Spatiotemporal aspects of silica buffering in restored tidal marshes
Introduction
Estuaries are biogeochemical hot-spots and are amongst the most productive ecosystems of the world (Costanza et al., 1993). As the interface between terrestrial and coastal waters, they support processes that are central to the planet's functioning (Costanza et al., 1997). Estuaries are characterized by steep chemical gradients and complex dynamics, resulting in major transformations in the amount, the chemical nature and the timing of material fluxes.
Eutrophication is one of the most important problems that confronts these systems. Eutrophication phenomena in estuaries are related to the balance between N, P and Si in river loading, and are thus dependent on the interactions between human activity and natural processes in the watershed, which ultimately determine the riverine nutrient delivery into the marine environment (Officer and Ryther, 1980, Billen and Garnier, 1997, Lancelot et al., 1997, Cugier et al., 2005). Eutrophication can cause anoxia, extreme turbidity and even toxicity in coastal areas and lakes, mostly provoked by shifts in plankton community following excessive inputs of N and P compared to Si. Decreases in the availability of silica relative to N and P in estuaries may result in a shift in the phytoplanktonic community from a dominance of diatoms to other phytoplankton forms as cyanobacteria or toxic dinoflagellate, affecting zooplankton and fisheries (see also Chícharo et al., 2006, Wolanski et al., 2006). Losses of diatom production, resulting from silica limitation, have not only been blamed for these toxic algal blooms, but for the reduction in ability of coastal food webs to support higher trophic levels (Treguer et al., 1995, Cugier et al., 2005, Kimmerer, 2005). Estuarine and marine foodwebs are based essentially on diatoms (Irigoien et al., 2002, Kimmerer, 2005). Dissolved silica concentrations have since long been known to control diatom populations (Wang and Evans, 1969), diatom blooms (Tessenow, 1966, Schelske and Stoermer, 1971, Davis et al., 1978), and seasonal succession in plankton communities (Kilham, 1971). In fact, the availability of dissolved silica (DSi) has been shown to control diatom silica production rates, at least seasonally, in every natural system examined to date (Nelson and Brzezinski, 1990, Nelson and Treguer, 1992, Brzezinski and Nelson, 1996, Nelson and Dortch, 1996, Brzezinski et al., 1998, Bidle and Azam, 2001).
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). The interaction between tidal marshes and estuaries or coastal zones received much attention through numerous exchange studies (e.g. Valiela et al., 2007, Spurrier and Kjerfve, 1988, Whiting et al., 1989, Childers et al., 1993), with emphasis on C, P and N. Dominant questions were whether marshes were importing or exporting N, P, C or particulate matter, often testing the “outwelling” hypothesis (e.g. Dame et al., 1986), which states that a large part of the organic matter produced in the intertidal marshes is not used in internal trophic chains but is transported into the adjacent sea areas and increases their productivity. Only a limited number of mass balance studies have targeted freshwater tidal marshes (e.g. Childers and Day, 1988, Gribsholt et al., 2005, Struyf et al., 2006). The freshwater systems are characterized by botanical properties resembling inland freshwater wetlands and by more direct contact with human-impacted river water. These characteristics make freshwater tidal marshes potentially important process interfaces. Struyf et al. (2006) have shown the importance of advective seepage water fluxes of dissolved silica (DSi) from freshwater marshes to pelagic waters during moments of riverine Si-limitation. 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). Export is the result of consequent dissolution of this amorphous silica (ASi) in marsh pore water from litter and sediments, and advective export of marsh pore- and puddle water between tidal flooding events (Struyf et al., 2007a, Struyf et al., 2007b). Silica limitation of diatoms (Conley et al., 1993, Smayda, 1997) and the consequent negative effects on food web structure may be avoided. However, data are available only from few tidal freshwater wetlands and conclusions are presently only applicable on a local scale. Furthermore, a recent review stresses the need for more research on silica cycling in wetlands, as it rivals their impact on other biogeochemical cycles and, to date, this topic has not received sufficient attention (Struyf and Conley, in press).
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 (CRTs) (Cox et al., 2006, Maris et al., 2007). Along the Schelde estuary, more than 50% of marsh area will eventually be located in such areas, and may result in international application. This article focuses on the silica biogeochemistry within these new systems and aims to explore spatiotemporal patterns of deposition and dissolution in recently flooded formerly agricultural polder areas. In the first implemented CRT, an intensive spatiotemporal sampling scheme was carried out during the first 16 months of development. This research expands the growing awareness that ecosystems and associated biogenically fixed amorphous Si rather than geological weathering control silica availability in the aquatic environment on a shorter, biological timescale (Conley, 2002, Humborg et al., 2004, Derry et al., 2005).
Section snippets
Materials and methods
Nine mass-balance studies were conducted in a newly constructed CRT in the freshwater Schelde estuary: on May 16, July 3, September 10 and 11 and October 10, 2006, and on March 20 and 21 and June 4 and 5, 2007. During nine complete tidal cycles both DSi and ASi concentrations were monitored at the entrance culverts as well as in different habitats in the marsh.
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. Outstream concentration profiles are highly variable and show increases, decreases or both: at starting concentrations below 2 mg DSi l−1, concentrations increase with 125.0% and 126.6% (June 2007 in Fig. 4) or even with a factor 17 (July 2006 in Fig. 4) at final concentration. For
Discussion
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.
Acknowledgements
The authors wish to thank the Flemish Government, Environment and Infrastructure department, W&Z for the financing of the SIGMA and OMES project, which made this research possible. Thanks to Eva De Bruyn and Godelieve Clement for the ICP-analysis, and especially to Tom Van Der Spiet for lab work. We wish to express our gratitude to the Schelde research team; Stefan Van Damme, Tom Cox, Olivier Beauchard, Johnny Teuchies and Katrijn Van Renterghem for support during field and lab work and
References (79)
- et al.
Nutrient exchange across the sediment–water interface in the Potomac River Estuary
Estuarine, Coastal and Shelf Science
(1982) - et al.
Use of a hydrotechnical infrastructure (Alqueva Dam) to regulate planktonic assemblages in the Guadiana estuary: basis for sustainable water and ecosystem services management
Estuarine Coastal and Shelf Science
(2006) - et al.
A dynamic nutrient budget of subsystem interactions in a salt marsh estuary
Estuarine, Coastal and Shelf Science
(1993) - et al.
Flood control areas as an opportunity to restore estuarine habitat
Ecological Engineering
(2006) - et al.
Modelling the eutrophication of the Seine Bight (France) under historical, present and future riverine nutrient loading
Journal of Hydrology
(2005) - et al.
Biogenic silica dissolution in seawater - in vitro chemical kinetics
Progress in Oceanography
(2001) Nutrient-phytoplankton production relationships in a groundwater-influenced tropical coastal lagoon Aquatic Ecosystem Health & Management
(1998)- et al.
Early diagenesis of biogenic opal, dissolution rates, kinetics, and paleoceanographic implications
Deep-Sea Research II
(1995) - et al.
Submarine groundwater discharge of nutrients to the ocean along a coastal lagoon barrier, Southern Brazil
Marine Chemistry
(2007) - et al.
The role of aggregation for the dissolution of diatom frustules
FEMS Microbiology Ecology
(2003)