Benthic oxygen fluxes in a coastal upwelling system (Ria de Vigo, NW Iberia) measured by aquatic eddy covariance

Abstract

Organic carbon mineralization and nutrient cycling in benthic environments are critically important for their biogeochemical functioning, but are poorly understood in coastal upwelling systems. The main objective of this study was to determine benthic oxygen fluxes in a muddy sediment in the Ria de Vigo (NW Iberian coastal upwelling), by applying the aquatic eddy covariance (AEC) technique during 2 campaigns in different seasons (June and October 2017). The main drivers of benthic fluxes were studied and compared among days in each season and between seasons. The 2 campaigns were characterized by an upwelling-relaxation period followed by a downwelling event, the last of which was due to the extratropical cyclone Ophelia in October. The mean (±SD) seasonal benthic oxygen fluxes were not significantly different for the 2 campaigns despite differences in hydrodynamic and biogeochemical conditions (June: -20.9 ± 7.1 mmol m-2 d-1 vs. October: -26.5 ± 3.1 mmol m-2 d-1). Benthic fluxes were controlled by different drivers depending on the season. June was characterized by sinking labile organic material, which enhanced benthic fluxes in the downwelling event, whereas October had a significantly higher bottom velocity that stimulated the benthic fluxes. Finally, a comparison with a large benthic chamber (0.50 m2) was made during October. Despite methodological differences between AEC and chamber measurements, concurrent fluxes agreed within an acceptable margin (AEC:benthic chamber ratio = 0.78 ± 0.13; mean ± SD). Bottle incubations of water sampled from the chamber interior indicated that mineralization could explain this difference. These results show the importance of using non-invasive techniques such as AEC to resolve benthic flux dynamics.

A high‐resolution submersible oxygen optode system for aquatic eddy covariance

“…Similarly, and despite using the more robust optical fiber sensors used in this study, 42% of recorded data were excluded from measurements in a eutrophic freshwater embayment (Koopmans et al 2021). Also, sometimes 50% of the data are excluded when planar optode sensors are used in tidal systems as the larger sensor tip may disrupt velocity measurements (Berg et al 2016; Amo‐Seco et al 2021). In comparison with these studies, 17% of the recorded data were excluded in our four deployments.…”

“…Typically, these O 2 measuring systems are tested in the lab in water that is close to atmospheric saturation or O 2 ‐undersaturated. These conditions are indeed representative of some ecosystems where AEC has been used, such as unvegetated muddy sediments at depths below the photic zone (Berg et al 2009; Attard et al 2014; Amo‐Seco et al 2021). However, the water column of ecosystems dominated by photosynthesizing vegetation such as seagrass meadows (Rheuban et al 2014 b ; Berger et al 2020) or macroalgal canopies (Volaric et al 2019; Attard et al 2019 a ) can be highly supersaturated with O 2 during the daytime.…”

“…However, representative in situ fluxes, determined under naturally varying environmental conditions, are difficult to obtain due to various limitations of standard flux methodologies (Cook et al 2007; Glud 2008). Benthic flux chambers separate an enclosed area from the surrounding environment, which excludes in situ hydrodynamics and disturbs in situ light conditions (Tengberg et al 2004; Amo‐Seco et al 2021; Polsenaere et al 2021). In situ microelectrode profiling and subsequent profile interpretations exclude the effects of local macrofauna (Archer and Devol 1992; Glud 2008) and may produce artifacts in permeable sediments due to interactions with bottom flows (Huettel and Gust 1993).…”

“…To increase robustness and thus, deployment success, some studies use multiple microelectrodes or fiber-optic optodes (Attard et al 2014(Attard et al , 2019bMerikhi et al 2021). An alternative to these sensing systems is the microplanar optode sensor, which is more robust because of its larger tip (8 mm diameter) that does not break and is rarely affected by floating debris (Berg et al 2016;Amo-Seco et al 2021). However, its relatively large tip can disturb the current flow, so recorded data may be compromised when the current flow comes from behind the sensor (Huettel et al 2020).…”

Wave Regime and Wave‐Current Coupling in an Upwelling–Driven Bay: Seasonal and Inter‐Annual Variability

“…Strong resuspension events in the BBL of shallow areas of the ria have a fundamental role in the seabed biogeochemistry (Amo‐Seco et al., 2021; Ibánhez et al., 2021). These events would enhance the remineralization of the organic matter that sank under the previous highly productive upwelling season (Froján et al., 2016, 2018; Zúñiga et al., 2011) by combining the key driving factors: warmer temperatures near the bottom associated with downwelling negative circulation (Nogueira et al., 1997; Pardo et al., 2001), intense resuspension (Almroth et al., 2009) and benthic ventilation and advection (Ibánhez & Rocha, 2016, 2017; Precht & Huettel, 2003).…”