Phytoplankton are photosynthetic microbes that form the base
of the marine food web and play a major role in the global
cycling of carbon, nitrogen, phosphorus and other elements and
the regulation of earth’s climate. Cell diameters of
phytoplankton span three orders of magnitude, but reduction of
this diversity into just two groups – picoplankton, with diameters
of 2 micrometers or less versus larger cells – represents a
dichotomy relevant to their ecological and biogeochemical role.
This thesis describes global patterns in small phytoplankton
abundance, both in absolute number and relative to the biomass
of total phytoplankton, and seeks to explain these patterns.

A simple ecosystem model forced with periodic and stochastic
variability in nutrient supply can produce a wide range of
possible contributions of small phytoplankton to total
phytoplankton biomass under a fixed set of model parameters,
only varying the forcing. It is illustrated how time scales intrinsic
to the ecological system interact with time scales characteristic
of the environment to produce emergent behavior in the form of
a trophic cascade: when forced at time scales on the order of
months, but not at shorter forcing intervals, the trophic link
between large and small zooplankton becomes important,
permitting episodic blooms of small phytoplankton in response
to perturbations lasting on the order of days. Such blooms
contradict the background-state paradigm that the biomass of
small phytoplankton is relatively constant while large cells
contribute most to the variability of total phytoplankton
biomass.

The exploration of a large observational data set shows that
escapes from the background state, such as those found in
model simulations, exist in nature: small phytoplankton
contribute to the variability of total phytoplankton biomass in
several regions of the ocean. Also, the contribution of
picoplankton to total phytoplankton at the scale of
biogeochemical provinces is significantly related to the variance
of wind speed. Both, model- and observation-based results
show that the background-state hypothesis is not a globally
applicable concept and support the hypothesis that variability in
environmental factors is an ecosystem-structuring mechanism.

More more information, please visit http://
mlehman.ocean.dal.ca/ or email mlehmann@dal.ca