Matthew E.S. Bracken


Donald Cheney, Carol Thornber

Date of Award


Date Accepted


Degree Grantor

Northeastern University

Degree Name

Master of Science

Department or Academic Unit

College of Science. Department of Biological Sciences.


Chondrus crispus, Fucus vesiculosus, Nitrogen, nutrient uptake, Phosphorus, rocky intertidal


Marine Biology


Environmental fluctuations associated with both natural events and anthropogenic impacts can appreciably alter the functioning of ecosystems. For example, in the Gulf of Maine, there is substantial spatial and temporal variation in availability of nutrients that fuel primary production. Macroalgae are important primary producers in intertidal and shallow benthic habitats, and they play essential roles in absorbing nutrients (e.g., inorganic C, N, and P) and mediating their availability (e.g., as organic C, N, and P) to the communities that they support. The ability of macroalgae to take up nutrients depends on nutrient availability. However, sometimes a nutrient in short supply can restrict access to another abundant nutrient, resulting in co-limitation. Macroalgae can exhibit remarkable changes in tissue quality due to tidal elevation and seasonal nutrient variability. These changes in tissue quality may influence herbivore preference, consumption rates, and the flow of materials through marine systems. Here, I evaluated potential biotic and abiotic factors mediating rates of nutrient uptake and transfer in marine communities. I used an observational approach based on long-term, weekly sampling of ambient nutrient levels and macroalgal tissue quality at two tidal elevations and across taxa. I coupled the observational approach with laboratory experiments to investigate nutrient interactions such as co-limitation, and the impact on primary producers. Additionally, I conducted feeding assays to assess herbivore impacts on macroalgae from different positions on the shore and how these impacts vary with season. Observational data revealed strong seasonal patterns in ambient and macroalgal tissue nutrient levels and indicate that ambient nitrate levels may be restricting algal ability to access ambient phosphate. Laboratory results confirmed these observations of co-limitation, illustrating that increased N availability enhances P uptake efficiency in seaweeds. Results of feeding assays demonstrated varying net effects of herbivores on macroalgae, depending on tidal elevation and season via a combination of consumption and facilitation via nutrient recycling. These results highlight how both top-down and bottom-up processes are influenced by, and in turn contribute to, nutrient availability. My research illustrates how species interactions and abiotic conditions interact to mediate nutrient cycling in intertidal ecosystems.

Document Type

Master's Thesis

Rights Holder

Valerie Perini

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