Strong gradient of benthic biogeochemical processes along a macrotidal temperate estuary: focus on P and Si cycles

Abstract

International audienceThis study aims to investigate the role of spatial and temporal physical, biological and biogeochemical gradients on sediment biogeochemistry along a macrotidal and Si-rich estuary. Scanning and biogeochemical analyses were performed in the inner, mid and outer Aulne Estuary (France) at four seasons. The inner estuary shows high diagenetic activity linked to fluid mud dynamics and river loads. The highest authigenic phosphorus (Aut-P) concentrations ever found in the literature are observed in the inner estuary (18 µmol g-1 PS sediment). This is explained by a combination of favorable factors, i.e. the high organic matter and nutrient loads, the reductive conditions, the freshwater properties (low pH, OH-, sulfate and Mg2+ concentrations), the increase of particle residence time by the upward convergence of particles due to residual currents, and allochthonous riverine Aut-P. We suggest that the high Si(OH)4 concentrations (> 400 µM) may even increase Aut-P precipitation through the increase of Fe-P formation in these low salinity conditions. In the mid estuary, erosion-deposition dynamics dominate in point bars and lead to the succession of poor and rich organic and authigenic phosphorus layers, recording thus the seasonality of matter loads and its seasonal translocation from the inner estuary. In the outer estuary, deposition rates are high and constant and biogeochemical properties are characteristic of marine environments. The precipitation of Aut-P from free phosphate (PO43-) is lower than in the inner estuary and might be limited by higher Mg2+ concentrations in saline waters. This study highlights that small macrotidal estuaries, and especially their freshwater sediments, may constitute an important phosphorus sink through the precipitation of Aut-P. This precipitation could even be enhanced in fresh or brackish environments, thus increasing long term phosphorus storage and altering benthic fluxes of PO43- to the pelagic ecosystem

Silicon cycle in Indian estuaries and its control by biogeochemical and anthropogenic processes

“…et al, 2013;Raimonet et al, 2013). In the present study, the observed DSi concentration of the upper estuaries are similar to those reported earlier for Indian rivers Gurumurthy et al, 2012;Meunier et al, 2015;Frings et al, 2015) and other tropical rivers (Liu et al, 2009;Hughes et al, 2011Hughes et al, , 2012Ran et al, 2015).…”

“…It is known that ASi dissolution at the sediment -water interface can be a significant supply of DSi in estuaries Delvaux et al, 2013;Raimonet et al, 2013). Our data do not allow quantifying such benthic flux, but we suggest that it should contribute significantly to the high DSi concentration in this cluster.…”

Phosphorus speciation, transformation, and preservation in the coastal area of Rushan Bay

“…The transformation, preservation, and recycling of P at the sediment-water interface are largely controlled by the reactivity of different forms of P (Ruttenberg, 1992;Slomp et al, 1996a;Mort et al, 2010), and driven by a number of biological, physical and geochemical processes (Schenau and De Lange, 2001;Slomp et al, 2013). Numerous studies have shown that estuarine and coastal areas are important P sinks and function as P filters (Fang et al, 2007;Raimonet et al, 2013;Bonaglia et al, 2014) due to their fairly high sedimentation rate and phosphorus burial efficiency. However, sediments can also provide a significant source of P for the water column (Lewandowski et al, 2003;Krom et al, 2010;Zaaboub et al, 2014).…”

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

“…However, some other processes such as Auth-Ca-P formation could counterbalance the decrease in dissolved Fe/P and N/P ratios in scavenging dissolved phosphate (Ruttenberg et al 1992;Raimonet et al 2013). The enhanced liberation of pore water HPO 2À 4 arising from Orga-P mineralization or from Fe-P dissolution should promote Auth-Ca-P precipitation (Slomp et al 1996;Raimonet et al 2013).…”

“…The enhanced liberation of pore water HPO 2À 4 arising from Orga-P mineralization or from Fe-P dissolution should promote Auth-Ca-P precipitation (Slomp et al 1996;Raimonet et al 2013). In the Aber Benoît, Auth-Ca-P precipitation and Fe-P precipitation in superficial sediments in May (beneath oysters) and in June and at the end of July in deeper layers (Reference) could partly explain the sharp rise in the ratio between dissolved ammonium and phosphate (NH þ 4 /HPO 2À 4 ) and dissolved iron and phosphate (Fe 2?…”