C.J. Smith

Fate of riverine nitrate entering an estuary: I. Denitrification and nitrogen burial

Fate of riverine nitrate entering a well defined turbid estuary receiving discharges from the Atchafalaya River, a distributary of the Mississippi River, was determined. Seasonal distribution of NO3 and its transformations were measured in Four League Bay (9,300 ha). Denitrification was estimated by incubating wet samples in the presence of acetylene and monitoring N2O production. The annual sediment accumulation of N was also determined within the bay and within the adjacent marshes. Nitrogen accumulation ranged from 6.0 to 23 gN per m2 per yr on the marsh and 6.1 to 11.2 gN per m2 per yr in the bay. Denitrification in this system was controlled by the availability of NO3− with fluxes ranging from 2 to 70 ngN per g per hr. The annual (N2O +N2)-N emission was equivalent to 142 and 120 μg per g or 2.1 and 1.7 gN per m2 from the 5 bay and 5 marsh stations, respectively. Approximately 1.95×105 kgN, predominantly as N2, is being returned to the atmosphere via denitrification. We estimate this to be equivalent to 50% of the riverine NO3− entering this estuary. A significant amount was also assimilated within the estuary.

Carbon dioxide emission and carbon accumulation in coastal wetlands

Abstract Direct measurements of CO 2 fluxes were made in salt, brackish and freshwater marshes and parallel adjacent open water areas in Barataria Basin, Louisiana. Vertical flux density was determined by monitoring the accumulation of CO 2 in aluminum chambers placed over the water or sediment surfaces. Annual CO 2 fluxes were 418, 180 and 618 g Cm −2 from the salt, brackish and freshwater marsh, respectively. Water bodies adjacent to the marsh evolved 103, 54 and 242 g CO 2 -Cm −2 yr −1 to the atmosphere from saline, brackish and freshwater lakes, respectively. The role these marshes play in serving as a major carbon sink was determined from the carbon content of the sediment, vertical accretion rates and the bulk density of the sediment. Accretion rates were calculated from the depth in the sediment of the 1963 horizon, the year of peak 137 Cs fallout. Net carbon accumulation was essentially the same in all three marshes; 183, 296 and 224 g Cm −2 yr −1 from the salt, brackish and fresh marsh, respectively. Data presented suggest a limited net export of carbon from these coastal marshes. A large percentage of fixed carbon remained on the marsh, being immobilized in accretionary processes or lost to the atmosphere as CO 2 .

Nitrous oxide emission from Gulf Coast wetlands

Nitrous oxide evolution may contribute to partial destruction of the ozone layer in the stratosphere. A two year study of the release of N2O from adjoining salt, brackish, and fresh marsh sediment indicates that the annual emission was 31, 48, and 55 mg N m−2 respectively. Emission from open water area was less than the corresponding emission from the marsh sediment. In vitro experiments indicate that the N2O emission was increased when the sediment was drained for extended periods of time. The addition of NO3− significantly increased the rate of N2O evolution, indicating that a large potential for denitrification exists in the anoxic sediment. Appreciable losses of N2O would only be expected when the marshes receive an extraneous source of nitrate such as sewage and/or wastewater. The contribution of the Gulf Coast wetlands to the atmospheric N2O balance is estimated to be 3.3 × 109 g N2O. The maximum average daily emission was equivalent to 1.5 g N2O-N ha−1, which is less than the measured emission from uncultivated soils (Mosieret al., 1981) but greater than the estimates from noncropped land (CAST, 1976).

Effect of oil on salt marsh biota: Methods for restoration

Abstract South Louisiana crude was applied to replicated plots in a Louisiana Spartina alterniflora salt marsh. Various marsh restoration methods were evaluated for mitigating the impact of crude oil on the marsh biota. Oiling the marsh caused no reduction in macrophyte production as compared with the non-oiled plots. Thus the cleanup treatment showed no beneficial effects to S. alterniflora. Likewisw, there was no oil-induced mortality for the marsh macrofauna or meiofauna. In Louisiana Gulf Coast salt marshes, which have a low sensitivity to oil as shown in this study, the best response is no cleanup action at all.

The effect of sediment redox potential on nitrogen uptake, anaerobic root respiration and growth of Spartina alterniflora loisel

Abstract A system developed for growing plants in estuarine sediment at controlled redox potential (Eh) was used to study the effect of Eh on anaerobic root respiration, uptake of added labelled nitrogen and photosynthetic rates of Spartina alterniflora Loisel. Plant growth, estimated by CO 2 fixation and nitrogen uptake, was not affected by sediment redox potential ranging from oxidized (+500 mV) to strongly reducing (−200 mV). Anaerobic root respiration, evident by alcohol dehydrogenase activity, increased with decreasing sediment redox potential. The results suggest that neither redox potential per se nor anaerobic root respiration can account for the growth differences of S. alterniflora observed in Louisianas coastal marshes.

Rejuvenated marsh and bay-bottom accretion on the rapidly subsiding coastal plain of U.S. Gulf coast: a second-order effect of the emerging Atchafalaya Delta

Abstract Sedimentation processes in marshes and bays under the influence of the emerging Atchafalaya delta are described. The Atchafalaya delta is a major geological event in the Holocene history of the Mississippi River delta system because it represents the initial stages of a new delta cycle. The delta has resulted from the capture of the Mississippi River flow by the hydraulically more efficient Atchafalaya River. Using 137 Cs and 210 Pb dating techniques, maximum sedimentation of delta-flanking environments was found to occur in the bay bottoms and marshes closest to the emerging delta. Marshes directly under the influence of the emerging delta were accreting at rates as great as 1·4 cm y −1 with appreciable mineral sediment inputs. In addition to increasing shoreline progradation, the added sediment is providing nutrients for increased net plant productivity which provides the organic source needed for vertical marsh accretion. Results show that the major area of coastal progradation will be in the immediate vicinity of the delta and along down-drift coasts. Updrift marshes (East Terrebonne marshes) away from the delta are accreting at a slower rate with smaller mineral sediment input and a larger percentage of organic material. These marshes are likely to continue experiencing rapid rates of deterioration.

A method for determining stress in wetland plant communities following an oil spill

Abstract Gaseous exchange chambers were used to determine the degree of sublethal stress in wetland plant communities. Air samples were collected in syringes and the CO 2 concentration determined on a gas chromatograph equipped with a catalytic conversion unit to quantitatively reduce CO 2 to CH 4 , and a flame ionisation detector. The system provides an appraisal of stress by measuring the relative decrease in gross photosynthesis. The described technique is portable, rapid and can be used in remote areas. It was developed to determine the effect of oil spills on a saltmarsh, but can be used to determine the effects of other man-induced—as well as natural—stresses.

Nitrogen losses from a Louisiana Gulf Coast salt marsh

Abstract Losses of 15 N labelled nitrogen in a Spartina alterniflora salt marsh was measured over three growing seasons. Labelled NH 4 + N equivalent to 100 μg 15 N g −1 of dry soil was added in four instalments over an eight week period. Recovery of the added nitrogen ranged from 93% 5 months after addition of the NH 4 + N to 52% at the end of the third growing season which represented a nitrogen loss equivalent to 3·4 gNm −2 . The availability of the labelled NH 4 + N incorporated into the organic fraction was estimated by calculation of the rate of mineralization. The time required for mineralization of 1% of the tagged organic N increases progressively with succeeding cuttings of the S. alterniflora and ranged from 152 to 299 days. Only 2% of the nitrogen applied as 15 N labelled plant material to the marsh surface in the fall could be accounted for in S. alterniflora the following season.

Methane production in Mississippi River deltaic plain peat

Abstract Seasonal methane production in a coastal peat deposit was studied. Dissolved methane content in interstitial water increased with increasing depth. Methane production rate was related to temperature in the 0–40 cm depth, but was relatively constant at depths below 40 cm. Maximal methane content was found in spring and lowest content in November to January. Production rates were pH dependent, with an optimum rate of methanogenesis at pH 7.7. The accumulation of metabolic end products produced within the peat-water matrix appear to be important in limiting methanogenesis in wetland ecosystems. The reserve of methane in the interstitial water of the brackish marsh was estimated to be 15.8 g CH 4 m −2 or 221 CH 4 per m −2 . Results suggest that the vast peat deposits in the Mississippi deltaic plain could possibly be a potential source of renewable energy if in situ method for removal of the produced methane could be developed.

Heavy metal concentrations along the Louisiana coastal zone

Abstract Cores were taken from seven locations in southern Louisiana and analyzed for concentrations of heavy metals. Sedimentation rates for the locations were determined using the 137 Cs dating technique. Correlations of metals with depth were calculated using absolute, aluminum normalized, and iron normalized concentrations. Correlations indicated recent increases at several sites (Lake Palourde, Manchac Pass, and Wax Lake Outlet) for several metals (Pb, Cd, Cr, Ni, Zn) when profiles were normalized to aluminum. Metal profiles from rapidly-accreting areas (Atchafalaya and Four League Bay) did not show historical increases comparable to areas accreting less rapidly (e.g., Wax Lake Outlet, Manchac Pass, and Lake Palourde).