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The rate of carbon fixation by phytoplank- ton in marine surface waters is often tied to the supply of growth-limiting nutrients such as iron (Fe). While average cellular requirements and ratios for various elements are well known in the literature, especially through laboratory culture work, the plasticity of these relationships in natural plankton communities has been less explored. To gauge how changes in the biological availability of dissolved Fe might influence cellular nutrient ratios of marine phytoplankton (and thus their physiology), we carried out incubation assays during a research expedition off the east coast of New Zealand. Trace-metal clean collection of plankton communities were amended with a continuum of concentrations of either Fe (as FeCl3) or desferroxamine B (to reduce bioavailable Fe) and then maintained for 72 h under in situ conditions. Along with standard assays (Fv/Fm, chlorophyll, nutrient drawdown), we measured ele- mental ratios in the bulk community by inductively coupled plasma mass spectrometry and within individ- ual plankton using synchrotron X-ray fluorescence. Our observations demonstrate that changes in the physiological ecology of the community (biomass, photosynthetic efficiency) were mirrored in changes in elemental ratios, including a 3-fold change in Fe stoi- chiometry and a 13-fold change in Zn stoichiometry when Fe-replete and Fe-depleted communities were compared. We present this information in consideration of the hypothesis that flexibility in elemental quotas influences the interactions between nutrient availabil- ity and planktonic physiological status, subsequently altering C flow through marine surface waters. X-ray fluorescence maps of P, Fe and Zn in an autotrophic flagellate overlaid on an X-ray fluorescence spectrum for the cell. Scale bar = 2 µm. Data collected by: B. S. Twining

Elemental quotas and physiology of a southwestern Pacific Ocean plankton community as a function of iron availability