1. Pulsed emission of methane (CH 4 ) from a small eutrophic lake Arianto Santoso David Hamilton photo credit: http://www.rotorualakes.co.nz/ Joint Conference of The NZFSS & NZHS: 24 th - 28 th November 2014

2. Ciais et al., (2013) 8.91.02.34.3 Introduction: Global carbon balance

3. Introduction: Climate change and eutrophication Moss et al. (2011) Nutrients and organic matter inputs Greenhouse gas pathway Climate change effect

4. Questions 1.How is CH 4 dynamics in a eutrophic lake: How is the production how is the emission How is the oxidation rate 2.Is physical properties of the lake controlling the release of CH 4 3.How climate change and eutrophication control CH 4 dynamics

5. Method: Study area, Lake Okaro Eutrophic, current TLI = 4.75 Surface area: 0.3 km 2 Catchment area: 3.89 km 2 (~95% pasture) Low Schmidt stability value means the lake is mixed Depth

6. Monthly sampling : October 2013 – October 2014 Dissolved gas analysis by GC after headspace equilibration Method: Sampling design Floating chamber Sediment porewater peeper Bubble trap CTD, water profile

7. Method: Parameter calculation Vertical turbulent diffusivity (Kz) : Hondzo & Stefan (1993) Sediment-water flux (J): Fick’s law Air-water flux (F): Cole & Caraco (1998) CH 4 oxidation: Michaelis-Menten with V max and K CH4 from Liikanen et al. (2002) Atmospheric flux (CH 4 ) D flux, K z CH 4 CH 4 ox CO 2 CH 4 J F O2O2

8. Results and discussion: Temperature and O 2 dynamics The lake mixes thoroughly around June Temperature Dissolved oxygen

9. Results and discussion: Dissolved CH 4 dynamics and pulsed atmospheric flux CH 4 starts to accumulate in the hypolimnion when stratification is built up High concentration of emission occurs during winter over turn CH 4 is “disappeared” just days after mixing started

10. Results and discussion: Coefficient of vertical eddy turbulent diffusion (Kz) Kz increases as the lake is mixed

11. Results and discussion: Diffusion vs. oxidation rate Diffusion Oxidation Diffusion rate might be higher than oxidation rate

12. Porewater CH 4 Hypolimnetic CH 4 (at 16 m depth) Results and discussion: Hypolimnetic accumulation Sediment-water CH 4 flux: 20.98 mmol m -2 d -1 Hypolimnetic CH 4 accumulation: 0.73 mmol m -2 d -1

13. 14.81 10 -4 ~ 10 -2 7. 10 -5 ~20 20.98 6 0.1 0.73 Results and discussion: CH 4 mass balance in Lake Okaro Diffusion flux Oxidation Hypolimnetic accumulation Rate: mmol m -2 d -1 stratifiedmixed

14. Conclusions Although can escape as net emission, most CH 4 produced in the sediment is oxidized in the water column The decrease in oxidation rate during the mixing period results in an increased CH 4 release from the lake Higher rate of diffusivity during winter overturn lifts CH 4 from hypolimnion to the surface

15. Outlook Lakes emissions will be simulated by hydrodynamic-ecological modelling with response to climate change and eutrophication http://aed.see.uwa.edu.au/research/models/GLM/

16. Acknowledgement Photo credit: Deniz Özkundakci Field sampling support: Joseph Butterworth Theodore Kpodonu Ryan Mallett Lab. analysis: Janine Ryburn

17. Simplified conceptual CO 2 Atmospheric flux (CO 2, CH 4 ) Algae Organic carbon burial River input: Nutrients, organic + inorganic carbon River output: Nutrients, organic + inorganic carbon D flux CO 2, CH 4 CH 4 ox CO 2 CH 4 CO 2