Floating Marshes in Louisiana: Substrate and Hydrolic Characterization

Substrate characteristics and vertical mat movement were determined for maidencane (Panicum hemitomon), bulltongue(Sagittaria falcata) and wire-grass (Spartina patens) dominated marshes located progressively closer to the Gulf of Mexico in Barataria Basin, Louisiana, at Lake Boeuf, Lake Salvador and Bayou des Rigolettes, respectively. The near-surface substrate of the marshes at Lake Boeuf and Lake Salvador was characterized by high mean organic matter content (\u3e 90%, \u3e80%, gig dry weight x 100) and low mean mineral densities (0.007, 0.013 glee) respectively. Mean dry bulk density at these two marshes was low (0.065, 0.068 glee, respectively). Mean dry bulk densities were highest at Bayou des Rigolettes (0.14 glee); the shallow substrate contained approximately equal densities of mineral and organic material. Rubbed and unrubbed fiber contents in the upper 40 to 50 centimeters of mat were highest at Lake Boeuf (83%,73%), intermediate at Lake Salvador (68%,38%) and lowest at Bayou des Rigolettes (50%,29%). Buoyancy among the three sites roughly paralleled the gradient of increasingly fibric underground biomass. The Lake Boeuf marsh surface was above the water table and responded freely to changes in ambient water levels; the Lake Salvador marsh, while still responding freely, floated below the water surface. The wire-grass dominated marsh adjusted to increasing ambient water levels only to a small degree (3 - 4 em). Water levels and marsh-flooding events were compared among the three sites. Absolute water levels were high during the study interval. Under these conditiqns a trend of increasing duration of continuous flooding and total flooding was evident in an inland direction. The depth of flooding was greatest at the Lake Salvador marsh (measured relative to lowest marsh mat levels). The results presented in this study support the hypothesis of buoyant detachment of attached marsh from the subsiding solid substrate (O\u27Neil, 1949) as a response to marsh flooding and an absence of mineral sediment. This suggests that floating marshes are an integral and possibly large part of the Louisiana wetlands

Mat Movement in Coastal Louisiana Marshes: Effect of Salinity and Inundation on Vegetation and Nutrient Levels

The present research compared and contrasted the physical structure of floating and rooted marshes, their differing responses to open-water salinities and inundation, as well as the nutrient distribution in the porewaters and sediment. The effects of the physical differences in the two marsh types on the ocurrence of the dominant emergent vegetation was discussed. The main difference in physical structure of the two marsh types was the presence of a mineral, non-buoyant layer at 25-45 cm depth in the rooted marsh, which could serve as an anchor for the overlying highly organic mat layer found in both marsh types. Porewater salinities in floating marshes tracked open-water salinities more closely than they did in rooted marshes. Under the prevailing, mostly fresh conditions, porewaters in the rooted marsh contained significantly higher salt levels. Here also, there was a more pronounced vertical gradient in salt levels than found in floating marshes. With the three years of data it was possible to demonstrate the more extensive exchange of below-ground water with open waters in the floating marshes, rather than the rooted marsh. Surprisingly, the different hydrodynamics of floating and rooted marshes did not appear to affect inorganic porewater nutrient levels. It appeared that dominant above-ground vegetation determined these levels. The two dominant species of emergent vegetation have clearly contrasting tolerances to ambient salinites and flooding. Thus, the continued persistance of the bore salt-tolerant species in this mostly fresh area is thought to be attributable to the recurring, but infrequent years of high salinity. The significance of floating marshes in the rapidly subsiding Mississippi River Deltaic Plain, with concomittant increases in ambient salinities is obvious. Their potentially unique responses to these environmental forcing functions deserve closer attention when mitigation measures are conceptualized and implemented. It is quite possible that a majority of the low-salinity marshes in the deltaic plain may be floating

Soil Shear Strength Losses In Two Fresh Marshes With Variable Increases In N And P Loading

We measured soil shear strength (SSS) from 2009 to 2018 in two hydrologically distinct freshwater marshes dominated by Panicum hemitomon after nitrogen (N) and phosphorous (P) were applied to the surface in spring. The SSS averaged over 100-cm depth in the floating and anchored marshes declined up to 30% throughout the profiles and with no apparent differences in the effects of the low, medium, and high N + P dosing. Plots with only N or P additions exhibited significant changes in SSS at individual depths below 40 cm for the anchored marsh, but not the floating marsh. The average SSS for the anchored marsh over the entire 100 cm profile declined when N and P were added separately or together. At the floating marsh, however, the SSS decreased when N and P were added in combination, or P alone, but not for the N addition. Increasing nutrient availability to these freshwater marsh soils makes them weaker, and perhaps lost if eroded or uplifted by buoyant forces during storms. These results are consistent with results from multi-year experiments demonstrating higher decomposition rates, greenhouse gas emissions, and carbon losses in wetlands following increased nutrient availability

The magnitude and origin of groundwater discharge to Eastern U.S. and Gulf of Mexico coastal waters

Author Posting. © American Geophysical Union, 2017. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geophysical Research Letters 44 (2017): 10,396–10,406, doi:10.1002/2017GL075238.Fresh groundwater discharge to coastal environments contributes to the physical and chemical conditions of coastal waters, but the role of coastal groundwater at regional to continental scales remains poorly defined due to diverse hydrologic conditions and the difficulty of tracking coastal groundwater flow paths through heterogeneous subsurface materials. We use three-dimensional groundwater flow models for the first time to calculate the magnitude and source areas of groundwater discharge from unconfined aquifers to coastal waterbodies along the entire eastern U.S. We find that 27.1 km3/yr (22.8–30.5 km3/yr) of groundwater directly enters eastern U.S. and Gulf of Mexico coastal waters. The contributing recharge areas comprised ~175,000 km2 of U.S. land area, extending several kilometers inland. This result provides new information on the land area that can supply natural and anthropogenic constituents to coastal waters via groundwater discharge, thereby defining the subterranean domain potentially affecting coastal chemical budgets and ecosystem processes.National Science Foundation Grant Number: EPS-1208909; NASA Carbon Cycle Science Grant Number: NNX14AM37G2018-04-2

Processes Contributing to Resilience of Coastal Wetlands to Sea-Level Rise

The objectives of this study were to identify processes that contribute to resilience of coastal wetlands subject to rising sea levels and to determine whether the relative contribution of these processes varies across different wetland community types. We assessed the resilience of wetlands to sea-level rise along a transitional gradient from tidal freshwater forested wetland (TFFW) to marsh by measuring processes controlling wetland elevation. We found that, over 5 years of measurement, TFFWs were resilient, although some marginally, and oligohaline marshes exhibited robust resilience to sea-level rise. We identified fundamental differences in how resilience is maintained across wetland community types, which have important implications for management activities that aim to restore or conserve resilient systems. We showed that the relative importance of surface and subsurface processes in controlling wetland surface elevation change differed between TFFWs and oligohaline marshes. The marshes had significantly higher rates of surface accretion than the TFFWs, and in the marshes, surface accretion was the primary contributor to elevation change. In contrast, elevation change in TFFWs was more heavily influenced by subsurface processes, such as root zone expansion or compaction, which played an important role in determining resilience of TFFWs to rising sea level. When root zone contributions were removed statistically from comparisons between relative sea-level rise and surface elevation change, sites that previously had elevation rate deficits showed a surplus. Therefore, assessments of wetland resilience that do not include subsurface processes will likely misjudge vulnerability to sea-level rise

Processes Contributing to Resilience of Coastal Wetlands to Sea-Level Rise

The objectives of this study were to identify processes that contribute to resilience of coastal wetlands subject to rising sea levels and to determine whether the relative contribution of these processes varies across different wetland community types. We assessed the resilience of wetlands to sea-level rise along a transitional gradient from tidal freshwater forested wetland (TFFW) to marsh by measuring processes controlling wetland elevation. We found that, over 5 years of measurement, TFFWs were resilient, although some marginally, and oligohaline marshes exhibited robust resilience to sea-level rise. We identified fundamental differences in how resilience is maintained across wetland community types, which have important implications for management activities that aim to restore or conserve resilient systems. We showed that the relative importance of surface and subsurface processes in controlling wetland surface elevation change differed between TFFWs and oligohaline marshes. The marshes had significantly higher rates of surface accretion than the TFFWs, and in the marshes, surface accretion was the primary contributor to elevation change. In contrast, elevation change in TFFWs was more heavily influenced by subsurface processes, such as root zone expansion or compaction, which played an important role in determining resilience of TFFWs to rising sea level. When root zone contributions were removed statistically from comparisons between relative sea-level rise and surface elevation change, sites that previously had elevation rate deficits showed a surplus. Therefore, assessments of wetland resilience that do not include subsurface processes will likely misjudge vulnerability to sea-level rise

Water Level Observations in Mangrove Swamps During Two Hurricanes in Florida

Little is known about the effectiveness of mangroves in suppressing water level heights during landfall of tropical storms and hurricanes. Recent hurricane strikes along the Gulf Coast of the United States have impacted wetland integrity in some areas and hastened the need to understand how and to what degree coastal forested wetlands confer protection by reducing the height of peak water level. In recent years, U.S. Geological Survey Gulf Coast research projects in Florida have instrumented mangrove sites with continuous water level recorders. Our ad hoc network of water level recorders documented the rise, peak, and fall of water levels (6 0.5 hr) from two hurricane events in 2004 and 2005. Reduction of peak water level heights from relatively in-line gages associated with one storm surge event indicated that mangrove wetlands can reduce water level height by as much as 9.4 cm/km inland over intact, relatively unchannelized expanses. During the other event, reductions were slightly less for mangroves along a river corridor. Estimates of water level attenuation were within the range reported in the literature but erred on the conservative side. These synoptic data from single storm events indicate that intact mangroves may support a protective role in reducing maximum water level height associated with surge

Elevated temperature and nutrients lead to increased N2O emissions from salt marsh soils from cold and warm climates

Salt marshes can attenuate nutrient pollution and store large amounts of ‘blue carbon’ in their soils, however, the value of sequestered carbon may be partially offset by nitrous oxide (N2O) emissions. Global climate and land use changes result in higher temperatures and inputs of reactive nitrogen (Nr) into coastal zones. Here, we investigated the combined effects of elevated temperature (ambient + 5℃) and Nr (double ambient concentrations) on nitrogen processing in marsh soils from two climatic regions (Quebec, Canada and Louisiana, U.S.) with two vegetation types, Sporobolus alterniflorus (= Spartina alterniflora) and Sporobolus pumilus (= Spartina patens), using 24-h laboratory incubation experiments. Potential N2O fluxes increased from minor sinks to major sources following elevated treatments across all four marsh sites. One day of potential N2O emissions under elevated treatments (representing either long-term sea surface warming or short-term ocean heatwaves effects on coastal marsh soil temperatures alongside pulses of N loading) offset 15–60% of the potential annual ambient N2O sink, depending on marsh site and vegetation type. Rates of potential denitrification were generally higher in high latitude than in low latitude marsh soils under ambient treatments, with low ratios of N2O:N2 indicating complete denitrification in high latitude marsh soils. Under elevated temperature and Nr treatments, potential denitrification was lower in high latitude soil but higher in low latitude soil as compared to ambient conditions, with incomplete denitrification observed except in Louisiana S. pumilus. Overall, our findings suggest that a combined increase in temperature and Nr has the potential to reduce salt marsh greenhouse gas (GHG) sinks under future global change scenarios

Submarine groundwater discharge to Tampa Bay : nutrient fluxes and biogeochemistry of the coastal aquifer

This paper is not subject to U.S. copyright. The definitive version was published in Marine Chemistry 104 (2007): 85-97, doi:10.1016/j.marchem.2006.10.012.To separately quantify the roles of fresh and saline submarine groundwater discharge (SGD), relative to that of rivers, in transporting nutrients to Tampa Bay, Florida, we used three approaches (Darcy's Law calculations, a watershed water budget, and a 222Rn mass-balance) to estimate rate of SGD from the Pinellas peninsula. Groundwater samples were collected in 69 locations in the coastal aquifer to examine biogeochemical conditions, nutrient concentrations and stoichiometry, and salinity structure. Salinity structure was also examined using stationary electrical resistivity measurements. The coastal aquifer along the Pinellas peninsula was chemically reducing in all locations sampled, and that condition influences nitrogen (N) form and mobility of N and PO43−. Concentrations of NH4+, PO43− and ratio of dissolved inorganic N (DIN) to PO43− were all related to measured oxidation/reduction potential (pε) of the groundwater. Ratio of DIN: PO43− was below Redfield ratio in both fresh and saline groundwater. Nitrogen occurred almost exclusively in reduced forms, NH4+ and dissolved organic nitrogen (DON), suggesting that anthropogenic N is exported from the watershed in those forms. In comparison to other SGD studies, rate of PO43− flux in the seepage zone (μM m− 2 d− 1) in Tampa Bay was higher than previous estimates, likely due to 1) high watershed population density, 2) chemically reducing conditions, and 3) high ion concentrations in fresh groundwater. Estimates of freshwater groundwater flux indicate that the ratio of groundwater discharge to stream flow is not, vert, similar 20 to 50%, and that the magnitudes of both the total dissolved nitrogen and PO43− loads due to fresh SGD are not, vert, similar 40 to 100% of loads carried by streams. Estimates of SGD based on radon inventories in near-shore waters were 2 to 5 times greater than the estimates of freshwater groundwater discharge, suggesting that brackish and saline SGD is also an important process in Tampa Bay and results in flux of regenerated N and P from sediment to surface water.This work was supported by a USGS Mendenhall Postdoctoral Fellowship to K.D.K. and by the USGS Coastal and Marine Geology Program's (CMGP) Tampa Bay Project