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.
Highlights
Coastal regions are active biogeochemical areas which play an important role in the global carbon cycle
Mar Ecol Prog Ser 670: 15–31, 2021 or biogeochemical transformations, which promote the benthic mineralization of organic carbon and benthic regeneration of nutrients (Wollast 1998)
The mean benthic oxygen flux of −23.1 ± 6.4 mmol m−2 d−1 obtained by aquatic eddy covariance (AEC) from 13 deployments over the muddy sediments of Ria de Vigo (Table 1) is comparable to those (−32.9 mmol m−2 d−1) derived from global-scale sediment−water flux data for coastal regions (Boynton et al 2018)
Summary
Coastal regions are active biogeochemical areas which play an important role in the global carbon cycle. Mar Ecol Prog Ser 670: 15–31, 2021 or biogeochemical transformations, which promote the benthic mineralization of organic carbon and benthic regeneration of nutrients (Wollast 1998). These recycled nutrients support the large primary production of coastal areas (Grenz et al 2010). Coastal upwelling regions constitute the most biologically productive areas due to the upwelling of cold and nutrient-rich subsurface waters These regions are highly dynamic and are characterized by the occurrence of upwelling/downwelling events, which potentially influence benthic remineralization. In these regions, it is important to measure benthic oxygen fluxes over different time scales to integrate the most important biogeochemical processes (Berelson et al 2013, Reimers et al 2016)
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