The biological dynamics of pelagic marine ecosystems are strongly influenced by the size structure and ecological
succession of phytoplankton, which in turn modifies photosynthetic efficiency. Variability in photosynthetic
rates is closely coupled with changes in community structure, but it is difficult to obtain coincident
data at high enough resolution to characterise these changes. In this study, we employ hierarchical cluster
analysis on chlorophyll-normalised high performance liquid chromatography (HPLC) pigment concentrations
from the North West Atlantic, to identify seasonal successional trends amongst phytoplankton populations.
Changes in phytoplankton community were also analysed as a function of mean equivalent spherical diameter
(MESD) derived from absorption measurements, photosynthetic rates, water-column stratification and temperature.
Well-mixed conditions in spring to early summer were associated with populations of large cells
containing high concentrations of fucoxanthin, chlorophyll-c1 and chlorophyll-c2 relative to chlorophyll-a (Chl
a). As stratification increased over the course of the summer, these cells were replaced by populations dominated
by chlorophyll-b, 19'-hexanoyloxyfucoxanthin, 19'-butanoyloxyfucoxanthin and divinyl chlorophyll-a, indicative
of small picophytoplankton. As stratification decreased in autumn, MESD and alloxanthin increased, suggesting
the presence of cryptophytes. Positive relationships were found between MESD and the quantum yield of
photosynthesis (φm) for 7 out of the 8 phytoplankton clusters identified, while negative relationships between
mean mixed layer photosynthetically active radiation and φm and the light limited slope of photosynthesis (αB)
were observed for 4 clusters, as a result of nutrient limitation and photo-protection. The highest photosynthetic
rates were associated with a pico & nanophytoplankton communities, which increased from spring to late
summer as stratification intensified. By contrast, diatom communities had the lowest photosynthetic rates
throughout the year. These successional patterns in the dominant phytoplankton size-class and phenology
support Margalef's mandala in terms of the relationship between turbulence and community structure. The study
sheds new light on assemblages dominated by smaller cells, under warm, stratified conditions, having higher
photosynthetic efficiencies, which has implications for the carbon flux in the NW Atlantic
