The effect of riverine discharge on biogeochemical processes in estuarine sediments

Abstract

A recently developed autonomous benthic lander equipped with an underwater potentiostat and a micromanipulator for in situ voltammetric depth profiling of main redox species in pore waters was deployed for 3 yr at multiple stations along the Satilla River estuary (Georgia, U.S.A.). These measurements revealed that biogeochemical processes in estuarine sediments vary seasonally and are influenced by riverine discharge. A prolonged drought decreased river discharge, altered the salinity gradient in the estuary, and profoundly affected anaerobic respiratory processes in the underlying sediments. Under normal hydrologic conditions, iron reduction was the dominant anaerobic respiratory process across the estuary, likely due to the significant supply of iron from the coastal plane and low salinity of the estuary. Under drought conditions, the salinity of the estuary increased to full seawater strength and carbon remineralization was enhanced significantly. Yet, sulfate reduction was only observed near the mouth of the estuary, whereas a substantial increase in iron reduction was distinctive upriver. Evidence indicates that the increase in iron-reducing activity during the drought resulted mainly from the deposition of mineral particles in the upper estuary when the salinity gradient increased. Altogether, this study demonstrates that the biogeochemical response of estuarine sediments to natural perturbations is rapid and that respiration processes are controlled by a combination of temperature, supply of inorganic and organic substrates, and hydrological processes. These findings also suggest that an increase in the frequency of droughts as a result of climate change may enhance the resiliency of iron-reducing bacteria in river-fed estuarine sediments.

The flux of soluble organic‐iron(III) complexes from sediments represents a source of stable iron(III) to estuarine waters and to the continental shelf

“…Sediment cores were characterized by oxygen penetration depths that did not exceed 0.1-0.3 cm (Meiggs and Taillefert 2011). In general, soluble Fe(III) concentrations were low at the surface of the sediment, rose until a maximum was reached within the first few centimeters of the sediment, and then either fell or remained at their maximum value (Fig.…”

“…1) between July 2007 and January 2008, at the end of a prolonged period of drought (Meiggs and Taillefert 2011). Surface (1 m below surface) and bottom (1 m above sediment) water samples were collected twice (November 2007 andJanuary 2008) along the salinity gradient of the lower 30 km of the Satilla River Estuary.…”

A global perspective on wetland salinization: ecological consequences of a growing threat to freshwater wetlands

“…Moreover, salinization itself can introduce new Fe to the system via influx of Fe-rich saline groundwater (Table 1) or enhanced sediment deposition (see Ionic effects). Meiggs and Taillefert (2011) showed that saltwater intrusion into riverine freshwater wetlands resulted in seasonally-enhanced Fe(III) reduction, as a direct result of ion-mediated enhanced mineral deposition. Although Fe reduction is important in regulating microbial C cycling (Neubauer et al 2005b) the process is rarely assessed in salinizing wetlands.…”

“…In organic soils, increased decomposition (see Carbon metabolism) and reduced root biomass may further accelerate subsidence, increasing rates of flooding with feedbacks to plant communities (Mac Nally et al 2011, Pittock andFinlayson 2011). Salinization may also increase sedimentation (Ionic change, above) as the peak of fluvial sediment trapping often occurs at the head of the salt wedge when saltwater intrudes upriver (Meiggs andTaillefert 2011, de Nijs andPietrzak 2012). The net outcome will depend on soil composition (mineral versus organic) and mineral sediment loads, among myriad other factors.…”

“…3A-i ).-Wetland soils contain an estimated 45-70% of all terrestrial C (Mitra et al 2005) and the accumulation of C in wetland soils can play an important role in reducing greenhouse gas concentrations and mitigating climate change (Mcleod et al 2011). Salinization increases the concentration of terminal electron acceptors (Fe(III), Mn(IV), SO 4 2À ), theoretically stimulating CO 2 production via increased microbial mineralization of organic matter (Chambers et al 2011, Meiggs and Taillefert 2011, Weston et al 2011, Marton et al 2012, Neubauer 2013) and shifting the dominant pathway of anaerobic metabolism from methanogenesis towards higher energy-yielding pathways (e.g., SO 4 2À reduction). The best studied effect of increased salinity on microbial C cycling in freshwater wetlands is the suppression of methanogenesis with the increased availability of SO 4 2À (Bartlett et al 1987, Boon and Mitchell 1995, Weston et al 2006, Chambers et al 2011, Poffenbarger et al 2011, Neubauer 2013.…”