Impact of Oyster Farming on Diagenetic Processes and the Phosphorus Cycle in Two Estuaries (Brittany, France)

Abstract

This study aims to compare the impact of oyster cultures on diagenetic processes and the phosphorus cycle in the sediments of the Aber Benoît and the Rivière d'Auray, estuary of Brittany, France. Our results showed clear evidence of the seasonal impact of oyster cultures on sediment characteristics (grain size and organic matter parameters) and the phosphorus cycle, especially in the Aber Benoît. At this site, seasonal variations in sulfide and Fe concentrations in pore waters, as well as Fe-P concentrations in the solid phase, highlighted a shift from a system governed by iron reduction (Reference) to a system governed by sulfate reduction (beneath oyster). This could be partly explained by the increase in labile organic matter (i.e., biodeposits) beneath oysters, whose mineralization by sulfate led to high sulfide concentrations in pore waters (up to 4,475 lmol l -1 ). In turn, sulfide caused an enhanced release of phosphate in the summer, as adsorption sites for phosphate decreased through the formation of iron-sulfide compounds (FeS and FeS 2 ). In the Aber Benoît, dissolved Fe/PO 4 ratios could be used as an indicator of phosphate release into oxic water. Low Fe/PO 4 ratios in the summer indicated higher effluxes of phosphate toward the water column (up to 47 lmol m -2 h -1 ). At other periods, Fe/PO 4 ratios higher than 2 mol/mol indicated very low phosphate fluxes. In contrast, in the Rivière d'Auray, the occurrence of macroalgae, stranding regularly all over the site, clearly masked the impact of oyster cultures on sediment properties and the phosphorus cycle and made the use of Fe/PO 4 ratios more difficult in terms of indicators of phosphate release.

Are benthic nutrient fluxes from intertidal mudflats driven by surface sediment characteristics?

“…present study, the significant and positive correlations between the phaeopigment content and that of the TOC, TN and Org-P contents indicated that the organic matter was enriched in the surface sediment due to the accumulation of algal detritus. In the Auray River (Brittany, France), Andrieux-Loyer et al [2014] also observed that the increase in the phaeopigment, TOC, TN and Org-P contents in the surface sediment was fueled by the occurrence of green macroalgae. An accumulation of algal OM most likely results in intense SOM mineralization, leading to both oxygen depletion in porewater and a shift in anaerobic mineralization from iron to sulfate reduction thereby enabling iron-sulfide formation [Lehtoranta et al, 2009].…”

Phosphorus Dynamics Associated With Organic Carbon Mineralization by Reduction of Sulfate and Iron in Sediment Exposed to Fish Farming

“…In organic-rich sediment in particular, the regeneration of inorganic P from C org remineralization was the predominant process for Aut-P precipitation (Anschutz et al, 2007;Tsandev et al, 2012;Joshi et al, 2015;Kraal et al, 2017). The enhancement of C org mineralization rate and reductive dissolution of Fe oxides resulting from the reaction of FeOOH with sulfide in the organicrich farm sediment were directly responsible for the strong surface enrichment of Aut-P (102 µmol g −1 , 40% of total P) in the farm sediment (Figure 5; van der Zee et al, 2002;Brock and Schulz-Vogt, 2011;Slomp, 2011;Andrieux-Loyer et al, 2014; Supplementary Table 1). In particular, the accumulation of S 0 in the sediments may result from the reaction between FeOOH and H 2 S (Figure 2H and Supplementary Table 1).…”

“…The high proportion of Aut-P together with the high PO 4 3− concentration in surface sediment at the farm site is primarily related to mineralization of organic matter (i.e., P-rich fish feed). The P released during C org remineralization resulted in the P supersaturation in pore water with respect to apatite, which ultimately forms Aut-P (Slomp et al, 1996;van der Zee et al, 2002;Andrieux-Loyer et al, 2014). In organic-rich sediment in particular, the regeneration of inorganic P from C org remineralization was the predominant process for Aut-P precipitation (Anschutz et al, 2007;Tsandev et al, 2012;Joshi et al, 2015;Kraal et al, 2017).…”

“…In shallow coastal ecosystems, sediment serves either as a source (i.e., regeneration and release) or a sink (i.e., adsorption and precipitation) of P for the water column (Slomp, 2011;Kraal et al, 2015;An et al, 2019). P dynamics in coastal sediments are tightly coupled to the rate and partitioning of C org mineralization and the resulting interaction between iron and sulfur (Rozan et al, 2002;Canfield et al, 2005;Kraal et al, 2013;Slomp et al, 2013;Andrieux-Loyer et al, 2014;An et al, 2019). P release from sediments into pore water first proceeds from C org mineralization, relying on various terminal electron-accepting processes, such as aerobic respiration, denitrification, and reduction of Mn and Fe oxides and sulfate (Canfield et al, 2005;Holmer et al, 2005).…”

How the origin of sedimentary organic matter impacts the benthic nutrient fluxes in shallow coastal mudflats

“…Although the amount of SOM has been reported as an important driver of mineralization activity in coastal areas [Sloth et al, 1995, Mesnage et al, 2007, the composition of SOM has also been shown to play a key role in benthic nutrient cycling [Andrieux-Loyer et al, 2014, Albert et al, 2021. The composition of SOM in coastal areas is tightly linked to its origin and is the result of a large diversity of allochthonous and autochthonous primary producers (such as phytoplankton, benthic macroalgae, microphytobenthos, seagrasses andwoody plants), as well as anthropogenic OM [Bianchi and Bauer, 2011, and references therein].…”