Salt Marsh Dieback in Coastal Louis

Survey of Plant and Soil Conditions 

Barataria and Terrebonne Basins,

June 2000-September 2001

Open-File Report 2006-1167

U.S. Department of the Interior

U.S. Geological Survey
Salt Marsh Dieback in Coastal Louisiana: Survey of Plant 

and Soil Conditions in Barataria and Terrebonne Basins, 

June 2000-September 2001 

By Karen L. McKee, Irving A. Mendelssohn, and Michael D. Materne 

Open-File Report 2006-1167 

U.S. Department of the Interior 

U.S. Geological Survey 
U.S. Department of the Interior 

DIRK KEMPTHORNE, Secretary 

U.S. Geological Survey 

P. Patrick Leahy, Acting Director 

U.S. Geological Survey, Reston, Virginia 2006 

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Suggested citation: 

McKee, K.L., Mendelssohn, I.A., and Materne, M.D., 2006, Salt marsh dieback in coastal Louisiana: Survey 

of plant and soil conditions in Barataria and Terrebonne Basins, June 2000-September 2001: U.S. Geological 

Survey Open File Report 2006-1167. 71 pp. 

Any use of trade, product, or firm names is for descriptive purposes only and does not imply 

endorsement by the U.S. Government. 

Although this report is in the public domain, permission must be secured from the individual 

copyright owners to reproduce any copyrighted material contained within this report. 

ii
 Contents 

List of Figures................................................................................................. v 

List of Tables.................................................................................................. viii 

Executive Summary...................................................................................... 1 

 Biotic Agents....................................................................................... 2 

 Excessive Soil Waterlogging and Sulfide....................................... 2 

 Drought and Low Water Levels........................................................ 3 

 Direct Lack of Water............................................................... 3 

 Hypersalinity............................................................................. 3 

 Soil Acidification and Metal Toxicity.................................... 3 

Abstract............................................................................................................ 5 

Introduction..................................................................................................... 6 

Study Sites and Experimental Design......................................................... 7 

 Preliminary Survey at Bay Junop...................................................... 7 

 Intensive Survey across Terrebonne and Barataria Basins......... 7 

Methods........................................................................................................... 10 

 Vegetative Condition........................................................................... 10 

 Physicochemical Conditions.............................................................. 10 

 Leaf and Soil Elemental Concentrations.......................................... 12 

 Soil Acidification Potential................................................................. 12 

 Statistical Analyses............................................................................. 12 

Results and Discussion................................................................................ 13 

 Scale and Pattern of Dieback........................................................... 13 

 Physicochemical Conditions............................................................. 27 

iii
 Bay Junop.................................................................................. 27 

 Intensive Sites 1-21.................................................................. 27 

 Soil Elemental Concentrations........................................................... 27 

 Soil Acidification Potential.................................................................. 38 

 Plant Elemental Concentrations......................................................... 38 

 Vegetative Recovery............................................................................. 51 

 Causation................................................................................................ 51 

 Hypothesis 1............................................................................... 57 

 Hypothesis 2............................................................................... 59 

 Hypothesis 3............................................................................... 60 

Conclusions....................................................................................................... 66 

 Biotic Agents........................................................................................... 67 

 Excessive Soil Waterlogging and Sulfide........................................... 67 

 Drought and Low Water Levels............................................................ 67 

 Direct Lack of Water.................................................................. 67 

 Hypersalinity................................................................................ 68 

 Soil Acidification and Metal Toxicity....................................... 68 

Acknowledgements.......................................................................................... 69 

Cited Literature.................................................................................................. 69 

iv
 Figures 

Figure 1. Map of study area and location of study sites....................................... 8 

Figure 2. Experimental design for intensive survey............................................... 9 

Figure 3. Views of dieback site at Bay Junop in June 2000................................. 14 

Figure 4. Aerial and ground views of a dieback Spartina patens 

marsh in August 2000........................................................................................... 17 

Figure 5. Spatial patterns of marsh dieback.......................................................... 18 

Figure 6. Ground views of healthy shoreline and dieback interior 

stands of Spartina alterniflora in August 2000................................................. 19 

Figure 7. Relative cover by condition measured in the shoreline zone 

at sites 1-21 in August/October 2000, April 2001, and September 2001....... 20 

Figure 8. Relative cover by condition measured in the transition zone 

at sites 1-21 in August/October 2000, April 2001, and September 2001........ 21 

Figure 9. Relative cover by condition measured in the interior zone 

at sites 1-21 in August/October 2000, April 2001, and September 2001........ 22 

Figure 10. Viability of Spartina alterniflora rhizomes 

in August-October 2000......................................................................................... 23 

Figure 11. Aerial view of Pelican Island showing healthy Avicennia germinans 

stands alongside dying Spartina alterniflora marsh......................................... 25 

Figure 12. Aerial and ground views of Juncus roemerianus (black 

needlerush) patches growing within areas of dead Spartina 

 alterniflora (smooth cordgrass) ........................................................................ 26 

Figure 13. Soil conditions measured at Bay Junop in June 2000......................... 28 

Figure 14. Soil redox potential measured in shoreline, transition, and 

interior zones at dieback sites.......................................................................... 30 

v
Figure 15. Soil salinity, pH, and sulfide measured in shoreline, transition, 

and interior zones at dieback sites................................................................... 31 

Figure 16. Acid-extractable Al and Fe concentrations in different 

vegetation zones in April 2001........................................................................ 43 

Figure 17. Soil pyrite concentrations in different vegetation zones 

in August 2000................................................................................................... 45 

Figure 18. Acidification potential of soils from dieback and control 

marshes.............................................................................................................. 46 

Figure 19. Leaf elemental ratios in Spartina alterniflora growing in 

dieback and control marshes.......................................................................... 53 

Figure 20. Aerial and ground views of a dieback area (site 6) that 

showed little vegetative recovery in September 2001................................. 54 

Figure 21. Example of a Spartina alterniflora marsh (site 17) that 

recovered from dieback.................................................................................... 55 

Figure 22. Aerial and ground views of a Spartina alterniflora marsh 

(site 2) recovering from dieback..................................................................... 56 

Figure 23. Sequence of shoot degradation in dieback marshes and role 

of snails .............................................................................................................. 58 

Figure 24. Primary rooting depth and Spartina alterniflora rooting 

depth in dieback and control marshes in August-October 2000................ 61 

Figure 25. Views of Avicennia germinans and Juncus roemerianus root 

 systems................................................................................................................ 62 

Figure 26. Reddish-orange coloration observed in the shoots, roots/rhizomes, 

and soil at some sites during early stages of dieback (August 2000)......... 64 

vi
Figure 27. Distant and close-up views of dry, cracked soil at a Spartina 

 alterniflora dieback area.................................................................................... 65 

vii
 Tables 

Table 1. Summary of plant and soil variables measured on each 

sampling date............................................................................................ 11 

Table 2. Summary of plant characteristics at a dieback site near Bay 

Junop in June 2000................................................................................... 15 

Table 3. Results of a repeated-measures ANOVA for plant (Spartina 

 alterniflora) variables measured at dieback and control sites......... 24 

Table 4. ANOVA results for physicochemical variables measured at 

Bay Junop in June 2000............................................................................ 29 

Table 5. Results of a repeated-measures ANOVA for soil variables 

measured at Spartina alterniflora-dominated dieback sites............... 32 

Table 6. Summary of physicochemical conditions (averaged over site 

and time) measured in stands dominated by Spartina alterniflora..... 33 

Table 7. Summary of physicochemical conditions (averaged over site 

and time) measured in stands dominated by Juncus roemerianus 

and Spartina patens................................................................................... 34 

Table 8. Water-extractable soil elemental concentrations (mg/kg) 

measured in August 2000 at dieback and reference marshes 

dominated by Spartina spp....................................................................... 35 

Table 9. Ammonium acetate-extractable soil elemental concentrations 

(mg/kg) measured in August 2000 at dieback and reference 

marshes dominated by Spartina spp....................................................... 36 

Table 10. Factor analysis of soil elemental concentrations measured 

at dieback and control sites in August 2000............................................ 37 

Table 11. Water-extractable soil elemental concentrations (mg/kg) 

viii
measured in April 2001 at dieback and reference marshes 

dominated by Spartina spp........................................................................ 39 

Table 12. Ammonium acetate-extractable soil elemental concentrations 

(mg/kg) measured in April 2001 at dieback and reference 

marshes dominated by Spartina spp........................................................ 40 

Table 13. Acid-extractable soil elemental concentrations (mg/kg) 

measured in April 2001 at dieback and reference marshes 

dominated by Spartina spp........................................................................ 41 

Table 14. Factor analysis of soil elemental concentrations measured 

at dieback and control sites in April 2000................................................. 42 

Table 15. Results of ANOVA for soil pyrite (August 2000) and 

acid extractable Al and Fe concentrations (April 2001).......................... 44 

Table 16. Elemental concentrations (μg/g) measured in Spartina 

 alterniflora leaves in August 2000 at dieback and reference 

marshes........................................................................................................... 47 

Table 17. Elemental concentrations (μg/g) measured in Spartina 

 alterniflora leaves in April 2001 at dieback and reference 

marshes........................................................................................................... 48 

Table 18. Elemental concentrations (μg/g) measured in Spartina 

alterniflora (sites 2-21) and S. patens (site 1) leaves in 

September 2001 at dieback and reference marshes............................... 49 

Table 19. Elemental concentrations (μg/g) measured in Spartina 

 patens (site 1) and Juncus roemerianus (site 16) leaves in 

August 2000 (A) and April 2001 (B).............................................................. 50 

Table 20. Factor analysis of plant elemental concentrations measured 

ix
at dieback and control sites in August 2000, April 2001, and 

September 2001.............................................................................................. 52 

x
Salt Marsh Dieback in Coastal Louisiana: Survey of 

Plant and Soil Conditions at 23 Sites in Barataria and 

Terrebonne Basins, June 2000-September 2001 

By Karen L. McKee1, Irving A. Mendelssohn2, and Michael D. Materne3

Executive Summary 

Sudden and extensive dieback of the perennial marsh grass, Spartina alterniflora

Loisel (smooth cordgrass), which dominates regularly flooded salt marshes along the 

Gulf of Mexico and Atlantic coastlines, occurred in the coastal zone of Louisiana. The 

objectives of our study were to assess soil and plant conditions in dieback areas of the 

Barataria-Terrebonne estuarine system as well as vegetative recovery during and after 

this dieback event. We examined dieback marshes from June 2000 until the end of the 

event when signs of vegetative recovery were evident (September 2001). Information 

was collected directly from multiple dieback sites along 100 km of shoreline from the 

Atchafalaya River to the Mississippi River. In addition to ground assessment of dieback 

conditions and vegetative recovery, we evaluated potential causes of dieback by using 

data collected from these surveys, historical records, and known tolerance limits of salt 

marsh vegetation to environmental stress factors. 

Intensive examination of plant and soil characteristics at multiple dieback sites 

provided some important insights into the dieback process, vegetative recovery, and 

causation. Some areas of salt marsh were completely dead by June 2000 (Bay Junop), and 

plant shoots were substantially decomposed in most dieback sites by August 2000. This 

timeline of vegetative condition supports the hypothesis that the dieback event began in 

early spring of 2000. The pattern of dieback was distinctive and suggested a correlation 

with tidal flushing; i.e., the greatest mortality occurred in the marsh interior, whereas 

fringing vegetation along shorelines of bays and tidal creeks often retained some living 

shoots. In salt marshes, only S. alterniflora appeared to be affected; sympatric species 

such as Avicennia germinans (L.) Stearn (black mangrove), Batis maritima L. (saltwort), 

Distichlis spicata (L.) Greene (seashore saltgrass), and Juncus roemerianus Scheele 

(needlegrass rush) showed no visible signs of stress, even though they were growing in 

mixture or immediately adjacent to dead stands of S. alterniflora. Brackish marsh dieback of Spartina patens (Ait.) Muhl. (marshhay cordgrass) was observed at one site 

(Point Au Fer). This study confirmed that these spatial patterns of dieback and 

differential species susceptibility occurred repeatedly across the Barataria-Terrebonne 

estuarine system. The documentation of consistent patterns is important to concurrent 

research on causation and supports the hypothesis that the cause was a regional factor 

(e.g., drought) that interacted with local patterns of soil chemistry and/or hydrology. 

 

1

USGS-National Wetlands Research Center, 700 Cajundome Blvd, Lafayette, LA 70506 

2

Wetland Biogeochemistry Institute, Louisiana State University Campus, Baton Rouge, LA 70803 

3

USDA-Natural Resources Conservation Service, Louisiana State University Campus, Baton Rouge, LA 

70803; Current address: Department of Agronomy, Louisiana State University, Baton Rouge, LA 70803 

1
Little or no expansion of dieback occurred subsequent to the initial event, and 

areas with moderate dieback (~50 percent mortality) had completely recovered by April 

2001. These data demonstrate the ephemeral nature of the dieback agent, which 

apparently had no residual effects on plant recovery. Rapid recovery of moderately 

impacted marsh was due to survival of S. alterniflora rhizomes. Slow recovery of 

interior marshes that experienced greater than 90 percent shoot mortality was attributable 

to low survival of rhizomes and variable recolonization by seedlings and/or vegetative 

expansion from edges. However, regenerating plants in dieback areas were robust (culm 

heights ~1.5 m), and reproductive output was high, indicating that post-dieback 

conditions were actually promoting growth of S. alterniflora. Vigorous growth in 

dieback areas was likely due to a greater availability of nutrients and/or light. Stands of 

J. roemerianus, D. spicata, and A. germinans within or near some dieback sites remained 

largely unchanged or expanded into the dead S. alterniflora marsh during the observation 

interval. Although further monitoring is needed to confirm the extent of recovery, our 

observations indicate a high potential for natural recovery from the dieback event. 

However, some areas that showed no recovery by September 2001 (one full growing 

season after dieback) may convert to open water. 

The cause of the dieback is currently unknown, primarily because the causative 

agent left no conclusive signature in plant or soil variables. Soil conditions were 

generally within normal ranges for salt marshes during the dieback event (June to 

October 2000) and during vegetative recovery (April to September 2001). However, our 

data either eliminate or are inconsistent with some proposed causes. We specifically 

examined three potential causes of marsh dieback in terms of consistency with the spatial 

pattern of dieback, the differential susceptibility of marsh species and their known stress 

tolerances, and the temporal sequence of dieback and recovery. 

Biotic Agents 

We found no evidence in support of a biotic agent as the primary cause of marsh 

dieback. Although potential fungal pathogens were isolated from S. alterniflora, these 

were species that typically infect previously stressed vegetation and are therefore unlikely 

to be the sole cause of dieback. Fungal or other pathogens also cannot explain the 

consistent spatial pattern of dieback. Fungal pathogens may have played a secondary role 

in plant mortality through interaction with an abiotic stress factor, but further experiments 

are required to test this possibility. 

Outbreaks of insects or other herbivores were also ruled out as potential causes, 

based primarily on the spatial pattern and extent of dieback as well as the condition of 

dead and dying vegetation. Our observations specifically do not support the contention 

that dieback was caused by excessive grazing by Littoraria irrorata (marsh periwinkle). 

The data instead indicate that snails responded to the dieback and played an important 

role in rapid degradation of standing dead plants in some areas. 

Excessive Soil Waterlogging and Sulfide 

Although soil waterlogging and sulfide toxicity have been implicated in historical 

dieback of S. alterniflora, our data do not support this explanation with respect to acute 

marsh dieback. Historical records do not indicate that water levels were abnormally high 

2
in interior marshes during spring 2000 but instead show that this was a period of 

unusually low water levels. Elevated sulfide concentrations were found in dieback 

marshes, but they may have occurred because death of the vegetation stimulated sulfate 

reduction. In addition, we found no evidence of sulfide concentrations that exceeded the 

tolerance limits of S. alterniflora during the study period (June 2000 to September 2001) 

and generally found conditions that were typical of interior marshes. Also, this 

explanation does not account for the dieback of Spartina spp. and survival of other 

species that are known to be sensitive to sulfide (e.g., Avicennia germinans) (McKee, 

1993). Finally, sulfide concentrations remained elevated or even increased over time in 

areas where revegetation occurred. However, waterlogging and elevated sulfide in 

interior marshes may have increased the susceptibility of Spartina spp. to another stress 

factor that precipitated the dieback event. 

Drought and Low Water Levels

The dieback event was coincident with an extreme drought condition, low river 

discharge, and low sea level. Drought, combined with less tidal flooding, may have 

decreased water availability directly, increased salinity by concentrating salts in the pore 

water, and/or caused oxidation and acidification of soils. 

Direct Lack of Water 

We found evidence of dry, cracked soils at a few sites, indicating that these 

marshes can become desiccated under certain conditions. However, most sites were 

flooded at the time of our surveys. Our data thus cannot eliminate the possibility that a 

direct lack of water caused plant mortality. However, greenhouse experiments were 

unable to duplicate this hypothesized effect on S. alterniflora, even when water was 

withheld for 30 d (Mendelssohn and others, 2005). 

Hypersalinity 

We found no evidence in support of hypersalinity as a cause of marsh dieback. 

Soil salinities measured at dieback sites from June 2000 to September 2001 were within 

the ecological range of S. alterniflora and well below the lethal limit for this species. In 

addition, salinity did not have a significant effect on mortality of S. alterniflora in 

experiments designed to simulate dieback (see Mendelssohn and others, 2005). 

However, the most convincing evidence against this hypothesis is the fact that J. 

roemerianus, a species with lower salt tolerance growing within S. alterniflora dieback 

areas, not only survived the event, but also showed little or no signs of stress. The 

differential salt tolerance of these species has been confirmed in controlled laboratory 

experiments (see Mendelssohn and others, 2005). 

Soil Acidification and Metal Toxicity 

When saline sediments are aerated during drying, microbial oxidation of pyrite 

and sulfide generates sulfuric acid, which leads to soil acidification and ultimately to the 

release of potentially toxic metals such as aluminum and iron. Our data are consistent 

with this explanation. Dieback sites had higher concentrations of pyrite and acid3
extractable aluminum and iron than did reference marshes that did not die. Plants from 

dieback marshes also contained significantly higher ratios of aluminum and iron, 

indicating increased uptake of potentially toxic metals, than did those growing along 

nearby shorelines and in reference marshes where plant mortality was low. The 

subsequent decline of tissue metal ratios in recovering vegetation further substantiates 

this explanation. In addition, laboratory trials demonstrated that soils from dieback 

marshes acidified upon oxidation, whereas those from reference sites did not. The fact 

that salt marsh sediments accumulate more trace and heavy metals because of mineral 

input from rivers compared to intermediate and freshwater marshes with more organic 

substrates and that they show greater decreases in pH upon oxidation is also consistent 

with the regional pattern of dieback primarily in saline marshes. 

A better understanding of this acute dieback event and its consequences relative to 

historical dieback in Barataria-Terrebonne salt marshes is needed to aid in conservation 

and restoration efforts. Just as important are the implications of this event for global 

change models that focus mainly on sea-level rise as the dominant variable controlling 

vegetation patterns in coastal ecosystems. The sudden dieback of salt marshes in 

Louisiana suggests that periodic climatic extremes may cause rapid, widespread mortality 

of vulnerable species. The implications of this event for other coastal systems are 

unclear, partly because the precise causal mechanism for the acute dieback in the 

Mississippi River deltaic plain is not known. Also, preexisting stress conditions may 

play a role in acute dieback, but how and to what degree is not currently understood. The 

Mississippi delta is perhaps an extreme case since it has been undergoing high rates of 

apparent sea-level rise (8.5 to 9.5 mm/yr) and marsh deterioration. Nonetheless, the 

sudden dieback of S. alterniflora marshes in Louisiana suggests that large-scale changes 

in coastal vegetation may occur much more rapidly than current models predict and as a 

consequence of weather extremes acting in concert with sea-level fluctuations and 

preexisting soil conditions. Management plans usually do not take disturbance into 

account because of unfamiliarity or a belief that the disturbance cannot be managed. 

However, even if a disturbance cannot be controlled, the resilience and recovery of the 

system may be altered by management (Dale and others, 1998). Such considerations will 

become increasingly important as global climate changes and as human pressures in the 

coastal zone grow. 

4
 Abstract

Sudden and extensive dieback of the perennial marsh grass, Spartina alterniflora

Loisel (smooth cordgrass), which dominates regularly flooded salt marshes along the 

Gulf of Mexico and Atlantic coastlines, occurred in the coastal zone of Louisiana. The 

objectives of this study were to assess soil and plant conditions in dieback areas of the 

Barataria-Terrebonne estuarine system as well as vegetative recovery during and after 

this dieback event. Multiple dieback sites were examined along 100 km of shoreline 

from the Atchafalaya River to the Mississippi River during the period from June 2000 

through September 2001. The species primarily affected was S. alterniflora; sympatric 

species such as Avicennia germinans (L.) Stearn (black mangrove) and Juncus 

roemerianus Scheele (needlegrass rush) showed no visible signs of stress. The pattern of 

marsh dieback was distinctive with greatest mortality in the marsh interior, suggesting a 

correlation with local patterns of soil chemistry and/or hydrology. Little or no expansion 

of dieback occurred subsequent to the initial event, and areas with 50 percent or less 

mortality in the fall of 2000 had completely recovered by April 2001. Recovery was 

slower in interior marshes with 90 percent or greater mortality initially. However, 

regenerating plants in dieback areas showing some recovery were robust, and 

reproductive output was high, indicating that the causative agent was no longer present 

and that post-dieback soil conditions were actually promoting plant growth. Stands of 

other species within or near some dieback sites remained largely unchanged or expanded 

(A. germinans) into the dead salt marsh. 

The cause of the dieback is currently unknown. Biotic agents and excessive soil 

waterlogging/high sulfide were ruled out as primary causes of this acute event, although 

they could have contributed to overall plant stress and/or interacted with the primary 

agent to cause plant mortality. Our observations over the 15 month study specifically do 

not support the contention that dieback was caused by excessive grazing by Littoraria 

irrorata (marsh periwinkle). Instead, the data show that snails were responding to plant 

mortality and played an important role in rapid degradation of dead material in some 

areas. The dieback event was coincident with an extreme drought, low river discharge, 

and low sea level. These conditions could have caused plant mortality by directly 

decreasing water availability, increasing salinity, and/or causing oxidation and 

acidification of soils. The latter scenario was supported by findings of higher pyrite and 

acid-extractable aluminum and iron, higher acidification potential of dieback soils, and 

higher concentrations of aluminum and iron in dieback plant tissues (indicating uptake of 

potentially toxic metals) when compared to reference marshes showing no dieback. The 

implication of these findings is that periodic weather extremes may play a greater role in 

shaping coastal plant communities than has previously been recognized. Although such 

events may not be controlled directly, the resilience and recovery of the system may be 

altered by management. Such considerations will become increasingly important as 

global climate changes and human pressures in the coastal zone grow.

5
Introduction 

Sudden and extensive dieback of the perennial marsh grass, Spartina alterniflora

Loisel. (smooth cordgrass), which dominates regularly flooded salt marshes along the 

Gulf of Mexico and Atlantic coastlines, occurred in the coastal zone of Louisiana. First 

observations of “browning” marsh were made in May 2000 by personnel of the Louisiana 

Department of Wildlife and Fisheries (G. Linscombe and R. Chabreck, public 

communication, www.brownmarsh.net, accessed 21 August 2003) and later confirmed by 

aerial surveys conducted by the U.S. Geological Survey National Wetlands Research 

Center (NWRC) (T. Michot, public communication, www.brownmarsh.net, accessed 21 

August 2003). These aerial surveys indicated that extensive areas of salt marsh were 

moderately to severely damaged with only 35 percent of the total showing no signs of 

dieback. The dieback was initially most evident in the Fourleague Bay area but 

subsequently was reported throughout the Barataria and Terrebonne basins. The dieback 

appeared to occur over a short period of time. Preliminary ground surveys revealed that 

anywhere from 50 to 100 percent mortality of plant shoots had occurred as early as June 

2000 in some areas of southeastern Louisiana. The pattern of dieback was distinctive in 

some areas and suggested a correlation with tidal flushing; i.e., the greatest mortality 

occurred in the marsh interior, whereas fringing vegetation along shorelines of bays and 

tidal creeks often retained some living shoots. In salt marshes, only S. alterniflora

appeared to be affected; sympatric species such as Avicennia germinans (L.) Stearn 

(black mangrove) and Juncus roemerianus Scheele (needlegrass rush) showed no visible 

signs of stress, even though they were growing in mixture or immediately adjacent to 

dead stands of S. alterniflora. Brackish marsh dieback (Spartina patens (Ait.) Muhl. 

(marshhay cordgrass) was reported in Rockefeller State Wildlife Refuge and Game 

Preserve but was small in areal extent (Nyman and others, 2001) compared to that 

observed in salt marshes. 

This recent dieback of salt marsh was unprecedented in the region’s recorded 

history. Sudden marsh dieback has been reported elsewhere in the Gulf of Mexico and 

the Atlantic coast (Florida: Carlson and others, 2001; Georgia: Georgia Coastal Research 

Council, public communication accessed 24 April 2003, http://alpha.marsci.uga.edu/ 

coastalcouncil/ marsh_dieback.htm; South Carolina: J.T. Morris, personal 

communication), but these occurrences do not approach the large-scale occurrence 

observed in Louisiana. The large extent and rapidity of this event also distinguish it from 

the long-term phenomenon previously described (Smith, 1970; Mendelssohn and others, 

1983; Turner, 1990). Historically, coastal wetlands in the Mississippi River deltaic plain 

have been gradually converting to open water because of a combination of natural 

processes related to the Mississippi River deltaic cycle (Scruton, 1960) and human 

activities (Turner, 1990). Historical dieback of S. alterniflora is caused by excessive soil 

waterlogging, which limits aeration of plant roots and allows buildup of soil phytotoxins 

such as sulfide (Mendelssohn and others, 1981; Mendelssohn and McKee,1988; Koch 

and others, 1990; Wilsey and others, 1992; Webb and others, 1995). However, historical 

dieback occurs gradually and typically requires years for a marsh to completely die. 

The objectives of our study were to assess soil and plant conditions in dieback 

areas as well as vegetative recovery during and after sudden dieback (June 2000-

September 2001). Information was collected directly from multiple dieback sites along 

6
100 km of shoreline from the Atchafalaya River to the Mississippi River. In addition to 

ground assessment of dieback conditions and vegetative recovery, we evaluated potential 

causes of dieback by using data collected from surveys, historical records, and known 

tolerance limits of salt marsh vegetation to environmental stress factors. We tested three 

hypotheses that might explain the dieback event: 

1. Dieback was caused by a biotic agent, such as fungal pathogens or excessive 

grazing or “eat-outs” by snails, nutria, or other herbivores. 

2. Dieback was caused by unusually high water levels that either (a) submerged 

plants, particularly those at low elevations in interior marshes, and directly 

caused root hypoxia and plant death and/or (b) caused a soil buildup of 

sulfide or some other fermentative product toxic to plants. 

3. Dieback was caused by low water levels in combination with extreme 

drought, which led to mortality of the vegetation because of a direct lack of 

water, hypersalinity, and/or soil acidification and release of toxic metals 

(e.g., Fe, Al, Mn). 

Study Sites and Experimental Design 

Preliminary Survey at Bay Junop 

A preliminary ground survey of a dieback marsh was made in the Bay Junop area 

in June 2000 just after the phenomenon was reported in coastal Louisiana (fig. 1). Three 

transects were established perpendicular to the shoreline and traversed three marsh zones: 

healthy shoreline and dead interior zones dominated by S. alterniflora and a healthy stand 

of A. germinans. Live and dead stem densities of S. alterniflora and soil conditions 

(redox potential, bulk density, salinity, pH, and sulfide; see detailed methods below) were 

determined at 1, 5, 10, 15, 30, 45, 60, and 75 m intervals from the shoreline. A reference 

marsh (showing no signs of dieback in June 2000) near Old Oyster Bayou was 

simultaneously examined.

Intensive Survey across Terrebonne and Barataria Basins 

A more detailed survey of multiple dieback sites was undertaken in late summer 

of 2000. Dieback sites were selected based on maps derived from prior aerial surveys (G. 

Linscombe, personal communication) and were accessed by helicopter on August 30-31, 

2000; October 4-5, 2000; April 26-29, 2001, and September 17-18, 2001. Eighteen 

dieback sites (fig. 1) of varying size were identified along 100 km of shoreline in the 

Mississippi River deltaic plain based on the occurrence of a distinct spatial pattern of 

mortality: healthy shoreline (less than 10 percent mortality), transition (50 percent 

mortality), and dead interior (greater than 90 percent mortality) (fig. 2). Nine sites were 

established in Terrebonne basin during August 2000 and nine sites in Barataria basin 

during October 2000 with funding from the Louisiana Sea Grant program. 

7
5

9 1 2

1

N

50 25 0 50 100 km

2 1

3 4

5

6

21 7

8

9

10

20

11

19

12

14

13

15

16

18 17

Figure 1. Location of 23 study sites in the Mississippi River deltaic plain. 

Preliminary dieback (Bay Junop) and control (Old Oyster Bayou) sites surveyed in 

June 2000 are indicated by red and green stars, respectively. Intensive survey sites 

established in August and October 2000 are indicated by circles. Intensive sites 1-18 

(red) were in dieback areas, and sites 19-21 (green) were in control areas (no visible 

signs of dieback). 

8
 

Basic Experimental Design:

18 dieback sites with 3 zones 

(shoreline, transition, interior)

3 control sites with 2 zones 

(shoreline, interior)

3 replicate plots per zone

Total plots: 180

Shoreline

(<10% mortality)

Transition

(~50% mortality)

Interior

(>90% mortality)

Basic Experimental Design:

18 dieback sites with 3 zones 

(shoreline, transition, interior)

3 control sites with 2 zones 

(shoreline, interior)

3 replicate plots per zone

Total plots: 180

Shoreline

(<10% mortality)

Transition

(~50% mortality)

Interior

(>90% mortality)

Shoreline

(<10% mortality)

Transition

(~50% mortality)

Interior

(>90% mortality)

Figure 2. Experimental design used in the intensive survey of dieback and control 

marshes. Photograph illustrates the spatial arrangement of zones sampled at dieback 

marshes. In addition to the 180 plots in the basic design, an additional 9 regenerating 

plots (sites 4-6), 3 Juncus roemerianus plots (site 16), and 6 dieback shoreline plots 

(sites 15-16) were sampled for a total of 194 plots overall. 

9
A regenerating zone with new shoot growth emerging within an older dieback 

area, a dead shoreline zone, and patches of J. roemerianus were additionally selected for 

sampling at a few of the sites. Three control sites were also established in areas showing 

no dieback either along the shoreline or in the marsh interior. Initially, only interior plots 

were sampled at control marshes (August-October 2000), but both shoreline and interior 

plots were sampled on subsequent trips (April and September 2001). At all 21 sites, a 

permanent marker (PVC pipe) was installed in each of the three main zones (shoreline, 

transition, interior) to mark the sampling area. In each sampling area, three replicate 

plots were randomly selected by first marking 10 stations in the vicinity of the PVC pipe 

with numbered flags. Three of the flagged stations were then randomly selected on each 

sampling date by using a random numbers table. 

Methods 

Table 1 summarizes the plant and soil variables measured on each sampling date. 

Vegetative cover by species and condition (percent live/dead and status of standing dead 

stems) and leaf tissue cation and metal concentrations were measured on every sampling 

trip (intensive survey). 

Vegetative Condition 

Percent cover by condition and species was visually estimated inside a 0.25 m2

quadrat. Plants were identified to species, and condition of shoots was categorized as 

live, standing dead with leaves, standing dead without leaves, stubble (less than 5 cm of 

stem remaining), or bare (no vegetative cover). A 0.5-m deep core was extracted with a 

Russian peat corer, and primary and maximum rooting depths were measured. Rhizome 

viability was assessed in August-October 2000 based on staining with triphenyltetrazolium chloride (1 percent w/v) solution. Five rhizomes per plot were extracted from 

root cores, washed free of sediment, cut into 1 cm sections, and placed into staining 

solution in the field and kept on ice and in the dark until analysis (within 24 h). 

Rhizomes were sectioned longitudinally in the laboratory and examined under 

magnification to affirm staining of meristems. Additional samples were stained with 

indigocarmine (0.05 percent w/v) to confirm viability when the tetrazolium procedure 

was equivocal. 

Physicochemical Conditions 

Shallow soil cores were collected from each plot with a piston corer (54.3 cm3, 12 

cm long x 1.2 cm radius) and stored in water-tight plastic bags until analysis. The soil 

was weighed, dried at 80°C to constant mass, and reweighed to determine bulk density 

(mass of dry soil per volume) and relative saturation (%). The dried soil was ground with 

a mortar and pestle, and a subsample was ashed at 550°C for 6 h to determine mineral 

mass after organic loss on ignition. 

Replicate redox potential (Eh) measurements were made in situ at two (1, 15 cm; 

Bay Junop) or three (1, 15, and 30 cm; Intensive Survey) soil depths in each plot to 

10
characterize reducing conditions in the soils (McKee and others, 1988). Bright platinum 

electrodes were inserted to the desired depth and allowed to equilibrate for a minimum of 

30 min before measurement. 

Table 1. Summary of variables measured by sampling date. 

________________________________________________________________________ 

Jun 2000 Aug-Oct 2000 Apr 2001 Sep 2001 

________________________________________________________________________ 

Vegetative Condition

Percent Mortality X X X 

Stem Density X 

Rhizome Viability X 

Rooting Depth X X 

Leaf Elements X X X 

Soil Condition

Salinity X X X X 

pH X X X X 

Redox Potential X X X X 

Sulfide X X X 

Percent Organic Matter X X 

Water Content X X 

Bulk Density X X X 

Elemental Analysis X X 

Acidification Potential X 

Fungal Pathogens

Shoot X 

Root/Rhizome X 

Voltage (mV) between the platinum electrode and a calomel reference electrode was 

measured with a hand-held pH/mV meter (Cole-Parmer, model 5938-00, Vernon Hills, 

Ill.) and corrected for the calomel potential by adding 244 mV to the reading. Pore water 

samples were collected and processed according to McKee and others (1988). A sipper 

apparatus consisting of a rigid plastic tube connected to a 60 ml syringe was used to 

extract interstitial water from the soil. The first 5 ml of each extraction was discarded to 

remove debris, sediment, and gas. An aliquot of water for sulfide determination was 

added to an equal volume of antioxidant buffer (electrode operating instructions) upon 

collection, and all samples were analyzed with a sulfide electrode (Lazar Model IS-146 

sulfide electrode, Lazar Research Laboratories, Los Angeles, Cal.). The remaining water 

was stored in glass vials for analysis of salinity (refractometer, Vista Model A366 ATC) 

and pH (pH/mV meter, model 5938-00, Cole-Parmer, Vernon Hills, Ill.). 

11
Leaf and Soil Elemental Concentrations 

Leaf and soil samples for nutrient and metal analysis were kept cold until 

extraction. Leaf samples were rinsed thoroughly with deionized water, dried at 70°C, 

ground in a Wiley Mill to pass a #40 mesh sieve, and digested in concentrated nitric acid 

(Lachat BD-46 block digester, Lachat Instruments, Milwaukee, Wisc.). Soil samples 

were extracted sequentially with deionized water, ammonium acetate, and acid (1 M HCl 

at pH 3.5). Extracts of soil and digested leaf samples were analyzed for total 

concentrations of Ca, Mg, Na, K, Mn, Fe, Cu, Zn, Al, S, and P with an inductively 

coupled plasma emission spectrometer (Soil Testing and Plant Analysis laboratory, 

Department of Agronomy, Louisiana State University). 

Soil Acidification Potential 

Acidification potential of soils was evaluated by using a subset of dieback (6 

sites) and control (3 sites) soil samples from the intensive field survey. Soil (~10 g wet 

wt) was added to flasks containing 250 ml of artificial seawater. The flasks were covered 

to exclude light, fitted with stoppers, and purged with either N2 gas or air at a rate of 5 

ml/min. The flasks were maintained at room temperature (21-22 °C), and pH of the 

solution was measured at intervals of 0, 6, 13, and 23 d. 

Statistical Analyses 

Since sites, zones, and sampling dates were preselected, a fixed effects model was 

used to analyze the data (SAS Institute, 2003). However, because sampling frequency 

and zone by site combinations were not the same for all variables measured, the specific 

model differed among plant and soil datasets. For example, dieback sites had three zones 

(healthy shoreline, transition, and dieback interior), and control sites had only two zones 

(shoreline and interior). Data from the preliminary survey at Bay Junop (single site and 

sampling date) were analyzed as a one-way ANOVA with zone as the grouping factor. 

Variables measured once at all 21 sites, but in different combinations of zones within site 

(e.g., rhizome viability), were analyzed with a one-way ANOVA with site and zone 

(nested within site) as the grouping factors. Variables measured on every sampling date, 

but in different zones within sites (e.g., vegetative condition and tissue elemental 

concentrations), were analyzed with a repeated measures ANOVA with site, zone (nested 

within site), and time as grouping factors. For physicochemical variables, data from three 

zones (shoreline, transition, and interior) at all dieback sites dominated by S. alterniflora

(sites 2-18), and three sampling dates were analyzed with a repeated measures ANOVA 

with site, zone, and time as grouping factors. Soil acidification (August 2000) was 

analyzed witht a two-way, repeated measures ANOVA with soil source (dieback or 

control), treatment (reduced or oxidized), and incubation time as grouping factors. 

Data were log (ln[x + 1]) or arcsin transformed prior to analysis where necessary 

to reduce heterogeneity of variance and to reduce deviations from normality. Post-test 

comparisons among sites and zones were described with one degree of freedom contrasts 

for single comparisons of interest between two levels. A priori contrasts were 

constructed to make comparisons among zones at dieback marshes (i.e., healthy shoreline 

vs. transition or dieback interior zones) or between dieback interior and control interior 

12
marsh zones when a significant main effect of zone was found. Dieback (sites 1-18) 

marshes were contrasted with control (sites 19-21) marshes when a significant site effect 

was found. A significant interaction of time with either zone or site was interpreted as a 

difference in temporal patterns across levels of the grouping factor. However, because 

measurements made in the same plots on different dates are not independent samples, 

post-test comparisons of zone by time or site by time combination levels could not be 

conducted. 

A factor analysis was performed on elemental concentrations in soil and plant 

tissue to reduce the number of variables and problems associated with multicollinearity. 

The factor extraction method was Principal Components Analysis (SAS Institute, 2003). 

A varimax rotation of factor scores was performed by using the JMP “factor rotation” 

procedure. The number of factors selected for rotation was based on the number of 

eigenvalues greater than unity for the correlation matrix of the test scores extracted from 

the data. Individual factor scores were then subjected to ANOVA to test for main effects 

of site and zone (nested within site). Data from each sampling date were analyzed 

separately because each dataset resulted in a different correlation matrix and factor 

loadings. If rotated factors extracted from the correlation matrix of elemental 

concentrations were interpretable and there was also a significant effect of site and/or 

zone on rotated factors, then individual elements with high loadings on that factor were 

examined further. Because the elements Na, Al, Fe, and Mn were hypothesized to be 

causative agents in dieback (i.e., directly toxic to plants), these elemental concentrations 

(in soil and plant tissues) were particularly targeted for individual scrutiny. Tissue 

concentrations of these elements were tested as ratios of K+ (Na:K, Fe:K, Al:K, and 

Mn:K). 

Results and Discussion 

Scale and Pattern of Dieback 

Aerial surveys conducted by other investigators(http://www.brownmarsh.net, 

accessed 21 August, 2003) and us indicated that over 100 km of coastline and more than 

100,000 ha of salt marsh were affected to some degree, with 43,000 ha severely damaged. 

We examined dieback marsh from the beginning of the phenomenon (June 2000) until 

the end of the event when signs of vegetative recovery were evident (September 2001). 

Our aerial observations covered 100 km of shoreline, and ground observations were 

conducted at a total of 23 sites established in Barataria and Terrebonne basins. 

Preliminary examination of a dieback site near Bay Junop in June 2000 found 

standing dead but largely intact shoots of S. alterniflora adjacent to a healthy stand of A. 

germinans (black mangrove) (fig. 3, table 2). A narrow fringe zone (< 1 m in width) 

contained about 50 percent live shoots but transitioned abruptly into dead marsh. The 

height and density of dead stems in the dead marsh at Bay Junop were not different from 

those in the shoreline zone with live shoots, a fact suggesting that shoots had died 

suddenly and after attaining their full stature. Dead shoots exhibited no signs of damage 

other than desiccation; i.e., leaves and stems were mostly intact with no fraying, necrotic 

lesions, or visible evidence of herbivory, pathogens, or insect infestations. 

13
A

B C

Figure 3. Views of preliminary survey site at Bay Junop in June 2000. (A) Aerial view 

of dieback marsh (brown) and Avicennia germinans (black mangrove) (dark green) 

(photo by Tommy Michot). (B) Ground views of dead Spartina alterniflora (smooth 

cordgrass) (foreground) and healthy A. germinans (background). (C) Healthy seedling of 

A. germinans growing within dead marsh. 

14
Table 2. Summary of Spartina alterniflora characteristics at a dieback site near Bay 

Junop in June 2000. 

__________________________________________________________________ 

Stem Density (m-2) Stem Height (cm)

 Live Dead Live Dead 

__________________________________________________________________ 

Shoreline 290 ± 130 305 ± 75 39.1 ± 4.1 37.3 ± 2.9 

Transition 305 ± 25 510 ± 131 41.0 ± 1.0 34.6 ± 2.3 

Interior 0 ± 0 500 ± 30 - 44.4 ± 0.6 

__________________________________________________________________ 

15
A more extensive examination of 18 dieback areas in Terrebonne and Barataria 

Basins demonstrated that the pattern of marsh dieback was distinctive both on regional 

and local scales. Although dieback areas occurred in the Barataria basin, they were not 

as frequent or as large as in the Terrebonne basin, farthest from the Mississippi River. A 

consistent local pattern suggested a correlation with tidal flushing; i.e., the greatest 

mortality occurred in the marsh interior where tidal flushing of soil is minimal, while 

fringing vegetation along shorelines of bays and tidal creeks often had less than 75 

percent live shoots (figs. 4-9). However, some fringing marshes along interior bays and 

tidal creeks did experience dieback in some areas (fig. 5B). The contrast between the 50 

to 100 percent mortality of plant shoots in transition and interior zones and visually 

healthy stands along most shorelines (figs. 4-9, table 3) was similar to that observed 

earlier at Bay Junop (table 2) and indicated the widespread nature of this spatial pattern. 

Shoot condition in dieback plots indicated a more advanced state of decay by late 

summer 2000, with many shoots without leaves or reduced to stubble (fig. 9, table 3). 

Examination of rhizomes, the main perennating organs, indicated that few belowground 

structures were viable in areas with high aboveground mortality (fig. 10). The marsh 

type primarily affected was salt marsh, and only two species appeared to be involved in 

the dieback: S. alterniflora and S. patens to a lesser extent (fig. 4). Sympatic species 

such as A. germinans (fig. 11), Batis maritima L. (saltwort) (fig. 5), Distichlis spicata

(L.) Greene (seashore saltgrass), and J. roemerianus (fig. 12) survived the event and 

exhibited little or no visible signs of stress. 

16
 A

B

Figure 4. Aerial (A) and ground (B) views of a dieback Spartina patens (marshhay 

cordgrass) marsh (site 1) in August 2000. Note the spatial pattern of dieback in the 

marsh interior and healthy, green shoots along the shoreline. The arrow indicates a salt 

pan with crusts of salt on the dry soil surface. 

17
A

B

1

2

 

Figure 5. Spatial patterns of marsh dieback. Panel A illustrates an interior dead 

marsh with healthy vegetation along a tidal creek. Panel B shows Spartina 

alterniflora (smooth cordgrass) dieback along a shoreline as well as in the marsh interior. 

Arrows indicate (1) healthy Avicennia germinans (black mangrove) (dark green 

vegetation) and (2) Batis maritima (saltwort) (light green vegetation) growing in the 

midst of dead S. alterniflora. 

18
A

B

 

Figure 6. Ground views of (A) healthy shoreline and (B) dieback interior 

stands of Spartina alterniflora (smooth cordgrass) in August 2000. 

19
 

Figure 7. Relative cover (percent of total) by condition measured in the shoreline zone of 

marshes dominated by Spartina patens (marshhay cordgrass) (site 1) or S. alterniflora

(smooth cordgrass) (sites 2-21) on 3 dates. Values are the mean ± 1 SE (n = 3 plots at 

dieback sites 1-18 and at control sites 19-21). 

0%

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21

April 2001

September 2001

August - October 2000

Site 

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Condition:

Dead + Leaves

Dead Stem

Stubble

Bare Ground

Other Species

Relative Cover (%)

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April 2001

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August - October 2000

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21

April 2001

September 2001

August - October 2000

Site 

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Condition:

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Dead Stem

Stubble

Bare Ground

Other Species

Live

Condition:

Dead + Leaves

Dead Stem

Stubble

Bare Ground

Other Species

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1 2 3 4 5 6 7 8 9 10 11 13 14 15 16 17 18

April 2001

September 2001

August - October 2000

Site 

Live

Condition:

Dead + Leaves

Dead Stem

Stubble

Bare Ground

Other Species

Figure 8. Relative cover (percent of total) by condition measured in the transition zone of 

marshes dominated by Spartina patens (marshhay cordgrass) (site 1) or S. alterniflora

(smooth cordgrass) (sites 2-21) on 3 dates. Values are the mean ± 1 SE (n = 3 plots at 

dieback sites 1-18). Rel

ative Cover (%)

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April 2001

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August - October 2000

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April 2001

September 2001

August - October 2000

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Bare Ground

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Condition:

Dead + Leaves

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%)

ative Cover ( Rel

21
Live

Condition:

Dead + Leaves

Dead Stem

Stubble

Bare Ground

Other Species

0%

20%

40%

60%

80%

Figure 9. Relative cover (percent of total) by condition measured in the interior zone of 

marshes dominated by Spartina patens (marshhay cordgrass) (site 1) or S. alterniflora

(smooth cordgrass) (sites 2-21) on 3 dates. Values are the mean ± 1 SE (n = 3 plots at 

dieback sites 1-18). 

100%

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August - October 2000

April 2001

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%

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September 2001

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Condition:

Dead + Leaves

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Bare Ground

Other Species

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Condition:

Dead + Leaves

Dead Stem

Stubble

Bare Ground

Other Species

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August - October 2000

April 2001

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August - October 2000

April 2001

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40

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Relative Cover (%)

September 2001

22
23

Figure 10. Viability of Spartina alterniflora (smooth cordgrass) rhizomes in AugustOctober 2000. Rhizomes were treated with triphenyltetrazolium chloride, which stains 

live cells a dark pink color. Meristematic tissue (arrow in left panel) was examined 

under magnification to affirm staining of rapidly growing root cells. Slight 

staining of root surface (right panel) was due to presence of microflora. Percent Viable izomes Rh

Viable Nonviable

0

20

40

60

80

100

Healthy Shoreline Transition Dieback Interior

Percent Viable izomes Rh

Viable Nonviable

0

20

40

60

80

100

Healthy Shoreline Transition Dieback Interior
24

 ____________________________________________ ________________________Table 3. Results of repeated-measures ANOVA for plant variables of Spartina alterniflora (smooth cordgrass) m

and control sites where site, zone (nested within site), and time were grouping factors. Where a significant zone was found, 1 degree of freedom contrasts were conducted for healthy shoreline (HS) versus dieback interior (DI) and for DI versus control interior (CI). Dieback (D) sites (1-18) were contrasted with control sites (19-21) where effect was found. Values are the F-ratio, and significance is indicated by * (p ≤ 0.05), ** (p ≤ 0.01), *** (p ≤ 0.

0.0001), or ns (not significant). __________________________________________________________________________________________

Source of Variation: Contrasts: 

 Site Zone(Site) Time Zone*Time HS vs DI HS vs T DI __________________________________________________________________________________________Relative Cover by Shoot Condition:%Live 21.03**** 67.23  8.10** ** 5.65  5.66**  67.25**** 1%Stubble 24.50  20.72** ** 136.8  15.46** ** 171.1**** 16.21* 41%Dead stem 11.76**** 6.72  85.44** ** 6.27  21.22** ** 34.55  0%Dead with leaves 15.48** * 10.46**** 35.06  7.68** ** 3.12* 11.87**** 16%Bare ground 14.46  57.43** ** 34.33  4.51** ** 21.35  12.78** ** 10%Other species 8.82  3.41** ** 8.70*** 2.97* 2.80ns 4.04* 3Leaf Elemental Ratios:Na:K 29.47**** 12.84  93.24** ** 7.66  11.02**  3.84* 3Fe:K 234.3**** 157.1  235.8** ** 56.38*** 218.8  0.785ns 46Al:K 58.74** ** 44.62  250.6** ** 61.43*** 276.4  0.183ns 60Mn:K 56.93** ** 20.28** 94.08  18.30** ** 19.61  10.90** ** 4__________________________________________________________________________________________
A. August 2000

B. September 2001

Figure 11. Aerial view of Pelican Island, La. showing healthy Avicennia germinans

(black mangrove) stands alongside dying Spartina alterniflora (smooth 

cordgrass) marsh (brown color in A). Note transformation of S. alterniflora 

marsh to mudflat (B) one year after dieback. 

25
Figure 12. Aerial (A) and ground (B) views of healthy Juncus roemerianus (needlegrass 

rush) patches, indicated by arrows in upper panel, growing within areas of dead Spartina 

alterniflora (smooth cordgrass). Inset photograph in panel B illustrates a healthy but 

shallow root system of J. roemerianus excavated from the patch shown above. 

A

B

26
Physicochemical Conditions

Bay Junop 

Salinity varied from 27 to 32 ppt across healthy and dieback marsh zones at Bay 

Junop and was similar to that in the bay water and slightly higher than at Old Oyster 

Bayou (OOB) (fig. 13, table 4). The highest salinity (46 ppt) was found in the healthy 

mangrove zone (fig. 13). Soil pH remained near neutrality in all vegetation zones (fig. 

13). Soil bulk density varied from 0.3 to 0.5 g cm-3 across marsh and mangrove zones 

and was higher on average than that at OOB (fig. 13). Soils became more reducing (i.e., 

less oxidized) with higher concentrations of sulfide with distance from open water (fig. 

13). 

Intensive Sites 1-21 

Soil physicochemical characteristics varied over sites, zones, and time (figs. 14-

15, tables 5-7). Averaged over site and time, salinity varied from 20 ppt in shoreline 

control marshes to 38 ppt in regenerating marshes. Although significant, salinity was 

only slightly higher in interior dieback marshes (30 ppt) compared to adjacent shorelines 

with live vegetation (28 ppt) and was well within the tolerance limits of S. alterniflora

(Hester and others, 1996). Porewater pH was near neutrality at all S. alternifloradominated marshes but was low (~4) in the interior dieback marsh at site 1 (dominated by 

S. patens). 

Soils at S. alterniflora sites were generally more reducing with higher sulfide 

concentrations in the marsh interior where the vegetation was dead compared to that at 

the shoreline. However, sulfide concentrations in interior dieback zones were similar to 

those at control marshes. Also, high redox potentials and low sulfide concentrations at 

the dieback site dominated by S. patens (site 1) indicated oxidizing conditions. Soil bulk 

density was lower and organic matter content was higher at control marshes compared to 

that at the dieback marshes (tables 6-7). 

Soil Elemental Concentrations 

Many of the soil elemental concentrations measured in August-October 2000 

(tables 8-9) had correlations that exceeded 0.5, a fact indicating that factor analysis could 

be used to assess patterns at dieback and control marshes. The first two eigenvalues for 

water- and acetate-extractable elements exceeded unity and explained 68 and 63 percent, 

respectively, of the variation in the overall data. For water-extractable elements, the first 

factor accounted for 53 percent of the total variation in the data and included elements 

associated with seawater Ca, K, Mg, Na and S (table 10). The second factor, which 

accounted for an additional 15 percent of the variation, had high loadings of Al, Fe, and P 

(table 10). For acetate-extractable elements, the first factor, had high loadings of Ca, K, 

Mg, Na, S, and Al and accounted for 48 percent of the total variation (table 10). The 

second factor, which accounted for an additional 15 percent of the variation, included two 

redox elements, Fe and Mn (table 10). 

27
28

Figure 13. Soil conditions measured at Bay Junop site in June 2000. Transects traversed 

a dieback salt marsh dominated by Spartina alterniflora and terminated in a healthy 

mangrove stand. Solid lines indicate average value measured at a control marsh (Old 

Oyster Bayou). 

20

25

30

35

40

45

50

0 1 5 1 0 1 5 30 45 60 75

0

0.5

1

1.5

0 1 5 10 15 30 45 60 75

5

6

7

8

9

0 1 5 10 15 30 45 60 75

-250

-150

-50

50

150

0 1 5 1 0 1 5 30 45 60 75

0

0.1

0.2

0 1 5 10 15 30 45 60 75

0.3

0.4

0.5

0.6

Salinity (‰) pH

nsity

-3)

Bulk De

(g cm Eh (mV)

1 cm

15 cm

Sulfide (mM)

Distance from Shoreline (m)

20

25

30

35

40

45

50

0 1 5 1 0 1 5 30 45 60 75

0

0.5

1

1.5

0 1 5 10 15 30 45 60 75

5

6

7

8

9

0 1 5 10 15 30 45 60 75

-250

-150

-50

50

150

0 1 5 1 0 1 5 30 45 60 75

0

0.1

0.2

0 1 5 10 15 30 45 60 75

0.3

0.4

0.5

0.6

Salinity (‰) pH

nsity

-3)

Bulk De

(g cm Eh (mV)

1 cm

15 cm

Sulfide (mM)

Distance from Shoreline (m)
29

Table 4. ANOVA results for physicochemical variables measured at Bay Junop (BJ) and Old Oyster Bayou (OOwhere site (BJ and OOB) and zone within site (healthy shoreline (HS), dieback interior (DI), and mangrove (M)) 

Values are the F-ratio or t-value (1 degree of freedom contrasts); significance is indicated by * (p ≤ 0.05), ** (p ≤ 0.001),   (p ≤ 0.0001), or ns (not significant). 

__________________________________________________________________________________________

Source of Variation: Contrasts: 

 _______________________ ________________________________________ 

Site Zone (Site) HS vs DI HS vs M DI vs M 

__________________________________________________________________________________________Soil Variables:Bulk density 22.13** * 3.16ns - - - Eh1 12.87** 16.81  -9.55** ** 6.07  0.408ns Eh15 3.72ns 9.13** * -5.30*** 3.31** 0.982ns Moisture 6.37* 2.49ns - - - Salinity 361**** 153  8.25** ** -35.2  -29.8** ** pH 29.09  13.7** ** 7.72  -1.46ns 3.29**  Sulfide - 45.49**** 16.15  -0.487ns 9.44** ** __________________________________________________________________________________________
-300

-200

-100

0

100

200

300

400

500

600

700

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22

-300

-200

-100

0

100

200

300

400

500

600

700

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22

-300

-200

-100

0

100

200

300

400

500

600

700

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22

08/00 Shoreline

04/01 Shoreline

09/01 Shoreline

08/00 Transition

04/01 Transition

09/01 Transition

08/00 Interior

04/01 Interior

09/01 Interior

Site

A.

B.

C.

Date/Zone:

Figure 14. Soil redox potential measured at 1 cm (A), 15 cm (B), and 30 cm (C) depths 

in shoreline, transition, and interior zones of marshes dominated by Spartina patens

(marshhay cordgrass) (site 1) or S. alterniflora (smooth cordgrass) (sites 2-21) on 3 dates. 

Values are the mean ± 1 SE (n = 6 at dieback sites 1-18 and at control sites 19-21) in 

August 2000 (08/00) and April 2001 (04/01), and n = 3 in September 2001(09/01). Redox

Potential (mV)

-300

-200

-100

0

100

200

300

400

500

600

700

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22

08/00 Shoreline

04/01 Shoreline

09/01 Shoreline

08/00 Transition

04/01 Transition

09/01 Transition

08/00 Interior

04/01 Interior

09/01 Interior

-300

-200

-100

0

100

200

300

400

500

600

700

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22

-300

-200

-100

0

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300

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700

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22

08/00 Shoreline

04/01 Shoreline

09/01 Shoreline

08/00 Transition

04/01 Transition

09/01 Transition

08/00 Interior

04/01 Interior

09/01 Interior

Site

A.

B.

C.

Date/Zone:

 (mV) Redox

Potential

30
31

Figure 15. Soil salinity (A), pH (B), and sulfide (C) measured in shoreline, transition, 

and interior zones of marshes dominated by Spartina patens (marshhay cordgrass) (site 1) 

or S. alterniflora (smooth cordgrass) (sites 2-21) on 3 dates. Values are the mean ± 1 SE 

(n = 3 plots at dieback sites 1-18 and at control sites 19-21) in August 2000 (08/00) and 

April 2001 (04/01), and single observations in September 2001(09/01). 

0

10

20

30

40

50

0123456789

10111213141516171819202122

2

3

4

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8

0123456789

10111213141516171819202122

9

0

1

2

3

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5

6

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22

08/00 Shoreline

04/01 Shoreline

09/01 Shoreline

08/00 Transition

04/01 Transition

09/01 Transition

08/00 Interior

04/01 Interior

09/01 Interior

pH Sulfide (mM)

Site

A.

B.

C.

Date/Zone:

Salinity (‰)

0

10

20

30

40

50

0123456789

10111213141516171819202122

2

3

4

5

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7

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0123456789

10111213141516171819202122

0

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6

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22

08/00 Shoreline

04/01 Shoreline

09/01 Shoreline

08/00 Transition

04/01 Transition

09/01 Transition

08/00 Interior

04/01 Interior

09/01 Interior

08/00 Shoreline

04/01 Shoreline

09/01 Shoreline

08/00 Transition

04/01 Transition

09/01 Transition

08/00 Interior

04/01 Interior

09/01 Interior

pH Sulfide (mM)

Site

A.

B.

C.

Date/Zone:

Salinity (‰)
Table 5. Results of a repeated-measures ANOVA for soil variables measured at Spartina alterniflora (smooth co

dieback sites (2-18) where site, zone (healthy shoreline [HS], transition [T], dieback interior [DI]), and time (Aug

September 2001) were grouping factors. 1 df contrasts are given where a significant Zone or Zone*Time effect ware the F-ratio, and significance is indicated by * (p 

≤ 0.05), ** (p 

≤ 0.01), *** (p 

≤ 0.001), **** (p 

≤ 0.0001), o

__________________________________________________________________________________________

Source of Variation: Contrasts:

 ___________________________________________________ _______________

Site Zone Time Zone*Time HS 

__________________________________________________________________________________________Soil Variables:Bulk density 46.60**** 7.12** 8.09** 3.24* 8.22** 12Eh1 14.23  1.83ns 115.6** ** 1.05ns - Eh15 8.45  1.76ns 24.40** ** 2.17ns - Eh30 6.10  6.98**  19.81**** 1.29ns 7.08** 12Salinity 34.32  9.17**  41.60**** 2.98* 14.64** 12pH 1.14ns 0.57ns 47.28  0.74ns - Sulfide 6.31** * 0.11ns 52.77**** 3.82* 8.91** 3__________________________________________________________________________________________

32
Table 6. Summary of physicochemical conditions (averaged over site and time) measured in stands dominated by 

alterniflora. Values are given by site; zone (shoreline [S], transition [T], or interior [I]); and vegetative condition [D], regenerating [R], or control [C)]. Values are the mean and (1 SE). __________________________________________________________________________________________Site Zone Condition n pH Salinity Sulfide Eh1 Eh15 Eh30 Bulk D (‰) (mM) (mV) (mV) (mV) (g/c__________________________________________________________________________________________2-18 S H 306 7.03 28 0.947 29 -69 -118 0.3 (0.04) 1 (0.107) (10) (8) (5) (0.02-18 T D 306 7.24 30 1.338 -8 -113 -162 0.3 (0.03) 1 (0.106) (10) (7) (4) (0.02-18 I D 306 7.25 31 1.312 19 -95 -156 0.3 (0.04) 1 (0.104) (10) (7) (4) (0.015-16 S D 36 6.77 31 0.129 237 32 -126 0.3 (0.09) 1 (0.021) (31) (27) (30) (0.019-21 S C 54 6.79 20 0.863 90 -5 -129 0.2 (0.06) 1 (0.267) (35) (21) (10) (0.019-21 I C 54 7.05 23 1.579 -70 -116 -154 0.1 (0.04) 1 (0.202) (17) (13) (8) (0.04-6 I R 18 7.32 38 0.565 136 -78 -140 -  (0.07) 1 (0.169) (28) (25) (20) __________________________________________________________________________________________

33
Table 7. Summary of physicochemical conditions (averaged over site and time) measured in stands dominated by 

(needlegrass rush) and Spartina patens(marshhay cordgrass). Values are given by dominant species, site; zone (stransition [T], or interior [I]); and vegetative condition (healthy [H] or dieback [D]). Values are the mean and (1 __________________________________________________________________________________________Site Zone Condition n pH Salinity Sulfide Eh1 Eh15 Eh30 Bulk D (‰) (mM) (mV) (mV) (mV) (g/c__________________________________________________________________________________________

Juncus roemerianus 

16 I H 24 7.11 31 1.217 189 -116 -169 0.2 (0.11) 0 (0.178) (42) (31) (8) (0.0Spartina patens 1 S H 18 6.57 23 0.014 371 208 115 0.4 (0.22) 1 (0.011) (52) (51) (34) (0.01 T D 18 5.41 26 0.026 207 304 200 0.5 (0.54) 1 (0.026) (57) (80) (40) (0.01 I D 18 4.29 30 0.005 213 303 229 0.5 (0.37) 2 (0.005) (62) (73) (45) (0.0__________________________________________________________________________________________

34
Table 8. Water-extractable soil elemental concentrations (mg/kg) measured in August 2000 at dieback and refere

dominated by Spartina spp. Values are given by site (1-20), zone (shoreline [S], transition [T], or interior [I]); ancondition (healthy [H], dieback [D], regenerating [R]) or control [C]). Three plots were sampled at each site/zon

combination. Values are the mean and (1 SE). 

__________________________________________________________________________________________Site Zone Condition n Al Ca Cu Fe K Mg Mn Na P 1-18 S H 54 0.56 436.6 0.013 0.25 321.0 808.3 0.85 8067 4.63  (0.14) (30.5) (0.003) (0.10) (19.0) (70.0) (0.17) (687) (0.661-18 T D 54 0.73 463.0 0.004 0.90 295.0 868.6 1.66 8208 5.39  (0.25) (19.1) (0.001) (0.43) (10.1) (42.1) (0.38) (509) (0.931-18 I D 54 0.67 482.8 0.021 0.13 430.3 1285.6 0.93 13251 6.41  (0.23) (24.6) (0.004) (0.06) (34.4) (129.9) (0.20) (1386) (1.186 I R 3 0.05 680.7 0.005 0.16 553.3 . 0.76 . 0.04  (0.02) (29.0) (0.001) (0.08) (31.7) . (0.36) . (0.0119-20 I C 6 1.46 618.9 0.070 0.00 799.3 2138.3 0.93 19430 20.99  (0.39) (65.9) (0.016) (0.00) (103.3) (237.8) (0.23) (2397) (1.87__________________________________________________________________________________________

35
Table 9. Ammonium acetate-extractable soil elemental concentrations (mg/kg) measured in August 2000 at dieba

marshes dominated by Spartina spp. Values are given by site (1-20), zone (shoreline [S], transition [T], or intericondition (healthy [H], dieback [D], regenerating [R] or control [C]). Three plots were sampled at each site/zone

combination. Values are the mean and (1 SE). Note that some elements were below detection limits (<DL). 

Site Zone Condition n Al Ca Cu Fe K Mg Mn Na P 1-18 S H 54 0.90 777.6 0.116 0.198 791.83 1306.7 3.65 4474 0.84  (0.11) (54.6) (0.020) (0.063) (57.67) (97.9) (0.70) (442) (0.101-18 T D 54 0.84 838.1 0.059 0.164 749.75 1212.6 4.52 4201 0.64  (0.08) (78.3) (0.012) (0.053) (62.81) (114.3) (0.66) (454) (0.061-18 I D 54 0.99 914.0 0.150 0.173 815.27 1488.9 4.29 4588 1.30  (0.11) (76.4) (0.041) (0.060) (61.13) (124.2) (0.68) (491) (0.226 I R 3 0.34 447.1 <DL <DL 536.84 870.3 6.45 2787 0.50  (0.11) (42.0) (23.05) (77.1) (0.78) (239) (0.2219-20 I D 6 2.01 1672.8 0.340 0.100 887.62 2680.0 3.06 7835 4.64  (0.22) (181.1) (0.083) (0.050) (48.98) (179.6) (0.42) (1097) (2.67__________________________________________________________________________________________

36
37

Table 10. Factor analysis of soil elemental concentrations measured at all sites (1-21) and zones (healthy shoreli[T], dieback interior [DI], dieback shoreline [DS], regenerating [R], control shoreline [CS], control interior [CI]) 

Principal component analysis was used to reduce the number of variables and multicollinearity. The rotated factwater- and ammonium acetate-extracted elements (see text for interpretation of factors). Individual factor scores ANOVA with site and zone (nested within site) as grouping factors. F-ratios for main effects and 1 DF contrasts 

factor patterns; significance is indicated by * (p ≤ 0.05), ** (p ≤ 0.01), *** (p ≤ 0.001),   (p ≤ 0.0001), or ns 

____________________________________________________________________________________ Extraction: Water Ammonium Acetate  ___________________________ _________________________ 

Factor 1 Factor 2 Factor 1 Factor 2 

____________________________________________________________________________________ Al 0.41 0.70 0.70 0.46 Ca 0.96 0.07 0.84 -0.17 Cu 0.18 0.51 0.50 -0.16 Fe -0.10 0.68 0.02 0.78 K 0.97 0.00 0.87 -0.20 Mg 0.98 0.09 0.93 -0.16 Mn 0.63 0.07 -0.27 0.79 Na 0.96 0.05 0.91 -0.20 P -0.05 0.57 0.56 0.13 S 0.98 0.06 0.73 0.18 Zn 0.68 0.34 0.65 -0.15 Cumulative percent of variance explained 53 68 48 63 ANOVA Source:Site 22.31** ** 3.10  38.18** ** 6.05  Zone (site) 1.01ns 1.84**  2.27*** 1.41ns Contrasts:HS vs. DI - 1.82ns 10.23** - HS vs. T - 4.71* 0.11ns - DI vs. CI - 5.38* 76.22**** - Dieback vs. Control29.84**** 4.95* 103.8**** 0.18ns _____
38

Water and acetate-extractable factor patterns for soil elements measured in April 

2001 (tables 11-13) were similar to those found in August 2000. Acid-extractable 

elements were additionally measured in April 2001, and two factors were extracted from 

the data: the first factor with high loadings of Ca, K, Mg, Na, and S accounted for 49 

percent of total variation and a second factor (23 percent of total variation) with high 

loadings of Al, Cu, and Fe (table 14). When factor scores were subjected to ANOVA, 

significant differences were found between dieback and control marshes for the factor 

with high loadings of metals (acid-extractable Al and Fe) (table 14). When acidextractable concentrations of Fe and Al were individually assessed, values were higher at 

dieback marshes compared to control marshes but were not different between healthy 

shoreline and dieback interior zones within dieback marshes (fig. 16, table 15). 

Similarly, pyrite concentrations were significantly higher at dieback compared to those at 

control marshes, but they were not different across zones within dieback marshes (fig. 17, 

table 15). These results demonstrate that the potential for soil acidification and release of 

potentially toxic metals such as Fe and Al was significantly higher at dieback marshes, 

but was similar within dieback marshes. The similarity of values within dieback marshes 

(i.e., shoreline, transition, and dieback interior) further suggests that if acidification 

and/or toxic metals caused plant mortality, spatial variation in hydrology or some other 

factor may have ameliorated effects along shorelines where plants were frequently 

inundated by tides. 

Soil Acidification Potential 

When soils from dieback marshes were subjected in the laboratory to oxidizing 

conditions, pH decreased significantly from near neutrality to 4.7 in 23 d, whereas soils 

from control marshes did not change significantly (fig. 18). The pH of both dieback and 

control soils maintained under reducing conditions remained above 6.5. These results 

confirmed the potential for acidification of dieback soils. 

Plant Elemental Concentrations 

Elemental concentrations in leaf tissues were also examined to determine if plant 

uptake of potentially toxic metals or cations was higher in dieback marshes (tables 16-

19). As for soil concentrations, factor analysis was used to reduce the number of 

variables and problems of multicollinearity. For elements measured in S. alterniflora

leaves in August-October 2000 and April 2001, three factors were extracted from the 

data, whereas four factors were extracted from the September 2001 data (table 20). 

Factor patterns of elements generally reflected processes controlling external soil 

concentrations and plant uptake. In August 2000, plant elemental composition was 

dominated by three factors: a factor dominated by divalent cations (Ca2+, Mg2+), a second 

factor dominated by metals (Fe2+, Al3+), and a third factor with a high loading of Na+

. In 

April 2001, the same factor patterns were extracted from the data, except that Al, Fe, and 

Zn negatively loaded on factor 1 and that K loaded with Na on factor 3. In September 

2001, three factors were extracted that were similar to previous dates plus a fourth factor 

with high loading of K+

. 
Table 11. Water-extractable soil elemental concentrations (mg/kg) measured in April 2001 at dieback and referen

dominated by Spartina spp. Values are given by site (1-21), zone (shoreline [S], transition [T], or interior [I]), ancondition (healthy [H], dieback [D], regenerating [R] or control [C]). Three plots were sampled at each site/zone

combination. Values are the mean and (1 SE). Note that some elements were below detection limits (<DL). 

Site Zone Condition n Al Ca Cu Fe K Mg Mn Na P 1-18 S H 48 1.01 492.9 0.07 0.37 521.2 1202 2.91 11358 0.70  (0.20) (62.7) (0.01) (0.14) (61.9) (142) (0.54) (1233) (0.091-18 T D 48 0.49 421.1 0.06 0.17 485.7 1074 2.55 10419 0.74  (0.09) (65.6) (0.01) (0.06) (66.4) (145) (0.67) (1329) (0.071-18 I D 54 0.82 524.1 0.07 0.20 563.8 1326 5.38 12546 0.79  (0.14) (73.8) (0.01) (0.09) (69.9) (177) (1.69) (1568) (0.1015-16 S D 6 0.65 527.7 0.09 <DL 611.5 1221 6.66 11945 0.26  (0.14) (53.6) (0.02) (47.4) (130) (1.36) (1125) (0.0619-21 S C 9 1.73 767.4 0.02 0.48 720.9 1532 6.47 15509 1.50  (0.46) (119.4) (0.00) (0.35) (69.4) (185) (1.67) (1497) (0.4419-21 I C 9 2.33 1088.9 0.08 <DL 958.0 2351 3.12 23289 1.93  (0.71) (171.2) (0.01) (152.4) (439) (1.00) (3689) (0.404-6 I R 9 0.37 181.87 0.09 0.27 240.9 583 0.82 5856 1.20  (0.19) (28.5) (0.02) (20.2) (52) (0.43) (348) (0.15___________________________________________________________________________________________________

39
Table 12. Ammonium acetate-extractable soil elemental concentrations (mg/kg) measured in April 2001 at dieba

marshes dominated by Spartina spp. Values are given by site (1-21), zone (shoreline [S], transition [T], or intericondition (healthy [H], dieback [D], regenerating [R] or control [C]). Three plots were sampled at each site/zone

combination. Values are the mean and (1 SE). Note that some elements were below detection limits (<DL). 

_____________________________________________________________________________________________________________Site Zone Condition n Al Ca Cu Fe K Mg Mn Na P 1-18 S H 48 0.33 892.6 <DL 0.22 702.0 1314 3.53 3963 3.98  (0.04) (63.8) (0.07) (52.6) (100) (0.54) (394) (0.491-18 T D 48 0.30 899.2 <DL 0.11 719.4 1293 4.56 3722 3.68  (0.04) (65.9) (0.05) (47.9) (103) (0.92) (358) (0.561-18 I D 54 0.44 1083.1 <DL 0.33 717.6 1433 5.43 4449 4.27  (0.07) (127.1) (0.14) (52.5) (131) (0.91) (509) (0.6315-16 S D 6 0.54 719.0 <DL 0.15 835.9 1434 5.70 4706 4.26  (0.24) (92.5) (0.09) (98.4) (162) (1.50) (984) (0.4719-21 S C 9 0.70 2035.8 <DL 0.46 1245.6 2614 6.97 9485 6.92  (0.17) (336.5) (0.04) (53.0) (122) (1.12) (633) (0.7719-21 I C 9 1.01 2239.4 <DL <DL 1594.0 3019 4.82 10267 9.28  (0.36) (489.5) (136.8) (483) (0.91) (2265) (1.504-6 I R 9 0.21 624.2 <DL <DL 502.2 851 2.25 2334 1.50  (0.04) (94.1) (30.8) (59) (0.91) (217) (0.20___________________________________________________________________________________________________

40
Table 13. Acid-extractable soil elemental concentrations (mg/kg) measured in April 2001 at dieback and referenc

by Spartina spp. Values are given by site (1-21), zone (shoreline [S], transition [T], or interior [I]), and vegetativ

[H], dieback [D], regenerating [R] or control [C]). Three plots were sampled at each site/zone/condition combin

mean and (1 SE). _____________________________________________________________________________________________________________Site Zone Condition n Al Ca Cu Fe K Mg Mn Na P __________________________________________________________________________________________1-18 S H 48 19.70 1205 0.25 8.06 1328 2577 10.28 23219 1.1 (4.82) (99) (0.03) (3.13) (91) (197) (1.43) (1573) (0.11-18 T D 48 12.33 1184 0.22 7.24 1394 2625 11.62 23921 2.2 (4.33) (114) (0.03) (3.23) (99) (227) (2.00) (1849) (0.91-18 I D 54 22.82 1417 0.33 12.23 1525 3165 14.29 27849 1.1 (6.63) (172) (0.07) (4.32) (139) (371) (2.75) (2783) (0.215-16 S D 6 79.82 996 0.45 30.23 1313 2325 14.53 21556 0.1 (39.64) (69) (0.20) (17.60) (67) (153) (3.83) (906) (0.119-21 S C 9 4.50 1779 0.28 0.03 1859 3681 21.18 33149 3.1 (1.29) (221) (0.05) (0.03) (147) (362) (4.03) (2771) (0.419-21 I C 9 4.34 3267 0.58 0.02 2605 5812 17.68 51633 6.5 (0.62) (889) (0.20) (0.02) (269) (773) (6.04) (6579) (2.34-6 I R 9 2.23 765 0.23 0.45 1077 1823 6.66 18056 0.7 (0.82) (88) (0.04) (0.21) (93) (158) (2.16) (1241) (0.2___________________________________________________________________________________________________

41
42

Table 14. Factor analysis of soil elemental concentrations measured at all sites (1-21) and zones (healthy shoreli[T], dieback interior [DI], dieback shoreline [DS], regenerating [R], control shoreline [CS], control interior [CI]) Principal component analysis was used to reduce the number of variables and multicollinearity. The rotated factwater-, ammonium acetate-, and acid-extracted elements. Individual factor scores were subjected to ANOVA wi(nested within site) as grouping factors. F-ratios for main effects and 1 DF contrasts are given below the factor pindicated by * (p ≤ 0.05), ** (p ≤ 0.01), *** (p ≤ 0.001),   (p ≤ 0.0001), or ns (not significant). ___________________________________________________________________________________________________Extraction: Water Ammonium Acetate Acid  ____________________ _____________________ ________________

Factor 1 Factor 2 Factor 1 Factor 2 Factor 1 

Al 0.41 0.70 0.38 0.57 -0.12 Ca 0.96 0.07 0.87 0.10 0.94 Cu 0.18 0.51 - - 0.20 Fe -0.10 0.68 - - -0.13 K 0.97 0.00 0.81 0.37 0.98 Mg 0.98 0.09 0.91 0.27 0.98 Mn 0.63 0.07 0.10 0.84 0.52 Na 0.96 0.05 0.86 0.38 0.96 P -0.05 0.57 0.82 0.17 0.15 S 0.98 0.06 0.61 0.62 0.98 Zn 0.68 0.34 0.15 0.65 0.56 Cumulative percent of variance explained 53 68 46 71 49 ANOVA Source:Site 22.31** ** 3.10  27.04** ** 9.55  30.77** ** Zone (site) 1.01ns 1.84** 1.89** 2.05** 1.94** 

___________________________________________________________________________________________________

Contrasts:

HS vs. DI - 1.82ns 0.39ns 2.29ns 5.23** HS vs. T - 4.71* 0.56ns 0.38ns 0.09ns DI vs. CI - 5.38* 104.5**** 5.03* 74.73**** Dieback vs. Control 29.84**** 4.95* 165.4**** 16.14  113.4** ** 
43

Figure 16. Acid-extractable Al and Fe concentrations in different vegetation zones in 

April 2001. Marsh zones dominated by Spartina spp. are healthy shoreline (HS), 

transition (T), dieback interior (DI), regenerating (R), control shoreline (CS), and 

control interior (CI). Values are the mean ± 1 SE (n = 47-54, HS, T, DI; n = 8-9, R, 

CS, CI). 

0

5

10

15

20

25

30

35

HS T DI R CS CI

Al

Fe

Concentration (mg kg-1 dry soil)

Dieback Control

0

5

10

15

20

25

30

35

HS T DI R CS CI

Al

Fe

Concentration (mg kg-1 dry soil)

Dieback Control
44

 ________ __________ __________________________________________Pyrite 12.28  4.38** ** 0.0069ns 2.36ns 12.71*** 14.81**Fe 6.30  1.78**  0.0051ns 1.54ns 9.63** 18.05**Al 5.95**** 1.70* 0.3029ns 3.16ns 0.51ns 1.10ns Table 15. Results of ANOVA for soil pyrite (August 2000), and acid extractable Al and Fe concentrations (Apri(healthy shoreline [HS], transition [T], dieback interior [DI], dieback shoreline [DS], control shoreline [CS], con

regenerating [R], and Juncus roemerianus [needlegrass rush]) where site and zone (nested within site) were the gValues are the F-ratio, and significance is indicated by * (p ≤ 0.05), ** (p ≤ 0.01), *** (p ≤ 0.001), **** (p ≤ 0.0

significant). ____________________________________________________________________________________________________________________________________________________________________________________

HS vs DI HS vs T DI vs CI Dieback vs 

ANOVA Source: Site Zone(Site) Contrasts 
0

1000

2000

3000

4000

5000

6000

HS T DI R DS CI J

0

1000

2000

3000

4000

5000

6000

HS T DI R DS CI J

e g (mg F -1) Pyri

Figure 17. Soil pyrite concentrations in different vegetation zones in August 2000. 

Marsh zones dominated by Spartina spp. are healthy shoreline (HS), transition (T), 

dieback interior (DI), regenerating (R), dieback shoreline (DS), and control interior (CI). 

Two stands of healthy Juncus roemerianus (J) were sampled in the dieback interior at 

site16. Values are the mean ± 1 SE (n = 46-51, HS, T, DI; n = 6, R, DS, CI; n = 2, J). te g F -1) e g (m Pyri

45
Figure 18. Acidification potential of soils from dieback and control marshes. Soil was 

collected in August 2000 and incubated under reduced (purged with N2 gas) or oxidized 

(purged with air) treatments. 

gure 18. Acidification potential of soils from dieback and control marshes. Soil was 

collected in August 2000 and incubated under reduced (purged with N2 gas) or oxidized 

(purged with air) treatments. 

4

5

6

7

8

9

0 5 10 15 20 25

pH

46

Time (d)

4

5

6

7

8

9

0 5 10 15 20 25

pH

Time (d)

Control/Reduced

Control/Oxidized

Dieback/Reduced

Dieback/Oxidized
Table 16. Elemental concentrations (μg/g) measured in Spartina alterniflora (smooth cordgrass) leaves in Augus

reference marshes. Values are given by site (2-21); zone (shoreline [S], transition [T], or interior [I]); and vegeta

(healthy [H], dieback [D], regenerating [R], or control [C]). Three plots were sampled at each site/zone/conditio

Values are the mean and (1 SE). 

__________________________________________________________________________________________Site Zone Condition n Al Ca Cu Fe K Mg Mn Mo Na Ni P 2-18 S H 51 91.71 3007 4.32 204.86 10534 2893 55.95 1.49 18492 41.39 1164  (13.45) (257) (0.97) (27.34) (449) (212) (10.21) (0.24) (416) (21.25 (582-18 T D 48 71.11 2871 3.80 179.61 11285 2809 35.16 1.22 20257 54.91 1057  (5.71) (189) (0.79) (25.44) (641) (223) (2.96) (0.26) (734) (34.88 (582-18 I D 48 164.63 2862 3.32 276.75 9957 2912 43.48 1.02 19890 15.39 942  (20.19) (175) (0.67) (29.19) (774) (203) (4.83) (0.17) (960) (9.91 (4715-16 S D 6 185.34 2505 2.04 314.71 12055 2584 14.53 0.82 9728 2.35 1230  (26.74) (114) (0.33) (98.19) (139) (452) (2.18) (0.14) (1677) (1.16 (3819-21 I C 9 146.25 2137 1.17 127.93 9485 3034 35.42 0.55 16493 0.55 805  (44.77) (367) (0.27) (30.22) (753) (608) (14.82) (0.05) (480) (0.13 (356 I R 3 111.60 2715 4.33 155.79 13411 1705 68.03 1.24 16855 . 1649  (5.90) (293) (0.78) (2.56) (269) (256) (4.16) (0.09) (1228) . (54

47
Table 17. Elemental concentrations (μg/g) measured in Spartina alterniflora (smooth cordgrass) leaves in April 2

reference marshes. Values are given by site (2-21); zone (shoreline [S], transition [T], or interior [I]); and vegeta

(healthy [H], dieback [D], regenerating [R], or control [C]). Three plots were sampled at each site/zone/conditio

Values are the mean and (1 SE). 

Site Zone Condition n Al Ca Cu Fe K Mg Mn Mo Na Ni P 2-18 S H 51 97.27 2505 3.39 127.50 12665 3029 60.50 0.63 17822 2.15 1450  (4.58) (98) (0.12) (7.96) (310) (95) (4.37) (0.05) (586) (0.21) (43) 2-18 T D 48 119.48 2529 3.35 194.53 12151 3080 73.87 0.45 16712 2.49 1299  (7.63) (85) (0.17) (19.10) (236) (114) (5.89) (0.05) (596) (0.22) (34) 2-18 I D 52 148.27 3317 4.12 217.88 12955 3655 84.17 0.52 18218 2.10 1502  (8.47) (91) (0.23) (15.84) (232) (108) (6.67) (0.06) (600) (0.21) (56) 15-16 S D 6 143.30 2725 5.81 225.93 13684 2772 98.48 0.23 17381 1.87 1655  (21.04) (237) (0.35) (26.24) (837) (159) (19.18) (0.10) (951) (0.59) (74) 19-21 S C 9 46.36 2769 3.94 98.84 13883 3082 86.79 0.46 17166 2.51 1232  (2.46) (389) (0.42) (10.47) (720) (390) (18.07) (0.06) (1210) (0.35) (118) 19-21 I C 9 38.90 2289 4.38 98.48 15951 2734 43.28 0.72 20368 2.36 1121  (1.58) (183) (0.56) (30.61) (622) (186) (6.26) (0.07) (1754) (0.17) (52) 6 I R 3 137.42 2763 3.71 196.42 11339 3682 133.76 0.65 14698 1.42 1416  (4.07) (162) (0.09) (40.03) (125) (100) (3.98) (0.11) (1057) (0.41) (9) __________________________________________________________________________________________

48
Table 18. Elemental concentrations (μg/g) measured in Spartina alterniflora (smooth cordgrass) (sites 2-21) and 

cordgrass) (site 1) leaves in September 2001 at dieback and reference marshes. Values are given by site (1-21); ztransition [T], or interior [I]); and vegetative condition (healthy [H], dieback [D], regenerating [R], or control [C]

sampled at each site/zone/condition combination. Values are the mean and (1 SE). ___________________________________________________________________________________________________Site Zone Condition n Al Ca Cu Fe K Mg Mn Mo Na 

Spartina alterniflora 2-18 S H 48 77.86 3439 5.08 92.82 11512 4022 30.28 1.59 14657 1 (3.83) (174) (0.25) (6.92) (371) (164) (3.26) (0.11) (416) 2-18 T D 48 93.99 3601 4.86 137.73 11301 3896 32.38 1.61 13596  (8.20) (105) (0.27) (22.02) (259) (140) (3.65) (0.10) (377) 2-18 I D 51 92.49 4161 6.05 151.77 10924 4045 40.60 1.44 15127 1 (6.39) (164) (0.49) (15.34) (327) (141) (6.46) (0.11) (544) 19-21 S C 9 44.23 3563 4.60 68.77 10770 3842 54.14 0.98 14124 1 (4.52) (552) (0.22) (4.13) (851) (568) (18.03) (0.11) (1243) (19-21 I C 9 61.23 2933 5.25 59.84 13085 3837 10.59 0.84 15138  (9.30) (129) (0.51) (7.04) (711) (200) (1.01) (0.16) (678) 6 I R 3 125.77 3305 5.52 125.33 7641 4364 22.14 1.28 15252  (10.63) (119) (1.08) (7.94) (130) (189) (2.29) (0.24) (205) S. patens 1 S H 3 28.57 2634 3.93 69.47 4830 1014 70.83 2.24 5386  (0.92) (221) (0.39) (8.93) (1045) (79) (6.98) (0.00) (544) 1 T D 3 30.63 1946 2.98 83.70 7709 1131 94.50 2.24 5205  (0.57) (90) (0.07) (2.07) (811) (158) (8.49) (0.00) (474) 1 I D 3 34.07 2154 3.21 99.33 8351 1396 118.47 2.25 7309 

 (2.83) (108) (0.09) (18.64) (1227) (264) (28.76) (0.00) (1154) 

49
50

Table 19. Elemental concentrations (μg/g) measured in Spartina patens (marshhay cordgrass) (site 1) and Juncus 

(needlgrass rush) (site 16) leaves in August 2000 (A) and April 2001 (B). Values are the mean and (1 SE). 

__________________________________________________________________________________________Site Zone Condition n Al Ca Cu Fe K Mg Mn Mo Na Ni P A. August 2000Juncus roemerianus 16 I H 3 517.14 2089 1.91 449.40 10374 3746 18.51 0.18 12918 1.67 1363  (81.21) (181) (0.07) (66.32) (565) (323) (0.86) (0.02) (1333) (0.40) (36) Spartina patens 1 S H 3 34.48 3292 3.11 109.62 13428 2815 517.36 0.74 5433 . 1206  (3.06) (167) (0.22) (5.45) (385) (140) (37.77) (0.11) (349) . (73) 1 T D 3 41.91 2702 2.48 89.75 13213 2042 356.48 1.44 5384 . 1061  (4.54) (134) (0.04) (4.35) (280) (138) (9.97) (0.21) (83) . (17) 1 I D 3 34.32 2333 2.80 182.98 12941 2353 454.09 2.97 5649 . 1097  (5.64) (15) (0.12) (83.79) (893) (60) (36.02) (0.26) (529) . (146) B. April 2001Juncus roemerianus 16 I H 3 117.05 918 3.17 130.83 7937 1682 90.05 0.34 8010 1.17 735  (17.43) (88) (0.10) (18.81) (118) (190) (9.71) (0.08) (932) (0.13) (14) S. patens 1 S H 3 65.82 1766 2.16 109.96 9014 1341 71.17 0.27 8258 1.93 1009  (2.37) (170) (0.20) (13.56) (545) (25) (5.07) (0.05) (336) (0.09) (161) 1 I D 3 751.08 1667 2.69 3237.04 1081 2107 198.98 0.02 3663 4.55 458  (39.92) (215) (0.10) (259.65) (52) (333) (28.88) (0.00) (940) (1.57) (14) 
51

When factor scores were subjected to ANOVA, significant differences among sites and 

across zones were found (table 20). This analysis indicated that there were spatial differences in 

tissue concentrations of correlated cations and metals. In particular, the factor patterns indicated 

differences in tissue elemental composition that might reflect spatial variation in uptake of 

potentially toxic elements that might be released upon acidification of soils (Fe, Al, Mn) or 

associated with elevated salinity (Na). Leaf concentrations of these elements were thus examined 

individually (as ratios of K) to compare relative uptake in dieback and control marshes on different 

sampling dates. Leaf ratios of Na, Fe, Mn, and Al in dieback interior marshes were significantly 

elevated compared to adjacent shorelines and to control marshes in August-October 2000 (fig. 19, 

table 3). On subsequent sampling of dieback areas, leaf ratios in recovering plants had decreased to 

control levels. These data indicate that plants growing in interior dieback marshes took up 

relatively more potentially toxic metals and cations during the dieback event in 2000 but not during 

recovery in 2001. 

Vegetative Recovery 

Surveys conducted in April and September 2001 showed substantial recovery in the 

transition zone and no evidence of dieback expansion at any of the original sites (fig. 8). The rapid 

recovery of the transition zone may be due to the high survival of rhizomes (fig. 10). Recovery of 

interior dieback zones was highly variable, however, and ranged from 0 to 73 percent live cover in 

experimental plots by September 2001 (fig. 9). At least half of the dieback sites remained almost 

completely unvegetated in September 2001 (fig. 20), indicating that S. alterniflora rhizomes had 

not survived and that revegetation would have to occur via seeding or vegetative expansion from 

edges. Seedlings of S. alterniflora were observed in several dieback sites (fig. 20C) and may 

ultimately lead to revegetation. Those sites that had substantially recovered by September 2001 

may have done so through vegetative expansion from surviving rhizomes (fig. 21), although a 

distinction could not always be made between seedling and rhizome regeneration after 1 year’s 

growth. Where there was recovery of dieback areas by S. alterniflora, the regenerating plants were 

robust (culm heights ≥ 1.5 m), and reproductive output was high (fig. 22). Vigorous growth in 

former dieback sites may be attributable to a greater availability of nutrients and/or light in the 

dieback areas and indicates that post-dieback soil conditions will not limit recovery of S. 

alterniflora. Stands of J. roemerianus, D. spicata, and A. germinans within or near some dieback 

sites remained largely unchanged or expanded into the dead S. alterniflora marsh during the 

observation interval. 

Causation 

The cause of the acute dieback of S. alterniflora is currently unknown. However, our data 

either eliminate or are inconsistent with some proposed causes. Based on the pattern of dieback 

and the large spatial extent, natural sources of plant stress or disturbance that could have a regionwide effect were the main focus of investigation. 
Table 20. Factor analysis of plant elemental concentrations measured at all sites (1-21) and zones (healthy shore

[T], dieback interior [DI], dieback shoreline [DS], regenerating [R], control shoreline [CS], control interior [CI]) 

April 2001, and September 2001. Principal component analysis was used to reduce the number of variables and Elements with values below detection limits (Mo, Ni, Pb) or missing values (S) were excluded from the analysispattern is given for each date. Individual factor scores were subjected to repeated measures ANOVA with site, zsite), and time as grouping factors. F-ratios for main effects and 1 DF contrasts are given below the factor patterindicated by * (p ≤ 0.05), ** (p ≤ 0.01), *** (p ≤ 0.001),   (p ≤ 0.0001), or ns (not significant). 

__________________________________________________________________________________________

August 2000 April 2001 September 20

 __________________________ ________________________ ____________________Element: Factor 1 Factor 2 Factor 3 Factor 1 Factor 2 Factor 3 Factor 1 Factor 2 FactAl 0.33 -0.73 -0.08 -0.85 0.06 -0.21 -0.03 0.04 0Ca 0.71 0.12 -0.06 -0.04 0.92 0.23 -0.92 -0.18 0Cu -0.81 -0.30 0.06 -0.27 0.04 0.75 -0.02 0.54 -0Fe -0.38 -0.68 0.03 -0.87 -0.14 -0.3 -0.04 -0.03 0K 0.28 0.75 0.02 0.50 -0.03 0.74 0.13 0.81 -0Mg 0.85 -0.07 0.16 0.01 0.89 0.13 -0.88 0.17 0Mn 0.03 0.09 -0.84 -0.55 0.39 -0.08 -0.45 -0.51 -0Na 0.01 0.02 0.87 0.31 0.24 0.69 -0.14 -0.04 0P 0.15 0.67 -0.04 0.07 0.35 0.61 -0.12 0.69 0Zn -0.78 -0.16 0.06 -0.68 -0.04 0.32 0.03 0.12 -0Cumulative percent of variance explained 28 50 65 27 46 69 19 36 5ANOVA Source:Site 40.78** **16.82 104.6** ** 22.74  13.05** **14.93  12.84** ** 34.68 1Zone (site) 6.55** **11.22 11.12** ** 15.18  7.17** **4.09*** 7.12  4.96** **5Contrasts:HS vs. DI 4.50* 60.18**** 4.82** 155.0 57.30** **6.60* 14.59****0.20ns 24HS vs. T 0.30ns 0.94ns 22.44* 17.51****1.03ns 0.01ns 0.15ns 6.61* 9DI vs. CI 2.10ns 0.22ns 0.43ns 57.79****61.85 12.36** * 10.68** 22.51**** 1Dieback vs. Control 4.73* 3.54* 0.69ns 31.40****15.21***15.70*** 0.68ns 4.66** 23

52
0

0.020.040.060.08

0.1

Fe/K

0

0.020.040.060.08

Al/K

0

0.0030.0060.0090.0120.0150.018

Healthy

Shoreline

Transition Dieback

Interior

Control

Shoreline

C

In

Mn/K

0

0.5

1

1.5

2

2.5

3

3.5

4

Healthy

Shoreline

Transition Dieback

Interior

Control

Shoreline

Control

Interior

Na/K

August 2000April 2001

0

0.020.040.060.08

0.1

Fe/K

0

0.020.040.060.08

Al/K

0

0.0030.0060.0090.0120.0150.018

Healthy

Shoreline

Transition Dieback

Interior

Control

Shoreline

C

In

Mn/K

0

0.5

1

1.5

2

2.5

3

3.5

4

Healthy

Shoreline

Transition Dieback

Interior

Control

Shoreline

Control

Interior

Na/K

August 2000April 2001

53

Figure 19. Elemental concentration ratios measured in Spartina alterniflora (smooth cordgrass) leaves in AugusApril 2001 at dieback and reference marshes. Three zones were sampled at dieback areas and two zones at control (no visible dieback) areas. Values are the mean ± 1 SE (n =48-51 dieback, n = 9 control). 
A

B C

Figure 20. Aerial and ground views of a dieback area (site 6) that showed little vegetative recovery 

in September 2001; however, small clumps (arrows) of Spartina alterniflora (smooth cordgrass) 

(A) have initiated from surviving rhizomes (B) or seedlings (C). 

54
0 % live cover

A. October 2000

B. September 2001

~ 6 0 % live cover

Figure 21. Example of a Spartina alterniflora (smooth cordgrass) marsh (site 17) that recovered 

from dieback. Arrows indicate the same position in both panels. 

55
September 2001

August 2000

A

C

B

D

September 2001

August 2000

A

C

B

D

Figure 22. Aerial and ground views of a Spartina alterniflora (smooth cordgrass) marsh (site 2) 

recovering from dieback. Note height and flowering of the plant shoots in panel D. Arrows 

connect aerial and ground views between two dates (August 2000 during the dieback and 

September 2001 during recovery). 

56
Each of the three hypotheses posed in the introduction was tested by using the data 

collected during preliminary and intensive surveys. Our approach was to consider potential factors 

in terms of consistency with the spatial pattern of dieback (primarily salt marsh; interior vs. 

shoreline vegetation), the relative vulnerability of marsh species (Spartina spp. vs. other marsh 

species), and the temporal sequence of dieback and recovery. 

Hypothesis 1: Biotic Agents 

Bacterial, fungal, or viral pathogens or insect outbreaks may cause widespread mortality of 

plants. In Florida, a fungal pathogen was identified in connection with Spartina dieback, but the 

direct demonstration of pathogenicity has not been made (Carlson and others, 2001). Examination 

of plant shoots and roots collected from dieback marshes in Louisiana revealed a variety of fungal 

species (e.g., Fusarium sp. and Macrophomina sp.; R. Schneider, Louisiana State University, 

personal communication). Although potential fungal pathogens were isolated from S. alterniflora, 

infection by these organisms usually occurs on previously stressed vegetation and is therefore 

unlikely to be an explanation by itself. Fungal or other pathogens also cannot explain the 

consistent local pattern of dieback with living vegetation along shorelines and creeks and abrupt 

transition to dead vegetation in the marsh interior. However, fungal pathogens may have played a 

secondary role in plant mortality through interaction with an abiotic stress factor, and experiments 

are needed to test the pathogenicity of isolates from plant material collected in this survey. 

Outbreaks of insects or other herbivores were also quickly ruled out as potential causes, 

primarily because of the spatial pattern and extent of dieback as well as the condition of dead and 

dying vegetation. No evidence of insect damage sufficient to cause dieback was observed at any of 

the dieback sites examined in June 2000 (Bay Junop) or August 2000 (intensive survey). In 

particular, plant shoots collected from Bay Junop early in the dieback sequence were completely 

dead (i.e., no green tissue) but were largely intact with unfrayed leaves and no consistent signs of 

necrotic lesions, abrasions, holes, or other damage indicative of excessive herbivory. 

The marsh periwinkle, Littoraria irrorata, has been specifically suggested as a cause of 

marsh dieback in Louisiana (Bertness and Silliman, 2002); however, our observations do not 

support this explanation. Although snails had heavily grazed the dead vegetation in some sites by 

August 2000, the mortality of the vegetation clearly preceded the grazing, as observed at Bay 

Junop in June 2000. The temporal sequence suggests instead that snails responded to dieback of 

vegetation (fig. 23). In June 2000, dieback sites had standing dead vegetation with no evidence of 

heavy grazing (figs. 3). By August 2000, seven of nine dieback marshes had been reduced to 

stubble or bare ground, but two sites still had standing dead shoots with leaves (fig. 9A). Site 1, 

which was dominated by S. patens, retained standing dead shoots much longer than did sites 

dominated by S. alterniflora (fig. 9). Qualitative observations indicated high variability among 

sites in snail densities and no obvious patterns relative to dieback. 

57
A. June 2000 B. August 2000

D. April 2001 C. October 2000

Figure 23. Littoraria irrorata (marsh periwinkle) responded to rather than caused marsh dieback. 

Spartina alterniflora (smooth cordgrass) shoots died in spring of 2000 and showed no signs of 

grazing by snails or other herbivores (A). By August 2000, snails had begun grazing on dead 

shoots (B), which were reduced to stubble by October 2000 (C) and to mudflats by April 2001 (D). 

Arrows indicate the sequence of stages in degradation of dead marsh plants. Inset photos in (B) 

and (C) show close-ups of L. irrorata. 

58
 

By October 2000, very little standing vegetation was left in interior dieback sites, but snails 

remained bunched around the stem remnants in some of these developing mudflats (fig. 23C) and 

had not moved into the nearby healthy stands along the shoreline. If overgrazing by snails were the 

cause of the dieback, then some expansion of plant mortality should have occurred as their food 

source dwindled. Dieback expansion not only did not occur, but by April 2001, the transition zone 

at all dieback sites had completely recovered (fig. 8). Snails were undoubtedly instrumental in the 

rapid degradation of dead shoots in dieback marshes and likely contributed to the variation among 

sites in dead shoot condition (figs. 7-9), but they were not the cause of the dieback. Thus, our data 

are inconsistent with Hypothesis 1 that marsh dieback was primarily caused by a biotic process. 

However, we cannot exclude the possibility that fungal pathogens interacted with an environmental 

stress factor to cause plant mortality. 

Hypothesis 2: Excessive Waterlogging and Sulfide Toxicity 

The pattern of interior dieback with surviving vegetation along the shoreline suggests that a 

hydroedaphic factor may have caused the dieback or, conversely, may have ameliorated the 

causative agent. Shoreline vegetation experiences more frequent and deeper soil flushing during a 

tidal cycle than does the marsh interior where sulfide and other toxins may accumulate and affect 

root metabolism (Mendelssohn and others, 1981). Although above-normal flooding depths were 

not indicated by water level records for the affected region (E. Swenson, public communication, 

http://www.brownmarsh .net), historical conditions of soil waterlogging could have increased 

overall plant stress in interior marshes and decreased ability to cope with additional stressors 

(Mendelssohn and others, 1981). Previous work has clearly demonstrated the role of excessive soil 

waterlogging and sulfide in reduced growth and historical dieback of S. alterniflora in interior 

marshes (see Mendelssohn and Morris, 2000 for a review). 

Although soil reduction-oxidation status (Eh) was not substantially different in the interior 

dead zones from that in healthy or transition zones (fig. 14, tables 4-6), sulfide concentrations were 

significantly elevated in the dead and transition zones at most sites (fig. 15, tables 4-6). Sulfide 

concentration in some dieback areas exceeded that known to cause growth reductions in S. 

alterniflora (~1 mM; Koch and others, 1990), but it is unknown if sulfide exceeded a lethal 

concentration prior to sampling. Observations at Bay Junop in June 2000, however, suggest that 

sulfide concentrations were not more elevated in dieback areas earlier in the year. At this site, 

sulfide concentrations were highest within the dead marsh zone and lowest in the healthy mangrove 

stand and along the shoreline where S. alterniflora shoots were still alive. However, this pattern 

cannot be interpreted as a cause and effect relationship between sulfide and marsh dieback for 

several reasons. First, there was a gradual increase in sulfide with distance from the shoreline, 

which is typical of salt marshes in Louisiana (Mendelssohn and others, 1981). This pattern does 

not match the spatial dieback pattern in which there was an abrupt transition from living plants to 

completely dead vegetation across a narrow transition zone. Second, the sulfide concentrations 

across much of the dead marsh in June 2000 were lower than that known to reduce growth of S. 

alterniflora (Koch and others, 1990). Third, because living vegetation influences soil aeration via 

oxygen leakage from roots, sulfide may have accumulated in the dead marsh simply because root 

aeration was not occurring. Similarly, the lower sulfide concentrations within the healthy 

mangrove stand can be attributed to the root aeration effect of healthy A. germinans, which has 

been demonstrated in tropical mangrove systems (McKee and others, 1988). Fourth, death of the 

59
vegetation may stimulate sulfate reduction, so that sulfide concentrations become elevated 

wherever the marsh plants have died. Fifth, sulfide concentrations remained elevated or even 

increased over time at dieback sites that ultimately recovered (e.g., sites 2, 7, and 17; fig. 15). If 

reducing conditions and elevated sulfide were responsible for the acute dieback of salt marshes, 

then it is unlikely that vegetative recovery would occur under those same soil conditions. Finally, 

although sulfide concentrations were elevated in dieback marshes dominated by S. alterniflora, 

they were extremely low in the dead S. patens marsh (site 1), and redox potentials indicated 

oxidizing, rather than reducing, conditions. For the reasons stated above, we reject Hypothesis 2, 

particularly with respect to sulfide as a primary cause of acute salt marsh dieback, but cannot 

eliminate excessive soil waterlogging as a factor that may have predisposed interior marshes to the 

causative agent. 

Hypothesis 3: Low Water Levels and Drought 

Marsh dieback coincided with extreme drought conditions and unusually low freshwater 

discharge from the Mississippi and Atchafalaya Rivers that prevailed during the previous year (E. 

Swenson, public communication, http://www.brownmarsh.net). These local events are related to La 

Niña, a phenomenon that is characterized by drier and warmer than normal conditions across the 

southern United States (Philander, 1990). Mean high tide levels in coastal Louisiana were also 

below normal during early spring 2000 (E. Swenson, public communication, 

http://www.brownmarsh.net), and this factor could have limited over-bank flooding of interior 

marshes. Drought, combined with less tidal flooding, may have decreased water availability 

directly and/or increased salinity by concentrating salts in the pore water. Salinity of surface water 

prior to and during dieback (E. Swenson, public communication—http://www.brownmarsh.net) 

and measured in pore water during our survey (figs. 13 and 15) did not exceed the tolerance limits 

of S. alterniflora (Hester and others, 1996). Leaf Na/K ratios in dieback sites were significantly 

higher than those in healthy shoreline plants (fig. 19) and higher than those of plants grown at 30 

‰ in greenhouse experiments (Hester, 1995). These results indicate that salinity was elevated in 

the interior marshes sufficiently to alter cation balance but do not demonstrate that hypersalinity 

caused mortality. Furthermore, although survival of more salt tolerant species (e.g., D. spicata and 

A. germinans) is consistent with the salinity hypothesis, survival of J. roemerianus, which is less 

salt tolerant than S. alterniflora, is not. Variation in species vulnerability to dieback could reflect 

differences in rooting depth and exposure to high salinity in surface soils. Decreased rooting depth 

of S. alterniflora from shoreline (37 ± 2 cm) to transition (28 ± 1 cm) to marsh interior (25 ± 1 cm) 

paralleled shoot mortality (fig. 24). However, species unaffected by the dieback, such as A. 

germinans and J. roemerianus, growing within Spartina dieback zones also had shallow root 

systems (14 ± 1 and 9 ± 1 cm, respectively) (fig. 25). 

60
-50

-45

-40

-35

-30

-25

-20

-15

-10

-5

0

8/004/018/004/018/004/018/00

SRD

PRD

8/004/018/004/018/004/01

8/004/018/004/018/004/018/004/01

Healthy

Shoreline

Transition Dieback

Interior

Dieback

Shoreline

Regenerating Control

Shoreline

Control

Interior

Depth (cm)

Dieback Marshes Control Marshes

-50

-45

-40

-35

-30

-25

-20

-15

-10

-5

0

8/004/018/004/018/004/018/00

SRD

PRD

8/004/018/004/018/004/01

8/004/018/004/018/004/018/004/01

Healthy

Shoreline

Transition Dieback

Interior

Dieback

Shoreline

Regenerating Control

Shoreline

Control

Interior

Depth (cm)

-50

-45

-40

-35

-30

-25

-20

-15

-10

-5

0

8/004/018/004/018/004/018/00

SRD

PRD

8/004/018/004/018/004/01

8/004/018/004/018/004/018/004/01

Healthy

Shoreline

Transition Dieback

Interior

Dieback

Shoreline

Regenerating Control

Shoreline

Control

Interior

Depth (cm)

Dieback Marshes Control Marshes

Figure 24. Primary rooting depth (PRD) and secondary rooting depth (SRD) in various zones at 

dieback and control marshes in August 2000. Values are the mean ± 1 SE (n = 48-54 at healthy 

shoreline, transition, and dieback interior marshes, n = 3-9 at other dieback zones and control 

marshes). 

 

61
A B

C D

Figure 25. Views of Avicennia germinans (black mangrove) (A) and Juncus roemerianus

(needlegrass rush) (B) growing within Spartina alterniflora (smooth cordgrass) dieback areas and 

their respective root systems (C and D). The ruler in C is 0.5 m.

62
Thus, a shallow root system cannot explain the greater vulnerability of S. alterniflora in 

interior marshes to acute dieback but may indicate a preexisting stress condition in interior marshes 

such as elevated sulfide or salinity that predisposed plants to dieback. Thus, although salinity may 

have been elevated in interior marshes prior to or during the dieback event and increased plant 

stress, the species pattern of response does not support hypersalinity as the primary causative agent. 

Besides salinity, other chemical changes may occur upon soil drying. Sediments that 

develop under an estuarine influence may contain pyrite or other readily oxidizable metal-sulfide 

compounds (Howarth, 1979). When saline sediments are aerated, e.g., during drainage or 

dredging, microbial oxidation of pyrite and metal sulfides generates sulfuric acid, which leads to 

soil acidification (Nordstrom, 1982). Aeration and acidification of sediments (to pH 3-4) may also 

occur naturally through oxygen advection by falling water, a process observed during early spring 

in Louisiana coastal marshes (Feijtel and others, 1988a). By the time of our initial surveys, all but 

one site was flooded and showed no evidence of drying or cracking. However, this one site 

(dominated by S. patens) showed evidence in August 2000 of soil desiccation in dead and transition 

zones with low soil pH (~4), high soil Eh (>600 mV) and root coatings of ferric iron, indicating 

oxidation (fig. 26). Another site (dominated by S. alterniflora) also showed dry, cracked soil in the 

dead, interior marsh zone as well as along a creekbank in April 2001 (fig. 27). These observations 

indicate that soil drying can occur in these marshes under certain conditions. Subsequent 

reflooding and mixing with alkaline seawater as well as reduction reactions would cause pH to rise 

(Gambrell and Patrick, 1978), which could explain why flooded sites had near-neutral pH at the 

time of our surveys. 

Dieback soils consistently had higher concentrations of pyrite and acid-extractable Fe and 

Al and lower organic matter contents compared to control marshes (figs. 16-17, Tables 8-10). 

Solubility of Fe, Mn, and Al increases several fold with decreases in pH and may reach toxic levels 

(Gambrell and Patrick, 1978). Similar concentrations of pyrite and acid-extractable Fe and Al in 

healthy shoreline and interior dieback zones indicate that the potential for soil acidification and 

metal release was the same but that differences in hydrology may have prevented the streamside 

soils from becoming acidified and/or metals reaching toxic concentrations. 

Lower concentrations of pyrite in control marshes would decrease the potential for soil 

acidification upon drying and oxidation, and higher organic matter would promote formation of 

insoluble complexes with metals (Adriano, 1986). The significantly higher leaf ratios of Fe:K and 

Al:K measured in interior dieback marshes in August 2000 and subsequent return to control levels 

in April and September 2001 is consistent with this scenario (fig. 19). The orange to reddish 

coloration of dying marsh vegetation (fig. 4) was also reminiscent of the leaf discoloration 

(“bronzing”) described in rice (Oryza sativa) affected by Fe toxicity (Jugsujinda and Patrick, 

1993). Salt marsh sediments accumulate more trace and heavy metals because of mineral input 

from rivers than do intermediate and freshwater marshes with more organic substrates (Feijtel and 

others, 1988b), and salt marsh sediments also show greater decreases in pH upon oxidation (Feijtel 

and others, 1988a). These factors could explain why acute dieback was more prevalent in saline 

marshes. 

63
A

B

Figure 26. Reddish-orange coloration observed in the shoots (A) and soil (B) at some sites during 

early stages of dieback (August 2000). Arrows point to ferric iron deposits on the water surface 

and roots (upper and lower panels, respectively), indicative of oxidation. 

64
A

B

Figure 27. Distant (A) and close-up (B) views of dry, cracked soil at a Spartina alterniflora

(smooth cordgrass) dieback area (site 16). 

65
 

The most compelling evidence for soil acidification and possible involvement in Spartina

spp. mortality, however, is that soils from dieback areas acidified upon oxidation, whereas those 

from control sites did not (fig. 18). The acidification phase in early spring could have been 

prolonged or exaggerated by drought conditions sufficiently to affect plants, either by acidity per se 

or by toxic metals released at low pH. 

Our findings are thus most consistent with Hypothesis 3 that soil acidification as a 

consequence of drought and low tide levels and release of toxic metals caused dieback of saline 

marshes. However, a direct lack of water and/or hypersalinity could have contributed to the overall 

stress on plants and their potential vulnerability to low pH and metal toxicity. These hypotheses 

still require further testing to evaluate the relative tolerance limits of S. alterniflora and other salt 

marsh species to low pH and metals. 

Conclusions 

Intensive examination of plant and soil characteristics at multiple dieback sites provided 

some important insights into the dieback process, vegetative recovery, and causation. Some areas 

of salt marsh were completely dead by June 2000, and plant shoots were substantially decomposed 

in most dieback sites by August 2000. This timeline of vegetative condition supports the 

hypothesis that the dieback event began in early spring of 2000. This study also confirmed that 

spatial patterns of dieback (interior vs. shoreline marsh) and differential species susceptibility 

(Spartina spp. vs. other salt marsh species) occurred repeatedly across the Barataria-Terrebonne 

estuarine system. The documentation of consistent patterns is important to concurrent research on 

causation and supports the hypothesis that the cause was a regional factor (e.g., drought) that 

interacted with local patterns of soil chemistry and/or hydrology. 

Little or no expansion of dieback occurred subsequent to the initial event, and areas with 

moderate dieback (~50 percent mortality) had completely recovered by April 2001. These data 

demonstrate the ephemeral nature of the dieback agent, which apparently had no residual effects to 

limit plant recovery. Rapid recovery of moderately impacted marsh was due to survival of S. 

alterniflora rhizomes. Slow recovery of interior marshes that experienced greater than 90 percent 

shoot mortality was attributable to low survival of rhizomes and variable recolonization by 

seedlings and/or vegetative expansion from edges. However, regenerating plants in dieback areas 

were robust (culm heights ≥ 1.5 m), and reproductive output was high; these factors indicate that 

post-dieback conditions were actually promoting growth of S. alterniflora. Vigorous growth in 

dieback areas was likely due to a greater availability of nutrients and/or light. Stands of J. 

roemerianus, D. spicata, and A. germinans within or near some dieback sites remained largely 

unchanged or expanded into the dead S. alterniflora marsh during the observation interval. 

Although further monitoring is needed to confirm the extent of recovery, our observations indicate 

a high potential for natural recovery from the dieback event. However, some areas that showed no 

recovery by September 2001 (one full growing season after dieback) may convert to open water. 

The cause of the dieback is currently unknown, primarily because the causative agent left 

no conclusive signature in plant or soil variables. Soil conditions were generally within normal 

ranges for salt marshes during the dieback event (June to October 2000) and during vegetative 

66
recovery (April to September 2001). However, our data either eliminate or are inconsistent with 

some proposed causes. We specifically examined three potential causes of marsh dieback in terms 

of consistency with the spatial pattern of dieback, the differential susceptibility of marsh species 

and their known stress tolerances, and the temporal sequence of dieback and recovery. 

Biotic Agents 

We found no evidence in support of a biotic agent as the primary cause of marsh dieback. 

Although potential fungal pathogens were isolated from S. alterniflora, infection usually occurs on 

previously stressed vegetation and is therefore unlikely to be an explanation by itself. Fungal or 

other pathogens also cannot explain the consistent local pattern of dieback. Fungal pathogens may 

have played a secondary role in plant mortality through interaction with an abiotic stress factor, but 

further experiments are required to test this possibility. 

Outbreaks of insects or other herbivores were also ruled out as potential causes. This 

conclusion was based primarily on the spatial pattern and extent of dieback as well as the condition 

of dead and dying vegetation. Our observations specifically do not support the contention that 

dieback was caused by excessive grazing by the Littoraria irrorata (marsh periwinkle) (Bertness 

and Silliman, 2002). The data instead indicate that snails responded to the dieback and played an 

important role in rapid degradation of standing dead plants in some areas. 

Excessive Soil Waterlogging and Sulfide 

Although soil waterlogging and sulfide toxicity have been implicated in historical dieback 

of S. alterniflora, our data do not support this explanation with respect to acute marsh dieback. 

Historical records do not indicate that water levels were abnormally high in interior marshes during 

spring 2000 but that this was a period of unusually low water levels. Elevated sulfide 

concentrations were found in dieback marshes but may have occurred because death of the 

vegetation would stimulate sulfate reduction. In addition, we found no evidence of sulfide 

concentrations that exceeded the tolerance limits of S. alterniflora during the study period (June 

2000 to September 2001) and generally found conditions that were typical of interior marshes. 

Also, this explanation does not account for the dieback of Spartina spp. and survival of other 

species that are known to be sensitive to sulfide (e.g., A. germinans) (McKee, 1993). Finally, 

sulfide concentrations remained elevated or even increased over time in areas where revegetation 

occurred. However, waterlogging and elevated sulfide in interior marshes may have increased the 

susceptibility of Spartina spp. to another stress factor that precipitated the dieback event. 

Drought and Low Water Levels 

The dieback event was coincident with an extreme drought condition, low river discharge, 

and low sea level. Drought, combined with less tidal flooding, may have decreased water 

availability directly, increased salinity by concentrating salts in the pore water, and/or caused 

oxidation and acidification of soils. 

Direct Lack of Water

We found evidence of dry, cracked soils at a few sites, indicating that these marshes can 

become desiccated under certain conditions. However, most sites were flooded at the time of our 

67
surveys. Our data thus cannot eliminate the possibility that a direct lack of water caused plant 

mortality. However, greenhouse experiments were unable to duplicate this hypothesized effect on 

S. alterniflora, even when water was withheld for 30 d (R.R. Twilley and others, unpublished data). 

Hypersalinity 

We also found no evidence in support of hypersalinity as a cause of marsh dieback. Soil 

salinities measured at dieback sites from June 2000 to September 2001 were within the ecological 

range of S. alterniflora and well below the lethal limit for this species. However, the most 

convincing evidence against this hypothesis is the fact that a species (J. roemerianus) with lower 

salt tolerance growing within S. alterniflora dieback areas not only survived the event but also 

showed little or no signs of stress. The differential salt tolerance of these species has been 

confirmed in controlled laboratory experiments (Mendelssohn and others, 2005). 

Soil Acidification and Metal Toxicity 

When saline sediments are aerated during drying, microbial oxidation of pyrite and sulfide 

generates sulfuric acid, which leads to soil acidification and ultimately to release of potentially 

toxic metals such as aluminum and iron. Our data are consistent with this explanation. Dieback 

sites had higher concentrations of pyrite and acid-extractable aluminum and iron than did reference 

marshes that did not die. Plants from dieback marshes also contained significantly higher ratios of 

aluminum and iron, indicating increased uptake of potentially toxic metals, than did those growing 

along nearby shorelines and in reference marshes where plant mortality was low. The subsequent 

decline of tissue metal ratios to control levels during vegetative recovery further substantiates this 

explanation. In addition, laboratory trials demonstrated that soils from dieback marshes acidified 

upon oxidation, whereas those from reference sites did not. The fact that salt marsh sediments 

accumulate more trace and heavy metals because of mineral input from rivers compared to those of 

intermediate and freshwater marshes with more organic substrates and that those salt marsh 

sediments also show greater decreases in pH upon oxidation is also consistent with the regional 

pattern of dieback primarily in saline marshes. 

A fuller understanding of this acute dieback event and its consequences relative to historical 

dieback in Barataria-Terrebonne salt marshes is needed to aid in conservation and restoration 

efforts. Just as important are the implications of this event for global change models that focus 

mainly on sea-level rise as the dominant variable controlling vegetation patterns in coastal 

ecosystems (Day and others, 1993). Numerous studies of sea-level rise and potential impacts on 

wetland areas have been conducted along the Atlantic and gulf coasts (Day and others, 1993). Sealevel rise gradually increases submergence and salinity in coastal areas, and these effects are most 

pronounced in deltas and lagoons that are undergoing regional subsidence and/or human alterations 

(Day and others, 1993). The sudden dieback of salt marshes in Louisiana suggests another scenario 

in which periodic climatic extremes cause rapid, widespread mortality of vulnerable species. 

Ecologists now recognize the importance of large, infrequent disturbances in shaping 

landscapes and structuring plant communities (Romme and others, 1998; Turner and Dale, 1998). 

Foster and others (1998) argue, for example, that such disturbances produce long-lasting changes in 

the physical and biological structure of forests that influence ecosystem functioning for decades or 

centuries. Large-scale, infrequent disturbances may also be so qualitatively different from small, 

frequent disturbances that the rates and patterns of recovery may differ (Romme and others, 1998). 

68
However, the fundamental characteristics of an ecosystem are usually not altered even by extreme 

events, since organisms have evolved in the presence of disturbance. Permanent changes typically 

occur only with multiple disturbances that together exceed the rate of recovery by organisms (Paine 

and others, 1998; McKee and Baldwin, 1999). Some dieback marshes have recovered naturally 

and will require no restoration. In other areas, S. alterniflora may ultimately be supplanted by 

other species such as A. germinans or J. roemerianus. At a minimum, there will be expansion of 

other species into areas formerly dominated by S. alterniflora. Lack of rapid revegetation and loss 

of soil structure in some dieback marshes could accelerate land loss. Even so, further monitoring 

of these areas will be necessary to determine the extent of permanent change. 

The implications of this event for other coastal systems are unclear partly because the 

precise causal mechanism for the acute dieback in the Mississippi River deltaic plain is not known. 

Also, preexisting stress conditions may play a role in acute dieback, but how and to what degree is 

not currently understood. The Mississippi delta is perhaps an extreme case since it has been 

undergoing high rates of apparent sea-level rise (8.5 to 9.5 mm/yr) and marsh deterioration (Day 

and others, 1993). Nonetheless, the sudden dieback of S. alterniflora marshes in Louisiana 

suggests that large-scale changes in coastal vegetation may occur much more rapidly than current 

models predict and as a consequence of weather extremes acting in concert with sea-level 

fluctuations and preexisting soil conditions. Management plans usually do not take disturbance 

into account because of unfamiliarity or a belief that the disturbance cannot be managed (Dale and 

others, 1998). However, even if a disturbance cannot be controlled, the resilience and recovery of 

the system may be altered by management (Dale and others, 1998). Such considerations will 

become increasingly important as global climate changes and human pressures in the coastal zone 

grow. 

Acknowledgments 

The Saltwater Marsh Dieback (aka Brown Marsh phenomenon) was declared an emergency 

by Louisiana Governor Mike Foster in the form of Executive Proclamation #55-MJF-2000 issued 

on October 23, 2000. In response, Congress allocated approximately three million dollars in funds 

through the National Atmospheric and Oceanic Administration to Louisiana Department of Natural 

Resources (LDNR) to examine causes and effects of the Brown Marsh phenonmenon. This 

investigation was funded by a grant from LDNR. Additional funding was provided by the 

Louisiana Sea Grant Program, USGS National Wetlands Research Center, and the Louisiana 

Governor’s Office of Coastal Activities. 

Field and laboratory assistance was provided by Andrea Anteau, Tommy McGinnis, and 

Lee Stanton. Plant and soil elemental analyses were conducted by the laboratories of Dr. Wayne 

Hudnell and Kurt Hutchinson of the Department of Agronomy, Louisiana State University. 

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