Top-Down Effects of Keystone Grazers on Benthic Macroalgae in Eastern Salt Marshes

Abstract

Both bottom-up and top-down forces can shape plant communities. In southern salt marshes, macroalgal growth is thought to be primarily controlled by bottom-up forces such as nutrients and physical factors. However, I noticed that Ulva lactuca, the green sea lettuce, showed heavy damage from grazing when it grew in the lower intertidal in salt marshes. In this study, I used experiments and observation work to test if two commonly occurring snails, Littoraria irrorata and Ilyanassa obsoleta, could control U. lactuca biomass in salt marshes. To test for top-down control, I employed field surveys, conducted feeding experiments, and analyzed data from a previously conducted field experiment. Lab experiments showed that both snails commonly graze on U. lactuca. Field experiments that excluded snails showed snails exert top-down control of marcoalagal growth. When snails were removed, biomass and percent cover increased throughout the summer, reaching a high of 91.33% and 64.16 g/m2 in August, respectively. In cages where snails were multiplied, biomass and percent cover of algae decreased throughout the summer, falling to 1.67% and 5.8 g/m2 in August, respectively. These results show that macroalgae in salt marshes are under strong top-down control, and suggest grazers, rather than physical stress, could account for the lower abundance of U. lactuca in salt marshes.