Master of Science (MS)


Biological Sciences

Document Type



A combination of natural abundances of stable carbon and nitrogen isotopes and tracer-addition experiments were employed to determine the relative contributions of three potential food sources (phytoplankton, benthic microalgae, and Spartina alterniflora) to benthic invertebrate fauna of Louisiana salt marshes. Samples of the major food sources and the consumers were collected quarterly (summer, fall, winter and spring) to analyze the ƒÔ13C and ƒÔ15N ratios. A simultaneous tracer-addition experiment, in which benthic microalgae (BMA) were labeled with NaH13CO3 and 15NH4Cl, was conducted in all quarters to determine the relative importance of the BMA to consumers. Results indicated that the carbon isotopic ratios of food sources were distinct except for an overlap in standard deviations of carbon isotopic values of BMA and phytoplankton in fall. Temporal variation in carbon isotopic ratios of phytoplankton and BMA was observed. A two-source mixing-model approach using carbon isotope data indicated a dependence of harpacticoid copepods on phytoplankton in summer (when phytoplankton biomass was highest) and BMA in fall, winter, and spring. However, this conclusion was not supported by the tracer-addition data, which showed minimal 13C uptake from BMA by copepods. Nematodes (summer, winter, and fall) and ostracods (summer, fall, and winter) were primarily supported by BMA for most part of the year. Marsh periwinkles (Littoraria irrorata) were partially or completely supported by Spartina detritus. The suspension feeding polychaete (S. benedicti) and oyster (Crossastrea virginica) populations were supported primarily by phytoplankton. Chironomid larvae (Tanypus clavatus) showed evidence of strong seasonal switching between BMA and Spartina. These observations indicate significant temporal variation among taxa and within taxa in exploitation of food sources. Finally, data from this study suggest that BMA, phytoplankton and Spartina, all contributed to the nutrition of consumers in this Louisiana salt marsh food-web.



Document Availability at the Time of Submission

Release the entire work immediately for access worldwide.

Committee Chair

Kevin R Carman