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.
Type
Honors thesisDepartment
BiologyPermalink
https://hdl.handle.net/10161/14302Citation
Loftus, Kathryn (2017). Top-Down Effects of Keystone Grazers on Benthic Macroalgae in Eastern Salt Marshes.
Honors thesis, Duke University. Retrieved from https://hdl.handle.net/10161/14302.Collections
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