Marsh Elevation Response to Hurricanes Katrina and Rita and the Effect of Altered Nutrient Regimes

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Our scientific understanding of the marshes along the north shore of Lake Pontchartrain, Louisiana, is limited in terms …

Our scientific understanding of the marshes along the north shore of Lake Pontchartrain, Louisiana, is limited in terms
of the processes required to sustain them and how to best manage them in the face of predicted rising sea levels. Subject
to localized subsidence and urban development, these marshes may also be affected by increased nutrient loading in the
future from proposed Mississippi River diversions and continued urbanization. This study presents data on marsh
surface elevation change across a series of experimental plots located in Big Branch Marsh National Wildlife Refuge,
Louisiana, that were subject to varying additions of phosphorus and nitrogen as well as a lethal herbicide treatment.
These plots were also affected by Hurricanes Katrina and Rita in 2005. The rate of marsh elevation change prior to the
storm suggests these marshes were maintaining elevation in the face of sea-level rise. A dramatic increase in elevation
occurred following the storms but was followed by a proportional decrease in elevation. Soil data indicate the increase
was caused by an influx of highly organic material at all plots. The results show how both storm and nonstorm processes
contribute to elevation change and the maintenance of these marshes in the face of sea-level rise.

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  • 1. Journal of Coastal Research SI 54 166–173 West Palm Beach, Florida Fall 2009 Marsh Elevation Response to Hurricanes Katrina and Rita and the Effect of Altered Nutrient Regimes Denise J. Reed{, Ann M. Commagere{, and Mark W. Hester{ { Pontchartrain Institute for { Department of Biology Environmental Sciences University of Louisiana at Lafayette University of New Orleans Box 42451, Lafayette, LA 70504, U.S.A. 2000 Lakeshore Drive GP 1065, New Orleans, LA 70148, U.S.A. ABSTRACT REED, D.J.; COMMAGERE, A.M., and HESTER, M.W., 2009. Marsh elevation response to Hurricanes Katrina and Rita and the effect of altered nutrient regimes. Journal of Coastal Research, SI(54), 166–173. West Palm Beach (Florida), ISSN 0749-0208. Our scientific understanding of the marshes along the north shore of Lake Pontchartrain, Louisiana, is limited in terms of the processes required to sustain them and how to best manage them in the face of predicted rising sea levels. Subject to localized subsidence and urban development, these marshes may also be affected by increased nutrient loading in the future from proposed Mississippi River diversions and continued urbanization. This study presents data on marsh surface elevation change across a series of experimental plots located in Big Branch Marsh National Wildlife Refuge, Louisiana, that were subject to varying additions of phosphorus and nitrogen as well as a lethal herbicide treatment. These plots were also affected by Hurricanes Katrina and Rita in 2005. The rate of marsh elevation change prior to the storm suggests these marshes were maintaining elevation in the face of sea-level rise. A dramatic increase in elevation occurred following the storms but was followed by a proportional decrease in elevation. Soil data indicate the increase was caused by an influx of highly organic material at all plots. The results show how both storm and nonstorm processes contribute to elevation change and the maintenance of these marshes in the face of sea-level rise. ADDITIONAL INDEX WORDS: Pontchartrain Basin, oligohaline marshes, sedimentation, organic soils, hurricanes. INTRODUCTION which they occur is necessary to determine relative sea level rise rates (eustatic sea level rise plus subsidence) in The future of tidal marshes in Louisiana and throughout Louisiana and thus the rate at which coastal marshes must the United States is threatened by sea-level rise and local accrete vertically to survive. factors such as erosion and subsidence. Moreover, coastal Rapid coastal land loss in Louisiana during the twentieth systems that are sediment starved and constrained on the century (Barras et al., 2003) underscores the need to landward margin by anthropogenic barriers are at increased understand the processes that maintain elevation in coastal risk (Day et al., 2008; Nicholls et al., 2007). Global climate marshes. In addition to sediment supply, organic matter models predict eustasic sea level to rise 0.18 to 0.59 m by 2100 accumulation from plant production is an important compo- (Meehl et al., 2007); however, recent estimates indicate these nent in maintaining marsh accretion processes (Nyman, predictions are too conservative and suggest global sea level DeLaune, and Patrick, 1990). The emergent marshes along will rise a meter or more (Holgate et al., 2007; Pfeffer, Harper, the north shore of Lake Pontchartrain are subject to and O’Neel, 2008; Rahmstorf, 2007a, 2007b; Rohling et al., subsidence and sea-level rise but little information is 2007; Schmith, Johansen, and Thejll, 2007). These rates are available regarding the processes required to sustain them compounded in Louisiana by local rates of subsidence caused and how to best manage them in the face of rising relative sea primarily by Holocene sediment compaction (Roberts, Bailey, level. In the future these marshes may be subject to increased and Kuecher, 1994; Tornqvist et al., 2008), faulting (Dokka, nutrient loading due to proposed diversions of freshwater 2006; Gagliano et al., 2003), and fluid withdrawal (Morton, from the Mississippi River but information on current loading Bernier, and Barras, 2006). Within the Pontchartrain Basin, rates is limited. Cramer, Day, and Conner (1981) calculated subsidence estimates based on releveling surveys of the average annual nutrient concentrations in a Spartina patens National Geodetic Survey’s benchmarks are between 5 and 10 marsh on the north shore of Lake Pontchartain equal to 0.69 mm y21 (Shinkle and Dokka, 2004). Identifying the processes mg L21 for total nitrogen and 0.06 mg L21 for total that contribute to subsidence as well as the degree and rate at phosphorus. Similarly, Waldon and Bryan (1999) calculated average annual concentrations within Lake Pontchartrain DOI:10.2112/SI54-003.1. equal to 0.65 mg L21 of total nitrogen and 0.06 mg L21 of total
  • 2. Marsh Elevation Response 167 phosphorous, and predicted an increase to 0.86 mg-N Ll21 and 0.071 mg-P L21 following a Mississippi River diversion with a maximum flow of 850 m3 s21. Higher nutrient loading rates may result in increased aboveground productivity (DeLaune, Pezeshki, and Jugsujinda, 2005), but may alter patterns of belowground-to-aboveground allocation, accelerate decompo- sition, as well as reduce species diversity (Cizkova-Koncalova, Kvet, and Thompson, 1992; Mendelssohn et al., 1999; Ulrich and Burton, 1985); all of which may influence marsh elevation and decrease the likelihood of sustainability. In addition to proposed Mississippi River diversions, the north shore of Lake Pontchartrain is subject to continued urbani- zation and development. In St. Tammany Parish, Louisiana, residential and urban land use increased over 218% between 1982 and 2000, resulting in the conversion of 22 km2 of marsh to urban development (Beall, Penland, and Cretini, 2001). The combined effects of multiple sources of nutrient loadings are poorly understood. This represents a critical data gap in our ability to successfully manage the remaining north shore marshes in a sustainable manner for the valuable functions that they provide, including water quality improvement, storm protection, and wildlife and fisheries habitat. Under- standing how marsh plant communities respond to present and altered nutrient regimes in the Lake Pontchartrain Basin, especially in terms of how these responses relate to long-term sustainability, is an essential component for the Figure 1. Study site: Big Branch Marsh National Wildlife Refuge, Louisiana. development of sound management strategies and priorities. The north shore of Lake Pontchartrain is also subject to hurricanes and tropical storm impacts. Storm sedimentation tropical storms in coastal marsh sedimentation (Cahoon, has been identified as an important component of landscape 2006; Cahoon et al., 1995b; Nyman, Crozier, and DeLaune, sustainability in coastal Louisiana (Reed, 2002). During the 1995; Reed, Peterson, and Lezina, et al., 2006; Tornqvist et al., conduct of this study, the area was affected by both 2007; Turner et al., 2006), few have been able to examine the Hurricanes Katrina and Rita in August and September of storm effect in the context of both pre- and poststorm changes 2005, respectively. Hurricane Katrina was a Category 5 and across marshes influenced by varying nutrient inputs. cyclone (maximum sustained winds 150 knots at 0900 CDT on August 28, 2005) as it traveled over the warm waters of the METHODS Gulf of Mexico. Although it weakened as it approached the northern Gulf Coast, Katrina still had maximum sustained Study Site winds of 115 knots (Category 4) just two hours before making landfall at Buras, Louisiana. Katrina was also unusually Our sites were established in a brackish marsh community large and the extent of its tropical storm-force and hurricane- dominated by Spartina patens and Schoenoplectus amer- force winds remained nearly the same as its maximum winds icanus within Big Branch Marsh National Wildlife Refuge weakened (Boesch, 2006; Day et al., 2007; Knabb, Rhome, and (Big Branch) along Bayou Lacombe (Figure 1). The refuge is Brown, 2006). Katrina generated a storm surge over 8 m (25 located in St. Tammany Parish, Louisiana, and is the largest, ft) along the Mississippi coast and 5 m (16 ft) in St. Tammany undeveloped natural area on the north shore of Lake Parish (Knabb, Rhome, and Brown, 2006). Hurricane Rita Pontchartrain. Formed in 1994, the refuge was established made landfall near Sabine Pass at the Louisiana–Texas to protect and conserve wildlife and their respective habitats border on September 24, 2005, as a Category 3 hurricane with amidst ongoing development and urbanization in St. Tam- sustained wind speed of 100 knots and a storm surge of at many Parish (Penland, Maygarden, and Beall, 2002). The least 6 m (20 ft) near landfall, 1–3.5 m (4–12 ft) along the coastal marshes at Big Branch Marsh NWR are hydrologi- Louisiana coast and 2 m (6.5 ft) in Lake Pontchartrain cally connected to Lake Pontchartrain via several bayous and (Knabb, Brown, and Rhome, 2006). a network of tidal creeks. Tides in Lake Pontchartain have The results presented here are part of a larger study that mean ranges of approximately 15 cm (Carillo et al., 2001), but examined vegetation productivity and community responses water levels are highly influenced by wind generated to nutrient additions and storm sedimentation. The focus of activities (Li et al., 2008; Signell and List, 2002). Conse- this article is to examine the effect of Hurricanes Katrina and quently, salinity values range considerably across the lake Rita on marsh surface elevation in the oligohaline marshes of with the freshest waters entering via Lake Maurepas (from the Pontchartrain Basin in the context of nonstorm conditions. the west) and the Tangipahoa River (from the north) and While many studies have noted the role of hurricanes and higher salinity waters entering via Lake Borgne and the Journal of Coastal Research, Special Issue No. 54, 2009
  • 3. 168 Reed, Commagere, and Hester release nitrogen (in the form of urea) and phosphorus (Humaphos) pellets were applied via broadcast dispersal during the first 2 years of the study (June 2004, March 2005, and June 2005). Nutrient treatments were not applied in 2006. The lethal herbicide treatment was applied in June 2004 and March 2006 using a backpack sprayer until leaves were lightly covered resulting in 100% mortality. The solution consisted of Rodeo herbicide diluted to 1.5% with active ingredients glycophosphate and N-(phosphonomethyl 1) gly- cine (Meert, 2008). Field Sampling To assess the dynamics of marsh surface elevation under the various scenarios of nutrient loading and disturbance, we established surface elevation tables (SET). The SET is a portable, leveling device designed to sit on a permanent aluminum pipe (i.e., benchmark) that is driven to refusal (approximately 4–6 m) into the ground using a vibracore (Cahoon et al., 2002). In the vicinity of our study site, the Holocene layer is relatively thin and vibracore data (McCarty, 2001) supports our assumption that the SET pipe was anchored into the Pleistocene. The SET consists of an arm that extends outward from the pipe and nine pins at the end of the arm that manually lower to the marsh surface. The pins measure marsh response to changes occurring between the marsh surface and the base of the benchmark, and does not account for processes occurring below the base. The SET is capable of moving in four positions, 90u apart, for Figure 2. Location of the three experimental blocks (triangles) within Big a total of 36 measurements. A total of 15 SETs were Branch Marsh National Wildlife Refuge, Louisiana. Each block consists of established (one in each of the experiment plots). A baseline five SETs, each applied with one of the nutrient treatments: control (C), measurement (in millimeters) was taken in August 2004 at nitrogen only (N), phosphorous only (P), nitrogen and phosphorous (NP), all 15 sites. Subsequent measurements continued biannual- and lethal (L). ly until April 2008 to detect changes relative to the baseline measurement. Mississippi River Gulf Outlet (from the southeast; O’Connell, In August 2005, prior to the passage of Hurricanes Katrina Cashner, and Schieble, 2004). Average monthly rain fall and Rita, surface sediment samples were taken using a 30- varies from 10 to 18 cm and temperatures from 4uC to 33uC mL syringe corer. Four core samples were taken at each SET (Peters and Beall, 2002). site, adjacent to each of the SET positions. Cores were kept on ice for return to the laboratory where they were dried and Experimental Design weighed. Samples were then incinerated in a muffle furnace (450uC for 16 hours) to determine organic matter content by In 2003, we established three blocks, each consisting of five loss-on-ignition. In May 2006, peat cores, 40 cm in length and plots, approximately 25 m east of Bayou Lacombe (Figure 2). 5 cm in diameter, were collected near each of the SET sites Each plot was 4.4 m2 and approximately 6 m apart from one using a Russian peat borer. This device is commonly used another. Permanent boardwalks were built around the blocks (McKee, 2001; Orson, Simpson, and Good, 1990; Watson, to limit marsh disturbance. Additional nitrogen (40 g N m22 2008) to extract cores because compaction is minimized and y21) and phosphorus (30 g P m22 y21) were added in a the core remains relatively undisturbed. Cores were then cut completely cross-classified manner to create four nutrient into 2-cm segments and were processed and analyzed using loading combinations (including the control). In addition, one the same method as syringe cores. The bulk density and lethal treatment (Rodeo herbicide) was used to simulate a organic matter content of the top 2 cm of the Russian peat lethal disturbance. Using a randomized block design, each of borer cores were compared with the data from the syringe the five treatments (control, N only, P only, N + P, and lethal) cores. had three true replicates, yielding a total of 15 experimental plots. The nutrient loading rates were based on reported rates Statistical Analysis from the Caenarvon river diversion project (Lane, Day, and Thibodeaux, 1999). For each SET, the nine pins were averaged at each position, The nutrient loading regime and lethal disturbance resulting in four measurements per SET. SETs were then treatments were implemented in the summer of 2004. Slow- grouped by treatment. Four separate time series were tested Journal of Coastal Research, Special Issue No. 54, 2009
  • 4. Marsh Elevation Response 169 Table 1. Rate of elevation change plus the standard error pre-Katrina, Katrina, post-Katrina, and the full time series for each treatment. Rate of Elevation Change Pre-Katrina Katrina Post-Katrina Full Time Series (7/15/04–8/22/05) (8/22/05–10/21/05) (10/21/05–4/10/08) (7/15/04–4/10/08) Treatment mm y 21 N mm y 21 N mm y 21 N mm y21 N Control 22 6 8 44 1307 6 267 16 249 6 10 60 50 6 5 104 P 5 6 8* 44 743 6 81 24 231 6 5 60 29 6 3 104 N 11 6 8* 44 232 6 158* 16 236 6 10 40 1 6 4* 84 N+P 57 6 15 44 652 6 147 16 243 6 9 60 39 6 4 104 Lethal 44 6 10 44 477 6 95 16 239 6 6 60 31 6 3 104 All 27 6 5 220 688 6 76 88 240 6 4 280 32 6 2 500 An asterisk indicates the slopes failed to be statistically significantly different from zero at the 5% confidence interval. independently of one another: pre-Katrina, Katrina, post- considerable variation, especially among the control plots. Katrina, and entire study period. The net change in surface Furthermore, the mean elevation increase was largest in the elevation relative to the initial baseline was calculated at control plots (215 6 64 mm) and was significantly different ( p each position for each period. Regression analyses were used , 0.05) from the lethal and N treatments (Figure 3b). The to test the linear fit of the elevation data for each treatment lethal and N treatments were also significantly different ( p , over time. The regression statistics tested whether the slope 0.05) from the P treatment. was significantly different from zero, i.e., whether there was a For the post-Katrina measurement period (October 21, significant change in elevation over each of the periods. 2005–April 10, 2008), each treatment showed a significant Univariate analysis of variance conducted at the 5% confi- trend of decreasing elevations (Table 1). For all but N + P and dence level was used to determine if treatment contributed a lethal treatments, the degree in which the rate of elevation significant effect to change in elevation and a post hoc Tukey’s decreased in the posthurricane period was higher than the test was used to test for significance between treatments. degree in which the rate increased in the prehurricane period. Analysis of the total elevation change for this period shows no RESULTS significant differences between treatments (Figure 3c). How- ever, it is worth noting that the greatest change occurred for Analysis of all the treatments combined showed a statisti- treatments that received the greatest elevation increase cally significant trend of increasing elevation during the pre- during the hurricane period, the control and N + P plots. Katrina period followed by a decreasing trend post-Katrina, Figure 4 shows that most of the elevation decrease occurred and an overall increasing trend for the entire period. Further between October 2005 and May 2006, leveling off thereafter. analysis revealed treatment type had a significant effect on The lethal plots did not show signs of recovery (i.e., stem these rates of elevation change. During the pre-Katrina growth) during this period. measurement period (July 15, 2004–August 22, 2005), control, N + P, and lethal treatments exhibited statistically The overall time series (July 15, 2004–April 10, 2008) significant (p , 0.05) positive trends of marsh surface presents significant trends of increasing elevation for all but elevation change (Table 1). The N + P treatment experienced the N treatment (Table 1). Furthermore, for the control and P the largest rate of elevation change, more than twice that of treatments, the overall rate of elevation change is higher than the control treatment. Analysis of the total change in prior to Hurricane Katrina. The total marsh elevation from elevation relative to the baseline measurement revealed July 2004 to April 2008 increased for all treatments, except significant differences between treatment types (Figure 3a). for N treatment, which showed a slight decrease of 24 6 29 Both N and P treatments were statically different from N + P mm (Figure 3d). but were not statically different from one another. Further- Table 2 shows the results of the surface soil analyses from more, lethal was significantly different from the P treatment. before and after the passage of the 2005 storms. The mean The increase in elevation that occurred between July 15, bulk densities of the surface sediments are low, as is usual in 2004, and August 22, 2005, is notably smaller than the fresher marshes in Louisiana (Nyman, Crozier, and DeLaune, change that occurred between August 22 and October 21, 1990). Also similar to oligohaline marshes in other parts of 2005, a period that included the influence of Hurricanes Louisiana, the soil organic matter content is high. The higher Katrina and Rita. The site experienced prolonged flooding value for bulk density and lower organic matter content for and wrack deposition as a result of the hurricanes although the P treatment in August 2005 is driven by one sample with observations suggest the wrack deposition was not distribut- a high bulk density and low organic matter (as reflected in the ed uniformly across all of the experimental plots. The rates of high standard error; Table 2), possibly associated with some elevation change during the hurricane period for all but the N extraneous mineral material on the marsh surface. In treatment were determined significant ( p , 0.05); however, general, poststorm bulk densities are lower and organic high standard error values shown in Table 1 illustrate matter content is more consistent among sites. Journal of Coastal Research, Special Issue No. 54, 2009
  • 5. 170 Reed, Commagere, and Hester Figure 3. Total change in elevation for individual periods: (a) pre- Figure 4. Change in elevation over time relative to the initial measure- Katrina, July 15, 2004–August 22, 2005, (b) Katrina, August 22, 2005– ment. The specific dates reflect the times in which the SETs were October 21, 2005, (c) post-Katrina, October 21, 2005–April 10, 2008, and (d) measured. full time series, July 15, 2004–April 10, 2008. Letters above the standard error bars indicate statistical differences between treatments for each period. Treatments with the same letter failed to be significantly different The magnitude of the increase in elevation on the control (p , 0.05). plots associated with the storm period, 215 6 64 mm between August 22, 200,5 and October 21, 2005, is substantially greater than any of the elevation increases documented by DISCUSSION Cahoon (2006) in his summary of storm influences on Elevation Change vegetation. Most of the marsh sites included in his summary are salt marshes dominated by Spartina alterniflora or The control plot data collected in this study represent the Juncus romerianus. Few studies have previously documented response of Big Branch marshes to natural processes during storm-associated changes in elevation in oligohaline marshes. the period of study. For both the period prior to the storms The surface sediment samples from the control plot for the and for the entire record, including the period of elevation August 2005 sampling (Table 2) show that surficial sediments decrease, the plots not influenced by nutrient additions were similar to those expected in oligohaline marsh soils. It increased by 22 6 8 mm y21 and 49 6 10 mm y21, respectively. does not appear that a significant surface influx of mineral These rates of elevation change are notably greater than the material was associated with the storm impact at this site—in estimates of current relative sea-level rise of the Pontchar- general there is a decrease in the bulk density measurements train Basin, noted as 4.5 mm y21 by Penland and Ramsey 7–8 months after the storm. This is in contrast with the (1990) based on the U.S. Army Core of Engineers tide gauge effects of surface sediment deposition found in previous at Mandeville, Louisiana. Consequently, although the input studies in Louisiana (Cahoon et al., 1995; Turner et al., of materials from the 2005 storms increased elevation, the 2006). Visual observations at the study site suggested that marshes were likely keeping pace with local relative sea-level disrupted marsh soils from other locations were left on many rise even during nonstorm periods. Previous studies have of the plots although no data are available to identify the suggested that periodic storm inputs can maintain marsh source of the material. Additionally, the poststorm data in elevations in Louisiana coastal wetlands remote from riverine Table 2 show remarkable consistency among treatments, influence, allowing vegetation to stay within its elevation implying that the surface sediments of all plots were altered range relative to tidal fluctuation (Baumann, Day, and Miller, by the storm with an influx of low bulk density, moderate 1984; Cahoon and Reed, 1995; Cahoon, Reed, and Day, 1995; organic matter material. Reed, 2002). However, the data collected for this study prior Elevation measures alone cannot point specifically to the to the 2005 storm season show that substantial elevation cause of elevation change. However, the subsequent decrease increase can occur during periods without direct storm in elevation during the 2.5-year poststorm period at the impact. Further analyses of soil composition from other control plots (Figure 3c) suggests that whatever caused the aspects of this study will enable elucidation of the processes initial increase in elevation could not be sustained. This is contributing to the elevation change. consistent with the deposition of highly organic material on Table 2. Bulk density and organic matter content (mean 6 1 standard error) of pre- and poststorm surface sediments. August 2005 May 2006 Treatment Bulk Density (g/cm3) % Organic Matter Bulk Density (g/cm3) % Organic Matter Control 0.17 6 0.05 49.4 6 2.8 0.10 6 0.02 48.1 6 2.8 P 0.24 6 0.05 38.6 6 4.4 0.09 6 0.01 47.7 6 2.1 N 0.15 6 0.05 46.9 6 2.0 0.10 6 0.01 47.6 6 0.2 N+P 0.16 6 0.03 46.2 6 2.5 0.09 6 0.02 41.7 6 7.4 Lethal 0.15 6 0.05 47.9 6 3.3 0.12 6 0.01 47.3 6 4.4 Journal of Coastal Research, Special Issue No. 54, 2009
  • 6. Marsh Elevation Response 171 the marsh surface and its subsequent decomposition. The relatively small areas devoid of vegetation that lie within increase in elevation (over 200 mm) associated with Katrina larger vegetated marsh areas are subject to depositional and Rita would have raised the marsh surface above the level processes, e.g., accumulation of storm rafted and deposited of regular tidal inundation. The tidal range in Lake materials, associated with the broader marsh landscape. Pontchartrain is approximately 15 cm (Carillo et al., 2001), More detailed analysis of the soils data collected for the larger meaning an increase in elevation of only a few centimeters study will be required to clarify the pre- and poststorm status can dramatically change flooding patterns on the marsh of any vegetative growth on those plots. surface (Reed and Cahoon, 1992). Lack of regular flooding would allow aerobic decomposition to gradually reduce the SUMMARY AND CONCLUSIONS volume of the surface soils, thus gradually reducing the elevation of the marsh surface as measured by the SET. This study has documented the effects of Hurricanes Notably, this effect would occur with such an elevation Katrina and Rita on elevation change in oligohaline marshes increase whether the cause of the elevation change was from on the north shore of Lake Pontchartrain. The rates of surface deposition or below ground processes because the elevation change measured prior to the storm period show organic surface soils would be susceptible to such decompo- that the marshes were likely increasing in elevation at a pace sition. that would keep up with local relative sea-level rise. The period including the hurricanes resulted in short-term dramatic increases in elevation. Despite the poststorm loss Influence of Nutrient Treatments of elevation, for the entire study period (July 2004 to April The prestorm patterns of change among treatment plots 2008), the surface elevation of these marshes increased by an shown in Figures 3 and 4 indicate that there may be some average of 32 mm y21. The effects of nutrient additions to the effect of nutrients on elevation change, but the pattern is marshes resulted in a complex pattern of elevation increase complex and will require further elucidation of plant biomass and decrease that cannot be explained without additional and soil physicochemical characteristics from other aspects of data from other aspects of the study. the study to more fully explain. Nevertheless, Table 2 shows Many previous studies have pointed to the importance of that the surface soils in the N plot showed higher bulk density vegetative growth and organic matter accumulation to marsh and lower organic matter content. This may reflect the elevation increases in fresher coastal marshes. This study deposition of more mineral sediment or some hurricane debris supports those assessments by showing that elevation of different character than other sites. However, without increase occurs both with and without storms in marshes further information on the character of the soil and vegetative with high soil organic matter content. Such observations response, it is difficult to account for the addition of nitrogen point to the need for additional study of the specific role of as a causative factor in the lower storm-related elevation storms in marsh elevation change. While previous studies change. Despite these pre- and poststorm differences among have elucidated the role of mineral sediment deposition in plots, all treatments lose elevation during the poststorm saline marshes, the findings of this study have shown the monitoring period, and there are no significant differences potential role of imported organic material in contributing to among treatments. The N plots shower higher amounts of a net increase in marsh elevation. How such material elevation loss (Figure 3c) than the P and lethal treatments. contributes to longer-term marsh sustainability will depend It might be expected that the lethal treatment plots, where upon the elevation of the surface and the rate of decomposi- vegetation had been killed as part of the experiment, would tion of the added material. The role of hurricanes in show lower elevation increases due to the lack of below- contributing mineral sediment to salt marshes has been ground production and the lack of aboveground structure to elucidated in recent years. This study shows that storm ‘‘trap’’ sediments in suspension. Conversely, during the effects in fresher marshes can include the import of organic prestorm period, the lethal treatment plots actually have material and that this can also increase marsh elevations, at the greatest net change in elevation and for the storm period least in the short term. the change in elevation is similar to N + P and P treatments. For the period of the entire record, the lethal treatments show ACKNOWLEDGMENTS significantly lower elevation change than control plots but significantly higher change than the N plots. Cahoon (2006) This work was funded by NOAA Grant #NA06NOS4630026. has pointed to substrate swelling as a potential factor in Thanks to U.S. Fish and Wildlife Service for site access. elevation increase independent of vegetative growth or Brendan Yuill, Carol Wilson, Reginald Graves, Laura Dancer, sediment trapping. However, surface soils in the lethal Matt Kaller, Mark Ford, Justina Horan, Chris Schieble, and treatment showed high organic content post-Katrina, imply- Dana Watzke are thanked for their assistance with boardwalk ing that organic marsh soils were rafted onto the lethal construction and field assistance during the course of this treatment plots and this material then decayed over time as project. Jen Roberts assisted with the statistical analysis. suggested for the control plots. For the entire period of study, the lethal treatment plots increased in elevation at an annual LITERATURE CITED rate of over 30 mm y21, indicating that in situ vegetative Barras, J.; Beville, S.; Britsch, D.; Hartley, S.; Hawes, S.; Johnston, J.; growth may have a limited contribution to elevation change. Kemp, P.; Kinler, Q.; Martucci, A.; Porthouse, J.; Reed, D.; Roy, K.; It is also possible that surface accumulation processes in Sapkota, S., and Suhayda, J., 2003. Historical and projected coastal Journal of Coastal Research, Special Issue No. 54, 2009
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