The Influence of Nutrient Availability on Trophic Energy Transfer Using Two-Stage Continuous Cultures

Areas of extreme nutrient limitation are expected to expand in the surface ocean as a consequence of climate change. Phytoplankton are the base of the marine food web and their physiology is strongly influenced by nutrient availability. Two-stage continuous cultures were used to investigate trophic energy and nutrient transfers between phytoplankton and microzooplankton under conditions simulating high and low nutrient availability. The first stage contained either fast (μ=1.2 d^-1) or slow (μ=0.2 d^-1) growing nitrogen-limited prey organisms: a green alga (Dunaliella tertiolecta) or a diatom (Thalassiosira pseudonana). The second stage contained the dinoflagellate Oxyrrhis marina, which was fed a constant biomass of prey delivered from the first stage. Cultures were allowed to reach steady state before the physiologies of both prey and predator were analyzed. The population of O. marina was five-fold higher when fed with the fast growing D. tertiolecta compared to slow-growing D. tertiolecta. D. tertiolecta also supported a population of O. marina that was an order of magnitude higher than the diatom prey type. The fitness of the prey and O. marina under each nutrient limiting condition were further assessed using a suite of physiological measurements including chlorophyll, C:N:P ratios, lipid body formation, cell volume, motility, and fecal pellet accumulation. Regardless of prey type, O. marina were consistently larger and less abundant when fed prey growing at 0.2 d^-1. These results suggest that major shifts in ecosystem food web structure are likely as surface waters become increasingly depleted of nutrients.