Thesis

Plankton commuity size structure in UK coastal waters : observations and modelling

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Awarding institution
  • University of Strathclyde
Date of award
  • 2026
Thesis identifier
  • T18160
Person Identifier (Local)
  • 202185655
Qualification Level
Qualification Name
Department, School or Faculty
Abstract
  • Plankton underpin marine food webs by facilitating the transfer of energy from primary producers to higher trophic levels. These organisms are incredibly diverse yet, among the many traits that influence ecological function, body size is regarded as a ‘master trait’ because physiological rates and trophic interactions can be described as an allometric function of size. While the seasonal cycles of biomass and taxonomic composition of the plankton community in UK coastal waters are well understood, comparatively little is known about size structure. This thesis addresses that gap by investigating the seasonal dynamics and drivers of plankton community size structure in UK coastal waters. Using weekly observational data spanning more than 20 years from three observatories, we assessed plankton community size structure by estimating biomass-size distributions, and subsequently computing summary statistics such as mean size and size diversity. In line with expectations derived from our review of the literature, we found that phytoplankton mean size is generally smaller in winter, and larger during the productive period of summer. On the other hand, zooplankton mean size decreases during summer, due to contributions from unicellular microzooplankton. Size diversity also exhibits a strong seasonal signal, although this is more complex and is perhaps better explained by its parabolic relationship with biomass. Having assessed seasonal patterns in plankton community size structure, we then scrutinised the validity of these results by assessing possible impacts of sampling bias on our estimates. Previous studies show that rare phytoplankton cells tend to be under-represented by traditional sampling methods and, since abundance decays exponentially with organism size widely in nature, concerns have been raised about the study of size structure using samples analysed by light microscopy. We addressed these concerns directly: firstly, by comparing the enumeration rates of cells by size between paired subsamples of two volumes; then, by conducting a simulated sampling experiment. We detected that bias significantly affects the estimation of biomass, mean size, and size diversity within the study of large phytoplankton. This effect varies seasonally, broadly tracking changes in biomass and size structure. However, our concerns do not extend to the overall study of community size structure in UK coastal waters because bias is most prevalent in winter, when cells tend to be smaller and are thus measured by flow cytometry. As such, winter size structure is largely driven by the extent in which small cells dominate the community, rather than by changes within the subset of larger cells. Our focus then shifted to investigating the mechanisms driving changes in plankton community size structure. To do so, we constructed a size-structured plankton ecosystem model and optimised unknown parameters using approximate Bayesian computation. Among a number of interesting results, a key finding is that light limitation through intracellular shading, which increases with cell size yet is often ignored in the modelling literature, plays a crucial role in driving the observed small mean size of phytoplankton in winter. In spring, larger phytoplankton cells are able to temporarily escape grazing, driving the increase in mean cell size. Furthermore, we investigated areas of poor performance of the model to construct hypotheses for future study. Firstly, the model’s inability to reproduce the spring rise and subsequent summer depression in zooplankton mean size appears to be a result of deviations from the allometric relationship for optimal prey size by dinoflagellates and meroplankton larvae. Additionally, the omission of mixotrophy from the model appears to drive the erroneous reduction in phytoplankton mean size in summer, particularly given the prominence of mixotrophic taxa in observations at UK coastal sites in summer and early autumn. We outline possible extensions to the model to improve its accuracy in this regard and, based on an extensive assessment of the model, believe that the model is a suitable tool for future development.
Advisor / supervisor
  • Eerkes-Medrano, Dafne
  • Chen, Bingzhang
Resource Type
DOI

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