James G. Gosselink

Primary Production in a Louisiana Golf Coast Spartina Alterniflora Marsh

Live and dead aboveground biomass of Spartina alternitflora Loisel in a Louisiana salt marsh was determined monthly over an annual cycle. Litterbags were used to determine loss rates of particulate vegetation from the marsh surface. Net aboveground primary produc- tion estimates ranged from 750 to 2,600 g m2 yr-1, depending on how data were handled. Analysis of these results suggested that true net production was probably much closer to the highest estimate than to the lowest.

Aboveground Production of Seven Marsh Plant Species in Coastal Louisiana

Results of a 2—yr evaluation of aboveground production of 7 plant species commonly found in Louisianas coastal marshes show a generally higher level of production compared to other studies. In 5 of the 7 species significant growth occurred throughout the year. Calculated annual production in grams per square metre for each species was; Distichlis spicata–3237; Juncus roemerianus–3416; Phragmites communis–2318; Sagittaria falcata–1501; Spartina alterniflora–2658; Spartina cynosuroids—1355; and Spartina patens–6043. Sampling variability, expressed as the ratio of the standard error to the mean x 100, averaged between 8 and 18 different species. Instantaneous loss rates (mg x g(—1) x day(—1)) of dead vegetation from the marsh, averaged over the year, ranged from 4.7 for P. communis to 25.2 for S. falcata. See full-text article at JSTOR

Vegetation dynamics in the emerging Atchafalaya Delta, Louisiana, USA.

1. The wetlands of the Atchafalaya Delta, Louisiana are characterized by a warm climate, highly fertile sediments, and an absence of moisture limitations. Consequently, vegetation succession (on islands that emerged in 1973) was expected to occur relatively rapidly. 2. From 1980 until 1986, annual surveys of vegetation cover and substrate elevation were conducted on 110 permanent plots along six transect lines crossing four islands. In addition, to evaluate the effect of herbivory (predominantly by Myocastor coypus, nutria or coypu), four 50-m × 40-m exclosures and four control plots were constructed on two of the islands

RELATIONSHIPS AMONG PHYSICAL CHARACTERISTICS, VEGETATION DISTRIBUTION AND FISHERIES YIELD IN GULF OF MEXICO ESTUARIES

The relationship of physical factors to vegetation distribution and fishery harvest was analyzed using data from the Gulf of Mexico. Data from 64 estuaries were used to investigate the relationships with vegetation. Fishery harvest in the southern Gulf of Mexico was analyzed using harvest statistics, estuarine area, and river discharge by state. Results show that the fishery harvest and area of an estuary are strongly related to freshwater input and physiography. Intertidal area is correlated to coastal land slope, length of coastline occupied by the estuary, and inshore open water area. The area of emergent vegetation is related to intertidal area and rainfall, but not to riverflow, because there are large areas of emergent vegetation with low riverflow (e.g., south Florida). Salt flat area is related to intertidal area and rainfall deficit. Fishery harvest per unit open water area in the southern Gulf is highly correlated to river discharge (r = 0.98).

Spatial and temporal changes in Louisiana's Barataria Basin marshes, 1945–1980

A computerized geographic information system with site-specific change-detection capabilities was developed to document amounts, rates, locations, and sequences of loss of coastal marsh to open water in Barataria Basin, Louisiana, USA. Land-water interpretations based on 1945, 1956, 1969, and 1980 aerial photographs were used as input, and a modified version of the Earth Resources Laboratory Applications Software developed by the National Aeronautics and Space Administration was used as a spatial data base management system. Analysis of these data sets indicates that rates of marsh loss have increased from 0.36% per year in the 1945–56 period, to 1.03% per year in 1956–69, and to 1.96% per year in 1969–80. The patterns of marsh loss indicate that the combination of processes causing degradation of the marsh surface does not affect all areas uniformly. Marsh loss rates have been highest where freshwater marshes have been subject to saltwater intrusion. The increase in the wetland loss rates corresponds to accelerated rates of subsidence and canal dredging and to a cumulative increase in the area of canals and spoil deposits.

The impact of vertebrate herbivores on wetland vegetation in Atchafalaya Bay, Louisiana

Delta islands in the Atchafalaya and Wax Lake deltas in Atchafalaya Bay, Louisiana, are in an extremely dynamic successional phase. These islands initially supported large marshes dominated by the pioneering plant species Sagittaria latifolia and Sagittaria platyphylla. A general decrease in vegetated areas has occurred in the delta island marshes in the Atchafalaya Delta since about 1980, while in the Wax Lake Delta portion of the complex the vegetation still flourished. The Atchafalaya Delta provides an interesting setting for the study of herbivory because of the complex interaction of biotic and physical factors operating in this delta. We hypothesized that grazing by herbivores has a marked effect on vegetation in these developing marshes. To test this hypothesis, exclosures were erected on islands in both deltas in September 1985 and January 1986. Each set of exclosure treatments included an openly-grazed control area, an ungrazed area, an area allowing nutria grazing, and one allowing waterfowl grazing in each site. Results of the experiment, based on field sampling of vegetation, indicated decreases in plant biomass and changes in plant species composition in grazed treatments. Waterfowl and nutria reduced biomass aboul equally, but there was a more marked effect in the openly grazed areas. These findings may be extrapolated to sediment diversion areas along the Mississippi River.

Modeling future trends in wetland loss and brown shrimp production in Louisiana using thematic mapper imagery

Abstract The land-water interface of coastal marshes may influence the production of estuarine-dependent fisheries more than the area of these marshes. To test this hypothesis, we created a spatial model to explore the dynamic relationship between land-water interface and degree of land loss in disintegrating coastal marshes of Louisianas Barataria, Terrebonne, and Timbalier basins. Calibrating our model with Landsat Thematic Mapper satellite imagery, we found a parabolic relationship between land-water interface and marsh disintegration. Aggregated simulation data suggested that the land-water interface in the study area will soon reach its maximum and then decline. We found a statistically significant positive linear relationship between brown shrimp catch and total interface length over the past 28 years. This relationship suggests that shrimp yields will decline when interface declines, possibly beginning about 1995.

Landscape conservation in a forested wetland watershed.

M ore than one-half of the 40 million ha of wetlands in the coterminus United States is forested (Frayer et al. 1983). Most of these wetlands (57%; Abernethy and Turner 1987) are in the southeastern United States. They are characterized as permanently, semipermanently, or intermittently flooded and are dominated by cypress (Taxodium spp.), tupelo (Nyssa spp.), and oak (Quercus spp.). The broad Mississippi River alluvial floodplain, which extends from the Gulf of Mexico to southern Illinois, historically supported the largest United States expanses of forested wetlands, but since the 1950s these areas have been rapidly converted to the production of cotton, corn, and soybeans (OTA 1984). Brinson et al. (1981) estimated the loss of riparian forest at more than 70% since presettlement days. Abernethy and Turner (1987) calcu-

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 (...)

Cumulative impacts on wetlands: Linking scientific assessments and regulatory alternatives

This article is an extension and application of Preston and Bedford (1988), especially as relevant to bottomland hardwood (BLH) forests of the southeastern United States. The most important cumulative effects in BLH forests result from incremental forest loss (nibbling) and from synergisms resulting from this nibbling. Present regulatory procedures are ineffective in preventing incremental forest loss because of the focus on permit site evaluation, rather than on large landscapes. Three examples are given to illustrate the need for a landscape focus. This perspective requires preplanning or goal-setting to establish the desired conditions to be maintained in the regulated landscape unit.Spatial and temporal scales are of particular concern for landscape impact assessment. Natural drainage basins of about 106 ha, as identified in U.S. Geological Survey hydrologic units, appear to appropriate spatial units: they have fairly natural boundaries, are of sufficient size to support populations of large, wide-ranging mammals, and are compatible with existing maps and databases. Time scales should be sufficiently long to include recovery of wetland ecosystems from human perturbations. In practice, available data sets limit analysis to no longer than 50 yr. Eight indicators of landscape integrity are identified, based on generally available long-term data sets.Linking technical information concerning cumulativeeffects on landscapes to the evaluation of cumulativeimpacts in regulatory programs (i.e., goal-setting) is a serious issue that can benefit from precedents found in the field of epidemiology, and in the establishment of clean air and clean water standards. We suggest that reference data sets must be developed, relating BLH function to structure (forest area). These can be used to set goals for individual watersheds, based on their present conditions and the magnitude and type of perceived development pressures. Thus the crucial steps in establishing a successful program appear to be (1) establish study unit boundaries, (2) assess the condition of study unit landscape integrity, (3) set goals, and (4) consider the impacts of permit proposals with both goals and the existing condition of the study unit landscape in mind.