Marsh Elevation Research Articles

The rôle of submerged aquatic vegetation in influencing the abundance of nekton on contiguous tidal fresh-water marshes

Field experiments were conducted in which submerged aquatic vegetation (SAV) was either removed or added to tidal freshwater marsh creeks and nekton was sampled on adjacent marshes with flume nets. These experiments were designed to determine whether the presence of SAV in tidal creeks influences the abundance of fishes and grass shrimp Palaemonetes pugio Holthuis on contiguous marshes. In experiments where SAV was removed from tidal creeks, the number of grass shrimp on adjacent marshes decreased, but the average density of fishes, mostly mummichogs Fundulus heteroclitus (Linnaeus) and banded killifish F. diaphanus (Lesueur), was not reduced. The abundance of grass shrimp increased on marshes where artificial SAV was added to adjacent tidal creeks without natural SAV, but the abundance of fish did not increase. The density of marsh plants and marsh elevation were investigated as additional factors that could influence the abundance of nekton on the marsh surface, but we found no evidence that these factors were important. We conclude that the proximity of SAV and the depth of adjacent tidal creeks are the most important factors that influence the abundance of nekton on tidal freshwater marshes.

Geomorphological control of subsurface hydrology in the creekbank zone of tidal marshes

A combination of field and numerical modeling methods was used to assess porewater movement in a narrow (20 m) Spartina marsh which was flooded regularly by tidal waters. Soil composition and soil hydraulic properties did not vary across the marsh or with depth. Hydraulic head was monitored on a transect perpendicular to the creekbank. During exposure of the marsh surface, hydraulic gradients were predominantly horizontal; vertical gradients were small or zero. Subsurface flow was directed from the marsh interior toward the creekbank. Approximately 141 of pore water were discharged laterally to the adjacent tidal creek per meter of creekbank over a complete tidal cycle. A numerical hydrological model was modified to simulate subsurface hydraulics in the creekbank vicinity of regularly flooded tidal marshes. The model was parameterized to represent soil conditions, tidal fluctuations and topography at the field site. Observed changes in hydraulic head over complete tidal cycles were accurately predicted by the model. Model simulations identified the vertical infiltration of creek water into the marsh surface at the onset of tidal flooding as the primary source (66%) for the replacement of water drained at the creekbank. Significant replacement (31%) also occurred as discharge from the interior marsh. Horizontal recharge at the creekbank was minimal (3%). A sensitivity analysis was conducted with the model to assess the relative importance of geomorphological factors and soil properties in controlling pore water export at the creekbank of tidal marsh soils. Each parameter was varied systematically over a realistic range for field conditions. Changes in marsh elevation exerted greater control over creekbank discharge than changes in soil hydraulic properties. More rapid turnover of pore water near creekbanks of higher elevation marshes is hypothesized.

Determinants of Pattern in a New England Salt Marsh Plant Community

In New England salt marshes, Spartina alterniflora dominates the low–marsh habitat, which is covered daily by tides. The high–marsh habitat, which is not flooded daily, is dominated on its seaward border by Spartina patens, and on its terrestrial border by Juncus gerardi. Each of these vegetation zones has a characteristic suite of physical factors associated with differences in tidal inundation. In particular, substrate redox increases and salinity decreases with decreasing marsh elevation. Although correlations between physical factors and the occurrence of specific marsh plants have been suggested to be causal, a 5–mo transplant experiment suggested that the distribution of perennials across the marsh does not correspond to their potential performance across the marsh in the absence of surrounding vegetation. While the high–marsh perennials appear to be restricted to the high–marsh habitat by harsh physical conditions in the low–marsh habitat, the low–marsh dominant, S. alterniflora, is capable of vigorous growth across the entire marsh and appears to be excluded from the high–marsh habitat by the high–marsh perennials. Throughout the high marsh, two other plant species, Distichlis spicata and Salicornia europaea, are found associated with areas that have been disturbed recently. Physical disturbance, in the form of mats of dead plant material (wrack) rafted by tides onto the marsh, is most severe in the spring and early summer, and decreases with increasing marsh elevation. Differential plant mortality results from short–term disturbance events. D. spicata and S. alterniflora are more tolerant of wrack burial than are the other marsh plants, and short–term disturbance increases the relative abundance of these species in the community. Longer lasting disturbance events kill all the underlying vegetation, leaving discrete bare patches throughout the high marsh. D. spicata rapidly colonizes these patches with vegetative runners, while S. alterniflora and Sa. europaea recruit to these patches by seed. The relative abundance of these plants in recently created bare patches exceeds greatly their relative abundance in the surrounding vegetation. Over time, however, these early colonizers are overgrown and displaced in high–marsh patches by S. patens and J. gerardi, which grow slowly, as dense turfs of roots, rhizomes, and tillers. Physical disturbance and interspecific competition appear to be major determinants of the spatial pattern of marsh plant communities. These processes will need to be considered in relation to edaphic factors in elucidating the underlying mechanisms of salt marsh plant zonation.

Factors affecting short-term variability in sediment pH as a function of marsh elevation in a Virginia mesohaline marsh

An analysis of 5 days of nearly continuously recorded sediment temperature, pH, and radiation measurements along a transect in a mesohaline marsh in Virginia suggests there was a shift in control over short-term pH variability from tidal inundation to radiation with height of the marsh surface at least in the surficial sediments. There was little evidence for tidal control of pH variability at depth in the sediment column. However, biological control of subsurface pH variability was evident both near the tidal creek and in the high marsh. Sediment pH means were generally highest toward the tidal creek and lower in the high marsh. The latter zone had the highest pH variability with diurnal pH excursions up to two units being observed during the experimental period. It is hypothesized that macrophyte transpiration or a series of interlocking mechanisms associated with photosynthesis and microbial activity were responsible for the pH excursions observed at depth in the marsh, since the rapid changes in pH were triggered at sunrise and sunset. The large excursions in sediment pH in the high marsh rhizosphere suggest that geochemical activity may be dynamic over the diurnal cycle.

Relationship of Marsh Elevation, Redox Potential, and Sulfide to Spartina alterniflora Productivity

AbstractData presented suggests that the short height form of Spartina alterniflora observed in inland areas of Louisiana Gulf Coast marshes is caused by toxic concentrations of sulfide, a result of slightly lower elevation and subsequently lower sediment redox potential than the adjacent productive streamside marsh. It is speculated that sulfide may limit growth by preventing nitrogen uptake and root development. Total sulfide concentrations as high as 250 µg g−1 of sediment and free sulfide concentrations in the order of 150 µg g−1 were found in the inland sediment. The inland sediment contained higher NH+4‐N concentrations at depth than the streamside sediment. The higher NH+4‐N concentrations closely paralleled the increasing sulfide concentrations and was inversely related to rooting depth. Maximum NH+4‐N concentrations of 60 and 16 µg N g−1 of dry soil was observed in the inland and streamside sediment, respectively. Redox potential was higher at the surface sediment of the more productive streamside marsh compared to the adjoining inland marsh. The higher redox potential is mainly due to the higher elevation of the streamside area that permitted greater drainage. This in turn allowed the development of more oxidized sediment containing lower sulfide levels. Observed differences in salinity and plant‐dissolved nutrients were not great enough to restrict plant growth.

Relationship of soil properties to standing crop biomass of Spartina alterniflora in a Louisiana marsh

Chemical and physical properties of salt marsh soils in Barataria Bay, Louisiana were measured and related to total standing crop biomass of Spartina alterniflora Loisel. Marsh sites on the natural levees alongside streams and bays supported taller and denser stands of S. alterniflora than sites inland from the streams. Soil density decreased with increasing distance from the streams and water bodies; it varied from 0·37 to 0·19 g cm −3. Organic carbon ranged from 6 to 16% on a weight basis. Aerial standing crop was positively correlated with soil density and negatively associated with soil carbon content and C N ratio. Neither extractable soil P, K, Mg, Ca, Na, or total soil N expressed on a dry weight soil basis was significantly related to growth of S. alterniflora. However, each of these soil constituents when converted to a soil volume expression (μg cm −3) was positively correlated with plant growth. This correlation was apparently a result of the association of these constituents with the content of mineral matter in the soil. Influx of sediments which helps maintain the marsh elevation presumably supplies the nutrient-rich mineral component in these soils.