| Summary: | The abundance, distribution, and nitrogen productivity of picoplankton were studied in upwelling and oceanic waters of the subtropical North Atlantic. The oceanic community was dominated by picoplankton, within which prochlorophytes and, to a lesser extent, heterotrophic bacteria accounted for most of the estimated carbon biomass. Based on size-fractionated $({$80%) at the oligotrophic stations, and to be responsible for a significant fraction ($\sim$70%) at the upwelling site. Given that prochlorophytes accounted for most of the picophytoplankton biomass at the oligotrophic stations, these results suggest that prochlorophytes accounted for most of the new and regenerated production in these waters. Bacterial uptake rates of inorganic nitrogen were estimated as the difference between total uptake rates of inorganic nitrogen (using $\sp $N) and phytoplankton uptake rates of nitrogen (based on protein synthesis). Bacteria contributed 25% on average to the total uptake of inorganic nitrogen. The errors in new production resulting from this bacterial uptake were $\sim$30% for NO$\sb3-$ uptake rates, and $\sim$10% for the f-ratio. These errors were small compared with some of the uncertainties associated with the $\sp $N technique. However, with the recent improvements in the $\sp $N measurement, bacterial utilization of nitrate and ammonium is likely to be one of the major problems associated with the estimation of autotrophic new and regenerated production. By not correcting new production estimates for a bacterial uptake of inorganic nitrogen, this may result in the C:N assimilation ratio being lower than the Redfield ratio. This can lead, in turn, to an overestimation of carbon-based new production when computed as the product of nitrate uptake and Redfield ratio, as is conventionally done. Thesis (Ph.D.)--Dalhousie University (Canada), 1995.
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