Freshwater marl lakes: the unsung hero of Blue Carbon


Blue Carbon refers to carbon captured and sequestered in marine and coastal environments, typically consisting of organic carbon stored in marine and estuarine sediments as well as that sequestered in cold water corals and salt marshes (Smeaton et al 2020, Shafiee 2021). The freshwater Blue Carbon component, with carbon stored in fluvial and lacustrine sediments, is less well defined. Freshwater marl lakes which actively accumulate calcium carbonate rich sediment are a potentially important contribution to freshwater Blue Carbon as in these settings carbon is both sequestered and stored
Marl lakes in their natural state are highly alkaline, carbonate rich lakes, with a diversity of macrophytes recognised in the European Union Habitats and Species Directive (Wiik et al 2015a). The accumulation of calcium carbonate as marl in these lake systems is through a combination of direct precipitation, driven by seasonal variations in a range of physicochemical parameters, and biochemical processes as the calcite forms as incrustations on macrophytes, notably Characeae and Potamogetonaceae (Duston, 1986). The marcophytic diversity of a healthy marl lake can lead to very rapid accumulation of marl sediment particularly during the summer months.
Pentacost (2009) points to relatively few marl lakes in the British Isles remaining as typical marl lake biology with eutrophication through human impact and reduction in lake size being the main drivers. Changes within the catchment and the littoral edge, in particular nutrient inputs, impact lacustrine productivity and greatly reduce the accumulation of marl (Wiik et al, 2014; 2015a). Co-precipitation of calcite with P may help to buffer the effects of eutrophication but the implications of this are contested (Wiik et al, 2014). Thus, the complex interplay of lake biogeochemical processes driving marl formation must be understood to assess carbon sequestration potential.

Wiik et al (2015b) use a range of palaeoecological and geochemical approaches to define the aquatic ecosystem changes in the recent past but faced a challenge in defining the nature and timing of human impact on the marl lake ecosystems, with many modern lake systems a product of environmental changes first initiated in prehistory. In this project, palaeoecological and geochemical analysis of the marl sediments accumulated during the Late Glacial and early Holocene will offer an insight into how these marl lake ecosystems adapted to rapid and extreme changes in the lacustrine and terrestrial environment. This long view of the response to changes in both biochemical and physiochemical parameters will define the health and resilience to environmental change of present day marl lake systems. In terms of natural based solutions, marl lakes are similar to saltmarshes in that with better understanding of how these ecosystems contribute to carbon sequestration and storage then we are better placed to actively conserve and restore them.


New analysis of previously generated highly resolved palaeoenvironmental data sets from a marl lake in Orkney will be used to refine the methodological approach used in this project. The generation of new palaeoecological and geochemical data sets will further define the nature of these phases of environmental change as well as indicate potential drivers, those that influence the biochemical and physiochemical parameters, switching on and off marl accumulation.
This refined methodological approach will be applied to other marl lakes, with sediment cores sampled from both active and recently inactive lake systems. The relationship between environmental change, lacustrine processes and aquatic ecology can be complex. Stable isotope analysis (Newcastle University), here with a focus on aquatic fossils will provide specific and detailed information on past ecosystems. This analysis will also provide habitat specific data sets on the biogeochemistry of marl lakes as well as data sets on climatic and hydrological changes. SEM and SEM-EDX (University of Stirling) will be used to explore more specifically the co-precipitation (and potential disassociation) of calcite, P and other metals as well as identifying evidence of dissolution of calcite, providing a fuller picture of changes in water quality, processes of marl formation and C sequestration. Lithostratigraphic analyses (organic content and micro-XRF geochemistry, supported by Dr Sarah Davies, University of Aberystwyth) will provide fine-scale evidence for the environmental induced changes in the water column and wider catchment. The palaeoenvironmental records will be constrained using radiocarbon dating (supported by NERC-RCF) and Pb210 (University of Stirling) to understand change on ecological timescales.
The methodology proposed here will integrate palaeoecological, ecological and geochemical techniques. This project will develop techniques and methodological approaches to overcome traditionally perceived challenges, bringing these disciplines together and gain a perspective that will connect past and future ecosystem dynamics.

Project Timeline

Year 1

Review of literature and palaeoecological data sets already generated from Late glacial and early Holocene marl lake system (Orkney). Refinements and development of methodological approach and site selection, with training in analytical techniques if required. Field sampling (Orkney) towards the end of year 1 (Spring/Summer).

Year 2

Analysis of lake sediment cores (those sampled in year 1) and subsequent field sampling for sites, both modern and palaeolacustrine settings, fieldwork to Isle of Lismore Argyll and Bute, and other sites that are active marl lakes (short cores) (Spring and Summer). Additional training in analysis and interpretation of newly generated data sets. Initial writing up of sites sampled and methods.

Year 3

Continued analysis of lake sediments sampled in year 2, final analysis to be completed by mid year 3 and then interpretation of data sets, with continuation of writing up of completed sites and analysis.

Year 3.5

Completion of thesis writing, viva preparation

& Skills

The PhD student will receive training in field techniques, develop expertise in palaeoenvironmental reconstruction techniques, specifically palaeolimnolgy and palaeoecological techniques and timeseries analysis. The project also allows the student to develop skills in a range of lithostratigraphic and geochemical techniques. The supervisory team will provide complementary expertise and deliver project-specific training in (1) sediment stratigraphic analyses, palaeolimnology and palaeoecological techniques (Tisdall) (2) Stable isotope analysis and limnological biogeochemical processes (Henderson) (3) lacustrine geochemistry (Wilson) (4) marl lake aquatic ecology (Willby/Henderson). The student will benefit from the vibrant conservation and aquatic ecosystem research community at Stirling as well as those established through the University of Newcastle. The student will also attend training and networking events within the IAPETUS2 DTP and offered via NERC.

References & further reading

Duston, N.M., Owen, R.M., Wilkinson, B.H. 1986 Water chemistry and sedimentological observations in Littlefield Lake, Michigan Implications for marl deposition. Environmental Geology and Water Sciences, 8, 229-236.
Pentacost, A. 2009 The marl lakes of the British Isles. Freshwater Reviews, 2, 167-197.
Wiik, E., Bennion, H., Sayer, C.D., Willby, N.J. 2014 Chemical and biological responses of marl lakes to eutrophication. Freshwater Reviews, 6. 35-62.
Wiik E., Bennion, H., Sayer, C.D., Davidson, T.A., McGowan, S., Patmore, I.R. and Clarke, S.J. 2015a Ecological sensitivity of marl lakes to nutrient enrichment: evidence from Hawes Water, UK. Freshwater Biology 60, 2226-2247.
Wiik E., Bennion, H., Sayer, C.D., Davidosn, T., Clarke, S.J., McGowan, S., Prentice, S., Simpson, G.L. and Stone, L. 2015b The coming and going of a marl lake:multi-indicator palaeolimnology reveals abrupt ecological change and alternative views of reference conditions. Frontiers in Ecology and Evolution 3, 82.
Shafiee, R. 2021 Blue Carbon. SPICe Briefing paper, The Scottish Parliament.
Smeaton, W, Austin, W., and Turrell, W.R. 2020 Re-Evaluating Scotland’s Sedimentary Carbon Stocks. Scottish Marine and Freshwater Science 11 (2), 20pp.

Further Information

Dr Eileen Tisdall, Biological and Environmental Sciences, Faculty of Natural Sciences, University of Stirling. Stirling FK9 4LA. Tel 01786 467771, email

Apply Now