Erick M. Swenson

Chromophoric dissolved organic matter (CDOM) variability in Barataria Basin using excitation―emission matrix (EEM) fluorescence and parallel factor analysis (PARAFAC)

Chromophoric dissolved organic matter (CDOM) variability in Barataria Basin, Louisiana, USA,was examined by excitation emission matrix (EEM) fluorescence combined with parallel factor analysis (PARAFAC). CDOM optical properties of absorption and fluorescence at 355nm along an axial transect (36 stations) during March, April, and May 2008 showed an increasing trend from the marine end member to the upper basin with mean CDOM absorption of 11.06 + or - 5.01, 10.05 + or - 4.23, 11.67 + or - 6.03 (m(-)(1)) and fluorescence 0.80 + or - 0.37, 0.78 + or - 0.39, 0.75 + or - 0.51 (RU), respectively. PARAFAC analysis identified two terrestrial humic-like (component 1 and 2), one non-humic like (component 3), and one soil derived humic acid like (component 4) components. The spatial variation of the components showed an increasing trend from station 1 (near the mouth of basin) to station 36 (end member of bay; upper basin). Deviations from this increasing trend were observed at a bayou channel with very high chlorophyll-a concentrations especially for component 3 in May 2008 that suggested autochthonous production of CDOM. The variability of components with salinity indicated conservative mixing along the middle part of the transect. Component 1 and 4 were found to be relatively constant, while components 2 and 3 revealed an inverse relationship for the sampling period. Total organic carbon showed increasing trend for each of the components. An increase in humification and a decrease in fluorescence indices along the transect indicated an increase in terrestrial derived organic matter and reduced microbial activity from lower to upper basin. The use of these indices along with PARAFAC results improved dissolved organic matter characterization in the Barataria Basin.

Organic and Inorganic Contributions to Vertical Accretion in Salt Marsh Sediments

The contribution of organic and inorganic constituents to recent vertical accretion rates (since 1963/4) was estimated for 141 salt marshes ranging from New England to the Gulf of Mexico. The range of vertical accretion and inorganic accumulation rates were 0.09 to 1.78 cm y−1, and 0.01 to 0.53 g cm y−1, respectively. The volume of the accumulated organic and inorganic in all salt marshes averaged 3.8 and 4.9%, respectively, of the total, which is relatively low among soil types. The remaining soil volume is water and air. There was a direct relationship between vertical accretion and organic accumulation that explained 59% of the variation for all samples combined. In contrast, the bulk density is strongly and directly related to inorganic content, but not the vertical accretion rate. A multiple regression equation describing the vertical accumulation as a function of mineral and organic accumulation suggests that organic accumulation is five times more important than inorganic accumulation for East coast salt marshes (n=19; weight basis), but that inorganic content is a statistically-insignificant factor for Gulf of Mexico salt marshes (n=122), or for all salt marshes examined (n=141). A simple linear regression showed that a 1 cm rise in salt marsh elevation was composed of 10.9 g of organic matter. A threshold level of 0.02 g organic matter y−1 accumulation can continue without inorganic accumulation. It appears that it is the accumulation of organic matter that controls inorganic accumulation in established marshes, not the reverse. These results document the dominant role of below ground plant material in maintaining salt marshes once they are established. When wetland hydrology is altered, it is the organic soil constituents that are affected (through oxidation or plant growth below ground), thus explaining salt marsh conversion to open water through indirect changes in hydrology. Salt marsh management and restoration efforts would do well to keep in mind the plant’s health, especially belowground, if the long-term and effective strategies are to be successfully implemented. The biological components, not the geological components, appear to control the fate of established salt marshes.

Spoil banks: Effects on a coastal marsh water-level regime

Abstract Above- and below-ground water-level fluctuations were measured in the marshes south of New Orleans, Louisiana, between November 1982 and December 1983. The purpose of the program was to define the basic marsh water-level regime and to investigate how canal spoil banks may influence the water-level regime. Two study areas were used: (1) a control area, defined as a section of marsh with unrestricted hydrologic connection to an adjacent bayou; and, (2) a partially-impounded area, defined as an area with limited hydrologic connection to an adjacent bayou due to the presence of dredged canal spoil banks. Data sources included marsh water levels from gages deployed at three sites within the study areas and water levels from the adjacent bayous obtained from the tide gages of U.S. Army Corps of Engineers. Data from all marsh gage sites showed a similar pattern with a distinct surface and subsurface diurnal tidal signal superimposed upon other, larger scale events. These larger scale events correspond to the passage of weather fronts. The data also indicated that a significant amount of water-level fluctuation in the marshes occurs below ground. A comparison of the control area and the partially-impounded site indicated that the spoil banks changed the response of the marsh water levels to the forcing from the bayou, with the result that the partially-impounded area: (1) was flooded 141 hours more per month than the control area; (2) had fewer, but longer flooding events; (3) had fewer but longer drying events; and (4) reduced water exchange, both above and below ground.

The Impacts of Pulsed Reintroduction of River Water on a Mississippi Delta Coastal Basin

Abstract During the twentieth century about 25% of the wetlands of the Mississippi delta was lost, partially a result of isolation of the river from the delta. River diversions are being implemented to reintroduce river water to the delta plain. We synthesize here the results of extensive studies on a river diversion at Caernarvon, Louisiana, one of the largest diversions in the delta.

Below-ground biomass in healthy and impaired salt marshes

Twelve salt marshes in south Louisiana (USA) were classified as either ‘impaired’ or ‘healthy’ before a summer sample collection of above- and below-ground biomass and determination of sediment accretion rates. The above-ground biomass of plant tissues was the same at both impaired and healthy salt marshes and was not a good predictor of marsh health. However, below-ground root biomass in the upper 30 cm was much lower in the impaired marshes compared to the healthy marshes. Compromises to root production apparently occur before there is an obvious consequence to the above-ground biomass, which may quickly collapse before remedial action can be taken. The subsequent change in vertical position of the marsh surface may be equivalent to many years of accretion, and be irreversible within decades without considerable effort. These results are consistent with the hypothesis that it is the plant’s below-ground accumulation of organic matter, not inorganic matter that governs the maintenance of salt marsh ecosystem in the vertical plane. Reversing the precursor conditions leading to marsh stress before the collapse of the above-ground biomass occurs is therefore a prudent management objective and could be easier than restoration.

Distribution and recovery trajectory of Macondo (Mississippi Canyon 252) oil in Louisiana coastal wetlands

We measured the concentration of petroleum hydrocarbons in 405 wetland sediment samples immediately before the April 2010 Deepwater Horizon disaster led to their broad-scale oiling, and on nine trips afterwards. The average concentrations of alkanes and PAHs were 604 and 186 times the pre-spill baseline values, respectively. Oil was distributed with some attenuation up to 100m inland from the shoreline for alkanes, but increased for aromatics, and was not well-circumscribed by the rapid shoreline assessments (a.k.a. SCAT) of relative oiling. The concentrations of target alkanes and PAHs in June 2013 were about 1% and 5%, respectively, of the February 2011 concentrations, but remained at 3.7 and 33 times higher, respectively, than in May 2010. A recovery to baseline conditions suggests that the concentration of alkanes may be near baseline values by the end of 2015, but that it may take decades for the PAH concentrations to be that low.

A PROPOSED COAST-WIDE REFERENCE MONITORING SYSTEM FOR EVALUATING WETLAND RESTORATION TRAJECTORIES IN LOUISIANA

Wetland restoration efforts conducted in Louisiana under the Coastal Wetlands Planning, Protection and Restoration Act require monitoring the effectiveness of individual projects as well as monitoring the cumulative effects of all projects in restoring, creating, enhancing, and protecting the coastal landscape. The effectiveness of the traditional paired-reference monitoring approach in Louisiana has been limited because of difficulty in finding comparable reference sites. A multiple reference approach is proposed that uses aspects of hydrogeomorphic functional assessments and probabilistic sampling. This approach includes a suite of sites that encompass the range of ecological condition for each stratum, with projects placed on a continuum of conditions found for that stratum. Trajectories in reference sites through time are then compared with project trajectories through time. Plant community zonation complicated selection of indicators, strata, and sample size. The approach proposed could serve as a model for evaluating wetland ecosystems.

Marsh mat flotation in the Louisiana delta plain

Vertical mat movement in relation to surface-water fluctuations was measured for 1 year at three marshes differing in dominant emergent vegetation and location in the Mississippi River delta plain of coastal Louisiana, U.S.A.. The freshwater marsh, dominated by Panicum hemitomon, floated directly with ambient water levels, provided they were high enough to float the mat. Water levels varied by c. 70 cm and mat movement by 55 cm. An intermediate-salinity marsh closer to the Gulf of Mexico and dominated by Sagittaria falcata moved 35 cm vertically during the study period, and water levels moved 70 cm. A brackish marsh, dominated by Spartina patens, moved only 3 cm in response to c. 40 cm of vertical water movement (...)

Tidal and subtidal water volume exchange in an estuarine system

Abstract Estuarine water movements occur over a broad range of time scales. In this study, moored current meter data were used to investigate water volume exchange in the tidal passes of Lake Pontchartrain, Louisiana on the tidal and subtidal time scales. A calibration technique, employing cross-channel measurements of water velocity, was used to calibrate the moored current meters, thus allowing for calculation of volume flow over a 35-day period. The local diurnal tide accounted for 50% of the total volume exchange, the rest being due to subtidal events (frontal passage). This subtidal exchange occurs primarily as large-scale events characterized by volume fluxes up to six times greater than the normal tidal prism. Neglecting this subtidal component in the determination of the volume fluxes for a system such as Lake Pontchartrain, could result in substantial underestimation (by as much as 50%) of these volume and their corresponding material fluxes.

Salinity trends in Louisiana estuaries

If long-term trends in the character of the estuarine salinity field are contributing to land-loss in coastal Louisiana, then evidence of the presence of these trends may exist in historical salinity records. Nonparametric tests for the presence of a trend in the time series of various salinity statistics were performed. The results appear to be strongly influenced by climatological variations of decadal scale. Statistically significant trends were found at many stations. Both increasing and decreasing trends occurred with no apparent underlying spatial pattern. The observed changes during the time covered by the available records were generally too small to be lethal to the dominant marsh plants.