Spatiotemporal aspects of silica buffering in restored tidal marshes
Abstract: Losses of pelagic diatom production resulting from silica limitation have not only been blamed for toxic algal blooms, but for the reduction in ability of coastal food webs to support higher trophic levels. Recent research has shown the importance of advective seepage water fluxes of dissolved silica (DSi) from freshwater marshes to pelagic waters during moments of riverine Si-limitation. In this study, we investigated the potential impact of recently installed new tidal areas along the Schelde estuary, located in former polder areas and characterized by so-called controlled reduced tidal regimes (CRT). Nine mass-balance studies were conducted in a newly constructed CRT in the freshwater Schelde estuary. During complete tidal cycles both DSi and amorphous silica (ASi) concentrations were monitored at the entrance culverts and in different habitats in the marsh. A swift DSi-delivery capacity was observed despite the shifted spatiotemporal frame of exchange processes compared to reference marshes. As silica-accumulating vegetation is not yet present, and difference with reference marshes' deliveries is surprisingly small, we indicate diatomaceous debris and phytoliths to be the main silica source. Although further research is necessary on the driving forces of the different processes involved, restoration of former agricultural areas under CRT-regime provide the potential to buffer silica in the estuary.
Summary (4 min read)
- Estuaries are biogeochemical hot-spots and are amongst the most productive ecosystems of the world (Costanza et al., 1993).
- Within the estuarine ecosystem, fringing tidal marshes act as a biogeochemical filter, removing inorganic and organic substances from the floodwaters and changing substance speciation (e.g. Gribsholt et al., 2005).
- Tidal freshwater marshes contain large amorphous silica stocks in marsh soils, built up through sedimentation of diatom shells and incorporation of silica in marsh vegetation (Struyf et al., 2005).
- In the first implemented CRT, an intensive spatiotemporal sampling scheme was carried out during the first 16 months of development.
2.1. Study area
- The Schelde estuary is one of the last European estuaries with a complete fresh- to saltwater tidal gradient, located in the Netherlands and Belgium.
- The rotation system was abandoned in 2003.
- During the two-year construction phase (2003–2005), crops were replaced by pioneer vegetation (mainly Epilobium hirsutum and Urtica dioica) (Fig. 1B).
- Because site elevation is several metres under mean high water level, reconstruction of spring-neap tide flooding variation required the construction of separate inlet culvert and outlet culvert (Maris et al., 2007).
- This results in a controlled reduced tidal area (CRT) with unique tidal features, such as a pronounced spring–neap variation and a prolonged stagnant phase (Fig. 3, for details see Cox et al., 2006; Maris et al., 2007).
- A total of 796 data points were obtained during the nine mass balance studies.
- This intensity was necessary to explore the spatial patterns in the marsh; however this exhaustive scheme was not entirely repeated during all campaigns.
- Samples were taken approximately 10 cm below the surface, and stored in dark incubators at 5 C for a maximum of 24 h. at typical neap (–), mean ($ $ $) and spring (–) tide outside and inside Lippenbroek CRT agnant phase, (220) start outstream ¼ stop stagnant phase and (580) stop outstream.
- For each of the samples analysed for ASi (Table 2), three sub samples of 25 ml each were filtered over 0.45-mm filters, from a well-mixed total sample of 150 ml.
- [ASi] (mg l 1) was then calculated by extrapolating the linear line through the three extraction points in a time-extracted silica plot (DeMaster, 1991).
2.3. Water and silica mass balances
- All calculations and statistical analyses were performed in R (R, 2006).
- Inlet and outlet culverts are the only exchange points with the river.
- Flow velocity was measured acoustically (Sontek ‘‘Argonaut’’).
- Water mass balances were calculated with an averaged discharge value throughout the water column for every 2 min, assuring accurate volume-weighing of concentration values during all tidal phases.
- General patterns were not seriously influenced by this effect (Figs. 5 and 6).
3.1. Concentration profiles
- DSi concentration profiles show different seasonal patterns (Fig. 4).
- Instream phases (see Fig. 3) are marked by steep concentration changes, whilst the fluctuations during stagnant phase do not exceed 0.2 mg l 1.
- Concentration profiles of ASi present a more variable pattern over a smaller concentration range (not shown).
- And differences are generally lower for ASi compared to DSi, there is a general evolution of increase or status quo (May, July, September 2006 and October 2006, not shown) towards strong ASi decrease in ASi- concentration profiles during later tidal cycle (March and June 2007b, not shown), with the exception of June 2007a.
3.2. Mass balances
- Calculated DSi mass balances indicate enrichment of exported water in summer months (July 2006 and June 2007, Fig. 5, upper left), but also in late autumn (October 2006) and during one spring campaign (March 2007).
- Absolute numbers (Fig. 5 upper right) are lower due to small water mass at neap tide, while the opposite is true for the 2007 campaigns.
- ASi mass balances confirm the transition from slight ASi delivery or status quo towards ASi caption by the marsh (Fig. 5, lower graphs).
- Epresented as percent, (out(g) in(g))/in(g)) (left graphs); and in absolute numbers (kg alance, represented as percent in both directions.
- Yet, despite large ASi retention in the March 2007b and June 2007a campaigns, DSi delivery can still be sufficient (Fig. 7, upper graphs) to provide the estuary with net silica (Fig. 6).
3.3. Ephemeral and diurnal aspects
- Non-numerical variable classes, being ‘‘day/night’’ show opposite differences in March vs. June 2007 campaigns.
- Variation of obtained numeric variables was maximized along two components (PCA correlation circle, see Section 2, Fig. 7).
- Parallel vectors indicate high correlation along the two components, whilst squared vectors are not correlated.
- Explicit Rsquared and p-values depend on the amount of variation explained by the components.
- Percent DSi delivery was negatively correlated with the logarithm of riverine DSi concentration (R2 ¼ 0.6397, p-value 0.0096), and the logarithm of mean water depth (R2 ¼ 0.52, p-value 0.02), and not correlated with temperature of outstream water, percent ASi delivery, or riverine ASi concentrations.
3.4. Spatial aspects
- Concentration profiles observed at other locations were not used for local mass balances, because water volumes and tidal phases were not measured separately for these locations.
- Concentrations appear unchanged during stagnant phase (minute 150 until 250 after instream in Fig. 8), but an hourglass pattern appears during outstream, when concentrations from different locations diverge strongly (Minute 300; Fig. 8).
- Typical retention habitat during the May 2006 campaign was the tidal pool (site 2 in Figs. 8 and 1C), whilst opposing behaviour is observed at the lower mudflat (site 3 in Figs. 8 and 1C).
- The hourglass structure returns, albeit less complete, in the other spatially sampled campaigns in the summer and autumn with net DSi delivery (not shown).
- A similar spatial sampling took place for ASi (Table 2).
- Numerous processes are involved in the silica exchange between tidal marsh and flooding water (Scheme 1).
- In the following overview, these processes are described and linked to the obtained data.
4.1. Diffusive and advective transport
- In several systems, the importance of advective ground water fluxes to the estuarine nutrient balance has been emphasized (i.e. Herrera-Silveira, 1998; Hays and Ullman, 2007; Niencheski et al., 2007).
- As previously explained, the key process in silica buffering by tidal marshes is the swift replenishing of dissolved silica (DSi) in the flooding water during seepage (e.g. Struyf et al., 2006).
- Other authors, however, have suggested the importance of bioturbation (Berner, 1980; Meile et al., 2005), resuspension (Mortimer et al., 1998), bioirrigation (Aller, 1965; Mortimer et al., 1998), advection and subsurface circulation patterns (Vanderborght et al., 1977) in diagenetic equations.
- In fact, the end of overmarsh tide and start of seepage coincides with a sudden increase in DSi concentrations in outstream water for almost all campaigns.
- The often observed decline in concentrations at the very end of the outstream phase is probably due to riverine water slowly entering the exit culvert.
4.2. Sedimentation and resuspension
- Sedimentation and recycling of suspended particulate silica are key processes in the marine and lacustrine silica cycle (Treguer et al., 1995; Bidle and Azam, 2001).
- In estuaries it has been shown that regeneration of silicic acid from particulate silica does not account for observed summer increases in silicic acid concentration (van Bennekom et al., 1974; Yamada and Delia, 1984).
- DSi delivery occurred equally from location 4 during the initial Urticadominated period, over the period of gradual decay of the Urtica vegetation, towards a rather bare mudflat stage.
- When discussing patterns of ASi delivery, it is necessary to emphasize that ASi concentrations are probably underestimated, as only surface samples were taken, and suspended solids concentrations are expected to be higher near the bottom.
- Throughout the first summer small creeks developed, and vegetation and algal layers began to consolidate marsh soil while sedimentation and erosion zones became apparent, as in reference marshes.
4.3. Diatom die-back and frustule dissolution
- PH, temperature, etc.) on the solubility of silicic acid have been well studied (Greenberg and Price, 1975; Hurd and Theyer, 1975; Kamatani and Riley, 1979), silicic acid is likely to be found at concentrations considerably below saturation in the water column and surface sediments of most estuaries (Yamada and Delia, 1984).
- Not only does increased temperature directly boost silica efflux from sediments (Yamada and Delia, 1984),it also enhances efficiency of bacterial removal of the organic carbon matrix from diatoms, increasing the surface area of naked silica exposed to dissolution and fastening chemical dissolution rates (Bidle and Azam, 2001).
- Also, the relationship could be magnified by temporal concurrence of low riverine DSi concentrations with periods of high temperature in summer.
- This suggests that day delivery might be lower, due to uptake of DSi by autochthonous diatoms.
- This diurnal aspect certainly deserves more attention.
4.4. Si uptake by diatoms
- The DSi retention in the tidal marsh, until now not observed in important quantities (Struyf et al., 2006), may be the most striking difference of this study with earlier findings.
- The cause is almost certainly diatom uptake.
- In later campaigns benthic diatom populations were observed throughout the marsh (Jacobs, personal observation), as were decreasing DSi profiles during stagnant and bulk phase (not shown).
- Settling of the sediments and very low depth compared to the pelagic optimizes light conditions and surface aeration.
- The uptake process is dominant in spring, as low ambient DSi concentrations disadvantage production and promote relative export numbers in summer campaigns.
- In contrast to other systems where silica fluxes are studied, processes influencing silica cycling from tidal marshes are mostly separated in space and/or time (Scheme 2).
- Specific tidal patterns in CRT’s influence the observed processes and their temporal distribution, most likely in favour of import processes (Scheme 2).
- When DSi deliveries are compared with earlier measured reference tidal marshes, the difference is surprisingly small (Fig. 9).
- The above-mentioned difference (Fig. 9) is mainly governed by two DSi-retention events, which are caused by DSi uptake by diatoms, a process that occurs throughout the marsh at stagnant phase and in the tidal pool throughout the whole tidal cycle.
- The typical tidal features of CRT areas influence their silica cycling, both through increased potential for uptake of DSi and enhanced sedimentation of ASi during the stagnant phase and in tidal pools.
- When DSi deliveries are compared with earlier measured reference tidal marshes, the difference is surprisingly small.
- Export processes do not seem strongly limited by their decreased time budget.
- This indicates that CRT areas are capable of fast build-up of Si-recycling capacity and swift DSi delivery at limitation events.
- The authors main conclusion is that recently constructed CRTs along estuaries are capable of a silica buffering role comparable to older, reference tidal marshes.
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Cites background from "Spatiotemporal aspects of silica bu..."
...…[Wamsley et al., 2010], the biogeochemical filtering of sediments, nutrients and contaminants [e.g., Almeida et al., 2011; Gribsholt et al., 2005; Jacobs et al., 2008; Struyf et al., 2006; Temmerman et al., 2004], and the provision of nursery areas to commercially important fish and shellfish…...
Cites background from "Spatiotemporal aspects of silica bu..."
...…and autochtonous BSi production in tidal marshes result in high concentrations of DSi in marsh pore water and tidal pools (100–600 μM; e.g., Norris and Hackney 1999; Jacobs et al. 2008; Struyf et al. 2005b; Querné et al. 2012) Tidal pools can show a strong decrease in DSi due to diatom production....
...The general importance of BSi accumulation and DSi delivery by tidal marshes has been confirmed in a growing number of studies (e.g., Struyf et al. 2005a, 2006; Jacobs et al. 2008; Vieillard et al. 2011)....
...The high BSi import and autochtonous BSi production in tidal marshes result in high concentrations of DSi in marsh pore water and tidal pools (100–600 μM; e.g., Norris and Hackney 1999; Jacobs et al. 2008; Struyf et al. 2005b; Querné et al. 2012) Tidal pools can show a strong decrease in DSi due to diatom production....
Cites background from "Spatiotemporal aspects of silica bu..."
...Besides their safety function, tidal marshes play an important role in the cycling of nutrients (e.g., Gribsholt et al., 2005; Jacobs et al., 2008; Struyf et al., 2006) and they are characterized by high habitat diversity....
...2009; Jacobs et al., 2008)....
...Estuaries are biogeochemical hot-spots and are amongst the most productive ecosystems of the world ( Costanza et al., 1993 )....
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Q1. What are the contributions mentioned in the paper "Spatiotemporal aspects of silica buffering in restored tidal marshes" ?
In this study, the authors investigated the potential impact of recently installed new tidal areas along the Schelde estuary, located in former polder areas and characterized by so-called controlled reduced tidal regimes ( CRT ). Although further research is necessary on the driving forces of the different processes involved, restoration of former agricultural areas under CRT-regime provide the potential to buffer silica in the estuary.