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UBC Theses and Dissertations

Phytoplankton-zooplankton interactions : data analysis and modelling (with particular reference to Ocean Station P (50⁰N, 145⁰W) and controlled ecosystem experiments) Parslow, John Stanley


The anomalous phytoplankton seasonal cycle in the Subarctic Pacific has been attributed to grazing control. In simple classical models of the phytoplankton-zooplankton interaction, grazing thresholds are found to be necessary to obtain this type of control. Weathership observations at O.S.P. are analysed to provide a basis for a more realistic model. Phytoplankton are present at O.S.P. in almost uniformly low concentrations (about 0.4 mg Chla.m⁻³), have low photosynthetic efficiency (<0.5 mg C.mg Chia⁻¹.ly⁻¹), adapt to seasonal changes in solar radiation and show most surface inhibition in the spring. A numerical production model based on these results and driven by physical time series from the weatherships yields low annual production levels compared with previous estimates. Predicted production levels are sensitive to the choice of respiration rate, and introduction of a rapid light response or 'Marra' effect results in a doubling of net production. Predicted year to year variation is low and might be higher if variation in Secchi depth could be accounted for. In a phytoplankton-zooplankton (biomass) model based on the production model, grazing thresholds and over-wintering strategies are both necessary for grazing control. Systems identification techniques are adapted to estimate population parameters for cohorts of the dominant grazers. Cohort structure is introduced into the phytoplankton-zooplankton model using these estimates. As a result, attention is shifted from the spring to late summer and fall where sensitivity and stability problems are associated with the over-wintering departure of the dominant grazers. An approximate mathematical analysis of Steele's (1974) nutrient-phytoplankton-zooplankton model allows the explanation and elaboration of previous authors' numerical results. Stable cyclic solutions are shown to exist under nutrient limitation for constant mortality rates in the absence of grazing thresholds. Attention is focused on the transient (spring bloom) approach to the nutrient-limited cycle and a broader (physiological and behavioural) framework for zooplankton response to declining food concentrations is proposed. Systems identification techniques are also used to estimate zooplankton feeding and growth parameters from CEPEX copepod time series. The estimates are compared with literature values and the statistical and deterministic limitations of the time series discussed with a mind to future experiments. A nutrient-phytoplankton-zooplankton model, based on the parameter estimates, provides a consistent explanation of the observed phytoplankton persistence at low densities as a stable nutrient-limited equilibrium. A mathematical solution in terms of Bessel functions is found for phytoplankton populations undergoing diffusion and sinking in the case of an exponential growth profile. Non-dimensionalization allows a relatively complete discussion of the effects of varying physical and biological parameters on profiles and population growth rates. Subsurface maxima for constant diffusivity and sinking rate, previously reported for an idealised step-function growth profile, are also obtained for the exponential growth profile. Solutions to coupled non-linear phytoplankton-nutrient equations corresponding to subsurface maxima of the nutrient-trap type are also obtained using boundary-layer techniques. The dependence of the depth, shape and magnitude of these maxima on parameters is explored. The approximate theory agrees well with previously published results from a complex simulation model.

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