Water salinity and inundation control soil carbon decomposition during salt marsh restoration: An incubation experiment

Coastal wetlands are a significant carbon (C) sink since they store carbon in anoxic soils. This ecosystem service is impacted by hydrologic alteration and management of these coastal habitats. Efforts to restore tidal flow to former salt marshes have increased in recent decades and are generally associated with alteration of water inundation levels and salinity. This study examined the effect of water level and salinity changes on soil organic matter decomposition during a 60‐day incubation period. Intact soil cores from impounded fresh water marsh and salt marsh were incubated after addition of either sea water or fresh water under flooded and drained water levels. Elevating fresh water marsh salinity to 6 to 9 ppt enhanced CO₂ emission by 50%?80% and most typically decreased CH₄ emissions, whereas, decreasing the salinity from 26 ppt to 19 ppt in salt marsh soils had no effect on CO₂ or CH₄ fluxes. The effect from altering water levels was more pronounced with drained soil cores emitting ~10‐fold more CO₂ than the flooded treatment in both marsh sediments. Draining soil cores also increased dissolved organic carbon (DOC) concentrations. Stable carbon isotope analysis of CO₂ generated during the incubations of fresh water marsh cores in drained soils demonstrates that relict peat OC that accumulated when the marsh was saline was preferentially oxidized when sea water was introduced. This study suggests that restoration of tidal flow that raises the water level from drained conditions would decrease aerobic decomposition and enhance C sequestration. It is also possible that the restoration would increase soil C decomposition of deeper deposits by anaerobic oxidation, however this impact would be minimal compared to lower emissions expected due to the return of flooding conditions.     

Effects of flooding and salinity on photosynthesis and water relations of four Southeastern Coastal Plain forest species

The influence of flooding and salinity on photosynthesis and water relations was examined for four common coastal tree species [green ash (Fraxinus pennsylvanica Marshall), water tupelo (Nyssa aquatica L.). Chinese tallow (Sapium sebiferum (L.) Roxb.), and baldcypress (Taxodium distichum (L.) Richard)]. Both chronic (as might be associated with sea level rise) and acute (similar to hurricane storm surges) exposures to these stresses were examined. Chronic freshwater flooding of green ash, water tupelo, and Chinese tallow seedlings reduced photosynthesis (A) relative to that of watered seedlings, while baldcypress was unaffected. Chinese tallow A declined with increasing length of flooding. A salinity increase of the floodwater to 2 ppt decreased A of baldcypress and water tupelo, but not A of green ash and Chinese tallow, which was already severely reduced by freshwater flooding. All seedlings of the four species died within 2 to 6 weeks when flooded with 10 ppt saltwater. Photosynthesis of all four species did not differ between 0 and 2 ppt watering. Watering with 10 ppt salinity initially reduced A of all four species, but the seedlings recovered over time. Photosynthesis was severely decreased for all species when flooded with 21 ppt salinity for 48 hours. Reduced A continued following the treatment. Photosynthesis of only green ash and water tupelo was reduced by watering with 21 ppt salinity for 48 hours. Flooding of low-lying areas with increased salinity would lead to shifts in species composition of coastal forests due to these differential tolerances.     

Combined effects of simulated tidal sea-level rise and salinity on seedlings of a mangrove species, Kandelia candel (L.) Druce

To investigate the effects of the simultaneous occurrence of salt stress and tidal sea-level rise on mangroves, potted Kandelia candel seedlings were treated under deep flooding (flooded 40 cm above the soil surface for 16 h per day, inundating the entire plant) and shallow flooding (flooded just above the soil surface for 8 h per day) at salinity levels of 5, 15, and 25 ppt over 14 months. Deep flooding enhanced stem elongations at all salinity levels but increased stem biomass only at 5 ppt. Deep flooding increased both leaf production and leaf fall; leaf biomass increased at 5 ppt, but decreased at 15 and 25 ppt. Biomass ratios of root/shoot (R/S) of deep flooding treatments were significantly lower than those of shallow flooding treatments. Under deep flooding, superoxide dismutase (SOD) activities did not show significant change between 5 and 15 ppt, but increased at 25 ppt. With increasing salinity level, peroxidase (POD) activities increased, and the difference between shallow and deep flooding was enhanced. Malonaldehyde (MDA) content significantly decreased at 25 ppt with 40 cm flooding, but was not affected by other treatments. These results demonstrated that the growth and physiological responses of K. candel seedlings under deep flooding conditions varied with salinity level; growth was enhanced at low salinity level but inhibited at high salinity level. It is therefore probable that K. candel will shift from downstream to upstream, where the influence of fresher river water resources will ameliorate the effects of increased salinities that accompany deeper tidal flooding in these mangrove ecosystems.     

Flooding and salinity effects on growth and survival of four common forested wetland species

The survival, growth, and biomass of baldcypress (Taxodium distichum (L.) Rich.), water tupelo (Nyssa aquatica L.), Chinese tallow (Sapium sebiferum (L.) Roxb.), and green ash (Fraxinus pennsylvanica Marsh.) seedlings were examined in an experiment varying water levels (watered, flooded) and salinity levels (0, 2, and 10 ppt, plus a simulated storm surge with 32 ppt saltwater). All seedlings, except for those flooded with 10 ppt saltwater, survived to the end of the experiment. In 10 ppt saltwater, flooded baldcypress, water tupelo, and green ash survived two weeks whereas Chinese tallow survived for 6 weeks. However, a second set of slightly older baldcypress, water tupelo, and Chinese tallow seedlings survived eight weeks of flooding with 10 ppt saltwater. When carried through the winter to the beginning of the second growing season, flooded baldcypress and Chinese tallow seedlings from the 0 and 2 ppt treatments leafed out, but only Chinese tallow recovered from the saltwater surge treatment. The diameter and growth (height) of each species was not affected when watered with 2 ppt saltwater, except for the effects of the height growth of baldcypress. Growth was reduced for all species when watered with 10 ppt saltwater. The diameter growth of green ash was reduced by freshwater flooding. The diameter growth of baldcypress and water tupelo was greater when flooded with fresh water. Flooding with 2 ppt saltwater caused a significant reduction in diameter growth in water tupelo, green ash, and Chinese tallow, but not in baldcypress. Root and stem biomass values were not significantly different for any species between the 0 and 2 ppt salinity watering treatments. However, seedlings watered with 10 ppt saltwater had significantly lower root and stem biomass values, except for baldcypress roots and green ash stems. Baldcypress was least affected by flooding with 0 and 2 ppt saltwater, although there were slight reductions in root biomass with increasing salinity. Because of the susceptibility of the seedlings of these four species to increases in flooding and salinity, their regeneration may be limited in the future, thereby causing shifts in species composition.     

Photosynthesis and water relations of four oak species: impact of flooding and salinity

 As global climate changes, sea level rise and increased frequency of hurricanes will expose coastal forests to increased flooding and salinity. Quercus species are frequently dominant in these forest, yet little is known about their salinity tolerance, especially in combination with flooding. In this study, 1-year-old seedlings of Quercus lyrata Walt. (overcup oak), Q. michauxii Nutt. (swamp chestnut oak), Q. nigra L. (water oak), and Q. nuttallii Palmer (Nuttall oak) were chronically (simulating sea level rise) and acutely (simulating hurricane storm surge) exposed to increased flooding and salinity, individually and in combination. The four species demonstrated two response patterns of photosynthesis (A), conductance, and leaf water potential, apparently related to their relative flood tolerance. In Q. lyrata, Q. nuttallii, and Q. nigra (moderately flood-tolerant), A was not immediately reduced after the initiation of the freshwater flooding, but was reduced as the duration of flooding increased. In the second pattern, demonstrated by the weakly flood-tolerant Q. michauxii, A was immediately reduced by freshwater flooding with an increasing impact over time. Watering with 2 parts per thousand (ppt) saline water did not consistently reduce A, but flooding with 2 ppt reduced A of all species, similar to the response with freshwater flooding. Photosynthesis of all species was reduced by 6 ppt watering or flooding, with the latter treatment killing all species within 8 weeks. When acutely exposed to 30 ppt salinity, A was quickly and severely reduced regardless of whether the seedlings were watered or flooded. Acutely flooded seedlings exposed to high salinity died within 2 weeks, but seedlings watered with 30 ppt saline water recovered and A was not reduced the following spring. As saline flooding of coastal areas increases due to sea level rise, photosynthesis of these species will be differentially affected based primarily on their flood tolerance. This suggests that increased flooding associated with sea level rise will impact these tree species to a greater extent than small increases in soil salinity. High salinity accompanying storm surges will be very harmful to all of these species. Received: 20 October 1997 / Accepted: 2 December 1998     

A casualty of climate change? Loss of freshwater forest islands on Florida's Gulf Coast

Sea level rise elicits short‐ and long‐term changes in coastal plant communities by altering the physical conditions that affect ecosystem processes and species distributions. While the effects of sea level rise on salt marshes and mangroves are well studied, we focus on its effects on coastal islands of freshwater forest in Florida's Big Bend region, extending a dataset initiated in 1992. In 2014–2015, we evaluated tree survival, regeneration, and understory composition in 13 previously established plots located along a tidal creek; 10 plots are on forest islands surrounded by salt marsh, and three are in continuous forest. Earlier studies found that salt stress from increased tidal flooding prevented tree regeneration in frequently flooded forest islands. Between 1992 and 2014, tidal flooding of forest islands increased by 22%–117%, corresponding with declines in tree species richness, regeneration, and survival of the dominant tree species, Sabal palmetto (cabbage palm) and Juniperus virginiana (southern red cedar). Rates of S. palmetto and J. virginiana mortality increased nonlinearly over time on the six most frequently flooded islands, while salt marsh herbs and shrubs replaced forest understory vegetation along a tidal flooding gradient. Frequencies of tidal flooding, rates of tree mortality, and understory composition in continuous forest stands remained relatively stable, but tree regeneration substantially declined. Long‐term trends identified in this study demonstrate the effect of sea level rise on spatial and temporal community reassembly trajectories that are dynamically re‐shaping the unique coastal landscape of the Big Bend.     

Tolerance of Pinus taeda and Pinus serotina to low salinity and flooding: Implications for equilibrium vegetation dynamics

Questions: 1. Do pine seedlings in estuarine environments display discrete or continuous ranges of physiological tolerance to flooding and salinity? 2. What is the tolerance of Pinus taeda and P. serotina to low salinity and varying hydrologic conditions? 3. Are the assumptions for ecological equilibrium met for modeling plant community migration in response to sea‐level rise? Location: Albemarle Peninsula, North Carolina, USA. Methods: In situ observations were made to quantify natural pine regeneration and grass cover along a salinity stress gradient (from marsh, dying or dead forest, to healthy forest). A full‐factorial greenhouse experiment was set up to investigate mortality and carbon allocation of Pinus taeda and P. serotina to low‐salinity conditions and two hydrology treatments over 6 months. Treatments consisted of freshwater and two salinity levels (4 ppt and 8 ppt) under either permanently flooded or periodically flushed hydrologic conditions. Results: Natural pine regeneration was common (5–12 seedlings per m²) in moderate to well‐drained soils where salinity concentrations were below ca. 3.5 ppt. Pine regeneration was generally absent in flooded soils, and cumulative mortality was 100% for 4 and 8 ppt salinity levels under flooded conditions in the greenhouse study. Under weekly flushing conditions, mortality was not significantly different between 0 and 4 ppt, confirming field observations. Biomass accumulation was higher for P. taeda, but for both pine species, the root to shoot ratio was suppressed under the 8 ppt drained treatment, reflecting increased below‐ground stress. Conclusions: While Pinus taeda and P. serotina are commonly found in estuarine ecosystems, these species display a range of physiological tolerance to low‐salinity conditions. Our results suggest that the rate of forest migration may lag relative to gradual sea‐level rise and concomitant alterations in hydrology and salinity. Current bioclimate or landscape simulation models assume discrete thresholds in the range of plant tolerance to stress, especially in coastal environments, and consequently, they may overestimate the rate, extent, and timing of plant community response to sea‐level rise.     

Physiological,morphological, and growth responses ofPlatanus occidentalis seedlings to flooding

Summary Flooding ofPlatanus occidentalis L. seedlings for up to 40 days induced several changes including early stomatal closure, greatly accelerated ethylene production by stems, formation of hypertrophied lenticels and adventitious roots on submerged portions of stems, and marked growth inhibition. Poor adaptation ofPlatanus occidentalis seedlings to soil inundation was shown in stomatal closure during the entire flooding period, inhibition of root elongation and branching, and death of roots. Some adaptation to flooding was indicated by (1) production of hypertrophied lenticels which may assist in exchange of dissolved gases in flood water and in release of toxic compounds, and (2) production of adventitious roots on stems which may increase absorption of water. These adaptations appeared to be associated with greatly stimulated ethylene production in stems of flooded plants. The greater reduction of root growth over shoot growth in flooded seedlings will result in decreased drought tolerance after the flood waters recede. The generally low tolerance to flooding of seedlings of species that are widely rated as highly flood tolerant is emphasized.     

The influence of vegetation,salinity, and inundation on seed banks of oligohaline coastal marshes

Sea level rise may alter salinity and inundation regimes and create patches of open water in oligohaline coastal marshes, potentially affecting the composition and germination of seed bank species. We conducted seedling emergence experiments to: (1) examine the effects of standing vegetation on the seed banks of three oligohaline marsh communities in coastal Louisiana (dominated by Paspalum vaginatum Sw., Sagittaria lancifolia L., or Spartina patens (Ait.) Muhl., respectively); and (2) investigate the effects of salinity and inundation regime on germination of seed bank species. We also studied the effect of a temporary increase in salinity (to simulate a salt water intrusion event) on the viability of buried seeds. We found that the presence or absence of vegetation within a community affected the abundance of some species in the seed bank but had little effect on species composition. Also, the seed banks of the three communities exhibited considerable overlap in species composition and had similar species richness (10–11) and diversity (antilog Shannon-Weaver diversity index = 6.5–7.1), despite differences in vegetation type. Higher salinities and flooding reduced seedling emergence for most species; few species emerged at salinities above four parts per thousand (ppt), and only Sagittaria lancifolia and Eleocharis parvula germinated well under flooded conditions. A temporary increase in salinity did not affect species richness or seedling emergence of most species. Our results suggest that differences in vegetation may have little effect on the composition of seed banks of oligohaline marshes. However, higher salinities and greater depth and duration of inundation (anticipated as global sea level continues to rise) may decrease recruitment of seed bank species, reducing their abundance in oligohaline marsh communities.     

Biogeochemical effects of simulated sea level rise on carbon loss in an Everglades mangrove peat soil

Saltwater intrusion and inundation can affect soil microbial activity, which regulates the carbon (C) balance in mangroves and helps to determine if these coastal forests can keep pace with sea level rise (SLR). This study evaluated the effects of increased salinity (+15 ppt), increased inundation (?8 cm), and their combination, on soil organic C loss from a mangrove peat soil (Everglades, Florida, USA) under simulated tides. Soil respiration (CO₂ flux), methane (CH₄) flux, dissolved organic carbon (DOC) production, and porewater nutrient concentrations were quantified. Soil respiration was the major pathway of soil organic C loss (94–98%) and was approximately 90% higher in the control water level than the inundated treatment under elevated salinity. Respiration rate increased with water temperature, but depended upon salinity and tidal range. CH₄ flux was minimal, while porewater DOC increased with a concomitant, significant decline in soil bulk density under increased inundation. Porewater ammonium increased (73%) with inundation and soluble reactive phosphorus increased (32%) with salinity. Overall, the decline in soil organic C mineralization from combined saltwater intrusion and prolonged inundation was not significant, but results suggest SLR could increase this soil’s susceptibility to peat collapse and accelerate nutrient and DOC export to adjacent Florida Bay.     

Sea-level rise and drought interactions accelerate forest decline on the Gulf Coast of Florida, USA

Sea-level rise threatens low-lying coastal ecosystems globally. In Florida, USA, salinity stress due to increased tidal flooding contributes to the dramatic and well documented decline of species-rich coastal forest areas along the Gulf of Mexico. Here, we present the results of a study of coastal forest stand dynamics in thirteen 400 m² plots representing an elevation gradient of 0.58–1.1 m affected by tidal flooding and rising sea levels. We extended previously published data from 1992–2000 to 2005 to quantify the full magnitude of the 1998–2002 La Niña-associated drought. Populations of the dominant tree species, Sabal palmetto (cabbage palm), declined more rapidly during 2000–2005 than predicted from linear regressions based on the 1992–2000 data. Dramatic increases in Juniperus virginiana (Southern red cedar) and S. palmetto mortality during 2000–2005 as compared with 1995–2000 are apparently due to the combined effects of a major drought and ongoing sea-level rise. Additionally, coastal forest stands continued to decline in species richness with increased tidal flooding frequency and decreasing elevation. Stable isotope (H, O) analyses demonstrate that J. virginiana accesses fresher water sources more than S. palmetto . Carbon isotopes reveal increasing δ ¹³C enrichment of S. palmetto and J. virginiana with increased tidal flooding and decreased elevation, demonstrating increasing water stress in both species. Coastal forests with frequent tidal flooding are unable to support species-rich forests or support regeneration of the most salt-tolerant tree species over time. Given that rates of sea-level rise are predicted to increase and periodic droughts are expected to intensify in the future due to global climate change, coastal forest communities are in jeopardy if their inland retreat is restricted.     

Effects of salinity and inundation regime on growth and distribution of Schoenoplectus triqueter

In Ireland, Schoenoplectus triqueter is confined to areas in the upper part of the Shannon estuary where average summer soil pore water salinity levels do not exceed 7.0 ppt. Soil-based and nutrient solution-based experiments showed that growth and reproduction of S. triqueter was significantly reduced at salinity of 10 ppt and significantly enhanced at 2.0 ppt compared to a freshwater control. Young plants were less tolerant of salinity than older plants. A transplantation trial showed that S. triqueter could grow at higher salinities in the field but that growth and reproduction were significantly inhibited at higher field salinities. The effect of simulated diurnal tidal inundation on the growth and reproduction of S. triqueter was examined by growing plants in a tank with fluctuating water levels. S. triqueter was able to grow and produce seed when inundated for up to 12 h per 24-h period, indicating a considerable capacity to withstand periodic inundation. Growth responses to simulated tidal inundation were also examined in Bolboschoenus maritimus. Long periods of daily inundation reduced growth of B. maritimus proportionately more than that of S. triqueter. It is concluded that S. triqueter occupies a narrow ecological niche in the Shannon Estuary that is circumscribed by competition from more robust emergent species but is facilitated by the ability of S. triqueter to tolerate lengthy periods of inundation by the tides.     

Response to long- and short-term salinity in populations of the C4 nonhalophyte Andropogon glomeratus Walter B.S.P.

Summary This research was undertaken to investigate differences in salt tolerance under conditions in which salinity is increased gradually and maintained for long periods or increased rapidly and maintained for shorter periods. The responses of populations of a C₄ nonhalophytic grass, Andropogon glomeratus, to long- and short-term salinity were measured under controlled environment conditions. Additionally, plants from a salt marsh population and an inland population were transplanted into a salt marsh and their survival compared. The relative growth reductions in the salt marsh and the inland populations under long-term salinity were similar. Survival of seedlings of 4 populations inundated with full-strength seawater over a relatively short period indicated differential capacities to tolerate soil salinities imposed in a manner similar to tidal inundation in a salt marsh. The greater survival of plants from the marsh population transplanted into the salt marsh further indicated genetic differentiation between the populations. These results indicate that genetic differentiation to salt tolerance in A. glomeratus is better reflected by survival after shortterm salinity events, rather than growth inhibition due to long-term salinity imposed gradually.     

Factors limiting the intertidal distribution of the mangrove species<Emphasis Type="Italic"> Xylocarpus granatum</Emphasis>

The tree species Xylocarpus granatum is commonly described as occurring in the upper intertidal zone of mangrove forests, but mature trees are occasionally found at lower elevations. In the Utwe River basin, on the Pacific island of Kosrae, we investigated the relative importance of several biotic and abiotic factors that may control the intertidal distribution of X. granatum. Factors we evaluated included differential seed predation across the lower, mid, and upper intertidal zones and seedling responses to salinity, tidal flooding, and shade. Seed predation was 22.4% over the first 34 days and varied little among zones or in gaps versus under the forest canopy. By day 161, there were still no differences in seed mortality, but a significant difference was found in seedling establishment, with much greater establishment in the upper intertidal plots. X. granatum seedlings in a greenhouse experiment exhibited greater growth in freshwater than seedlings in 23 ppt salinity, which is typical of salinity levels found in the mid intertidal zone in our field study sites in Micronesia, where mature X. granatum trees are generally absent. Seedlings grown in 23 ppt salinity, however, exhibited few visible signs of stress associated with patterns in growth. Seedlings grown in a simulated tidal flooding treatment (with 23 ppt salinity) also showed few signs of stress. Growth declined dramatically under 80% shade cloths, but there were few interactions of shading with either 23 ppt salinity or simulated tidal flooding. Differential seed predation is not likely to be the primary factor responsible for the intertidal distribution of X. granatum on Kosrae. However, seedling tolerance of flooding or salinity may be more important, especially relative to a potential contribution to secondary stress mortality. Other factors may ultimately prove to be more critical, such as physiological effects of salinity on seed germination, effects of tides on seed dispersal and rooting, or differential herbivory on seedlings.     

Differential responses to salinity help explain the replacement of native Juncus kraussii by Typha orientalis in Western Australian salt marshes

The influence of salinity on Typha orientalis and Juncus kraussii was documented in experiments on germination of seeds and on growth of seedlings and adult, rhizome-bearing plants. Juncus was more salt-tolerant than Typha at all three life-history stages, but salt tolerance increased with plant age for both species. Although seeds of both species germinated at 0 and 5 ppt, the germination data overestimated salt tolerance for Typha. Only the newly emerged seedlings of Juncus were capable of growth after removal from the 5 ppt NaCl solution to fresh water. Typha seedlings that initiated growth at 0 ppt grew well at 5 ppt but not at 10 ppt, while Juncus seedlings were tolerant of 10 ppt. Although the 20 ppt treatment caused high mortality of Juncus seedlings, the 10 ppt treatment mainly reduced growth. Adult plants of Typha, which were collected from the field, survived the 20 ppt treatment, while adult Juncus survived the 40 ppt treatment. The presence of salt (10–40 ppt) shortened the growing season for adult, rhizome-bearing plants of both Juncus and Typha, with a lower maximum and earlier peak in total leaf length and maximum leaf number. Thus, the greater biomass in fresh water was achieved primarily through a longer growth period, rather than a greatly accelerated growth rate. Interactions between the two species were explored in mixed-species culture of both seedlings and adult rhizome-bearing plants. Interspecific interactions were present at low salinity, but results differed for seedlings and adult plants. Typha seedlings failed to outgrow Juncus seedlings (at 5 ppt) but adult plants of Typha outgrew Juncus (at 0 ppt). Relative yields (biomass in mixed/pure pots) for Juncus and Typha seedlings were 0.85 and 0.26 at 5 ppt. Relative yields of adult plants were 0.24 for Juncus and 1.20 for Typha at 0 ppt. For both seedlings and adults, the species that ultimately dominated the mixed-species pots produced just as much total biomass as in pure-species pots, even though initial planting density was half as high. Extrapolating findings to the field situation, it appears that Typha has a narrow regeneration niche. The indication is that Typha could invade Juncus stands only following salinity reduction (allowing seed germination and early seedling growth) and after disturbance disrupts the native vegetation. The combined conditions of prolonged low salinity and open habitat occur where street drains are cut through the salt marsh. The probability of Typha becoming established would be highest in such areas. vegetative expansion would follow with continued freshwater influx, as rhizome-bearing plants gain an interspecific advantage.     

Mangrove inundation and nutrient dynamics from a GIS perspective

A digital elevation model describing topography, tide elevation and inundation degree and frequency of a mangrove forest in North Brazil is discussed in relation to existing phosphate and physicochemical data in waters of an adjacent tidal creek. Due to smooth topography, an increase of 20 cm in tidal height above average neap tides increases flooded area from about 50 to 80%. Analysis of the relationship between microtopography, tidal height and flooding rate showed that in the upper 60 cm of the mangrove forest, increases of 20 cm in topographical height resulted in a doubling of the inundation frequency. This can be particularly relevant for the analysis of nutrient mobilization and vegetation structure of infrequently inundated wetlands. Throughout the year, low-tide phosphate in creek water was inversely proportional to the maximum area flooded during high tide, this correlation being higher during the dry season. Similarly, the inverse relationship between flooded areas and low-tide/high-tide pH ratios was highly significant during the dry season and the beginning of the rainy season. Although the high correlations obtained are based on data pairs obtained at high and low tide, it has to clarified whether the association between inundation degree and creek water pH is relevant for the stability of P compounds in sediment on the short scale of a tidal cycle.     

Morphological and physiological responses of Scaevola sericea (Goodeniaceae) seedlings to salt spray and substrate salinity

The effects of substrate salinity and salt spray upon seedlings of Scaevola sericea were examined in this study. Three levels of substrate salinity: 0.0 ppt, 3.0 ppt, and 10.0 ppt were examined in conjunction with three levels of salt spray: zero, medium (200 mg m^-2mdd^-1), and high (1200–1500 mg-m^-2mdd^-1). Leaf surface area, root to shoot ratio, as well as leaf, stem, and root mass decreased significantly (P 0.05) with increasing substrate salinity. Biomass accumulation was very low at 10.0 ppt substrate salinity, suggesting that higher levels of substrate salinity cannot be tolerated by the seedlings. Salt spray had a substantial effect on several of these variables, however its effects were less pronounced than those of substrate salinity. Cell sap osmolarity, leaf thickness, and leaf specific mass increased significantly (P 0.05) with both increasing substrate salinity and salt spray levels. Leaf carbon isotope ratios (δ^l3C) became more positive with increasing salinity, indicating an enhancement of the intrinsic water use efficiency of the seedlings at higher salinities. Scaevola sericea is one of the dominant plants found at the leading edge of strand communities in the Hawaiian archipelago and throughout much of the tropical Pacific. Since substrate salinity and salt spray increase with proximity to the ocean, the two factors may act together to limit the seaward expansion of S. sericea in coastal habitats.     

Threshhold for recovery in the marsh halophyte Spartina alterniflora grown under the combined effects of salinity and soil drying

A study quantifying the physiological threshhold at which Spartina alterniflora plants are able to tolerate the interactive effects of salinity and soil drying was conducted in a climate controlled greenhouse. The experiment consisted of two levels of salinity (3-5 ppt, L and 35-38 ppt, H) as well as four dynamic water levels: flooding (water level maintained 3-5 cm above the soil surface at high tide and 10 cm below the soil surface at low tide for entire study duration, F), 8-day drought (water level maintained at least 20 cm below the soil surface at high tide for 8 days then flooded, 8 days), 16-day drought (water level maintained at least 20 cm below the soil surface at high tide for 16 days then flooded, 16 days), and 24-day drought (water level maintained at least 20 cm below the soil surface at high tide for 24 days then flooded, 24 days). Plant gas exchange and growth responses were measured along with soil conditions of redox potential and water potential. Significant decreases were seen in plant gas exchange and growth in response to increases in salinity and soil drying. Survival was 100% for all flooded treatments while increased salinity combined with soil drying decreased survival to 86% in both low salt/24-day drought plants (LD24) and high salt/16-day drought plants (HD16). The lowest survival rate was seen in the high salt/24-day drought treatment (HD24) at 29%. Therefore, it appears that the critical time for recovery from the combined effects of increased salinity and soil drying may greatly diminish after two weeks from the onset of stress conditions. Consequently, if salinity continues to increase along the MRDP, marshes dominated by S. alterniflora may be more susceptible to short-term drought and likewise large-scale marsh browning.     

Salinity effects on greenhouse gas emissions from wetland soils are contingent upon hydrologic setting: a microcosm experiment

Coastal forested wetlands provide important ecosystem services such as carbon sequestration, nutrient retention, and flood protection, but they are also important sources of greenhouse gas emissions. Human appropriation of surface water and extensive ditching and draining of coastal plain landscapes are interacting with rising sea levels to increase the frequency and magnitude of saltwater incursion into formerly freshwater coastal wetlands. Both hydrologic change and saltwater incursion are expected to alter carbon and nutrient cycling in coastal forested wetlands. We performed a full factorial experiment in which we exposed intact soil cores from a coastal forested wetland to experimental marine salt treatments and two hydrologic treatments. We measured the resulting treatment effects on the emissions of carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O) over 112 days. Salinity effects were compared across four treatments to isolate the effects of increases in ionic strength from the impact of adding a terminal electron acceptor (SO₄^2?). We compared control treatments (DI addition), to artificial saltwater (ASW, target salinity of 5 parts per thousand) and to two treatments that added sulfate alone (SO₄^2?, at the concentration found in 5 ppt saltwater) and saltwater with the sulfate removed (ASW-SO₄^2?, with the 5 ppt target salinity maintained by adding additional NaCl). We found that all salt treatments suppressed CO₂ production, in both drought and flooded treatments. Contrary to our expectations, CH₄ fluxes from our flooded cores increased between 300 and 1200% relative to controls in the ASW and ASW-SO₄^2? treatments respectively. In the drought treatments, we saw virtually no CH₄ release from any core, while artificial seawater with sulfate increased N₂O fluxes by 160% above DI control. In contrast, salt and sulfate decreased N₂O fluxes by 72% in our flooded treatments. Our results indicate that salinization of forested wetlands of the coastal plain may have important climate feedbacks resulting from enhanced greenhouse gas emissions and that the magnitude and direction of these emissions are contingent upon wetland hydrology.     

Salinity tolerance of non-native Asian swamp eels (Teleostei: Synbranchidae) in Florida,USA: comparison of three populations and implications for dispersal

Three populations of non-native Asian swamp eels are established in peninsular Florida (USA), and comprise two different genetic lineages. To assess potential for these fish to penetrate estuarine habitats or use coastal waters as dispersal routes, we determined their salinity tolerances. Swamp eels from the three Florida populations were tested by gradual (chronic) salinity increases; additionally, individuals from the Miami population were tested by abrupt (acute) salinity increases. Results showed significant tolerance by all populations to mesohaline waters: Mean survival time at 14 ppt was 63 days. The Homestead population, a genetically distinct lineage, exhibited greater tolerance to higher salinity than Tampa and Miami populations. Acute experiments indicated that swamp eels were capable of tolerating abrupt shifts from 0 to 16 ppt, with little mortality over 10 days. The broad salinity tolerance demonstrated by these experiments provides evidence that swamp eels are physiologically capable of infiltrating estuarine environments and using coastal waters to invade new freshwater systems.