| Summary: | The study of the distribution of biomass by size is a holistic approach to analyzing the structure of the pelagic ecosystem. This approach has been applied to the study of ecosystems energetics, pollution and fisheries. The main objectives of this thesis are: first, to examine the patterns of variation in the size-distribution of pelagic biomass in different kinds of ecosystems; and second, to determine if there is any regularity in the distribution of metabolic activity by size at the community level of organization in the pelagic system. The answer to these issues have important implications for understanding and modelling fluxes of energy and matter in the oceanic ecosystem. Normalized biomass size-spectra (NBS-spectra) covering a size-range from bacteria to zooplankton were constructed from plankton collected in Sargasso Sea, New England Seamounts area (Northwest Atlantic), Georges Bank, Northeast Channel, Gulf of Maine and Scotian Shelf. It is concluded that the planktonic biomass size-distribution at all stations sampled can be described by linear NBS-spectra. These results support the hypothesis that there is a regularity in the distribution of pelagic biomass by size. The NBS-spectrum in offshore waters presents a slope close to $-$1.0 when expressed in biovolume units and $-$1.1 when expressed in carbon units. Therefore, biovolume is roughly the same at all size classes (Sheldon's "linear biomass hypothesis"), however biomass expressed as carbon content slightly decreases with increasing size (Platt and Denman's model). In the highly productive Georges Bank, biomass increases considerably with body size in the zooplankton size-range. In Georges Bank, biomass size-diversity and evenness are highest in mixed waters, less in frontal waters and lowest in stratified waters. At several stations in the Northwest Atlantic metabolic activity of size fractions was determine using the ETS technique. The metabolic activity in the pelagic system (from bacteria to zooplankton) decreases as a power function of body size with an exponent of $-$0.22. Consequently, the slope of the normalized metabolic size spectrum (NMS-spectrum) is $-$1.22. At all stations the NMS-spectrum has a more negative slope than the NBS-spectrum, indicating that the smallest organisms play even a more important role than the larger ones from a metabolic point than from a biomass point of view. These results, along with an analysis of other published data, suggest that the linearity of the NMS-spectrum and the numerical value of its slope ($-$1.2) may be characteristics of the pelagic ecosystem. Thesis (Ph.D.)--Dalhousie University (Canada), 1992.
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