Tidal exchange between a freshwater tidal marsh and an impacted estuary: the Scheldt estuary, Belgium

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Abstract

Tidal marsh exchange studies are relatively simple tools to investigate the interaction between tidal marshes and estuaries. They have mostly been confined to only a few elements and to saltwater or brackish systems. This study presents mass-balance results of an integrated one year campaign in a freshwater tidal marsh along the Scheldt estuary (Belgium), covering oxygen, nutrients (N, P and Si), carbon, chlorophyll, suspended matter, chloride and sulfate. The role of seepage from the marsh was also investigated. A ranking between the parameters revealed that oxygenation was the strongest effect of the marsh on the estuarine water. Particulate parameters showed overall import. Export of dissolved silica (DSi) was more important than exchange of any other nutrient form. Export of DSi and import of total dissolved nitrogen (DIN) nevertheless contributed about equally to the increase of the Si:N ratio in the seepage water. The marsh had a counteracting effect on the long term trend of nutrient ratios in the estuary.

Introduction

It is generally thought that fringing marshes act as a filter for the estuarine water by removing inorganic and organic substances from the floodwaters or by changing the substance speciation (e.g. Cai et al., 2000, Tobias et al., 2001, Gribsholt et al., 2005). The marsh basically provides a large increase in reactive surface and enhances sedimentation. In the past, the interaction between tidal marshes and estuaries or coastal zones received much attention through numerous exchange studies (e.g. Valiela et al., 1978, Spurrier and Kjerfve, 1988, Whiting et al., 1989, Jordan and Corell, 1991, Childers et al., 1993). In these ‘classic’ interaction studies, fluxes were determined through the construction of mass balances. Dominant questions were whether marshes were importing or exporting substances, such as N, P, C or particulate matter (C and sediment), often testing the ‘outwelling’ hypothesis (e.g. Dame et al., 1986). Recently these studies have shifted their focus towards the underlying processes, using more refined techniques such as isotope labeling (e.g. Gribsholt et al., 2005, Gribsholt et al., 2006). However, there are several reasons why exchange studies within the frame of the eutrophication problem in estuaries and coastal seas are still important.

Firstly, while certain aspects such as P and N retention (e.g. Valiela et al., 1978, Dame et al., 1986, Whiting et al., 1989, Jordan and Corell, 1991, Troccaz et al., 1994) have been studied in detail, others such as Si have been covered less frequently (e.g. Dankers et al., 1984, Struyf et al., 2005a, Struyf et al., 2005b).The high input of N and P in estuaries can lead to potential Si limitation in diatom communities, which are then less available to the higher trophic levels than dominating non-diatom species (Schelske et al., 1983, Smayda, 1997). Silica has only exceptionally been incorporated in mass-balance studies (e.g. Dankers et al., 1984, Struyf et al., 2005a). Furthermore, while tidal salt marshes are relatively well studied (e.g. Troccaz et al., 1994) only few mass-balance studies have focused on freshwater tidal marshes (e.g. Simpson et al., 1983, Childers and Day, 1988, Bowden et al., 1991, Gribsholt et al., 2005, Gribsholt et al., 2006, Struyf et al., 2006, Struyf et al., 2005a, Struyf et al., 2005b).With their botanical properties resembling inland freshwater wetlands, and as they interact more with river hydrology and the corresponding water quality than saline marshes, freshwater tidal marshes are very specific process interfaces. Within these potentially strongly reactive areas, it has been shown through process studies that the seepage water, that usually contributes a minor part of the tidal water balance of the marsh, nevertheless can play a very important role in the processing capacity of a marsh. Yet the characterization of the seepage water has in the classic mass-balance studies only seldom been emphasized (e.g. Whiting and Childers, 1989).

Secondly, most tidal marsh exchange studies were performed in the 1970s and 1980s. For many estuaries, this period was characterized by peaking eutrophication problems. This is notably true for e.g. the Seine (Billen and Garnier, 1999), the Elbe (ARGE – Elbe, written communication) and the Scheldt estuary (Soetaert et al., 2006). In the 1990s, measures were generally taken to improve the water quality and, as a consequence, river scientists are now often studying “oligotrophication” (decreasing N and P loads) rather than eutrophication. Thus, potential nutrient limitation in estuaries has changed, especially in the Scheldt estuary, where the N:P Redfield ratio, charcterising the need for growth of diatoms, shifted from less than 20 in the seventies to over 50 in 2000 (Billen et al., 2005, Van Damme et al., 2005, Soetaert et al., 2006). In the Seine, a similar phenomenon was documented (Billen et al., 2001). It is interesting to investigate if such a trend in estuarine systems interacts with the processing potential of marshes. Therefore a re-assessment of tidal marsh exchange is required, illustrating the use of old unpublished data.

Nutrient regulation, oxygenation, sediment accretion, carbon production and processing and water storage are all directly or indirectly linked with ecological functions or goods and services of estuarine systems, as defined e.g. by De Groot et al. (2002). The use of ecosystem functions in estuarine restoration has the major advantage that it is not a static approach, as is much of the protective legislation. The aim of this study was to assess the interaction of a freshwater tidal marsh and the water column by means of mass balances, including a comparison between nutrients, carbon, suspended matter and other parameters. This comparison is a basic essential step in the quantification and decision making of priorities in estuarine restoration. Also, the difference between the bulk tidal exchange and seepage is scoped.

In this study fluxes of nutrients (N, P and Si), carbon and particulate matter were determined in a freshwater tidal marsh, together with the ambient estuarine conditions. Also the oxygenating potential, the fluxes of chloride and sulfate, and other supporting parameters were determined simultaneously. To our knowledge, this is a tidal marsh exchange study with the most comprising parameter list so far.

Section snippets

Material and methods

Four mass-balance studies were conducted in a freshwater tidal marsh of the Scheldt estuary: on 1 July 1997, 7 October 1997, 27 January 1998 and 29 April 1998. Each tidal cycle was monitored at the entrance of the main creek as well as in the river upstream of the marsh. The cycles were monitored from the point of low tide in the river to the next low tide. All 13 h measurement campaigns started between 12:30 and 16:00.

Water balances

The discharge profiles of the four monitored cycles showed a similar pattern (Fig. 3). It took a few hours before the rising tide reached the level of the breach where the creek starts. During this period, water from the previous tide still seeped out of the marsh. During flood the discharge increased to a maximum after which it slowed down till slack tide. Superimposed on the tidal discharge asymmetry pattern, as described by Postma (1967), the out flowing water showed an additional discharge

Discussion

Before addressing the aims of this study (the effect of the marsh, the comparison between parameters and the difference between the seepage and bulk phase) the quality of the data must be assessed. Indeed, the water vs. the chloride balances showed differences that could indicate error. However, the concentration profiles indicate that the marsh can to some degree load and unload salt. Sediment physical properties, evapotranspiration and elevation are important determinants of salinity

Acknowledgements

The results were sponsored by the Flemish Environmental Agency (VMM) and the Flemisch Administration for Waterways and Maritime Affairs, division Zeeschelde. We thank the Fund for Scientific Research for funding the Scientific Community ‘Ecological characterization of European estuaries, with emphasis on the Schelde estuary’ (project nr. W 10/5-CVW.D 13.816). This is Publication 4600 of the Netherlands Institute of Ecology (NIOO-KNAW). Eric Struyf is funded as a postdoc researcher by FWO

References (43)

  • W.B. Bowden et al.

    Transport and processing of nitrogen in a tidal freshwater wetland

    Water Resources Research

    (1991)
  • W. Cai et al.

    Intertidal marsh as a source of dissolved inorganic carbon and a sink of nitrate in the Satilla river–estuarine complex in the southeastern U.S

    Limnology and Oceanography

    (2000)
  • D.L. Childers et al.

    Spatial and temporal variability in marsh–water column interactions in a southeastern USA salt marsh estuary

    Marine Ecology Progress Series

    (1993)
  • R. Dame et al.

    The outwelling hypothesis and North Inlet, South Carolina

    Marine Ecology Progress Series

    (1986)
  • B. Gribsholt et al.

    Nitrogen processing in a tidal freshwater marsh: a whole ecosystem 15N labeling study

    Limnology and Oceanography

    (2005)
  • B. Gribsholt et al.

    Ammonium transformation in a nitrogen-rich tidal freshwater marsh

    Biogeochemistry

    (2006)
  • P. Meire et al.

    The Scheldt estuary from past to future: a description of a changing ecosystem

    Hydrobiologia

    (2005)
  • J.T. Morris

    The mass-balance of salt and water in intertidal sediments – results from North-Inlet, South Carolina

    Estuaries

    (1995)
  • H. Postma

    Sediment transport and sedimentation in the estuarine environment

  • C.T. Roman et al.

    Organic carbon flux through a Delaware Bay salt marsh: tidal exchange, particle size distribution and storms

    Marine Ecology Progress Series

    (1989)
  • J.E. Rooth et al.

    Increased sediment accretion rates following invasion by Phragmites australis: the role of litter

    Estuaries

    (2003)
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    Present address: Center for Geomicrobiology, Department of Biological Sciences, Aarhus University, Ny Munkegade, Building 1540, DK-8000 Aarhus C, Denmark.

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