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Graduate Research Fellowship
Macroalgal blooms in Waquoit Bay: the relative influence of nitrogen loads and grazers, and links to watersheds

Sophia Fox
Graduate Research Fellow 2003-2004
Waquoit Bay, MA
Boston University

Top-down control of macroalgal abundance and composition is well supported in the published literature. In spite of demonstrated control of macroalgae by grazers, blooms are widespread along the coasts of the world, and are correlated with anthropogenic inputs of nutrients to coastal waters. In the presence of large inputs of nitrogen, the relative influence of top-down controls may be overwhelmed by larger nitrogen supplies, through bottom-up control mechanisms. I proposed a three-part plan for research. First, I will carry out a survey to measure abundance and composition of macrophytes and of the associated herbivores in the estuaries of Waquoit Bay receiving nutrients from land-derived loads. Second, to test the relative importance of top-down control by grazers versus bottom-up control by nutrients, I will run experiments in which I will manipulate algae and grazer abundance and measure growth of algae and consumption by grazers. Two dominant species of macroalgae, Caldophora vagabunda and Gracilaria tikvahiae, will be collected from each of 3 estuaries with different nitrogen contents. I will run the experiments with the two dominant amphipods, Cymadusa compta and Microdeutopus gryllotalpa, and the dominant isopod, Idotea baltica, as the grazers. Third, to assess the strength of the link between land use mosaics on watersheds and food webs in Waquoit Bay's estuaries, I will measure stable isotopes of both macroalgae and grazers, and determine the relationship between isotopic signatures and abundance of macroalgae, grazers and nitrogen load.
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