The goal of this thesis is to study the relationship between the community structure of marine
microorganisms and nutrient availability. To this end, size spectra of microbes were studied over
a range of nutrient regimes, both natural and manipulated. Three transects in the Atlantic
provided a natural range of nutrient environments, especially because they captured seasonal
variations. The transects encompassed Sargasso Sea, Gulf Stream, and coastal waters, during
winter, spring, and summer. Nutrient regimes ranged from surface waters of the Sargasso Sea
during stratified periods (low-nutrient), to deeply mixed waters in all three regions of the
transects during winter and spring (high-nutrient). Complementing natural variations in
nutrients, two experiments were used to study the effects of enrichment on size structure. An in
situ iron-enrichment experiment conducted in the equatorial Pacific (IronEx II) provided a
unique opportunity to monitor changes in community structure following increased nutrient
availability. In a second experiment in the Sargasso Sea, enrichments with nitrogen and
phosphorus were conducted in bottles, because one or both are commonly thought to be limiting
in this region. In order to carry out the goal of this thesis, which depended on the use of flow
cytometry to characterize bacterio-, pico-, ultra-, and nanophytoplankton, advancements were
made in methods for enumerating a wide range of cell sizes and for estimating cell size from
forward angle light scatter. In addition, because ambient concentrations of inorganic nitrogen
and phosphorus are exceedingly low (<lOnM) in the Atlantic, especially during stratified
periods, low-level determinations of these nutrients were made to compliment the analyses of
community structure. Size structure varied systematically, although not necessarily as a function
of nutrient availability. Two parameters were explored: 1) spectral slope, which indicates the
relative contribution of large and small cells to total biomass, and 2) spectral shape, or adherence
of the spectra to relationships explained by a power law. The relative ranking of the slopes from
specific regions of the transects remained constant throughout different seasons. Shapes ranged
from discontinuous to those which adhered to a power law. It is hypothesized that only
microbial systems with abundant nutrient inputs and, perhaps, reduced grazing pressure, have
smooth spectra whose shapes conform to power laws.