Glud RN et al. (2016), Benthic Carbon Mineralization and Nutrient Turn...

ECB-ART-50017

Aquat Geochem 2016 Jan 01;225:443-467. doi: 10.1007/s10498-016-9300-8.

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Benthic Carbon Mineralization and Nutrient Turnover in a Scottish Sea Loch: An Integrative In Situ Study.

Glud RN , Berg P , Stahl H , Hume A , Larsen M , Eyre BD , Cook PLM .

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Based on in situ microprofiles, chamber incubations and eddy covariance measurements, we investigated the benthic carbon mineralization and nutrient regeneration in a ~65-m-deep sedimentation basin of Loch Etive, UK. The sediment hosted a considerable amount of infauna that was dominated by the brittle star A. filiformis. The numerous burrows were intensively irrigated enhancing the benthic in situ O2 uptake by ~50 %, and inducing highly variable redox conditions and O2 distribution in the surface sediment as also documented by complementary laboratory-based planar optode measurements. The average benthic O2 exchange as derived by chamber incubations and the eddy covariance approach were similar (14.9 ± 2.5 and 13.1 ± 9.0 mmol m-2 day-1) providing confidence in the two measuring approaches. Moreover, the non-invasive eddy approach revealed a flow-dependent benthic O2 flux that was partly ascribed to enhanced ventilation of infauna burrows during periods of elevated flow rates. The ratio in exchange rates of ΣCO2 and O2 was close to unity, confirming that the O2 uptake was a good proxy for the benthic carbon mineralization in this setting. The infauna activity resulted in highly dynamic redox conditions that presumably facilitated an efficient degradation of both terrestrial and marine-derived organic material. The complex O2 dynamics of the burrow environment also concurrently stimulated nitrification and coupled denitrification rates making the sediment an efficient sink for bioavailable nitrogen. Furthermore, bioturbation mediated a high efflux of dissolved phosphorus and silicate. The study documents a high spatial and temporal variation in benthic solute exchange with important implications for benthic turnover of organic carbon and nutrients. However, more long-term in situ investigations with like approaches are required to fully understand how environmental events and spatio-temporal variations interrelate to the overall biogeochemical functioning of coastal sediments.

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	Fig. 1. a Map showing the location of Loch Etive in West Scotland, UK. b An enlargement of Loch Etive including the locations of the major sills at Connel Narrows and Bonawe enclosing the study site in Airds Bay. c Bathymetry of Loch Etive along the deepest points. a, b is modified from Inoue et al. (2011), while c is modified from Overnell et al. (2002)
	Fig. 2. a One typical in situ image (89 × 63 cm) of the seabed at the study site showing a high density of arms from A. filiformis that are extending from the sediment surface. b A 3D, CT scan of burrow structures in an intact block of sediment containing actively ventilating brittle stars at natural densities. c One typical black-and-white image selected from a 30-h time series of images at the sediment water interface obtained at a frequency of 5 min. The image was obtained through a transparent O2 optode, and the central cavity with the disc and arms extending to the left and right from a single specimen of A. filiformis can be observed. d–f depict three selected images of the O2 distribution within and around the ventilated burrow system. One arm is used to channel air-saturated water into the central cavity while O2-depleted water is exhaled to the right (indicated by arrows). g The O2 concentration within the central cavity as extracted from 25 h of continuous O2 images. The full movie on animal activity in black-and-white and the concurrent O2 dynamics is available in the supplementary material
	Fig. 3. a–d Typical O2 microprofiles as measured by the transecting profiling instrument. The estimated sediment surface as reflected by a slight shift in the concentration slope is indicated by the thin horizontal line. Many profiles showed clear signs of irrigation with O2 peaks deep within the sediment presumably penetrating irrigated burrows (a, d) or larger cavities (b). e, f depict typical microprofiles of NO3 − distribution at the sediment–water interface. Some profiles reveal intense NO3 − production (nitrification) at the oxic surface (e–h). As for O2, irrigation clearly transported NO3 − deep into burrows (e, h) or larger cavities (f) of the sediment
	Fig. 4. Changes in O2, DIC and nutrient concentrations within a single in situ chamber incubation conducted at 5 November 2008 at 65 m water depth (deployment 6). Flux rates were derived from linear approximations of the concentration changes and by accounting for the enclosed volume of water
	Fig. 5. Changes in N2 within four separate in situ chamber incubations as derived from overall changes in the measured N2–Ar ratio. Samples containing small bubbles after sampling or storage were discarded. Flux rates were derived from linear approximations of the concentration changes accounting for the enclosed volume of water
	Fig. 6. a Flow rates as derived by the horizontal flow components measured by the ADV of the eddy tripod during a 42-h-long deployment. Values are averages of 14.5 min of recording. b The concurrent O2 exchange rates as derived by eddy covariance in bursts of 14.5 min. The example represents a deployment from 5 to 6 November 2008 at 68 m, and a parallel data set from 7 to 10 November 2008 at 55 m has been presented previously (Holtappels et al. 2013)
	Fig. 7. Benthic O2 exchange as resolved by three different in situ approaches. The error bars represent the standard deviation (For EOE the SE amounted to 0.5 mmol m−2 day−1)

References [+] :

Blair, The fate of terrestrial organic carbon in the marine environment. 2012, Pubmed