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.