Level of funding: UK/EU fees + stipend for 3 years
Background: The diurnally migrating phantom-midge larvae Chaoborus spp. are prevalent in lakes (down to 70 m) world-wide with up to 130,000 individuals m-2. They are key predators of zooplankton and prey for fish, and are widely used palaeolimnological indicators for reconstructing the ecological history of lakes. As they traverse from the sediment, where they reside during daytime, to the surface water to feed at night, Chaoborus exert influences on the entire water column plus the sediment-water interface on a daily basis. We have begun to investigate their migration energetics and the resultant sediment bioturbation.
Key outcomes include:
- Chaoborus use sediment methane to inflate their gas sacs, which provide buoyancy to aid migration. This unique adaptation can save the larvae up to 95% of their energy for migration.
- Daily movement into and out of the sediment leads to an estimated 1000-fold increase in sediment diffusivity.
- Transport of sediment methane by migrating Chaoborus can increase methane emission by up to 40%.
- We propose the concept of tipping point when sediment methane production reaches a threshold in eutrophication history for this migration mechanism to be feasible, thereby allowing successful colonization and expansion of Chaoborus habitat range, with corresponding change in the lake's foodweb structure and biogeochemical properties.
For the PhD project, the student will:
- Use imaging technology to characterise the fine-scale bioturbation behaviour at the sediment-water interface, and from which compute the apparent diffusivity. These data provide the physical basis for understanding bioturbation effects on lake ecology.
- Conduct laboratory experiments to quantify chemical release from sediment by bioturbation. The experiments will use different Chaborous species, sizes and chemicals. Data will be scaled up to ecosystem-level chemical fluxes from sediment. By combining with part 1 the student will produce a predictive model on chemical internal loading based on eutrophication history and Chaoborus abundance.
- Conduct lake-mesocosm experiments to study the effects of Chaoborus migration on water column chemistry. By manipulating the larvae's migration intensity (e.g. by changing the light environment or Chaoborus abundance), the student will monitor the corresponding changes in water chemistry. These data will complement parts 1 and 2 to improve the model.
- Study relationship between lake eutrophication and Chaoborus distribution. This will be based on data mining, field surveys and examining archived samples. Of particular interest are lakes undergoing eutrophication and lakes undergoing nutrient reduction (oligotrophication). This part provides quantitative data to test the eutrophication tipping point idea, and to compare model output from parts 1 and 2.
Significance: This interdisciplinary project combines biology, physics and modelling to advance our understanding of an environmentally important topic. Eutrophication and pollution increasingly threaten many lakes. We posit that once a lake has past the eutrophication tipping point, Chaoborus bioturbation will sustain internal loading of sediment nutrients and pollutants for an extended time despite efforts to manage or reduce external input. This project will provide a better scientific understanding into this "legacy effect" and inform more effective and feasible management practices.
Specify project DTC BIO 9
Application deadline: 30th April 2017