- Phytoplankton and microzooplankton comprise the base and the first link of the marine food web, respectively. These microbes are key drivers of marine carbon and nutrient cycles. Phytoplankton convert atmospheric CO₂ into organic carbon, and microzooplankton consume phytoplankton, packaging phytoplankton carbon into particulate forms that have a variety of fates: export into the deep ocean,release as dissolved organic carbon (DOC), food for higher trophic levels, or remineralization back to CO₂. Phytoplankton community composition is strongly influenced by seasonal mixing and stratification events, with resource availability determining the dominant phytoplankton species [Behrenfeld & Boss, 2014]. Phytoplankton also alter their physiology in response to the environment. Their growth rates, energy storage, population sizes, and elemental composition shift in response to nutrient limitation [Follows & Dutkiewics, 2011]. These changes are hypothesized to alter the quality of phytoplankton as prey for microzooplankton.
This thesis investigated the effects of phytoplankton prey species (a green alga, Dunaliella tertiolecta,and a diatom, Thalassiosira pseudonana) and nutrient limitation on the physiology of the microzooplankter, Oxyrrhis marina. Two stage continuous cultures were used to study the steady state physiology of both phytoplankton prey and the microzooplankton grazer without the limitations of batch culture studies, such as population fluctuations and starvation. This two-stage system enabled tight control of media flow to the prey and prey flow to the grazer. Prey were grown to two widely varying nutrient limited growth rates (0.2 and 1.2 d⁻¹), and each grazer treatment received prey (in carbon units) at approximately the same rate, regardless of prey species or their degree of nutrient limitation. O. marina regulated ingestion rates and elemental composition regardless of prey species and nutrient availability, with the exception of O. marinafed very slow growing T. pseudonana. O. marina fed this ‘slow growing’ T. pseudonana had the lowest C:N and carbon and nitrogen transfer efficiencies despite consuming prey at three times the rate of the other treatments. Carbon transfer efficiencies (CTE) decreased in O. marinafrom 25% when fed fast growing T. pseudonanato only 5% when fed the slow growing diatom. This low CTE was not balanced by an increase in fecal pellet or dissolved organic carbon production relative to the other treatments, so 95% of the ingested prey carbon was returned to the environment as CO₂. In contrast, the CTE in O. marinafed fast growing D. tertiolecta was 40%, and this treatment also resulted in the highest amount of fecal pellet production. Together with Nile Red staining of neutral lipids and confocal microscopy, these results were linked to nutrient-dependent fatty acid accumulation in phytoplankton and metabolism of these phytoplankton fatty acids by O. marina. As prey, nutrient limited diatoms severely shorten the marine carbon cycle because their predator respires the bulk of the prey carbon to CO₂. These results should be incorporated into marine ecosystem models to better predict the influences of nutrients and prey species on food web dynamics.