Integrating the Effects of Sea Level Rise and Salt Water Intrusion on Tidal Freshwater Marsh Stability
Coastal ecosystems have historically existed in constantly but gradually changing environments. Present and predicted future rates of environmental change, driven largely by human alterations to global C cycling, may be greater than rates of change in the recent geological past. Sea level exerts an especially powerful control on tidal wetland ecosystems (Morris et al. 2002), which are found at the interface of the aquatic and terrestrial environments. Sea levels have been rising globally over the past century by about 2 mm yr-1 (Fig. 1), but the rate of sea level rise is expected to increase by the end of this century, perhaps exceeding 5 mm yr-1 (Nakada and Inoue 2005; Church and White 2006; IPCC 2007). Climate change will also alter patterns of precipitation in the watersheds of rivers (Smith et al. 2005), resulting in increasing or decreasing river discharge to coastal systems (Milly et al. 2005). In addition, human population growth, development in coastal regions and land use change in the watersheds of rivers place increasing pressure on tidal marshes. It is unclear how coastal marshes will respond to climate change and other human activities.
Tidal freshwater marshes (TFMs) are found in the tidally influenced freshwater portions of many estuaries. Approximately 20% of total tidal marsh area along the East and Gulf Coasts of the United States is TFM (Odum et al. 1984; Mitsch and Gosselink 1993). TFMs and salt marshes are functionally similar ecosystems, but different plant communities and microbial processes are dominant in each type of system. Both TFMs and salt marshes are highly productive ecosystems that provide a number of important ecological services (Odum et al. 1984). Marshes serve as key habitat for many species of fish, shellfish, birds and other organisms (Mitsch and Gosselink 1993). Marshes are efficient nutrient filters, and can reduce the loading of nutrients from developed watersheds to coastal waters (Merrill and Cornwell 2000; Neubauer et al. 2002; Gribsholt et al. 2005). Additionally, tidal marshes absorb storm surge and wave energy (Barbier et al. 2008), minimizing flooding and damage to adjacent upland areas during hurricane landfall.
Tidal marshes must accrete vertically to keep pace with rising sea levels and avoid inundation (Reed 1995). Accretion in marshes depends on allochthonous watershed-derived sediment sources and autochthonous organic matter production via plant photosynthesis and subsequent accumulation of this organic matter in marsh soils (Reed 1995; Fig. 2). The processes regulating tidal marsh accretion are tightly interconnected and may be influenced by human activities (climate change and land use change) in a number of ways (Fig. 2). As sea levels rise, deposition of watershed-derived sediments and/or accumulation of organic matter must be sufficient to allow for marsh vertical accretion, or the tidal marsh will convert to a sub-tidal habitat. Rising sea levels as well as other aspects of climate change together with changing land use all impact TFM accretion rates (Fig. 2). The overall goal of our proposed research is to understand the processes that regulate accretion in TFMs in order to predict how they will respond to future climate change.
Most TFMs are poised to grow vertically at rates that approximate or exceed current rates of sea level rise. In a recent compilation of TFM accretion rates from several estuaries (Neubauer 2008), only 4% of the TFMs studied had vertical accretion rates that were lower than the current average 1.8 mm y-1 rate of global eustatic sea level rise since the 1960s. More than 20% of TFMs, however, have decadal-scale accretion rates that are less the forecast rate of 3.8 mm y-1 for 2100 [Neubauer 2008; Meehl et al. 2007 (SRES A1B scenario)]. This indicator of vulnerability to sea level rise is likely an underestimate since land subsidence is occurring throughout much of the range of TFMs, both in the United States and Europe (Emery 1980). As sea levels rise, the depth and duration of marsh flooding increase, leading to enhanced delivery of watershed-derived inorganic sediments (Pasternack and Brush 1988; Neubauer et al. 2002; Darke and Megonigal 2003; Morse et al. 2004). This feedback (Fig. 2) between sea level rise and accretion rates may allow TFMs to grow vertically in step with sea level. There are multiple interactions, however, between the biological and physical processes that affect TFM accretion, and various ways that climate change may impact these processes (Fig. 2). There have been no comprehensive studies of how TFMs will respond to multiple climate change stressors.
TFM plant communities respond to rising sea level and increased inundation in various, species-specific ways. Increased flooding may increase, decrease or have no effect on TFM plant production and biomass (Koch and Mendelssohn 1989; Pezeshki et al. 1991; Willis and Hester 2004; Spalding and Hester 2007). Rising sea levels and potentially lower river discharge due to changing precipitation patterns will result in the up-estuary migration of the freshwater-saltwater mixing zone (Hamilton 1990). Therefore, climate change will impact TFMs through both increased flooding duration and salt water intrusion. The introduction of salt water into previously freshwater marshes will likely alter plant and microbial community structure and function (Crain et al. 2004; Rysgaard 1999; Mondrup 1999; Dincer & Kargi 1999; Pattnaik et al. 2000; Mishra et al. 2003; Canavan et al. 2006; Weston et al. 2006), and therefore alter the cycling of C and other elements in these ecosystems. In TFMs, organic matter accumulation is a major mechanism of marsh accretion (Neubauer 2008; Fig. 2), and freshwater marshes contain large reservoirs of organic C in the soil. Salinity-induced stress decreases TFM plant production and biomass (Pezeshki et al. 1987; McKee & Mendelssohn 1989; Spalding and Hester 2007), and therefore both organic matter and vertical accretion rates are compromised following salt water intrusion (Fig. 2). In our previous work in the Delaware River Estuary, we have documented declines in TFM plant biomass above a critical salinity level of about 3 (conductivity of 5 mS cm-1) after cores were transplanted to down-estuary sites (Fig. 3). Corresponding declines in photosynthetic efficiency and primary production (C fixation) were also observed.
In addition to decreasing plant productivity, salt water intrusion drives shifts in microbial organic matter mineralization pathways (Weston et al. 2006). The availability of sulfate (SO42-) is usually low in freshwater marshes, and therefore microbially-mediated methanogenesis is often a major pathway of anaerobic organic matter mineralization (Capone & Kiene 1988). Sulfate reduction replaces methanogenesis as the dominant anaerobic microbial terminal C mineralization process in marine sediments (Jorgensen 1982; Capone & Kiene 1988; Howarth 1993) due to the greater availability of SO42- in seawater (approximately 28 mM) and the higher energy yield of organic C degradation coupled to sulfate reduction as compared to methanogenesis (Capone & Kiene 1988; Mishra et al. 2003). Sulfate reducers out-compete methanogens for organic matter substrates when SO42- is available, and hydrogen sulfide (H2S) produced by sulfate reducers is toxic to methanogens and can further inhibit methanogenesis in marine sediments (Visser et al. 1993). Rates of methane (CH4) production and emission from saline sediments therefore often are lower than those observed in freshwater sediments (Bartlett et al. 1987; Capone & Kiene 1988). The issue is further complicated because microbial reduction of Fe(III) can be more important than either SO42- reduction or methanogenesis in both freshwater and marine marshes (e.g., Kostka et al. 2002, Neubauer et al. 2005). The sulfide produced by sulfate reduction may also be toxic to the TFM plants (Pezeshki et al. 1991; Koch and Mendelssohn 1989), further limiting TFM production following salt water intrusion.
Microbial processes largely regulate the accumulation of organic matter in marsh soils, and organic content and therefore accretion rates are generally greater in TFMs than in salt marshes due to lower decomposition rates in freshwater soils (Craft 2007). We have documented increased rates of microbial sulfate reduction in TFM soils undergoing salt water intrusion (Weston et al. 2006; Weston et al. in prep). Results from our previous work have indicated that salt water intrusion into TFMs can increase organic matter mineralization via sulfate reduction and, surprisingly, methanogenesis. The flux of carbon dioxide (CO2) and CH4 from TFM soils exposed to salt water was significantly greater than soils under freshwater conditions, resulting in approximately 50% more organic C lost from salt-exposed soils over 1 year (Fig. 4). Results from these experiments suggest that, following salt water intrusion into TFMs, plant C fixation declines (Fig. 3) and organic C mineralization by microbial processes increases (Fig. 4). The net result of salt water intrusion will likely be a decrease in the organic C content of the soils and a reduction in marsh accretion potential (Fig. 2). The ability of freshwater marshes to keep pace with rising sea levels during periods of elevated C mineralization and reduced organic matter accretion will determine the fate of freshwater marshes undergoing salt water intrusion (Weston et al. 2006).
Substantial uncertainty remains about how climate change will impact the processes regulating TFM accretion. The factors influencing organic and inorganic sediment accretion rates are tightly interconnected (Fig. 2), and climate change will likely influence TFMs in myriad ways. Given the overall dependence of TFM accretion on both organic and mineral sources, forecasting how these marshes will respond to sea level rise requires an understanding of the effects flooding and changing salinity can have on both allochthonous particle deposition (i.e., sedimentation) and in situ organic matter production and retention. Both processes, as well as attendant feedbacks between flooding, sedimentation, plant productivity, organic matter production and decomposition will ultimately influence marsh accretion rates. In TFMs, both sediment deposition (Pasternack and Brush 1998, Neubauer et al. 2002, Darke and Megonigal 2003, Morse et al. 2004) and longer-term sediment accretion rates (Merrill 1999, Merrill and Cornwell 2000) are typically greater in sites that are flooded more frequently and for a longer duration, or on levees immediately adjacent to tidal creeks. Thus, as tidal flooding increases due to sea level rise, tidal marsh sedimentation may increase if water column particulate material is available (Fig. 2; Friedrichs and Perry 2001). Increased flooding can also affect TFM plant primary production, especially if coupled with salt water intrusion (Spalding and Hester 2007). Although small increases in marsh flooding due to interannual variations in sea level have a positive effect on plant production in salt marshes (Morris et al. 2002), similar studies have not been conducted thus far in TFMs. If rates of soil organic matter decomposition increase due to rising temperatures and salt water intrusion (Fig. 2; Weston et al. 2006), TFMs may become further dependent on allochthonous sediments for vertical growth. Wetlands in sediment-rich watersheds will be more resilient to higher rates of sea level rise (Morris et al. 2002), although patterns of sediment delivery will also likely be altered by climate change (Fig. 2). For instance, sea level rise effectively increases the distance of a marsh from the sediment source as the tidal influence moves up-river (Fig. 2), potentially decreasing sedimentation rates. Reliance on watershed sediment delivery poses a potential problem for TFMs in sediment-poor systems (e.g., northeast United States). Even in the relatively sediment-rich Delaware River, for instance, suspended sediment concentrations have been decreasing significantly over time from the 1950s to the present (-9.3 mg L-1 yr-1; p < 0.001; based on USGS water quality data at Trenton, NJ), likely due to changing land use in the watershed and storm activity (Orson 1990; Khan and Brush 1994). Thus, there may be a variable response of TFMs to climate change along a latitudinal gradient.
The response of TFMs to environmental change will be a complex interaction of the processes that drive plant production, microbial decomposition, sediment deposition and, ultimately, marsh accretion. Our research is examining the balance of processes that impact TFM accretion (Fig. 2), and how these processes change due to climate change, rising sea levels and salt water intrusion into TFMs.