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Responses of Tidal Freshwater and Brackish Marsh Macrophytes to Pulses of Saline Water Simulating Sea Level Rise and Reduced Discharge

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Responses of Tidal Freshwater and Brackish Marsh Macrophytes to Pulses of Saline Water Simulating Sea Level Rise and Reduced Discharge Wetlands (2018) 38:885–891 https://doi.org/10.1007/s13157-018-1037-2 ORIGINAL RESEARCH Responses of Tidal Freshwater and Brackish Marsh Macrophytes to Pulses of Saline Water Simulating Sea Level Rise and Reduced Discharge Fan Li1 & Steven C. Pennings1 Received: 7 June 2017 /Accepted: 16 April 2018 /Published online: 25 April 2018 # Society of Wetland Scientists 2018 Abstract Coastal low-salinity marshes are increasingly experiencing periodic to extended periods of elevated salinities due to the com- bined effects of sea level rise and altered hydrological and climatic conditions. However, we lack the ability to predict detailed vegetation responses, especially for saline pulses that are more realistic in nature than permanent saline presses. In this study, we exposed common freshwater and brackish plants to different durations (1–31 days per month for 3 months) of saline water (salinity of 5). We found that Zizaniopsis miliacea was more tolerant to salinity than the other two freshwater species, Polygonum hydropiperoides and Pontederia cordata. We also found that Zizaniopsis miliacea belowground and total biomass appeared to increase with salinity pulses up to 16 days in length, although this relationship was quite variable. Brackish plants, Spartina cynosuroides, Schoenoplectus americanus and Juncus roemerianus, were unaffected by the experimental treatments. Our ex- periment did not evaluate how competitive interactions would further affect responses to salinity but our results suggest the hypothesis that short pulses of saline water will increase the cover of Zizaniopsis miliacea and decrease the cover of Polygonum hydropiperoides and Pontederia cordata in tidal freshwater marshes, thereby reducing diversity without necessarily affecting total plant biomass. Keywords Salineintrusion .Freshwatermarsh .Brackishmarsh .Zizaniopsismiliacea .Polygonumhydropiperoides .Pontederia cordata Introduction increases and increases groundwater discharge and river dis- charge. Because the species composition and productivity of Global climate change is expected to affect temperature and tidal marshes is affected by salinity (Howard and precipitation patterns, the rate of sea level rise, and the fre- Mendelssohn 1999a), any of these scenarios will likely affect quency and intensity of hurricanes and tropical storms, there- plant productivity and composition (Sharpe and Baldwin by changing the delivery of fresh and saline water to coastal 2012;Neubauer2013). wetland ecosystems (Barendregt and Swarth 2013). Salinity in The response of estuarine biota to variation in salinity on estuaries may increase as saline water moves upstream due to the intensity and timescale of the variation (Webb and sea level rise or storm surges, increase as droughts reduce river Mendelssohn 1996; Baldwin and Mendelssohn 1998; discharge, or decrease if precipitation to the watershed Howard and Mendelssohn 1999a). Short pulses of elevated salinities may not cause permanent changes to the ecosystem, but longer pulses could temporarily alter productivity or com- Electronic supplementary material The online version of this article munity structure depending on the tolerance of each individ- (https://doi.org/10.1007/s13157-018-1037-2) contains supplementary ual species to salinity and interactions among plant species. material, which is available to authorized users. Once the pulse is withdrawn, the ecosystem may recover to baseline conditions after a period of time (Smith 2011). If, * Fan Li [email protected] however, the altered conditions become chronic, there will be a Btipping point^ beyond which species better suited to the new conditions will immigrate into the ecosystem, 1 Department of Biology and Biochemistry, University of Houston, Houston, Texas 77204, USA resulting in a state change as defined by a new community 886 Wetlands (2018) 38:885–891 + composition (Smith et al. 2009). Historically, wetland ecolo- ammonium (NH4 ) can be replaced from the cation exchange gists often manipulated salinity in experiments using a con- sites by the influx of sea salt cations; bio-available phosphate 3− stant low versus a constant elevated salinity treatment to test (PO4 ) can increase due to desorption by chloride; increases 2− the effects of saline intrusion (Pezeshki et al. 1987;McKee in sulfate (SO4 ) concentrations can enhance rates of organic and Mendelssohn 1989; De Leeuw et al. 1990;Duntonetal. matter mineralization through sulfate reduction (Weston et al. 2001;Więski et al. 2010; Hughes et al. 2012; Sharpe and 2006; Steinmuller and Chambers 2018). Shifts between fresh- Baldwin 2012; Woo and Takekawa 2012). However, in na- water and saline conditions could thereby liberate repeated ture, coastal wetlands typically experience fluctuating salinity fluxes of nutrients from soils (Weston et al. 2011). conditions rather than constant press changes. Lower precip- Therefore, tidal marsh plants might even benefit from pulses itation can lead to temporary reductions of freshwater input of saline water due to an increased nutrient availability into the estuaries, and therefore cause species composition (Weston et al. 2011; Ardón et al. 2013). shift to salt-tolerant plants (Visser et al. 2002;Whiteand The objective of this study was to determine the effect of Alber 2009). Similarly, human activities such as surface and duration of saline water exposure on the growth of both fresh- groundwater withdrawals also alter river discharge and salin- water and brackish marsh macrophytes. We used a mesocosm ity regimes in estuaries (Sklar and Browder 1998). Therefore, study because it allowed us to apply more treatment combi- determining how coastal wetlands respond to different saline nations than we could easily impose in the field, as well as to intrusion scenarios is fundamental to understanding and man- control other biotic and abiotic variables that might confound aging these systems. treatment effects. We tested the hypotheses that 1) increasing The vegetation composition of tidal marshes (freshwater, duration of exposure to saline water negatively affects tidal brackish and salt marshes) along an estuary is determined by freshwater and brackish marsh plants, but 2) the nature of the the responses of each individual species to salinity (varying response varies among species, both among and within salin- along the estuary from freshwater to euryhaline), interacting ity zones. with competition (Guo and Pennings 2012). Each type of marsh provides unique and important ecosystem functions (Więski et al. 2010). Situated in the upper estuary where river Materials and Methods freshwater discharge and the tidal wave from the sea combine, tidal freshwater marshes support high macrophyte diversity, We collected plants from the tidal marshes of the Altamaha productivity and nutrient retention (Costanza et al. 1998; River estuary, Georgia, USA (31.4° N, 81.4° W). Tidal fresh- Więski et al. 2010). Ongoing sea level rise alters hydrologic water marshes in this area are dominated by Polygonum over time resulting in increased saline water incursion into hydropiperoides, Pontederia cordata,andZizaniopsis previously freshwater marshes. Increased salinity can de- miliacea; and brackish marshes by Spartina cynosuroides, crease plant growth through osmotic effects and the accumu- Schoenoplectus americanus,andJuncus roemerianus lation of toxic ions in the soil and plant tissue (Adam 1990), (Więski et al. 2010). Water column salinities near the collec- and drives shifts in the composition of plant communities tion sites were recorded by the Georgia Coastal Ecosystems because different species differ in salinity tolerance (McKee Long-Term Ecological Research (GCE LTER) program using and Mendelssohn 1989; Knighton et al. 1991; Sharpe and moored hydrographic sondes (Sea-Bird Electronics model 37- Baldwin 2012; Sutter et al. 2015). Eventually, increases in SM MicroCATs) deployed in 2001 at the GCE7 site (approx- salinity will convert tidal freshwater marshes to brackish or imately 800 m upstream from the site where freshwater marsh salt marshes (Craft et al. 2009; Jarrell et al. 2016). Although plants were collected) and the GCE8 site (approximately these long term changes in salinity for multiple years can be 750 m downstream from the site where brackish marsh plants thought of as Bpresses^ (sensu Collins et al. 2011), on a shorter were collected). Porewater salinities measured at GCE sites 7 time scale of days to months there is constant variation in and 8 in October of 13 years averaged 0.72 ± .0.10 and 12.22 salinity (De Leeuw et al. 1990;Duntonetal.2001;Hughes ± 20.39. The hydrographic data sets (GCE-LTER et al. 2012). As one example, variation in the frequency and Hydrographic Monitoring in the Altamaha River) are avail- intensity of summer drought affects freshwater flow from the able on the GCE-LTER data portal (see Appendix, Table 1). river and causes pulses of saline water to penetrate further We collected thirty-five individual culms of each of the six upstream (Więski et al. 2010). In contrast to the press of species listed above (hereafter we referred to these specie by long-term saline intrusion, freshwater plants may be resilient their genus) on March 9–11, 2013. We collected culms spaced to saline pulses and able to recover once the pulse is with- at least 5 m apart to minimize the chances that they were drawn (Flynn et al. 1995; Howard and Mendelssohn 1999b; genetically identical (Richards et al. 2004). Within 24 h we Goodman et al. 2010). potted them individually in 12 L pots in sediments
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