Chalking up differences: plankton diversity and the calcium carbonate budget

Biogeochemical Cycles



The formation, surface fluxes to depth and deep-sea burial of calcium carbonate are all key processes in the global marine carbon cycle, with the combination of the first two providing an important feedback to atmospheric CO2. Around half of global calcium carbonate production occurs in shallow coastal environments (e.g. by corals), with the remainder occurring in the open-ocean by a diverse range of organisms; from small single-celled plants and animals to large free-swimming multicellular snails and slugs (Fig. 1), and even fish. Such diversity in size and ecology, with differing roles in marine ecosystems (from primary producers to grazers), underpins the global calcium carbonate budget – however, this diversity is not recognised in our understanding. Few studies have attempted to partition calcium carbonate production, fluxes or deep-sea burial between the organisms involved (see examples: Broecker and Clark, 2009; Manno et al., 2018). Importantly, the impact and sensitivity to future climate change (ocean warming, acidification and de-oxygenation) also varies between calcifying organisms, meaning we urgently need a better understanding of their relative roles in upper ocean production, export from the upper ocean to the deep-sea, and burial into sediments.

Export fluxes from the surface ocean to deep-sea sediments and subsequent burial are both ‘relatively’ well characterised in terms of global magnitude and regional trends. The scale of biological formation of calcium carbonate in the upper ocean, however, is poorly constrained, in terms of both its magnitude and biogeography, due to knowledge gaps in our ecological and physiological understanding of the organisms involved. Despite recent advances in our understanding of the biomass distribution of coccolithophores (e.g. Daniels et al., 2018; Schiebel, 2002), foraminifera and pteropods, and how these may relate to ocean chemistry, we still have very little idea of the relative magnitude (or biogeography) of their respective rates in terms of production or export. Hence, we have a relatively poor understanding of how upper ocean production translates into export or burial; how much surface production is lost through dissolution in the upper ocean, and how may this differ between the organisms involved?

Hence, there are still large global uncertainties in who is doing the majority of surface calcium carbonate production and export (coccolithophores, foraminifera or pteropods), what the seasonal and annual magnitudes of these fluxes from different calcifiers are, and where in the world’s oceans the fluxes by each of them are high or low.

The goal of this studentship is to tackle this global problem through synthesis of existing data on growth and calcification rates, upper ocean biomass, particulate matter fluxes to depth, and deep-sea sedimentation patterns for the different plankton groups. The student will also collect new comparative data through various methods; for example, examination of satellite data, biogeographical studies, analysis of water and net samples, sediment trap material, and allometric estimates. The outcomes of this studentship will have important implications for how we understand and model the current and future global ocean C-cycle.


The early stages of this studentship will involve an extensive literature review and synthesis of existing data and observations on production, export, and burial for the different planktonic calcifiers. This synthesis will be complimented through focussed discussion with national and international experts on the different organisms to identify key knowledge gaps for the different groups and design a strategy to overcome these gaps. This approach will allow the student to make targeted decisions as to where new observations, estimates and data can be generated and fill our knowledge gaps. Potential avenues for the student to address, all of which are supported with considerable experience and relevant facilities by the supervisory team, include:
• Analysis of sediment trap material to ascertain relative fluxes of calcium carbonate from different planktonic groups.
• Allometric estimates of growth rates and calcite production rates.
• Rate measurements using radioisotopes.
• Satellite estimates of calcite production for different hydrographic provinces.

Project Timeline

Year 1

Synthesis of existing data and observations, simple allometric estimates of growth and calcium carbonate production rates based on organism dimensions and ecology, gap analysis to identify the key bits of missing information and design of a methodological approach. Discussion with national and international experts on calcifier ecology and calcium carbonate production. Write up of synthesis and conclusions so far for peer-reviewed publication.

Year 2

Laboratory work: to include (e.g.) sample analysis, and satellite data. Presentation of year 1 conclusions at a national conference (e.g. Challenger Marine Sciences Conference).

Year 3

Further laboratory work, data analysis, and preparation of second publication. Presentation of year 2 and 3 conclusions at an international conference (e.g. American Geophysical Union).

Year 3.5

Completion of thesis, high-level synthesis of project conclusions into a new global calcium carbonate budget separated by calcifying organism and submission of this to a high-impact journal.

& Skills

Project support: The facilities, equipment 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.


Student support: The Lyell Centre has a large research student cohort (>30) 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, University of Stirling and British Antarctic Survey, providing an opportunity to learn about other techniques and research areas which may be applicable to their research. Additionally, the supervisors are all based in research-active institutes 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.:

  • 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.
  • 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

Broecker and Clark (2009) Ratio of coccolith CaCO3 to foraminifera CaCO3 in late Holocene deep-sea sediments. Paleoceanography, 24, doi: 10.1029/2009PA001731.

Daniels et al. (2018) A global compilation of coccolithophore calcification rates. Earth System Science Data,10, 1859-1876, doi: 10.5194/essd-10-1859-2018.

Manno et al. (2018) Threatened species drive the strength of the carbonate pump in the northern Scotia Sea. Nature Communications, 9, doi: 10.1038/s41467-018-07088-y.

Schiebel (2002) Planktic foraminiferal sedimentation and the marine calcite budget. Global Biogeochemical Cycles, 16, doi: 10.1029/2001GB001459.

Further Information

Dr Alex J Poulton,

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