Reoxidation of estuarine sediments during simulated resuspension events: Effects on nutrient and trace metal mobilisation
Graphical abstract
Introduction
Estuaries are highly dynamic coastal environments regulating delivery of nutrients and trace metals (TMs) to the ocean (Sanders et al., 1997; Statham, 2012). In most coastal ecosystems in the temperate zone, nitrogen controls primary productivity as it is usually the limiting nutrient; therefore an increased load flowing into such oligotrophic waters could lead to eutrophication, and the subsequent environmental impacts due to hypoxia, shifts in the biological community and harmful algal blooms (Howarth et al., 1996; Abril et al., 2000; Boyer and Howarth, 2002; Roberts et al., 2012; Statham, 2012). This has been the focus of attention because human activities over the last century have increased nitrogen fluxes to the coast due to intensive agricultural practices, and wastewater and industrial discharges (Howarth et al., 1996; Canfield et al., 2010).
River inputs are the main nitrogen sources to estuarine waters. Inorganic nitrogen is generally the major portion of the total dissolved nitrogen inputs to an estuary; however organic nitrogen may sometimes be significant (20–90% of the total nitrogen load) (Seitzinger and Sanders, 1997; Nedwell et al., 1999). The speciation and distribution of nitrogen along the salinity continuum will be controlled by complex dissimilatory and assimilatory transformations coexisting at a range of oxygen concentrations (Thamdrup, 2012); but denitrification is considered the major removal process to the atmosphere in shallow aquatic environments (Statham, 2012). Anammox and dissimilatory nitrate reduction to ammonium (DNRA) can also play a role in the nitrogen cycle, although their relative importance in different coastal environments is still a matter of debate (Song et al., 2013; Roberts et al., 2014). The organic nitrogen pool will be cycled during microbial metabolism, and thus it also plays an important role in estuarine geochemistry. However, this pool is difficult to characterise as it comprises a wide variety of compounds, mostly complex high molecular weight compounds that are more refractory and less bioavailable than low molecular weight compounds (Seitzinger and Sanders, 1997). Organic matter buried in the sediments will be involved in early diagenesis through a combination of biological, chemical and physical processes. In fact, high rates of organic matter oxidation are expected in estuaries due to the sediment accumulation rates, organic matter flux into the sediment and organic matter burial (Henrichs, 1992).
Estuarine sediments may also have accumulated contaminants such as TMs carried by river loads. Sediment geochemistry and dynamics will control the mobility and bioavailability of TMs, and therefore sediments subjected to reoxidation processes may be a potential source (Salomons et al., 1987; Di Toro et al., 1990; Allen et al., 1993; Calmano et al., 1993; Simpson et al., 1998; Saulnier and Mucci, 2000; Caetano et al., 2003). Trace metals can be in solution, sorbed to or co-precipitated with different mineral surfaces and organic matter, but in anoxic sediments, iron sulphides are thought to be the main solid phases controlling TM mobility (Salomons et al., 1987; Huerta-Diaz and Morse, 1990; Allen et al., 1993). When sediments are exposed to oxic conditions, dissolved Fe and Mn will precipitate rapidly as amorphous and poorly crystalline Fe/Mn oxyhydroxides, incorporating the released TMs by co-precipitation and/or adsorption (Burdige, 1993; Calmano et al., 1993; Simpson et al., 1998; Saulnier and Mucci, 2000; Gunnars et al., 2002; Caetano et al., 2003). These newly formed minerals will be transported, mixed, and maybe, eventually buried into the underlying anoxic sediment again.
In aquatic sediments, there is a vertical progression of metabolic processes determined by the use of the available electron acceptors during organic matter mineralization (Canfield and Thamdrup, 2009). The sequential utilization of the terminal electron acceptors is based on the thermodynamics of the process and the free energy yield (Stumm and Morgan, 1970; Froelich et al., 1979; Berner, 1980). At the surface, dissolved oxygen can diffuse a few millimetres into the sediments (the oxic zone), where aerobic respiration is the dominant metabolic pathway. Beneath, there is often a suboxic zone where nitrate is actively reduced and nitrite accumulates as its reduction intermediate (the nitrogenous zone). Below, zones dominated by metal reduction (the manganous and ferruginous zones), sulphate reduction (the sulphidic zone), and methanogenesis (the methanic zone) occur in sequence (Canfield and Thamdrup, 2009). Dissolved Fe normally accumulates below Mn in the sediment column since it is less mobile and more sensible to oxygen. In general, besides the effects of advection and bioturbation, Mn and Fe cycling in aquatic sediments imply vertical diffusion that depends on gradient concentrations and different environmental factors (pH, oxygen, hydrogen sulphide concentrations, organic matter, suspended particulate matter, etc.) (Canfield et al., 2005). Finally, in anoxic sediments, sulphate reduction, the major anaerobic mineralization process in coastal sediments, results in the accumulation of dissolved sulphide (Jørgensen, 1977, 1982; Middelburg and Levin, 2009).
However, in coastal and estuarine sediments, these geochemical zones, and the correspondent metabolic zones, are not normally well delineated and they tend to overlap because sediment profiles are often disturbed by mixing and bioturbation (Sørensen and Jørgensen, 1987; Aller, 1994; Postma and Jakobsen, 1996; Mortimer et al., 1998; Canfield and Thamdrup, 2009). Rapid redox changes at the sediment-water interface due to successive cycles of sediment suspension and settling will control the speciation and cycling of nutrients and trace elements on a tidal-cycle timescale (Morris et al., 1986). Yet, less frequently, seasonal or annual resuspension events can affect sediment to depths that are not disturbed normally, which will alter the biogeochemistry of the system (Eggleton and Thomas, 2004). The pairing of in situ hydrodynamic and erosion observations during a moderate storm and estimates of the magnitude of benthic nutrient release at increasing erosion thresholds show that resuspension events may significantly influence nutrient budget of shallow estuarine systems (Kalnejais et al., 2010; Couceiro et al., 2013; Percuoco et al., 2015; Wengrove et al., 2015). Nutrient release during resuspension can be associated to the entrainment of particles and porewaters into the water column and also to reactions of freshly suspended particles (Kalnejais et al., 2010; Couceiro et al., 2013).
In this study sediments from four different sites along the salinity range of the Humber Estuary (UK) were used in order to investigate the impact of sediment resuspension on the redox cycling and transport of the major elements and TMs to the coastal waters. The authors have worked in the Humber since 1994 (Mortimer et al., 1998, 1999; Burke et al., 2005) and have observed the frequency and magnitude of resuspension events. Small-scale resuspension of the upper 1–2 mm occurs on a tidal cycle; medium scale resuspension of the order of centimetres occurs during large flooding or moderate storm events which occur approximately twice a year. Very significant resuspension events that strip off the mud from intertidal areas occur on a timescale of several decades (a removal of about 10 cm deep in the intertidal mudflat was observed following a storm in early 1996) (Mortimer et al., 1998). Accordingly, for this experiment, two sediment depths (the mobile oxic/suboxic surface layer, 0–1 cm, and the suboxic/anoxic subsurface layer, 5–10 cm) were selected to simulate different timescales of resuspension and to analyse their effects on nutrient and TM behaviour.
Climate change-associated impacts will have effects on estuarine morphodynamics (Townend et al., 2007; Robins et al., 2016). For the UK, an increase in the extreme rainfall events (during the winter season) and long periods of low flow conditions have been predicted (Jones and Reid, 2001; Christensen et al., 2007; IPCC, 2013; Robins et al., 2016). This combined with the sea-level rise will increase estuarine flood risk and will have further implications on sediment transport patterns; on the position of the estuarine turbidity maximum (ETM); and on the retention time of river-borne substances (i.e. sediments and contaminants) (Robins et al., 2016). The aim of this work is to better understand the environmental impact of different sediment remobilisation events within the estuary. The more frequent disruption of subsurface sediments will affect the geochemistry of estuarine sediments; porewater profiles may not reach steady state between resuspension episodes, and there may be impacts on the nutrient and TM fluxes to the sea.
Section snippets
Field sampling
The Humber Estuary is a macrotidal estuary on the east coast of northern England (Fig. 1). It is 60 km in length, there are ∼115 km2 of mudflats, and is highly turbid (Pethick, 1990). The Humber is also considered a major source of nutrients for the North Sea (Pethick, 1990; Mortimer et al., 1998; Uncles et al., 1998a,b).
Samples of intertidal mudflat sediments and river water were collected at low tide during the same tidal cycle on the 15th July 2014 along the north bank of the Humber Estuary (
Site characterisation
The basic physicochemical parameters at the four sampling sites are reported in Table 1. During sampling, the light brown surface sediments contrasted visually with the underlying dark grey materials, except at S2 (Blacktoft), where there was no colour change but abundant plant material throughout. The full chemical characterisation of the river waters and porewaters is given in the SI.
Solid phase
The bulk mineralogy of the dried sediments was characterised and all sediments contained a mixture of quartz,
Geochemical character of river water and estuarine sediments
The four sites along the Humber estuary represent the gradual change from a typical freshwater environment to an intertidal mudflat with brackish waters. This salinity profile was similar to that measured in other surveys (NRA, 1995, 1996; Sanders et al., 1997; Mortimer et al., 1998). Along the salinity gradient, nitrate concentrations in the overlying waters decreased with increasing salinity and were inversely correlated with the ammonium concentrations. Previously nitrate has been described
Conclusions
This study gives an insight into the complex mosaic of processes that result from physical disturbances along the Humber estuary continuum. The position in the salinity gradient was the dominant control on sediment geochemistry with a change from a Mn/Fe-dominated redox chemistry in the inner estuary to a Fe/S-dominated system in the outer estuary. Therefore, understanding the system dynamics and sediment characteristics is key when studying nutrients and metal cycling along a salinity
Acknowledgements
The authors acknowledge funding from a University of Leeds Doctoral Training Award to A. Vidal-Durà. We are grateful to S. Reid, A. Stockdale, for technical support with the ICP-MS analysis, AA3 autoanalyser respectively; and to A. Connelly, F. Keay and D. Ashley (all from University of Leeds) for their help in the column-switching setting up for saline water analysis by IC. We also thank S. Poulton and his team for the iron extraction training, and the technical support from L. Neve in the XRF
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