Direct and Indirect Entry Pathways of Diatom Organic Matter into the Marine Food Web

Primary production in marine systems is dominated by microscopic phytoplankton. Most of this production is quickly consumed by zooplankton, which includes single-celled organisms (protists) and multicellular animals. Diatoms are one of the most productive phytoplankton groups and copepods, one of the most abundant animal groups in the ocean, are important zooplankton consumers of diatom production. Insight into how food webs are currently structured and how marine ecosystems function can be made by understanding the magnitude of energy flow, and the mechanisms that influence the efficiency of energy movement, between diatoms and copepods. The studies in this dissertation leverage copepod-diatom grazing experiments to investigate how copepods select diatoms to consume. Copepods must break through the diatom shell made of a biosilica composite to access the contents within the cell, and prior work has suggested it is preferable for the copepod to distinguish between cells with weak or strong shells (i.e., mechanical information). The results for the model system studied here suggest that the thickness of the shell (i.e., strength) does not significantly limit copepod consumption and therefore the diatom shell mechanical properties are not a primary selection factor for copepods. Chemical sensing is important and is likely used to distinguish between cells with higher organic content or detect specific classes of compounds. Diatoms were grown in different conditions to produce variable types including cells having thin shells with low protein content or thick shells with high protein. To test the combined effect of protein xii content and shell thickness on copepod preference, copepods were acclimated to one cell type, starved, and then fed several diatom blends mixed in different proportions. Copepods always consumed fewer of the diatoms on which they had been acclimated, instead targeting new diatoms even in blends where new diatoms were three times less abundant; neither protein content nor shell thickness provided predictive power in this model system. These trends strongly suggest that copepods prefer a varied diet and will search and target new food, even if it presents a stronger mechanical barrier or is less nutritious. This dissertation also demonstrates that copepods can assimilate diatom-derived compounds in a previously unknown way. Domoic acid (DA), a diatom-produced toxin, was used as a tracer to demonstrate the movement of dissolved organic matter into the food web. This work provides the first direct evidence that an algal toxin can be transferred into the food web via consumption of organic polymers instead of the producing organism; DA was measured in the dissolved phase, particulate phase, and in copepods which could only ingest the DA-laced organic polymers. This pathway has implications for how DA is cycled through pelagic and benthic food webs and the duration over which it can remain in marine systems. Collectively, these findings have relevance from food-web transfer of metabolites to factors affecting phytoplankton bloom dynamics and provide information on processes that should be considered when trying to predict how the ecology of this critical trophic interaction will change in the future ocean