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

Abstract

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 O 2 uptake by *50 %, and inducing highly variable redox conditions and O 2 distribution in the surface sediment as also documented by complementary laboratory-based planar optode measurements. The average benthic O 2 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 O 2 flux that was partly ascribed to enhanced ventilation of infauna burrows during periods of elevated flow rates. The ratio in exchange rates of RCO 2 and O 2 was close to unity, confirming that the O 2 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 O 2 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.

Oxygen dynamics in shelf seas sediments incorporating seasonal variability

“…Benthic oxygen uptake provides a good proxy for benthic mineralisation (Glud et al 2016;Jahnke et al 2000;Woulds et al 2007), however spatial and temporal variability of oxygen dynamics may present a bias in interpretation if careful consideration is not given to the methodology used and caveats of each approach, e.g. incubations favour diffusive processes so may lead to over or underestimation in permeable sediments (Lohse et al 1996;Tengberg et al 2004).…”

“…Benthic oxygen uptake is often used as a robust proxy for total benthic mineralisation (Glud et al 2016;Jahnke et al 2000;Stahl et al 2004b), and this method is particularly applicable in cohesive sediments (Glud 2008). The rate of oxygen uptake, or consumption, is determined by the type of sediment; the presence, quality (labile vs refractory) and quantity of organic matter in the sediment; and the organisms that live on and within the sediment (from microorganisms and meiofauna to macrofauna).…”

“…In contrast, Diffusive Oxygen Uptake (DOU) rates are representative of the diffusive oxygen exchange across the sediment-water interface (Fischer et al 2009;Jorgensen and Revsbech 1985;Rabouille et al 2003;Rasmussen and Jorgensen 1992), related to microbial respiration and chemical oxidation (Glud et al 2016;Revsbech 1989a). The difference between TOU and DOU can be used to ascertain the relative contribution of the fauna mediated oxygen uptake (FOU), which both refers to the respiration and activity of the macrofauna itself, as well as the stimulation of microbial respiration within the sediments due to the introduction of oxygen-rich water into deeper sediment layers through bioirrigation and bioturbation (Glud 2008;Glud et al 2016;Kristensen et al 2012). The contribution of FOU to TOU is typically higher in coastal, slope and shelf sediments, compared to deep-sea sediments where the abundance of macrofauna is lower Wenzhofer and Glud 2002).…”

A Tribute to Rick and Debbie Jahnke: From Deep Sea Pore Water to Coastal Permeable Sediments-Contributions that Cover the Oceans

“…Using similar in situ methods, Reimers et al (2016) provide new insights into the dynamics of benthic respiration by measuring oxygen exchange rates in mid-shelf sediments off the Oregon coast. Glud et al (2016) also use these in situ and other methods to understand carbon mineralization and nutrient turnover as a result of oxygen uptake and exchange across the water sediment interface. Organisms influence element cycling across the water-sediment interface via bioturbation, which has two major components [bio-mixing (solid particle transport) and bio-irrigation (enhance solute transport)] that Van de Velde and Meysman (2016) describe with a reactive transport model to further our understanding of iron and sulfur cycling.…”

Modelling Marine Sediment Biogeochemistry: Current Knowledge Gaps, Challenges, and Some Methodological Advice for Advancement

“…Biogeochemical models mostly consider bio-irrigation (biological enhancement of solute transfer) as a factor enhancing the diffusion coefficient (Blackford, 1997;Reed et al, 2011), resulting in a thicker oxic layer near the sediment surface. However, the transport of oxygen (and other solutes) will likely not occur homogenously, rather being concentrated along the network of burrows (Glud et al, 2016), which become an extension of the oxic-anoxic interface. It has been shown that burrowing activity of benthic macrofauna can lead to as much as 400% increase in denitrification rates (Gilbert et al, 1998;Webb and Eyre, 2004), mostly due to the high rates of nitrification occurring within the burrows (Howe et al, 2004).…”