange with adjacent wetlands and small side-bays. This flow-dependent contribution to river methane loading is shown to be negligible for flows less than 4 m3 s-1, and greater than 50% as flows exceed 7 m3 s-1. While the temperature - ebullition relationship is comparable to other systems, the flow rate dependency has not been previously demonstrated. In general, we found that about 80% of the total emissions were due to methane bubbles. Applying ebullition rates to global estimates for fluvial systems, which currently are not considered, could dramatically increase emission rates to ranges from lakes or wetlands. This work illustrates that small rivers can emit significant methane, and highlights the need for further studies, especially the link between hydrodynamics and connected wetlands.

Description: Inland waters transport and transform substantial amounts of carbon and account for 18% of global methane emissions. Large reservoirs with higher areal methane release rates than natural waters contribute significantly to freshwater emissions. However, there are millions of small dams worldwide that receive and trap high loads of organic carbon and can therefore potentially emit significant amounts of methane to the atmosphere. We evaluated the effect of damming on methane emissions in a central European impounded river. Direct comparison of riverine and reservoir reaches, where sedimentation in the latter is increased due to trapping by dams, revealed that the reservoir reaches are the major source of methane emissions (0.23 mmol CH4 m–2 d–1 vs 19.7 mmol CH4 m–2 d–1, respectively) and that areal emission rates far exceed previous estimates for temperate reservoirs or rivers. We show that sediment accumulation correlates with methane production and subsequent ebullitive release rates and may therefore be an excellent proxy for estimating methane emissions from small reservoirs. Our results suggest that sedimentation-driven methane emissions from dammed river hot spot sites can potentially increase global freshwater emissions by up to 7%

Description: Sea-ice ecosystems are among the most extensive of Earth’s habitats; yet its autotrophic and heterotrophic activities remain poorly constrained. We employed the in situ aquatic eddy-covariance (AEC) O2 flux method and laboratory incubation techniques (H14CO3−, [3H] thymidine and [3H] leucine) to assess productivity in Arctic sea-ice using different methods, in conditions ranging from land-fast ice during winter, to pack ice within the central Arctic Ocean during summer. Laboratory tracer measurements resolved rates of bacterial C demand of 0.003–0.166 mmol C m−2 day−1 and primary productivity rates of 0.008–0.125 mmol C m−2 day−1 for the different ice floes. Pack ice in the central Arctic Ocean was overall net autotrophic (0.002–0.063 mmol C m−2 day−1), whereas winter land-fast ice was net heterotrophic (− 0.155 mmol C m−2 day−1). AEC measurements resolved an uptake of O2 by the bottom-ice environment, from ~ − 2 mmol O2 m−2 day−1 under winter land-fast ice to~ − 6 mmol O2 m−2 day−1 under summer pack ice. Flux of O2-deplete meltwater and changes in water flow velocity masked potential biological-mediated activity. AEC estimates of primary productivity were only possible at one study location. Here, productivity rates of 1.3 ± 0.9 mmol O2 m−2 day−1, much larger than concurrent laboratory tracer estimates (0.03 mmol C m−2 day−1), indicate that ice algal production and its importance within the marine Arctic could be underestimated using traditional approaches. Given careful flux interpretation and with further development, the AEC technique represents a promising new tool for assessing oxygen dynamics and sea-ice productivity in ice-covered regions.