James T. Morris

RESPONSES OF COASTAL WETLANDS TO RISING SEA LEVEL

Salt marsh ecosystems are maintained by the dominant macrophytes that regulate the elevation of their habitat within a narrow portion of the intertidal zone by accumulating organic matter and trapping inorganic sediment. The long-term stability of these ecosystems is explained by interactions among sea level, land elevation, primary production, and sediment accretion that regulate the elevation of the sediment surface toward an equilibrium with mean sea level. We show here in a salt marsh that this equilibrium is adjusted upward by increased production of the salt marsh macrophyte Spartina alterniflora and downward by an increasing rate of relative sea-level rise (RSLR). Adjustments in marsh surface elevation are slow in comparison to interannual anomalies and long-period cycles of sea level, and this lag in sediment elevation results in significant variation in annual primary productivity. We describe a theoretical model that predicts that the system will be stable against changes in relative mean sea level when surface elevation is greater than what is optimal for primary production. When surface elevation is less than optimal, the system will be unstable. The model predicts that there is an optimal rate of RSLR at which the equilibrium elevation and depth of tidal flooding will be optimal for plant growth. However, the optimal rate of RSLR also represents an upper limit because at higher rates of RSLR the plant community cannot sustain an elevation that is within its range of tol- erance. For estuaries with high sediment loading, such as those on the southeast coast of the United States, the limiting rate of RSLR was predicted to be at most 1.2 cm/yr, which is 3.5 times greater than the current, long-term rate of RSLR.

Limits on the adaptability of coastal marshes to rising sea level

[1] Assumptions of a static landscape inspire predictions that about half of the world’s coastal wetlands will submerge during this century in response to sea‐level acceleration. In contrast, we use simulations from five numerical models to quantify the conditions under which ecogeomorphic feedbacks allow coastal wetlands to adapt to projected changes in sea level. In contrast to previous sea‐level assessments, we find that non‐linear feedbacks among inundation, plant growth, organic matter accretion, and sediment deposition, allow marshes to survive conservative projections of sea‐ level rise where suspended sediment concentrations are greater than ∼20 mg/L. Under scenarios of more rapid sea‐level rise (e.g., those that include ice sheet melting), marsheswill likelysubmerge neartheend ofthe 21stcentury. Our results emphasize that in areas of rapid geomorphic change, predicting the response of ecosystems to climate change requires consideration of the ability of biological processestomodifytheirphysicalenvironment.Citation: Kirwan, M. L., G. R. Guntenspergen, A. D’Alpaos, J. T. Morris, S. M. Mudd, and S. Temmerman (2010), Limits on the adaptability of coastal marshes to rising sea level, Geophys. Res. Lett., 37, L23401,

A 5-yr Record of Aerial Primary Production and Stand Characteristics of Spartina Alterniflora

The purpose of this paper is to document and explain the interannual variability in aboveground primary productivity from salt marshes at North Inlet, South Carolina. A census method of measuring production was applied to salt marsh sites vegetated by the grass Spartina alterniflora, and a statistically significant relationship between stem age and cumulative leaf loss was used to estimate leaf turnover. Aboveground productivity was 2.3 times as large as the positive increment in standing biomass density due to the turnover of stems and leaves, with stem turnover accounting for 33% and leaf turnover 23% of total aboveground productivity. A numerical simulation demonstrated the sensitivity of destructive harvest methods to sampling errors that are propagated by spatial variability. Monthly measurements, made at one site for >5 yr, document a twofold variation in annual aboveground production, which has important implications for the biogeochemistry and trophic dynamics of estauries. Positive correlations i...

How does vegetation affect sedimentation on tidal marshes? Investigating particle capture and hydrodynamic controls on biologically mediated sedimentation

[1] Plants are known to enhance sedimentation on intertidal marshes. It is unclear, however, if the dominant mechanism of enhanced sedimentation is direct organic sedimentation, particle capture by plant stems, or enhanced settling due to a reduction in turbulent kinetic energy within flows through the plant canopy. Here we combine several previously reported laboratory studies with an 18 year record of salt marsh macrophyte characteristics to quantify these mechanisms. In dense stands of Spartina alterniflora (with projected plant areas per unit volume of >10 m −1 ) and rapid flows (>0.4 m s −1 ), we find that the fraction of sedimentation from particle capture can instantaneously exceed 70%. In most marshes dominated by Spartina alterniflora, however, we find particle settling, rather than capture, will account for the majority of inorganic sedimentation. We examine a previously reported 2 mm yr −1 increase in accretion rate following a fertilization experiment in South Carolina. Prior studies at the site have ruled out organic sedimentation as the cause of this increased accretion. We apply our newly developed models of particle capture and effective settling velocity to the fertilized and control sites and find that virtually all (>99%) of the increase in accretion rates can be attributed to enhanced settling brought about by reduced turbulent kinetic energy in the fertilized canopy. Our newly developed models of biologically mediated sedimentation are broadly applicable and can be applied to marshes where data relating biomass to stem diameter and projected plant area are available.

Eco-Physiological Controls on the Productivity of Spartina Alterniflora Loisel

The intertidal salt marshes of the Atlantic and Gulf coasts of the United States are dominated by the perennial grass, Spartina alterniflora Loisel. The ecology of salt marshes in which this species dominates has been extensively investigated because of the documented biogeochemical functions that these ecosystems perform and the resulting societal values they provide. Since many of the salt marsh-derived values originate, either directly or indirectly, from the presence of a vegetated marsh and its primary productivity, it has long been a major goal of salt marsh ecology to elucidate the determinants of the growth of Spartina. This paper reviews the interaction of the abiotic environment with key eco-physiological processes controlling the growth of this important plant species. The productivity of Spartina can vary on both spatial and temporal scales. Spatial differences in productivity on a local scale are primarily determined by abiotic factors, particularly the interaction of soil anoxia, soluble sulfide, and salinity, with plant nitrogen uptake and assimilation. Also, Spartina can induce a positive feedback on productivity by enhancing substrate aeration. The growth enhancing effects of marsh infauna, e.g., fiddler crabs, are mediated through these interacting abiotic variables. Productivity differences on regional scales are largely dependent on geographical differences in climate, tidal amplitude, and soil parent material. Temporal variation results from seasonal and annual variation in climatic and tidal controls that may influence marsh salinity and/or inundation. The concerted research of a large number of scientists has provided one of the most comprehensive and ecologically-relevant analyses of determinants of the primary productivity of any nonagricultural plant species.

Influence of oxygen and sulfide concentration on nitrogen uptake kinetics in Spartina alterniflora.

The effects of sulfide concentration and hypoxia on NH4@?—uptake kinetics in Spartina alterniflora were examined in a laboratory culture experiment. Both factors significantly influenced the Michaelis—Menten parameters, Vmax and Km, which characterize the nitrogen uptake rate as a function of nitrogen concentration. Under oxygen—saturated conditions, the mean NH4@? Vmax per unit root dry mass (DM) was 12.91 @mmol°g—1°h—1 and the Km was 1.05 @mmol/L NH4@?+. Under hypoxia, Vmax decreased to 8.14 @mmol°g—1°h—1, while Km increased to 2.54 @mmol/L NH4@?+. Dissolved sulfide concentrations as low as 0.25 mmol/L inhibited the NH4@?+—uptake kinetics of S. alterniflora significantly, and to a greater extent than hypoxia alone. Exposure to 1.0 mmol/L sulfide resulted in a Vmax per unit root DM of 2.28 @mmol°g—1°h—1 and a Km of 8.79 mmol/L NH4@?+. No significant NH4@?+ uptake was found with exposure to 2.0 mmol/L sulfide. The results are consistent with a conceptual model of changes in productivity of S. alterniflora in the salt marsh as a function of environmental modification of NH4@?+—uptake kinetics.

Distribution and speciation of phosphorus along a salinity gradient in intertidal marsh sediments

We examined forms of solid phosphorus fractions in intertidal marsh sediments along a salinity (0–22%.) gradient in a river-dominated estuary and in a marine-dominated salt marsh with insignificant freshwater input. Freshwater marsh sediments had the highest ratio of organic N:P of between 28:1 and 47:1 mol:mol, compared to 21∶1 to 31∶1 mol∶mol in the saltmarshes, which is consistent with a trend toward P-limitation of primary production in freshwater and N-limitation in salt marshes. However, total P concentration, 24.7±11.1μmol P g dw−1 (±1 SD) averaged over the upper meter of sediment, was greatest in the freshwater marsh where bioavailablity of P is apparently limited. In the freshwater marsh the greatest fraction of total P (24–51%.) was associated with humic acids, while the importance of humic-P decreased with increasing salinity to 1–23%. in the salt marshes. Inorganic P contributed considerably less to total sediment P in the freshwater marsh (15–40%.) than in the salt marshes (33–85%.). In reduced sediments at all sites, phosphate bound to aluminum oxides and clays was an important inorganic P pool irrespective of salinity. Inorganic P associated with ferric iron [Fe(III)] phases was most abundant in surface sediments of freshwater and brackish marshes, while Ca-bound P dominated inorganic P pools in the salt marshes. Thus, our results showed that particle-bound P in marsh sediments exhibited changes in chemical association along the salinity gradient of an estuarine system, which is a likely consequence of changes in ionic strength and the availability of iron and calcium.

The Nitrogen Uptake Kinetics of Spartina Alterniflora in Culture

Rates of nitrogen uptake were measured in cultures of Spartina alterniflora growing outdoors in continuously flowing nutrient solutions. A Michaelis—Menten model with Vmax expressed as an exponential function of temperature was an accurate predictor of uptake rates during the growing season. Half—saturation constants were estimated to be 0.057 @+ 0.016 mg N/L for NH4 and 0.124 @+ 0.034 mg N/L for NO3. These half—saturation constants are too low in comparison to levels of inorganic NH4 in marsh pore water to account for nitrogen—limited growth in the field. It was suggested that an edaphic factors(s), possibly an oxygen deficiency, or a metabolic poison such as H2S, or competition from other ions for carriers, might inhibit nitrogen uptake in the marsh in such a way as to increase the half—saturation constant for uptake. A gradient of such an environmental factor could account for gradients in morphology and productivity in communities of Spartina alterniflora. See full-text article at JSTOR

Integrating LIDAR elevation data, multi-spectral imagery and neural network modelling for marsh characterization

Vertical elevation relative to mean sea level is a critical variable for the productivity and stability of salt marshes. This research classified a high spatial resolution Airborne Data Acquisition and Registration (ADAR) digital camera image of a salt marsh landscape at North Inlet, South Carolina, USA using an artificial neural network. The remote sensing‐derived thematic map was cross‐referenced with Light Detection and Ranging (LIDAR) elevation data to compute the frequency distribution of marsh elevation relative to tidal elevations. At North Inlet, the median elevation of the salt marsh dominated by Spartina alterniflora was 0.349 m relative to the North American Vertical Datum 1988 (NAVD88), while the mean high water level was 0.618 m (2001 to May, 2003) with a mean tidal range of 1.39 m. The distribution of elevations of Spartina habitat within its vertical range was normal, and 80% of the salt marsh was situated between a narrow range of 0.22 m and 0.481 m. Areas classified as Juncus marsh, dominated by Juncus roemerianus, had a broader, skewed distribution, with 80% of the distribution between 0.296 m and 0.981 m and a median elevation of 0.519 m. The Juncus marsh occurs within the intertidal region of brackish marshes and along the upper fringe of salt marshes. The relative elevation of the Spartina marsh at North Inlet is consistent with recent work that predicts a decrease in equilibrium elevation with an increasing rate of sea‐level rise and suggests that the marshes here have not kept up with an increase in the rate of sea‐level rise during the last two decades.

The influence of salinity on the kinetics of NH inf4 sup+ uptake in Spartina alterniflora

SummaryThe effects of short- and long-term exposure to a range in concentration of sea salts on the kinetics of NHinf4sup+uptake by Spartina alterniflora were examined in a laboratory culture experiment. Long-term exposure to increasing salinity up to 50 g/L resulted in a progressive increase in the apparent Km but did not significantly affect Vmax (mean Vmax=4.23±1.97 μmole·g−1·h−1). The apparent Km increased in a nonlinear fashion from a mean of 2.66±1.10 μmole/L at a salinity of 5 g/L to a mean of 17.56±4.10 μmole/L at a salinity of 50 g/L. These results suggest that the long-term effect of exposure to total salt concentrations within the range 5–50 g/L was a competitive inhibition of NHinf4sup+uptake in S. alterniflora. No significant NHinf4sup+uptake was observed in S. alterniflora exposed to 65 g/L sea salts. Short-term exposure to rapid changes in salinity significantly affected both Vmax and Km. Reduction of solution salinity from 35 to 5 g/L did not change Vmax but reduced Km by 71%. However, exposing plants grown at 5 g/L salinity to 35 resulted in an decrease in Vmax of approximately 50%. Exposure of plants grown at 35 g/L to a total sea salt concentration of 50 g/L for 48h completely inhibited uptake of NHinf4sup+. For both experiments, increasing salinity led to an increase in the apparent Km similar to that found in response to long-term exposure. Our data are consistent with a conceptual model of changes in the productivity of S. alterniflora in the salt marsh as a function of environmental modification of NHinf4sup+uptake kinetics.