Does form follow function in aquatic ecosystems?

Biogeochemical Cycles



Phytoplankton play key roles in both marine and freshwater ecosystems and exhibit an incredible diversity in cell morphology. Central to this studentship is asking:

  • Does cell morphology link to a species’ ecological function or the role they play in biogeochemical processes?
  • Does form follow function in aquatic environments?
  • Does diversity matter in aquatic biogeochemistry?

Predicting ecosystem responses to environmental change on regional and global scales requires a mechanistic, or functional, understanding of the ecological and biogeochemical role of individual species. The development of functional, rather than taxonomic, classifications of phytoplankton has led to major improvements in our ability to assess, understand and predict the function and metabolism of both marine and freshwater ecosystems. However, the development of these functional classifications differs between marine and freshwater systems; in marine systems functional groups relate to biogeochemical roles (e.g. Le Quere et al., 2005), whereas in freshwater systems functional groups relate to responses to environmental variability (e.g. Reynolds et al., 2002, 2014). In these schemes, only a few exemplar species are used to fully represent phytoplankton functional types, which can lead to an oversimplification of their ecologies or functions. In many ways, the unique characteristics of individual species are lost in these interpretations and there is the potential for the loss of key ecological and biogeochemical information.

A major development in our understanding of the ecology of phytoplankton has been the recent focus on ecological traits, species’ characteristics such as cell size, motility, life-history, and cellular physiology. Traits often link a species’ cell morphology to its ecological interactions and biogeochemical role; for example, cell size dictates resource acquisition, competition and fate, while cell silica-content in diatoms relates to their propensity for grazing, sinking and their role in carbon cycling (e.g. Fragoso et al., 2018).

What we propose here is an amalgamation of marine and freshwater functional ecology with species-specific traits related to the role of phytoplankton in the global carbon cycle. The project aims to build a bridge of understanding between marine and freshwater phytoplankton ecology, drawing on their different strengths, to provide insights into how individual species respond to environmental change combined with how this then influences key aquatic biogeochemical processes. Our primary focus is on carbon cycling – linking together species’ ecological traits and functional roles in regulating carbon fixation (primary production), carbon cycling (respiration, organic matter release), carbon export (sinking) and the transfer of organic matter between trophic levels (grazing losses).


Phytoplankton traits can now be assessed quantitively (see e.g. Fragoso et al., 2018), allowing more statistical power to be applied to the analysis of species ecology and biogeochemical roles, without the reliance on expert opinion. This allows species traits to be statistically linked to environmental drivers (resources, mortality factors) and biogeochemical processes.
Functional classifications in relation to the major C cycling processes will be derived through:

(1) Application of multivariate analysis to allow the student to exploit existing long-term (>25 years) time-series datasets from marine (L4, English Channel) and fresh (Loch Leven) waters and analyse the distribution of traits and trait assemblages (clusters of traits) relative to environmental change. Such analyses can allow the re-evaluation of time-series data for changes in system function in relation to carbon-cycling.

(2) Automated flow-cytometry and microscopic techniques to analyse morphological traits of a wide range of common phytoplankton species across strong environmental gradients in order to fill knowledge gaps in the literature.

(3) Experimental determination of photo-physiology, cell elemental content (e.g. C, N, P, Si), and other functional properties (e.g. release of dissolved organic carbon) of representative species from novel functional groups.

These activities (1-3) may also be linked to future field work opportunities in both marine (Indian Ocean, Equatorial Pacific, South Atlantic) and fresh (Loch Leven) waters. The student will also be encouraged to participate in some of the regular time-series data collection from both marine and freshwater systems to give them first-hand experience of the different sampling techniques, environmental considerations, and taxonomy of these different environments.

Project Timeline

Year 1

Analysis of species traits in time-series data from marine (L4, English Channel) and freshwater (Loch Leven) ecosystems. The first year of the studentship will involve preparation of a literature review on phytoplankton functional groups and traits, alongside construction of a database of species’ traits with which to compare to the time-series data. Training in multivariate and trait analysis will be provided and the student will examine the time-series for floral patterns in the context of seasonal environmental changes in both systems. At the end of year 1 the analysis and insights gained so far will be prepared for publication in the peer-reviewed literature.

Year 2

Experimental studies and field observations to examine/quantify species’ traits related to carbon-cycling. The second year of the studentship will focus on the insights gained in year 1 and will add to this understanding of trait distribution by linking traits with biogeochemical function through experimental work and future targeted field observations. Field work opportunities (e.g. Loch Leven, Indian Ocean) will be encouraged in year 2, as well as presentation of the results at international and national conferences.

Year 3

Translation and synthesis of existing data and information on phytoplankton ecology and carbon cycling across aquatic systems (e.g. How does the freshwater perspective of C-cycling relate to the marine equivalent?). The third year of the studentship will bring together the knowledge gained from time-series analysis and field work opportunities to compare across marine and freshwater systems. A synthesis of how species’ traits link to ecological and biogeochemical function will be prepared and applied to re-evaluate our understanding of system changes in time-series databases and how carbon is processed in different environments.

Year 3.5

The final half year of the studentship will be spent writing up the thesis, as well as finalising manuscripts and disseminating the findings.

& Skills

Project support: The facilities, equipment, datasets, and expertise available within the institutions and supervisory team provide a combination of world-leading field, analytical and laboratory capability and technical support that ideally fits this PhD project, maximising the expert training that will be available.


This project will equip the student with a range of skills, including data synthesis and meta-analysis, fieldwork, analytical science, numeracy, and translation of science for wider audiences.


The student will receive training in phytoplankton taxonomy and ecology, from both a marine and freshwater perspective, experimental design, physiological experimentation, and statistical analysis.


Student support: The Lyell Centre has a large research student cohort that will provide peer-support throughout the studentship, including participation in the annual post-graduate research conference. All project supervisors are also highly research-active: the student will interact with all members of their research groups through lab-group meetings at the Lyell Centre and UKCEH, both of which are in easy reach of Edinburgh city centre, providing an opportunity to learn about other techniques and research areas which may be applicable to their research. The supervisors are all based in research-active institutions that span a broad range of ecological, environmental and geoscience research, exposing the student to a range of other research areas. Active participation in these research groups will provide the opportunity to discuss cutting-edge topics in the field, review recent papers and to present current research plans to academics with a common research interest in an informal and supportive atmosphere.


Where required, and to maintain continued professional development, the student will be supported to attend specialist courses directly aligned to the project, e.g.:

  • Multivariate statistics analysis, Data analysis and time-series analysis
  • Elemental analysis via mass spectrometry and sample partitioning via selective extraction protocols.
  • Analytical training will be provided by the supervisors and / or specialist technicians for each piece of instrumentation required for analyses, including phytoplankton identification.
  • The project supervisors will also support and encourage the student’s attendance on transferable skills training such as data management, scientific writing and science communication. These are provided for free within Heriot-Watt University’s Research Futures Academy.
References & further reading

Reynolds et al. (2002) Towards a functional classification of the freshwater phytoplankton. Journal of Plankton Research 24, 417-428, doi: 10.1093/plankt/24.5.417.

Le Quéré et al. (2005) Ecosystem dynamics based on plankton functional types for global ocean biogeochemistry models. Global Change Biology 11, 2016-2040;doi: 10.1111/j.1365-2486.2005.1004.x

Reynolds et al. (2014) Predictive utility of trait-separated phytoplankton groups: A robust approach to modelling population dynamics. Journal of Great Lakes Research 40, doi: 10.1016/j.jglr.2014.02.005.

Fragoso et al. (2018) Diatom biogeography from the Labrador Sea revealed through a trait-based approach. Frontiers in Marine Science, doi: 10.3389/fmars.2018.00297.

Further Information

Dr Alex J Poulton,

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