Mark W. Hester

The effects of oil spill and clean-up on dominant US Gulf coast marsh macrophytes: a review.

The objective of this review was to synthesize existing information regarding the effects of petroleum hydrocarbons on marsh macrophytes in a manner that will help guide research and improve spill-response efficiency. Petroleum hydrocarbons affect plants chemically and physically. Although plants sometime survive fouling by producing new leaves, even relatively non-toxic oils can stress or kill plants if oil physically prevents plant gas-exchange. Plant sensitivity to fouling varies among species and among populations within a species, age of the plant, and season of spill. Physical disturbance and compaction of vegetation and soil associated with clean-up activities following an oil spill appear to have detrimental effects on the US Gulf coast marshes. Other techniques, including the use of chemicals such as cleaners or bioremediation, may be necessary to address the problem. Clean-up may also be beneficial when timely removal prevents oil from migrating to more sensitive habitats.

Species and population variation to salinity stress in Panicum hemitomon, Spartina patens, and Spartina alterniflora: morphological and physiological constraints

Abstract Panicum hemitomon , Spartina patens , and Spartina alterniflora are wide-spread dominant grasses of fresh, brackish, and salt marsh plant communities, respectively. Our previous research identified significant intraspecific variation in salt tolerance and morphology among populations within each species. In this study our objectives were to determine shorter-term physiological/biochemical responses to salinity stress and identify potential indicators of salt tolerance, with the ultimate goal of discerning similarities and differences in the mechanisms of salinity stress resistance. We subjected a subset of six populations within each species, ranging from high to low salt tolerance, to sublethal salinity levels (4, 20, and 30 ppt, respectively, for species) and monitored physiological and growth responses after 1 week (early harvest) and 5 weeks (late harvest). In all three species sublethal salinity levels generally resulted in significantly reduced net CO 2 assimilation, leaf expansion, midday leaf xylem pressure, water use efficiency, and live and total biomass; and significantly increased leaf Na + /K + ratio, leaf proline, leaf glycine betaine, leaf sucrose, root-to-shoot ratio, and dead:total aboveground biomass ratio. All three species displayed significant population (intraspecific) variation in net CO 2 assimilation, leaf expansion, water use efficiency, midday leaf xylem pressure, leaf proline, leaf glycine betaine (except Panicum , where it could not be accurately determined), leaf Na + /K + ratio, leaf sucrose, total plant biomass, dead:total aboveground biomass ratio, and root-to-shoot ratio. General indicators of salt tolerance (regardless of species) included high net CO 2 assimilation rates and water use efficiencies, and low ratios of root-to-shoot and dead:total aboveground biomass. Factor analysis and a-priori linear contrasts revealed some unique differences between species in terms of the relative importance of morphology and physiology in explaining intraspecific variation in salt tolerance. Plant morphology (size attributes) were strongly associated with salt tolerance in P. hemitomon , weakly associated with salt tolerance in S. patens , and not associated with salt tolerance in S. alterniflora . Highly salt-tolerant populations of Spartina alterniflora displayed the greatest ion selectivity (lower leaf Na + /K + ratios), which was not displayed by the other two species. These results suggest that plant size attributes can be very important in explaining population differences in salt tolerance in glycophytes, but may be independent of salt tolerance in halophytes, which have specialized physiological (and/or anatomical) adaptations that can confer salinity stress resistance through mechanisms such as selective ion exclusion and secretion.

Interactive effects of hydrology and salinity on oligohaline plant species productivity: Implications of relative sea-level rise

Sea-level rise is anticipated to alter hydrologic and salinity regimes of coastal wetlands. We conducted a mesocosm experiment to determine species-level responses to 12 sea-level rise scenarios. Both hydrologic regime (−10, +5, and +20 cm flooding depth) and salinity level (fresh, 2‰, 4‰ and 6‰) were interactively manipulated. Within these various sea-level rise scenarios, we sought to determine the effects of hydrologic regime, salinity level, and the interaction of these two stresses on the productivity ofPanicum hemitomon, Sagittaria lancifolia, andSpartina patens, which are dominant macrophytes of fresh, intermediate, and brackish marsh types, respectively, in coastal Louisiana and the southeastern coastal plain. We found that altered hydrologic regimes and increased salinity levels differentially affected edaphic conditions and species-level productivity. Increases in flooding depth were most detrimental toS. patens. Salinity levels greater than 4‰ resulted in mortality ofP. hemitomon, and salinity levels of 6‰ resulted in reduced growth and eventual death, ofS. lancifolia. The effects of elevated salinity levels onP. hemitomon andS. lancifolia were exacerbated when coupled with increased flooding levels. Although soil organic matter was shown to increase in all vegetative conditions, increases were dependent upon the productivity of the species under the different hydrologic regimes and salinity levels withP. hemitomon displaying tremendous potential to increase soil organic matter under fresh conditions, especially when coupled with moderate flooding. The results of this study indicate that as plant communities are subjected to long-term changes in hydrology and salinity levels, community productivity and sustainability ulimately will be determined by species-level tolerances in conjunction with species interactions.

Long-term recovery of a Louisiana brackish marsh plant community from oil-spill impact: vegetation response and mitigating effects of marsh surface elevation

Oil spills can have significant, short-term, negative impacts on coastal marshes, but the long-term effects and eventual recovery are not well documented, particularly in brackish marshes. The goals of this investigation were to: (1) document the long-term recovery of a Louisiana brackish marsh plant community impacted by a 1985 oil spill; (2) separate the effect of the oil spill on marsh deterioration from ambient rates of marsh deterioration; and (3) assess the relative importance of residual oil in the sediment and decreased marsh surface elevation in the failure of certain areas to recover. A total of 68 permanent plots previously established in 1985 were re-surveyed for plant and soil recovery in the fall of 1989. Although substantial (and near total) vegetative recovery was evident by significant increases in live and total vegetative cover, many of the plots that were initially heavily impacted by oil still displayed elevated levels of total saturated hydrocarbons in the soil. August 1990 measurements of plant photosynthetic response and edaphic variables revealed no significant differences between control plots and plots heavily impacted by oil that displayed vegetative regrowth. Rates of wetland land loss in the oiled marsh during an 8-year period that bracketed the time of the spill were within the historical range measured for this site and similar to the land loss rates of adjacent reference marshes. Results from a manipulative field transplant experiment indicated that the long-term failure of certain small areas to revegetate was primarily due to a decrease of marsh surface elevation (increased flooding stress), not a residual oil effect.

INTRASPECIFIC VARIATION IN SALT TOLERANCE AND MORPHOLOGY IN PANICUM HEMITOMON AND SPARTINA ALTERNIFLORA (POACEAE)

Nineteen clones of Panicum hemitomon, a fresh marsh dominant, and 25 clones of Spartina alterniflora, a salt marsh dominant, were collected from the coastal marshes of Louisiana and Texas. Plants were deacclimated from field conditions under uniform, nonsaline conditions in the greenhouse for four to six vegetative generations and morphological variables were then measured. Genotypes of each species were subjected to a salinity screening protocol where salinity was increased in weekly increments of 2 per mil (g salt/L solution) for P hemitomon and 10 per mil for S. alterniflora using a commercial sea salts mix. Plants were harvested when there was 50% death of aboveground tissue, defined as the lethal salinity level. Both species displayed highly significant intraspecific variation in lethal salinity level, ranging from 7.6 per mil to 12.0 per mil for P. hemitomon and from 83 per mil to 115 per mil for S. alterniflora. Both species also displayed significant intraspecific variation in many morphological variables, as well as in leaf-rolling index, leaf expansion rates, aboveground, belowground, and total plant dry weight, and belowground-to-aboveground biomass ratio. An ANOVA of principal component scores further illustrated intraspecific variation in both species expressed as a single principal component made up of lethal salinity level and covariable-adjusted total plant dry weight. In S. alterniflora none of the plant morphological variables was significantly correlated with salt tolerance, whereas leaf rolling at 35 per mil accounted for 38% of the variation in lethal salinity level among genotypes. Conversely, in P. hemitomon leaf rolling was not significantly correlated with salt tolerance, but two morphological variables, leaf length and leaf length x width, accounted for 44% and 46%, respectively, of the variation in lethal salinity level among genotypes. Therefore, morphological traits related to plant size appear more important in explaining genotypic variation in salt tolerance in the glycophyte P. hemitomon than in the facultative halophyte S. alterniflora. The ability to identify superior salt-tolerant genotypes within both species has significant implications for marsh restoration and creation projects throughout their ranges.

Population variation in growth response to flooding of three marsh grasses

Abstract Comparative studies were conducted to evaluate intraspecific variation in leaf elongation and biomass partitioning in response to flooding stress in populations of Spartina alterniflora, S. patens , and Panicum hemitomon . These populations were collected along the Louisiana and Texas coasts and grown for four to six vegetative propagations in the greenhouse. Plants were flooded to a maximum water level of 39 cm above the soil surface for approximately 2 months and leaf elongation rates, soil redox potentials, and final biomass of each population were measured. Tests of flooding response between the populations indicated significant ecotypic differentiation in biomass partitioning. Using a rotated principal components analysis, we identified the three most flood tolerant and the three least flood tolerant populations for each species, with greater below-ground and below-water biomass or total biomass production characterizing greater flood tolerance. The ability to select more flood tolerant populations has direct application in wetland creation and restoration.

Interactive effects of salinity, flooding, and soil type on Panicum hemitomon

It is well documented that Louisiana is experiencing wetland loss at rates greater than any other locale in the world. High rates of relative sea-level rise, a combination of eustatic sea-level rise and subsidence, is anticipated to compound this problem further in the future through increased flooding and encroachment of saline water into freshwater wetlands. The research presented in this paper examines the interactive effect of increased salinity level, flooding depth, and soil type on the growth responses of a dominant Louisiana fresh-water marsh plant, Panicum hemitomon, whose prevalence in Louisiana is currently in decline. This study was conducted under greenhouse conditions and employed a factorial design consisting of three salinity levels (0, 1.5, 3.0 ppt), three hydrologic regimes (0, 10, 20 cm), and two soil types (high organic content, low organic content). Panicum hemitomon productivity was significantly reduced even under the relatively small increases in salinity level (1.5 and 3.0 ppt) imposed in this study. Interestingly, moderate flooding tended to increase productivity, although this relationship was not statistically significant. Significantly greater productivity was observed for plants grown in mineral soil compared with organic soil. These results indicate that any degree of saline influx into P. hemitomon-dominated wetlands will result in decreased vigor and localized decline of this species. Moderate increases in the degree of freshwater inundation may not be as damaging as originally expected and, in fact, may actually stimulate production. However, if increased flooding is accompanied by increased salinity levels, which is anticipated to occur, then the overall effect on this species will be detrimental.

The effect of a louisiana crude oil discharge from a pipeline break on the vegetation of a Southeast Louisiana brackish marsh

A pipeline break on 23 April 1985 near Nairn, Louisiana, resulted in the release of approximately 300 barrels of Louisiana crude oil into a brackish marsh dominated by a vegetative mixture of Spartina patens, S. alterniflora and Distichlis spicata. Since the impact of oil spills on brackish marshes has received little attention, we initiated this investigation to assess the post-spill status of the vegetation. Sixty-eight randomly selected plots located on 15 transects which traverse the complete study area were sampled for various vegetative cover parameters. The major impact of the spill was confined to the 50-acre (20-ha) marsh located immediately around the pipeline rupture. The oil caused a 64% reduction in live vegetative cover (adjusted for differences in total percentage cover among plots) in this marsh 3 months after the spill. This high plant mortality from a relatively low oil dosage (estimated at 0·28 liters/m2) was probably due to the contact of the oil with a large percentage (about 30–70%) of the photosynthetic leaf surfaces of the vegetation and the penetration of the oil into the marsh substrate.

Salt marsh‐mangrove ecotones: using structural gradients to investigate the effects of woody plant encroachment on plant–soil interactions and ecosystem carbon pools

Summary 1.Changing winter climate extremes are expected to result in the poleward migration of mangrove forests at the expense of salt marshes. Although mangroves and marshes are both highly valued ecosystems, the ecological implications of mangrove expansion have not been fully investigated. 2.Here we examined the effects of mangrove expansion on below-ground properties related to peat development and carbon storage. We investigated plant-soil interactions in marshes and across mangrove forest structural gradients in three locations in the northern Gulf of Mexico (USA). We compared our results to those from terrestrial grasslands where the effects of woody plant encroachment are often influenced by rainfall and plant traits. 3.Abiotic conditions at our study locations differed, particularly in terms of physicochemical properties related to precipitation. Marsh species composition, marsh above-ground biomass, and mangrove forest structural complexity also varied across these locations. Marshes in the driest location (Central Texas) had higher salinities and were dominated by low biomass succulent plants and lower soil carbon pools. Marshes in the wetter, less saline locations (Louisiana and North Florida) contained high biomass grasses and higher soil carbon pools. 4.At all locations, above-ground biomass and above-ground carbon pools were higher in mangroves than marshes; however, below-ground soil carbon pools were only higher in mangroves than marshes in the driest location. In the wetter locations, the linkages between mangrove forest structure and soil properties were minimal or not significant. However, in the driest location, there was a significant increase in soil properties related to peat development and carbon storage with increased mangrove forest structural development. 5.Synthesis: Our results indicate that the ecological implications of woody plant encroachment in tidal saline wetlands are dependent upon precipitation controls of plant-soil interactions. Although the above-ground effects of mangrove expansion are consistently large, below-ground influences of mangrove expansion appear to be greatest along low-rainfall coasts where salinities are high and marshes being replaced are carbon poor and dominated by succulent plants. Collectively, these findings complement those from terrestrial ecosystems and reinforce the importance of considering rainfall and plant-soil interactions within predictions of the ecological effects of woody plant encroachment. This article is protected by copyright. All rights reserved.

A comparison of indicators of sublethal salinity stress in the salt marsh grass, Spartina patens (Ait.) Muhl.

Wetland plant communities in coastal Louisiana are degrading, resulting in the loss of live emergent vegetation and subsequent succession to open water. Saltwater intrusion has resulted from the construction of navigation canals through the marshes; the subsequent salinity increase is one of the potential sources of sublethal stress on plants. Greenhouse experiments were conducted on Spartina patens (Ait.) Muhl. to compare the usefulness of several indicators for the detection of salinity stress. CO2 uptake, leaf expansion, proline concentration and live aboveground biomass displayed significant responses to the salinity levels employed as treatments (0, 7, 14, 21 and 28 ppt.). CO2 exchange was the only indicator showing a significant response within 7 days of the initiation of treatments (measurements were made at 7, 14 and 42 days).