UBC Theses and Dissertations
Seasonality in the community composition and food web structure of plankton in the Strait of Georgia McEwan, Nicole
The Strait of Georgia (SoG) is a productive, temperate, and coastal ecosystem in the Northeast Pacific Ocean. Throughout a year in the SoG, plankton productivity and community structure have strong seasonal variations. However, the diverse range of plankton species, trophic processes, and interactions are often over-simplified in food web models. The objectives of this study were to: a) describe the seasonal trends in plankton community composition, biomass, and vertical distribution; b) represent the seasonal food web structure of the plankton community; and c) compare this complex and seasonal modelling approach to a more common and simplified approach. In the spring, diatoms dominate the plankton biomass in the SoG, leading to high primary productivity and fueling zooplankton growth. In summer, primary production is nutrient limited, leading to a decline in diatom biomass and increased mixotrophic flagellate dominance. Zooplankton biomass, dominated by large crustaceans including copepods, amphipods, and euphausiids, peaks and their diets shift to become more omnivorous. In winter, plankton biomass and productivity are low. Based on these seasonal trends, I constructed a food web model for each season (spring, summer, and winter), with the plankton community represented by ten mesozooplankton groups, two mixotrophic microzooplankton groups, one phytoplankton group, one heterotrophic bacteria group, and one detrital group. The spring food web is mostly fueled by high diatom primary production. However, the microbial loop, consisting of detritus, heterotrophic bacteria, and microzooplankton, shifts to become an important energy source for the system under limiting conditions in the summer and winter food webs. These structural changes throughout a year could have important implications and insights for higher trophic levels, such as into the seasonal food availability and quality for plankivorous fish. Finally, I compared this detailed seasonal approach to plankton modelling with an aggregated and year-averaged approach. The microbial loop is often excluded from coastal ecosystem models but is an important component, influencing trophic positions and transfer efficiencies. However, aggregating plankton groups appears to be an adequate approach to ecosystem modelling in the SoG, but modelling decisions should be driven by the research question.
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