Latitudinal diversity gradient of marine phytoplankton through the lens of similarity

Gray, Matthew Coster Fleet

Thesis

Latitudinal diversity gradient of marine phytoplankton through the lens of similarity

Download PDF

Creator

Gray, Matthew Coster Fleet

Rights statement

Strathclyde Thesis Copyright

Awarding institution

University of Strathclyde

Date of award

2026

Thesis identifier

T18112

Person Identifier (Local)

202082919

Qualification Level

Doctoral (Postgraduate)

Qualification Name

Doctor of Philosophy (PhD)

Department, School or Faculty

Department of Mathematics and Statistics

Abstract

Phytoplankton are one of the oldest living groups of organisms on Earth. All of life owes its start to these oxygenic photoautotrophs arising around 2.7 billion years ago. Their existence oxygenated the Earth’s atmosphere and their expansion from the ocean to land paved the way for modern plant-based agriculture and the advancement of humanity. Today, we know they are a speciose group inhabiting most aquatic ecosystems including the global ocean. Yet, general ecological understanding of their diversity distributions at the macroecological scale remains unclear despite their crucial role in global biogeochemical cycling and ecosystem productivity. This thesis focuses on examining the existence of the latitudinal diversity gradient (LDG) of marine phytoplankton and how this pattern responds to changes in how diversity is mathematically defined. While the LDG is observed across fauna and flora, exceptions are observed in both marine and terrestrial ecosystems. Alignment of a group to the LDG leads itself to predictable environmental relationships such as temperature which covary with latitude, enhancing predictive modelling approaches required for biological modelling of ecological function. Deviation from the expected LDG relationship can identify evolutionary adaptions to environmental conditions or the role of time and location in species distributions. The knowledge gained with respect to alignment and deviation from this exploration of the LDG pattern of marine phytoplankton, informs future modelling and sampling directions globally by characterising three different components of diversity. Here I use a recently compiled global dataset of marine phytoplankton to assess the global distribution of marine phytoplankton diversity. Typically, at macroecological scales, the definition of diversity is limited to species richness, particularly for latitudinal diversity gradients (LDGs). Here in Chapter 2, I found that the inclusion of community relative abundances using the Hill number index resulted in maximal diversity in the sub-tropical latitudinal regions. Following this, I further expand the mathematical definition of diversity in Chapter 3 to include taxonomic similarity using the Leinster and Cobbold diversity index (LCI) resulted in a singular peak of diversity in the southern hemisphere. Diversity assessments are always affected by sampling effort, and here this is particularly the case, rarefaction is used in Chapter 2 and 3 to assess the effect of effort on global distributions of marine phytoplankton diversity. Following the observational data research performed in Chapter 2 and 3, Chapter 4 focuses on modelling spatial distributions with limited observational data. In addition, in Chapter 4 I examine the diversity-productivity relationship of marine phytoplankton in the context of limited observational data availability and the modelling approach applied. All diversity distributions of marine phytoplankton are reviewed with respect to observed macroecological patterns. This thesis contributes the first descriptions of global marine phytoplankton diversity using the Hill number diversity index and the LCI while assessing the sampling effort issues therein. The novel methodological development of the code required to assess this global dataset will contribute to new research focused on assessing diversity beyond the species richness paradigm. The species distribution model (SDM) research adds to the ongoing evaluation of SDM applicability and in the field of ecology. While the diversity-productivity relationship observed in the modelling research, suggests that SDMs may provide a novel avenue of predictive modelling of chlorophyll-a while capturing community composition.

Advisor / supervisor