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Modelling a phytoplankton dichotomy in the eastern Subarctic Pacific : subtitle impact of atmospheric variability, iron surface flux, and life cycle dynamics of the calanoid copepods, Neocalanus spp. Jeffery, Nicole

Abstract

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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