Part of the Perez-Swarzenski team, Brad Segura uses a rod surface elevation table (RSET) to measure vertical marsh surface elevation change.
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A task related to the climate study is a field study directed by a USGS team led by Brian Perez and Chris Swarzenski. Together with Stephen Faulkner (USGS) and Robert Gambrell (LSU), they are investigating the hydrology and soil biogeochemistry of both impacted and unimpacted marshes in order to better understand the ways in which different soil types respond to drought conditions
Part of the Perez-Swarzenski team, Brad Segura uses a rod surface elevation table (RSET) to measure vertical marsh surface elevation change. |
While seemingly straightforward, this study, in fact, is a perfect illustration of the sorts of complexities that are confronted while investigating the brown marsh phenomenon as a whole. For instance, there is a gradual change, or gradient, in soil hydrology mechanisms the farther one moves away from surface water. Soils nearest the water tend to receive more lateral than vertical hydration; the farther one moves away from the water source, the balance tips the other direction, with some areas completely dependent on precipitation and runoff. Because it had already been established that smooth cordgrass plants along marsh edges were healthier, the initial problem in designing a study was to find near-neighboring tracts of brown and healthy marsh equidistant from surface water sources so that like settings could be compared.
Yet even this task had its complications, namely because of the compaction and subsidence already taking place in the severely affected brown marsh zones. In other words, not only did the distance from surface water have to be roughly equal, but the elevation of the healthy and afflicted tracts with respect to the surface water had to be selected for as well. In addition, because Perez and Swarzenski's team wanted to discover the role salinity played in the dieback, they were keen to have the paired sites occur in areas that were susceptible to broad fluctuations in salinity across the course of a year.
 Using a cryogenic coring technique, sediments are solidified for ready, accurate analysis by the Perez-Swarzenski team. Above, Troy Olney uses liquid nitrogen to freeze a sample prior to its extraction. |
In the end, the team found four suitable sites with a range of soil types between Bayou Lafourche and Fourleague Bay in which study pairs some 40-50 meters inland could be created: (1) Old Oyster Bayou, nourished by the Atchafalaya's freshwater pulses in the spring and characterized by a clay and fine-silt mineral marsh; (2) Bay Junop, also affected by the Atchafalaya, but characterized by a clayey mineral marsh; (3) Bayou Sale, a site that is affected by the hydrology associated with the Houma Navigation canal and characterized by a more organic marsh; and (4) Lake Felicity, a site entirely dependent upon precipitation and rainfall for its freshwater input and the site with the highest level of organic material (nearly 50%) in its substrate.
As far as the question of salinity and its role as the primary causative agent is concerned, according to Perez, the results from these four paired sites indicate that "salinity alone was not the causative factor." If that had been the case, a threshold event that would have caused any one tract to brown would have most certainly caused its adjacent counterpart--equidistant from surface water and at the same elevation--to do so as well. The question of soil type and its ability to store water, however, is more complicated than the teams findings regarding salinity.
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As far as the question of salinity and its role as the |
Based on preliminary results Faulkner and Gambrell have observed differences in the iron and sulfur chemistry, both of which are sensitive to changes in hydrology. The brown marshes generally have less pyrite and acid-volatile sulfides than the paired healthy marshes in the soils' upper 15 centimeters, a zone where the water table was lowered by the drought. This finding supports their original hypothesis that the drought caused reduced sulfides to oxidize with the potential to increase acidity and soluble metals. Since this is an observation of current field conditions, the results of the laboratory and greenhouse studies will be necessary to help verify the oxidation of sulfides as a primary causative agent.
Another field study is taking an approach that integrates the analysis of both soil characteristics and plant response. Headed by Thomas Michot, an ecologist with USGS, the study is taking place upon the same four pairs of sites that Perez and Swarzenski are studying. While Michot and his group are likewise interested in soil characteristics, they are approaching their task from a physiochemical standpoint. In addition, the team is looking at smooth cordgrass vigor and surveying transects across the eight shared sites to create broader plant species inventories intended to track the natural recovery process of dead and browned areas.
 One of the many fixed sample plots being employed by the Michot study team. |
The physiochemical variables the Michot group are tracking include salinity, pH, sulfides, nutrients (e.g., nitrates, phosphates, and ammonia), and what soil specialists call "redox," the reduction/oxidation potential of the soil (Eh). Their samples are being gathered monthly from four randomly selected one-square-meter plots within each site and are taken at three different depths: the soil surface, 15 centimeters, and 30 centimeters. Within these same plots, the group is also making an array of plant health measurements, including the ratio of live to dead plants, average plant height, maximum height, stem counts, and a three-fold quality assessment of overall plant health. The group is hoping to learn about the variety and associated effects, if any, of soil chemistries upon plant health, not only in the like-to-like soil-settings of the healthy and afflicted pairs, but across the range of soils found in Old Oyster Bayou, Bay Junop, Bayou Sale, and Lake Felicity.
 One of the several boardwalk transects that Michot's team constructed. The boardwalk allows the team to assess plant species makeup as afflicted areas, such as this one, revegetate. |
Finally, in what might be thought of as an extension of the "Status and Trends" subproject, the team is surveying eight transects, one for each site in the study's 2 x 4 design. The healthy sites, made up almost completely of smooth cordgrass, serve as references to the afflicted sites, some of which had turned to mud flats at the height of the dieback. As the afflicted areas revegetate, Michot's team is tracking both the rate of revegetation as well as the multispecies colonization and succession processes as the denuded areas return to largely homogenous tracts of smooth cordgrass.
Revegetation Graph
Click to enlarge.