| Summary: | The vertically resolved, process-based, numerical model presented in this work serves to critique
a planktonic paradigm of the eastern subarctic Pacific. The modelled phytoplankton consists
of a small (< 10 μm) size fraction of low-iron-adapted phytoplankton grazed by microzooplankton
and the iron-stressed > 20 μm diatoms grazed by ontogenetic mesozooplankton migrants,
primarily large calanoid copepods of Neocalanus spp. Two approaches are used to include the
effects of iron limitation: in the first, an iron dependence is implicit in diatom growth rate
parameters and, in the second, diatom growth is an explicit function of bioavailable soluble
iron concentration whose dynamic evolution is defined by a partial differential equation. A
new copepod life cycle model (LCM) is presented which couples dynamically to the ecosystem
model, thus, incorporating population migration patterns and weight and maturity distributions
in the omnivorous "predation closures" of the micro-plankton equations. As an additional
benefit of this novel approach, model predictions of population weight and weight at diapause
provide time-dependent diagnostic information about the health and fecundity of the pelagic
copepodid population. The LCM and ecosystem model, coupled with the KPP ocean boundary
layer model of Large et al. (1994), reproduces quasi-equilibrium annual cycles of observations
in the northeastern Pacific planktonic ecosystem while maintaining equation parameterizations
consistent with physical and biological processes.
Model experiments investigate ecosystem response to fluctuations in atmospheric forcing
(irradiance and wind), the mesopelagic population of nauplii, copepodid maturation, and the
surface iron flux. Model results indicate several robust features of the trophic structure which,
through experimentation, are potentially verifiable. The dual role of the mesozooplankton as
herbivores of diatoms and carnivores of microzooplankton creates antisymmetry in phytoplankton
biomass trends in all experiments; biomass trends of small-size phytoplankton are positively
correlated with trends in peak copepodid biomass. In general, increased carnivorous predation
amplifies the biomass and net primary production oscillations of all living plankton pools.
Model experiments investigating the transition between "iron stressed" and "iron replete" conditions
indicate a shift in trophic structure favoring the diatom-copepod food chain, which is
accompanied by decreased pelagic recycling, a doubling of f-ratios, increased new production,
and a 5-fold increase in 200 rn particulate nitrogen flux. However, in all quasi-equilibrium iron
replete scenarios, total yearly primary production decreases. Copepodid migration patterns and
development connect temporally and spatially isolated conditions of the mesopelagic and upper
ocean ecosystems. Model solutions indicate that changes in the pelagic prey environment or
the winter population of mesopelagic nauplii are evident as variations in copepodid population fecundity, and the resulting feedback provides a potential biological mechanism for interannual
oscillations in both copepodid and the lower trophic biomass.
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