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Wetlands (2018) 38:885–891 https://doi.org/10.1007/s13157-018-1037-2

ORIGINAL RESEARCH

Responses of Tidal Freshwater and Brackish Macrophytes to Pulses of Saline Simulating Level Rise and Reduced

Fan Li1 & Steven C. Pennings1

Received: 7 June 2017 /Accepted: 16 April 2018 /Published online: 25 April 2018 # Society of Scientists 2018

Abstract Coastal low- are increasingly experiencing periodic to extended periods of elevated due to the com- bined effects of 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 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 appeared to increase with salinity pulses up to 16 days in length, although this relationship was quite variable. Brackish plants, , Schoenoplectus americanus and 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 biomass.

Keywords Salineintrusion .Freshwatermarsh .Brackishmarsh .Zizaniopsismiliacea .Polygonumhydropiperoides .Pontederia cordata

Introduction increases and increases discharge and dis- charge. Because the species composition and 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 (Barendregt and Swarth 2013). Salinity in The response of estuarine biota to variation in salinity on 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 , 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, 77204, USA resulting in a state change as defined by a new community 886 (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 (Weston et al. 2011). conditions rather than constant press changes. Lower precip- Therefore, 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 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 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 ), 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 and plant tissue (Adam 1990), (Więski et al. 2010). 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 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, 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 collected Moreover, pulses of saline intrusion can also affect plants from the same site as the plants. Plants were acclimated for by altering the availability of nutrients. For example, 2monthsinfreshwatertominimizetransplantshock,andthen Wetlands (2018) 38:885–891 887 were exposed to seven salinity treatments during the three- Results month experiment, which ran from May 26 to August 28, 2013. Most of the Spartina individuals, however, died during The GCE7 site in the Altamaha River is located approximately the acclimation, for unknown reasons. We collected a second 20 km upstream from the . The water column salinities set of Spartina plants from the same location on June 13, at the GCE7 site were very low except for periods of drought. potted them within 24 h, and acclimated them in freshwater Between August 2001 and May 2014, 26% of the days expe- until June 27, 2013. The Spartina plants were consequently rienced a maximum salinity higher than salinity of 0.5, and exposed to salinity treatments for only two months, rather than only 3% of the days had peak salinities over 5 (Fig. 1). The three; however, they grew rapidly during the experiment, and GCE8 site is located approximately 15 km upstream from the were on a trajectory to outgrow the pots if the experiment had ocean, and water column salinities at this site are mostly continued. As a result, we considered the two-month period as oligohaline to mesohaline (5–18). Between October 2001 sufficient for effects of treatments to be manifest. and May 2014, 55% of the days experienced a maximum We placed individual potted plants inside dish pans that salinity higher than 5, and 12% of the days had salinities over had drain holes at mid-pot height (7.5 cm) to ensure that soil 18 (Fig. 2). Higher salinities at both sites happened more com- wetness was similar across all replicates, and to ensure that monly late in the year when river discharge was lower pore water in the lower half of the pots was maintained at a (Blanton et al. 2001). known value. We placed all plants outside near the University All plants in control treatments grew over the course of the of Georgia Marine Institute, about 70 m from a natural salt experiment. All of the freshwater marsh plant species exhib- marsh that is ~20 km downstream from the collection point, ited a reduced tissue biomass (both aboveground and below- and exposed them to ambient temperature, humidity and light ground) with permanent exposure to elevated salinities regimes. For salinity treatments, we watered plants once a day (31 days per month) relative to the control, but the response with a salinity of 5 (created using Instant Ocean Sea Salt, to the saline pulses varied with species (Fig. 3). Polygonum System Inc., Mentor, OH, USA) for 1, 2, 4, 8, 16, aboveground biomass did not respond to short saline pulses or 31 days per month, and then watered them on the remaining (up to 4 days in duration) and declined sharply thereafter days of the month with freshwater, except for the 31-days (Fig. 3a). In contrast, Polygonum belowground biomass de- treatment which represented a permanent saline press. We creased sharply in response to short salinity pulses, but largely used salinity values of 0 and 5 because both are frequently stabilized with pulses of 8 or more days (Fig. 3d). Because experienced by tidal fresh and brackish marsh plants (see above- and belowground biomass showed different trends, Results) and 5 represents the transition from oligohaline to total biomass, which summed the two, exhibited a fairly mesohaline salinity regimes. On days when the salinity treat- steady decline with increasing exposure to salinity (Fig. 3g). ment was not changed, plants were watered by pouring the Pontederia above-, belowground and total biomass declined water in the dishpan into the top of the pot. All plants were steadily with increased exposure to saline water (Fig. 3b, e watered daily. We adjusted the water in the dish pans to the and h). Zizaniopsis aboveground biomass did not respond to target salinity before each watering to ensure that porewater salinity was maintained as close as possible to the target values. On days when the salinity treatment was changed, water in the dishpan was discarded and replaced with new water of the correct salinity. For the control treatment, we watered plants with freshwater daily. We harvested plant biomass on August 28, 2013. We sep- arated individual plants into aboveground and belowground live biomass, washed them to remove soil, and dried them at 60 °C to constant mass. The plant data set (Pot experiment on freshwater and brackish marsh plants responses to salinity pulses in summer 2013) is available on the GCE-LTER data portal (see Appendix, Table 1). To test the responses of plant production to duration of saline water, we conducted piecewise regression on all spe- cies, with plant biomass (aboveground, belowground and to- tal) as the dependent variables and days of saline water addi- tion as the independent variable. The piecewise regression Fig. 1 Daily maximum water column salinity from August 10, 2001 to was performed with SigmaPlot 11.0 software (Systat May 17, 2014 at GCE 7. The dashed lines indicate transitions from fresh Software Inc.). to oligohaline (salinity 0.5–5) to mesohaline (salinity 5–18) salinity levels 888 Wetlands (2018) 38:885–891

treatments. A more complex analysis using multiple regres- sion, with initial size included as a predictor variable, indicat- ed that some of the variation in final brackish plant biomass was caused by differences in initial sizes of the plantings, but did not otherwise change the general conclusions of the anal- ysis (Appendix, Table 2).

Discussion

Understanding the responses of tidal marsh plants to different saline water intrusion scenarios is critical to predicting the fate of tidal marshes. Previous studies have primarily focused on plant responses to constant elevated salinities (Pezeshki et al. 1987; McKee and Mendelssohn 1989; Sharpe and Baldwin Fig. 2 Daily maximum water column salinity from October 26, 2001 – 2012; Woo and Takekawa 2012), but salinization can occur on May 17, 2014 at GCE 8. The dashed lines indicate transitions from many time scales (Visser et al. 2002;WhiteandAlber2009; – – euryhaline to mesohaline (salinity 5 18) to polyhaline (salinity 18 30) Goodman et al. 2010). We examined the effects of salinity salinity levels duration on different tidal freshwater and brackish plant spe- cies. Our results suggest that not only was Zizaniopsis more salinity treatments (Fig. 3c). Both belowground and total bio- tolerant of salinity pulses than Polygonum and Pontederia,its mass, however, increased with salinity pulses up to 16 days in belowground and total biomass may actually have increased length, and then declined with longer salinity pulses (Fig. 3f, i). with salinity pulses up to 16 days in length. We also found that In contrast to results from the freshwater plants, the above- long exposure to moderately elevated salinities (salinity of 5) ground, belowground and total biomass of the three brackish had negative effects on the biomass of all three freshwater marsh species did not respond to changes in salinity exposure species that we tested, but no effects on the three brackish (Fig. 4). The data showed considerable variation in all marsh species that we tested.

2 2 Fig. 3 Piecewise regression of aboveground, belowground and total biomass: R =0.55,F3,31 = 12.41, P < 0.01; belowground biomass: R = 2 biomass of Polygonum, Pontederia and Zizaniopsis as a function of 0.24, F3,31 =3.26, P = 0.03; total biomass: R =0.37,F3,31 =6.11,P< 2 salinity treatments. Solid lines represent significant relationships 0.01. Zizaniopsis aboveground biomass: R = 0.09, F3,31 = 098, P = 2 2 (P <0.05). Polygonum aboveground biomass: R =0.52,F3,31 = 11.27, 0.41; belowground biomass: R = 0.22, F3,31 = 3.01, P =0.041;total 2 2 P < 0.01; belowground biomass: R =0.49,F3,31 = 12.41, P < 0.01; total biomass: R =0.23,F3,31 = 3.12, P = 0.04 2 biomass: R = 0.58, F3,31 = 14.11, P < 0.01. Pontederia aboveground Wetlands (2018) 38:885–891 889

Fig. 4 No significant relationships (all P values >0.05) were found between aboveground, belowground and total biomass of Spartina, Schoenoplectus and Juncus and days of saline water addition

In our study, Zizaniopsis belowground biomass showed a duration of salinity pulses, indicating that they were unable to slight increase with salinity pulses up to 16 days. The rela- benefit in the same way as Zizaniopsis from short saline tionship was quite variable, with relatively low R2 values pulses. The relationship between salinity and biomass of (0.22) and was strongly influenced by one data point. After Polygonum was quite strong, with relatively high R2 values removing the most influential point, the hump-shaped pattern (0.49 to 0.58). The response to salinity of Pontederia was was still visually apparent but was no longer statistically sig- somewhat more variable than for Polygonum,withlowerR2 nificant. Thus, we must view the finding that moderate salin- values (0.24 to 0.55). Based on these results, we predict that, ity exposure benefits Zizaniopsis with some caution. in the field, these two species will be lost during salinization However, past work has shown that alternating between fresh well before Zizaniopsis disappears. and saline water can release nutrients from wetland soils These results come with one important caveat. Our exper- (Weston et al. 2011), and such salinity-induced nutrients iments focused on vegetative responses of adult plants, be- may benefit salt-tolerant species like Zizaniopsis (Ardón cause all the common species at this study site are perennials. et al. 2013). Adding a short duration pulse of higher salinity Many plants in tidal freshwater marshes, however, are an- water into freshwater marsh sediments can cause an increase nuals. For these species, it is likely very important to also + 3− of NH4 and PO4 efflux from sediments (Rysgaard et al. consider how the timing and extent of salinity pulses affects 1999; Sundareshwar and Morris 1999). Saline water carries seed germination among species (Flynn et al. 1995;Baldwin + many ions that compete for exchange sites with NH4 and et al. 1996). 3− PO4 (Seitzinger et al. 1991;Westonetal.2006). The salinity treatments had no effect on the three brackish 2− Simultaneously, the introduction of SO4 by saline water species. This may be because the sediments collected in the will shift the decomposition pathway from methanogenesis brackish marsh had adapted to the salinity fluctuations at sa- to sulfate reduction, which is more efficient in organic matter linity of 5 (Marks et al. 2016). Although we did not measure mineralization (Weston et al. 2006). There is also evidence to nutrients in the pots, van Dijk et al. (2015) found that subject- suggest that elevated salinity can inhibit rates of nitrification, ing brackish sediments to saline pulses did not significantly + leading to the increased release of NH4 (Ardón et al. 2013). alter nutrient concentrations. Therefore, we speculate that the Thus, there is a likely mechanism to explain the increase in treatments did not cause significant changes in nutrient con- Zizaniopsis biomass at intermediate salinity exposure, and we tent in the brackish sediments. As a result, plants did not view the result as reasonable. However, if the plants are too benefit from saline pulses because no additional nutrients stressed by salt, uptake of the nutrients will be disrupted. This were released and did not suffer because salinities were low is probably why longer saline pulses ceased to be beneficial compared to the range of conditions that they normally expe- to Zizaniopsis. rience (Fig. 2). We found that belowground and total biomass of The response of an entire plant community to saline Polygonum and Pontederia steadily declined with increasing pulses will depend not only on the characteristics of the 890 Wetlands (2018) 38:885–891 saline pulses and the tolerances of the individual plant spe- Acknowledgements This material is based upon work supported by the cies, but also on intraspecific and interspecific interactions National Science Foundation through the Georgia Coastal Ecosystems Long-Term Ecological Research program under Grant No. OCE- among plant species, and the composition of the seed 1237140. We thank Huy Vu, Wei-Ting Lin, Jacob Shalack, Caroline (Baldwin et al. 1996; Howard and Mendelssohn 1999b). Reddy, Tim Montgomery, Zachary Chejanovski, Carolyn Kilgore and Competition, for example, plays an important role in struc- George Wheeler for help with this project. We thank Wade Sheldon for turing variation in wetland plant communities along the assistance with the salinity data. This is contribution number 1068 of the University of Georgia Marine Institute. salinity gradient of an estuary (Crain et al. 2004;Engels and Jensen 2010; Guo and Pennings 2012). Therefore, we cannot confidently extrapolate our results directly to the field without additional work, as reduced competition due References to elevated salinity may benefit a given salt-intolerant spe- ciesatthesametimeassalinity has a direct negative effect, Alber M (2002) A conceptual model of estuarine freshwater inflow man- making the net outcome hard to predict. Future work, then, agement. Estuaries 25:1246–1261 needs to consider the responses of plants to salinity both Adam P (1990) Saltmarsh . 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