Effects of physical disturbance on marine ecosystem functioning


Physical pressures like bottom trawling and aggregate extraction modify the condition of benthic habitats, their species composition, and their ability to deliver essential ecosystem functions such as carbon sequestration that play a role in climate regulation. While our understanding of the effects of physical disturbance such as bottom trawling and deep sea mining on macrofaunal communities and their recoverability is considerably well developed [1 – 3], we understand far less the impacts on benthic biogeochemical processes such as carbon remineralization and nutrient cycling, and biological groups such as microbes and meiofauna that mediate these functions. Understanding these changes is critical if we are to understand the environmental cost of anthropogenic activities and manage these activities sustainably.

This PhD focuses on using systematic review as evidence synthesis tool and lab-based experiments to study the effects of physical disturbance from anthropogenic activities on benthic ecosystem functioning. The project will provide data into quantitative risk assessment methods [e.g. 4] for the purposes of assessing anthropogenic impacts and informing habitat conservation and ecosystem-based management.


In the first instance the student will quantify the effects of physical disturbance, in terms of initial change and recovery to pre-disturbance conditions, on different benthic biological groups (macrofauna, meiofauna, microbes) and biogeochemical properties (e.g. oxygen content) and processes (e.g. denitrification), using existing scientific literature and empirical datasets. Relevant studies from peer-reviewed and grey literature will be identified following standard systematic review methodology [5] and synthesized using quantitative methods such as meta-analysis [6]. The scope of the review will be global and will focus on anthropogenic activities that disturb the seafloor through direct physical contact (e.g. abrasion, ploughing, extraction), and include activities such as boat anchoring, fishing using bottom towed gear, marine aggregate extraction, hydraulic and suction dredging used in some deep sea mining practices. In addition to generating estimates of depletion and recovery for different biological groups and ecosystem functions following different types of bottom disturbance, this review will identify key evidence gaps that are not well-studied and limit our understanding of the effects of anthropogenic activities on benthic ecosystem functioning.

The second part of the PhD will examine how changes in the biological and physico-chemical characteristics of the sediment following physical disturbance effect organic matter remineralization. The student will conduct ‘physical disturbance simulation experiments’ in the lab using undisturbed sediment collected from the field (intertidal) and the advanced benthic flux chambers available at the Lyell Centre, to examine the effect of different disturbance frequencies and intensities. Changes between the pre- and post-disturbance macro-, meiofaunal and microbial (bacteria & archaea) assemblages will be measured in terms of biomass, density, community composition and mortality using standard benthic ecology and molecular analytical procedures at Heriot-Watt University (HWU) and University of Glasgow (UoG), respectively. Changes to ecosystem functioning responses such as oxygen consumption dynamics and organic matter processing will be measured using stable isotope tracer techniques (isotopically enriched 13C algae) at HWU, to trace carbon throughout the benthic assemblage and quantify the change of fresh organic carbon processing following disturbance [7].

Collaborating partners:
CEFAS will provide existing spatio-temporal datasets of macrofauna and biogeochemical data (e.g. oxygen penetration depth, apparent redox potential discontinuity depth, nutrients, organic carbon) across different gradients of physical disturbance in the UK. These datasets will be included in the meta-analysis of empirical data on the effects of physical disturbance on benthic ecosystem functioning.
Bangor University (BU): Models co-developed by Dr Sciberras (HWU) and Prof Hiddink (BU) to quantify initial macrofauna depletion (% loss of biomass), recovery rate and time to pre-disturbance levels following bottom trawling disturbance will be used to quantify impact of different physical disturbance activities on meiofauna and microbes. Training support will be available to the research student by Prof Hiddink.

Project Timeline

Year 1

The student will carry out literature scoping and searching following systematic review methodology (3.5 months). Articles will be screened for their relevance (2 months) and critically appraised (1 month). Data will be extracted from relevant articles and inputted into a database for subsequent quantitative and qualitative syntheses (4 months). The student will have the opportunity to receive training on systematic review methodology and meta-analysis and gain proficiency in undertaking benthic flux experiments and grow 13C-labeled algae (labelled isotope tracers) that will be used in experiments in Year 2.

Year 2

The student will carry out fieldwork to collect sediment cores from the intertidal and set up incubation chambers in the laboratory at HWU. Experiments will last for 1 month, after which work will begin on sample processing and analysis; changes in O2 consumption, CO2 production and nutrient analysis at HWU (4 months); taxonomic analysis of macro- and meiofauna and stable isotope analysis at HWU (4 months); microbial community analysis (quantitative PCR) at UoG (3 months).

Year 3

Data synthesis and thesis writing. The student will undertake meta-analysis whenever sufficient quantitative data are available to synthesize data among disparate studies, and generate systematic maps highlighting distribution of data and knowledge gaps. The student will apply statistical analysis such as inverse linear modelling to determine changes in carbon-flows through the entire benthos following a disturbance event.

Year 3.5

Completion of peer-reviewed papers and the PhD thesis, and attendance to international or national marine ecological conferences.

& Skills

• Training in evidence synthesis tools, systematic review, and statistical analysis that combines the results of multiple scientific studies, meta-analysis.
• Training in biogeochemical techniques for assessing diffusive O2 uptake and faunal mediated O2 consumption, dissolved inorganic carbon fluxes and particulate organic carbon concentrations; stable isotope analysis to determine pathways of C-uptake; taxonomic identification; molecular analysis to determine microbial community structure (qPCR)
• Skills in experimental design, lab-based incubation chamber experiments, big data analysis
• Statistical skills using the R statistical platform and inverse linear modelling
• Science communication skills through participation in international or national conferences, and presentation of research during seminars held at HWU, UoG, BU and CEFAS
• Transferable skills: data management, project management, scientific writing

Research student support: The PhD student will be based at the Lyell Centre under the supervision of Dr Marija Sciberras and Prof Andrew Sweetman at Heriot-Watt University and Dr Karen Cameron at the University of Glasgow. The student will also benefit from advisory input from collaborators Dr Ruth Parker and Dr Clement Garcia at CEFAS and Prof Jan Hiddink at Bangor University. All are based in research-active departments that span a broad range of ecological, environmental and geochemical research, exposing the research student to a range of research areas.

References & further reading

[1] Hiddink JG, Jennings S, Sciberras M, Szoztek CL, Hughes KM. et al. 2017. Global analysis of depletion and recovery of seabed biota following bottom trawling disturbance. PNAS doi: 10.1073/pnas.1618858114[2] Jones DOB, Kaiser S, Sweetman AK. et al. 2017. Biological responses to disturbance from simulated deep-sea polymetallic module mining. Plos One doi:10.1371/journal.pone.0171750[3] Sciberras M, Hiddink JG, Jennings S, Szostek CL, Hughes KM. et al. 2018. Response of benthic fauna to experimental bottom fishing: a global meta-analysis. Fish and Fish doi: 10.1111/faf.12283[4] Pitcher CR, Ellis N, Jennings S, Hiddink JG, Mazor T et al. 2017. Estimating the sustainability of towed fishing gear impacts on seabed habitats: a simple quantitative risk assessment method applicable to data-limited fisheries. Methods Ecol Evol 8: 472-480[5] Collaboration for Environmental Evidence, Guidelines and Standards for Evidence Synthesis in Environmental Management. Version 5.0, A.S. Pullin, et al., Eds. 2018[6] Borenstein M, Hedges LV, Higgins JPT, Rothstein HR. 2009. Introduction to MetaAnalysis, 1st ed. John Wiley & Sons, Ltd, Chichester, UK[7] Mevenkamp L, Stratmann T, Guilini K, Moodley L, van Oevelen D, Vanreusel A, Westerlund S, Sweetman AK. 2017. Impaired short-term functioning of a benthic community from a deep Norwegian Fjord following deposition of mine tailings and sediments. Front Mar Sci doi: 10.3389/fmars.2017.00169

Further Information

For further information, please contact:
1) Dr Marija Sciberras, Lyell Centre, Heriot-Watt University. Email: m.sciberras@hw.ac.uk

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