The effects of light and nutrients on the buoyancy of marine planktonic diatoms and the potential biogeochemical consequences of vertical movements of diatoms in coastal and open ocean ecosystems were examined. A study of a relatively small coastal diatom (THALASSIOSIRA WEISSFLOGII) in an experimental water column showed that under nitrate-replete conditions, T. WEISSFLOGII grew rapidly and exhibited nearly-neutral buoyancy, but that cells sank after depletion of ambient nitrate. Experiments showed that increased carbohydrate ballast in nitrate-depleted cells may have caused the increased sinking of cells in the tank, and that reversion of chemical composition upon re-introduction of nitrate can result in detectable increases in cell buoyancy. The biogeochemical consequences of nutrient-dependent changes in sinking rates of small diatoms include increased residence time of cells in the mixed layer of the ocean and enhanced transport of deep nutrients to the euphotic zone uncoupled from inputs of inorganic carbon.

Time-course experiments involving the large, buoyant diatom RHIZOSOLENIA FORMOSA examined changes in chemical composition and buoyancy during nitrate-replete growth, nitrogen-starvation, and recovery. Cells could maintain unbalanced growth for at least 53 hours after depletion of ambient nitrate. Increases in carbon:nitrogen (C:N) and carbohydrate: protein ratios observed during N-depletion reversed upon re-introduction of nitrate to culture medium. Buoyancy was related to nutrition: upon N-depletion, the percentage of positively buoyant cells decreased, but increased within 12 hours of nitrate re-addition. RHIZOSOLENIA FORMOSA took up nitrate in the dark at rates three times their N-specific growth rate, indicating the potential for luxury consumption of nitrate that can be stored for later use in N-depleted surface waters. These results are consistent with purported vertical migrations of RHIZOSOLENIA in nature. Cells may survive fairly long periods in N-depleted surface waters and will continue to take up carbon, then can resume nitrate uptake and will become more buoyant upon returning to deep water sources of N.

The potential contributions of migrations of RHIZOSOLENIA to open ocean new production were examined using a numerical model that predicted fluxes of carbon and nitrogen during a steady state migration cycle, specific rates of increase of biomass, total migration cycle times, and vertical distributions. Modelled fluxes of particulate organic carbon (POC) and particulate organic nitrogen (PON) normalized to integrated water column abundance were used to derive a factor which was then combined with literature estimates of RHIZOSOLENIA abundance to predict fluxes of POC and PON. New production estimated by the model was on order of 0.033 millimoles N square meter per day; this value represents at most 17% of new production that results from turbulent diffusive fluxes of nitrate into the euphotic zone.