Bottom-up factors such as nutrient availability have long been thought to be the primary regulators of plant growth in salt marshes. However, that paradigm has been challenged by investigations showing that grazing by the periwinkle snail Littoraria irrorata regulates Spartina alterniflora growth through top-down forces. Our investigation was conducted between November 2003 and March 2007 within the North Inlet Estuary (NIE) salt marsh to examine relationships between S. alterniflora and L. irrorata. The primary goal was to determine if observed in situ densities of L. irrorata significantly decreased S. alterniflora productivity at the study sites. The results indicate that S. alterniflora productivity positively correlated with L. irrorata snail density during winter. However, no correlation was observed during summer. Hence, there was no inverse relationship between snail density and primary productivity and no support for the hypothesis of ‘top-down’ control of marsh plant production. A significant relationship between S. alterniflora stem density and L. irrorata density was observed during summer and winter, suggesting that stem density may play a key role in determining the distribution of periwinkle snails in NIE. These results challenge the applicability of the L. irrorata-S. alterniflora top-down control model as a generalized phenomenon in southeastern salt marshes.
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Vol. 29 • No. 3