No abstract

The aquatic application of the eddy correlation (EC) technique is growing more popular and is gradually becoming a standard method for resolving benthic O 2 fluxes. By including the effects of the local hydrodynamics, the EC technique provides greater insight into the nature of benthic O 2 exchange than traditional methods (i.e., benthic chambers and lander microprofilers). The growing popularity of the EC technique has led to a greater demand for easily accessible and robust EC instrumentation. Currently, the EC instrumentation is limited to two commercially available systems that are still in the development stage. Here, we present a robust, open source EC picoamplifier that is simple in design and can be easily adapted to both new and existing acoustic Doppler velocimeters (ADV). The picoamplifier has a response time of < 0.1 ms and features galvanic isolation that ensures very low noise contamination of the signal. It can be adjusted to accommodate varying ranges of microelectrode sensitivity as well as other types of amperometric microelectrodes. We show that the extracted flux values are not sensitive to reduced microelectrode operational ranges (i.e., lower resolution) and that no signal loss results from using either a 16-or 14-bit analog-to-digital converter. Finally, we demonstrate the capabilities of the picoamplifier with field studies measuring both dissolved O 2 and H 2 S EC fluxes. The picoamplifier presented here consistently acquires high-quality EC data and provides a simple solution for those who wish to obtain EC instrumentation. The schematic of the amplifier's circuitry is given in the Web Appendix.

Cold-water coral (CWC) reefs are distributed globally and form complex threedimensional structures on the deep seafloor, providing habitat for numerous species. Here, we measured the community O 2 and dissolved inorganic nitrogen (DIN) flux of CWC reef habitats with different coral cover and bare sediment (acting as reference site) in the Logachev mound area (NE Atlantic). Two methodologies were applied: the noninvasive in situ aquatic eddy co-variance (AEC) technique, and ex situ whole box core (BC) incubations. The AEC system was deployed twice per coral mound (69 h in total), providing an integral estimate of the O 2 flux from a total reef area of up to 500 m 2 , with mean O 2 consumption rates ranging from 11.6 ± 3.9 to 45.3 ± 11.7 mmol O 2 m −2 d −1 (mean ± SE). CWC reef community O 2 fluxes obtained from the BC incubations ranged from 5.7 ± 0.3 to 28.4 ± 2.4 mmol O 2 m −2 d −1 (mean ± SD) while the O 2 flux measured by BC incubations on the bare sediment reference site reported 1.9 ± 1.3 mmol O 2 m −2 d −1 (mean ± SD). Overall, O 2 fluxes measured with AEC and BC showed reasonable agreement, except for one station with high habitat heterogeneity. Our results suggest O 2 fluxes of CWC reef communities in the North East Atlantic are around five times higher than of sediments from comparable depths and living CWCs are driving the increased metabolism. DIN flux measurements by the BC incubations also revealed around two times higher DIN fluxes at the CWC reef (1.17 ± 0.87 mmol DIN m −2 d −1 ), compared to the bare sediment reference site (0.49 ± 0.32 mmol DIN m −2 d −1 ), due to intensified benthic release of NH 4 + . Our data indicate that the amount of living corals and dead coral framework largely contributes to the observed variability in O 2 fluxes on CWC reefs. A conservative estimate, based on the measured O 2 and DIN fluxes, indicates that CWC reefs process 20 to 35% of the total benthic respiration on the southeasterly Rockall Bank area, which demonstrates that CWC reefs are important to carbon and nitrogen mineralization at the habitat scale.

We investigated the seasonal dynamics of in‐stream metabolism at the reach scale (∼ 150 m) of headwaters across contrasting geological sub‐catchments: clay, Greensand, and Chalk of the upper River Avon (UK). Benthic metabolic activity was quantified by aquatic eddy co‐variance while water column activity was assessed by bottle incubations. Seasonal dynamics across reaches were specific for the three types of geologies. During the spring, all reaches were net autotrophic, with rates of up to 290 mmol C m−2 d−1 in the clay reach. During the remaining seasons, the clay and Greensand reaches were net heterotrophic, with peak oxygen consumption of 206 mmol m−2 d−1 during the autumn, while the Chalk reach was net heterotrophic only in winter. Overall, the water column alone still contributed to ∼ 25% of the annual respiration and primary production in all reaches. Net ecosystem metabolism (NEM) across seasons and reaches followed a general linear relationship with increasing stream light availability. Sub‐catchment specific NEM proved to be linearly related to the local hydrological connectivity, quantified as the ratio between base flow and stream discharge, and expressed on a timescale of 9 d on average. This timescale apparently represents the average period of hydrological imprint for carbon turnover within the reaches. Combining a general light response and sub‐catchment specific base flow ratio provided a robust functional relationship for predicting NEM at the reach scale. The novel approach proposed in this study can help facilitate spatial and temporal upscaling of riverine metabolism that may be applicable to a broader spectrum of catchments.

We report on newly discovered mud volcanoes located at ~4500 m water depth ~90 km west of the deformation front of the accretionary wedge of the Gulf of Cadiz, and thus outside of their typical geotectonic environment. Seismic data suggest that fluid flow is mediated by a >400-km-long strike-slip fault marking the transcurrent plate boundary between Africa and Eurasia. Geochemical data (Cl, B, Sr, 87 Sr/ 86 Sr, d 18 O, dD) reveal that fluids originate in oceanic crust older than 140 Ma. On their rise to the surface, these fluids receive strong geochemical signals from recrystallization of Upper Jurassic carbonates and clay-mineral dehydration in younger terrigeneous units. At present, reports of mud volcanoes in similar deep-sea settings are rare, but given that the large area of transform-type plate boundaries has been barely investigated, such pathways of fluid discharge may provide an important, yet unappreciated link between the deeply buried oceanic crust and the deep ocean.

[1] A natural carbon dioxide (CO 2 ) seep was discovered during an expedition to the southern German North Sea (October 2008). Elevated CO 2 levels of ∼10-20 times above background were detected in seawater above a natural salt dome ∼30 km north of the East-Frisian Island Juist. A single elevated value 53 times higher than background was measured, indicating a possible CO 2 point source from the seafloor. Measured pH values of around 6.8 support modeled pH values for the observed high CO 2 concentration. These results are presented in the context of CO 2 seepage detection, in light of proposed subsurface CO 2 sequestering and growing concern of ocean acidification. We explore the boundary conditions of CO 2 bubble and plume seepage and potential flux paths to the atmosphere. Shallow bubble release experiments conducted in a lake combined with discrete-bubble modeling suggest that shallow CO 2 outgassing will be difficult to detect as bubbles dissolve very rapidly (within meters). Bubble-plume modeling further shows that a CO 2 plume will lose buoyancy quickly because of rapid bubble dissolution while the newly CO 2 -enriched water tends to sink toward the seabed. Results suggest that released CO 2 will tend to stay near the bottom in shallow systems (<200 m) and will vent to the atmosphere only during deep water convection (water column turnover). While isotope signatures point to a biogenic source, the exact origin is inconclusive because of dilution. This site could serve as a natural laboratory to further study the effects of carbon sequestration below the seafloor.

This study presents a novel approach resulting in the first cold-water coral reef biomass maps, used to assess associated ecosystem functions, such as carbon (C) stock and turnover. We focussed on two dominant ecosystem engineers at the Mingulay Reef Complex, the coral Lophelia pertusa (rubble, live and dead framework) and the sponge Spongosorites coralliophaga. Firstly, from combining biological (high-definition video, collected specimens), environmental (extracted from multibeam bathymetry) and ecosystem function (oxygen consumption rate values) data, we calculated biomass, C stock and turnover which can feed into assessments of C budgets. Secondly, using those values, we employed random forest modelling to predictively map whole-reef live coral and sponge biomass. The whole-reef mean biomass of S. coralliophaga was estimated to be 304 T (range 168–440 T biomass), containing 10 T C (range 5–18 T C) stock. The mean skeletal mass of the coral colonies (live and dead framework) was estimated to be 3874 T (range 507–9352 T skeletal mass), containing a mean of 209 T of biomass (range 26–515 T biomass) and a mean of 465 T C (range 60–1122 T C) stock. These estimates were used to calculate the C turnover rates, using respiration data available in the literature. These calculations revealed that the epi- and microbial fauna associated with coral rubble were the largest contributor towards C turnover in the area with a mean of 163 T C year−1 (range 149–176 T C year−1). The live and dead framework of L. pertusa were estimated to overturn a mean of 32 T C year−1 (range 4–93 T C year−1) and 44 T C year−1 (range 6–139 T C year−1), respectively. Our calculations showed that the Mingulay Reef overturned three to seven (with a mean of four) times more C than a soft-sediment area at a similar depth. As proof of concept, the supply of C needed from surface water primary productivity to the reef was inferred. Since 65–124 T C year−1 is supplied by natural deposition and our study suggested that a mean of 241 T C year−1 (range 160–400 T C year−1), was turned over by the reef, a mean of 117–176 T C year−1 (range 36–335 T C year−1) of the reef would therefore be supplied by tidal downwelling and/or deep-water advection. Our results indicate that monitoring and/or managing surface primary productivity would be a key consideration for any efforts towards the conservation of cold-water coral reef ecosystems.

In recent decades, the central North Sea has been experiencing a general trend of decreasing dissolved oxygen (O 2 ) levels during summer. To understand potential causes driving lower O 2 , we investigated a 3-day period of summertime turbulence and O 2 dynamics in the thermocline and bottom boundary layer (BBL). The study focuses on coupling biogeochemical with physical transport processes to identify key drivers of the O 2 and organic carbon turnover within the BBL. Combining our flux observations with an analytical process-oriented approach, we resolve drivers that ultimately contribute to determining the BBL O 2 levels. We report substantial turbulent O 2 fluxes from the thermocline into the otherwise isolated bottom water attributed to the presence of a baroclinic near-inertial wave. This contribution to the local bottom water O 2 and carbon budgets has been largely overlooked and is shown to play a role in promoting high carbon turnover in the bottom water while simultaneously maintaining high O 2 concentrations. This process may become suppressed with warming climate and stronger stratification, conditions which could promote migrating algal species that potentially shift the O 2 production zone higher up within the thermocline.