Oxygen transport and reactivity within a sandy seepage face in a mesotidal lagoon (Ria Formosa, Southwestern Iberia)

Abstract

Sandy sediments are now recognized as sites of high carbon turnover enhanced by porewater flow. However, it is unclear how coupling between porewater advection and benthic metabolism might permit O2‐saturated areas and suboxic environments to coexist in close proximity. Field sampling campaigns, flow‐through reactor (FTR) experiments and diagenetic modeling were used to study benthic O2 dynamics in surface sediments of a permeable intertidal seepage face (Ria Formosa; Southwestern Iberia). Field results demonstrated that the pressure gradient imposed by tidal oscillation at the seepage face permits seawater infiltration at high tide, and hence resupplies the benthos in O2 and organic matter (OM). Significant aerobic respiration rates (12.1–26.8 nmol cm−3 “bulk sediment” (bs) h−1) were derived from vertical O2 gradients taken from the top 28 cm of the sediment during active seepage. FTR experiments showed a vertical zonation of respiration rates following benthic OM availability (from 8.63 ± 1.88 nmol cm−3 bs h−1 at 12–32 cm depth to 53.55 ± 7.93 nmol cm−3 bs h−1 at 0–2 cm depth). Advection enhances O2 consumption rates, but respiration is ultimately limited by OM availability at high seepage rates. Diagenetic modeling of O2 distribution in the porewater suggests that even within a well‐oxygenated beach aquifer, local respiration is sufficiently high to lower the porewater O2 concentration near the sediment surface during active seepage. Our results indicate that high seepage rates, OM and O2 loading promoted by tidally‐driven seawater infiltration control the O2 consumption at the sediment surface, thus favoring the occurrence of suboxic biogeochemical processes near the sediment surface.

Spatial and Temporal Patterns of Pore Water Chemistry in the Inter-Tidal Zone of a High Energy Beach

“…However, Beck et al (2017) indeed found 60 -100% oxygen saturation in pore water from the top 5 cm of the ridge (corresponding to In-2 of our study) in May 2014. It should be noted that another study on a mesotidal beach at the Portugese Atlantic coast, reported high oxygen levels (>50%) even in the lowest exfiltration zone, which they attributed to an interplay of strong tidal oscillations, rapid advection, and low organic matter supply (Ibanhez and Rocha, 2016). In our study, we did observe some patches with relatively elevated oxygen values in the ridge-runnel area (light blue spots in Figure 4), but they always remained well below 50%.…”

Does Nitrate Enrichment Accelerate Organic Matter Turnover in Subterranean Estuaries?

“…At station A, the lower salinity values in NO 3 − was the predominant form of porewater DIN throughout the studied period (Figure 5), as expected given the near-saturation of oxygen inside the beach (see Supplementary Figure 3; Ibánhez and Rocha, 2016). Significant reductions in oxygen levels during active seepage are only found in association with particulate organic matter (POM) enrichments near the sediment surface (Ibánhez and Rocha, 2016). NO 3 − distribution within the seepage face follows two different patterns during the year: from December 2009 to July 2010, NO 3 − porewater concentrations were lower than 15 µM and generally peaked close to the sediment surface.…”

“…The exception occurred in January, where the lowest salinities within the beach are found at the shallower screened depths at station B (Figure 5.5). At station A, the lower salinity values in NO 3 − was the predominant form of porewater DIN throughout the studied period (Figure 5), as expected given the near-saturation of oxygen inside the beach (see Supplementary Figure 3; Ibánhez and Rocha, 2016). Significant reductions in oxygen levels during active seepage are only found in association with particulate organic matter (POM) enrichments near the sediment surface (Ibánhez and Rocha, 2016).…”

“…NO 3 − peaked within the surface 30-50 cm of sediment in station A from December 2009 to July 2010 (Figure 9) when large porewater flow rates were measured (Figure 2). Considering the predominant vertical direction of flow near the sediment surface at our site (Rocha et al, 2009;Ibánhez and Rocha, 2016), these results imply the active production of NO 3 − near the sediment surface at that point of the beach profile. The same is clearly observed at station B in July and October (Figure 10).…”

Molecular Traits of Dissolved Organic Matter in the Subterranean Estuary of a High-Energy Beach: Indications of Sources and Sinks

“…The question is then, whether too high disturbance amplitudes or frequencies, for example by storm events or high-energy waves, have a negative influence on the adaptation capacities of the microbial community. So far, a comparison of research literature from different types of environments indicates that frequent disturbances of flow paths and sediment structures enhance, rather than restrict, microbial remineralization processes in high-energy STEs (McLachlan, 1982;Charbonnier et al, 2013;Reckhardt et al, 2015;Ibánhez and Rocha, 2016;Waska et al, 2019b). If there is no upper limit to microbial resilience against physical disturbances (Novitsky and MacSween, 1989;Degenhardt et al, 2020), then meso-to macrotidal STEs with strong wave action should be particularly active sites of organic matter turnover along the world's coastlines.…”