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Modelling the trophodynamics of a coastal upwelling system Robinson, Clifford L. K.


Climate variability and its influence on the northeastern Pacific Ocean is of concern to fisheries oceanographers because of the potential impacts on fish production, and management implications about fishery potential. The main objective of my dissertation is to evaluate the interactions between oceanic variability, plankton production, and fish production in a coastal upwelling system off southwestern British Columbia, Canada. A simulation model is constructed that describes the feeding interactions among diatoms, copepods, euphausiids, juvenile and adult Pacific herring (Clupea harengus pallasi), Pacific hake (Merluccius productus), chinook salmon (Oncorhynchus tshawytscha), and spiny dogfish (Squalus acanthias). The trophodynamics model is forced by empirical seasonal patterns in upwelling, sea surface temperature, solar radiation, and by observed hake and herring biomass. The most important simulation result is that there is an imbalance between fish consumption and euphausiid production during the summer upwelling season. This highlights the requirement for improved spring estimates of zooplankton biomass, evidence of euphausiid import and aggregation mechanisms, and basic life history information for the dominant euphausiid Thysanoessa spinifera. Using observed environmental data and hake and herring biomasses from 1972 to 1990, the model estimates primary production to range from 250-500 g C m⁻² y⁻¹ , secondary from 5-50 g C m⁻² y⁻¹ , and tertiary from 0.5-4.5g C m⁻² y⁻¹. Interannual and longer-term variability in plankton production occurred, with some years favouring copepod versus euphausiid production and vice versa. The trends in plankton production are determined primarily by variability in the dynamics of spring and summer upwelling. The average annual transfer efficiency (TE) of production from diatoms-to-zooplankton ranged 5-14% during 1972-90, while the 19 y average seasonal TE ranged 0-70%. Oceanic factors strongly influence euphausiid production, and ultimately the transfer of secondary production to hake and herring. Simulations also indicate that hake predation only significantly effects zooplankton production, while upwelling rate determines production at all trophic levels, from diatoms to fish. This research provides insights into how the productivity of a coastal upwelling region may respond to temporal variability in ocean climate.

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