High contribution of methane in greenhouse gas emissions from a eutrophic lake: a mass balance synthesis

ABSTRACT

Lakes are ‘hotspots’ for greenhouse gas (GHG) emissions, primarily carbon dioxide (CO₂) and methane (CH₄). Understanding the processes regulating GHG emissions from lakes, and their temporal variability, is essential for more accurately quantifying the role of lakes in global GHG cycles. In this study, we identified the processes that affect CO₂ and CH₄ concentrations in a small (0.3 km²) eutrophic monomictic lake (Okaro, New Zealand). A mass balance model was used to calculate changes in CO₂ and CH₄ storage in the lake as a result of internal cycling and atmospheric fluxes. To support model computations, CO₂ and CH₄ concentrations profiles were measured monthly over a one-year period, in addition to temperature, dissolved oxygen and chlorophyll a. Annually, Lake Okaro acted as a sink of CO₂ from the atmosphere (425.4 mmol CO₂ m⁻² y⁻¹) and a source of CH₄ (553.4 mmol CH₄ m⁻² y⁻¹) equating to a net GHG emission (diffusive fluxes of CO₂ and CH₄ combined) of 0.22 kg CO₂-eq m⁻² y⁻¹. Although it may be viewed as conservative and applies only to diffusive fluxes, our study indicates that eutrophic lakes with high rates of primary production may act as a net source of GHGs.

KEYWORDS:

• Lake
• eutrophic
• greenhouse gas
• CO₂
• CH₄
• thermal stratification

Acknowledgement

We thank the Bay of Plenty Regional Council (BOPRC) for technical support, and Paul Scholes of BOPRC for data provision. Special thanks to Joseph Butterworth (BOPRC) for field work assistance and Janine Ryburn (The University of Waikato) for helpful support in laboratory analysis. ABS: Conceptualisation, Methodology, Software, Validation, Formal Analysis, Investigation, Writing – Original Draft, Writing – Reviewing and Editing, DPH: Funding acquisition, Supervision, Resources, Writing – Reviewing and Editing. LAS: Supervision, Resources, Reviewing and Editing. ISO: Writing – Reviewing and Editing. CHH: Supervision.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

This study was part of a PhD project supported by the NZ-ASEAN Scholarship. Research funding for this study was also supported through the Lake Biodiversity Restoration programme funded by the Ministry of Business, Innovation & Employment (UOWX0505) and the Bay of Plenty Chair in Lake Restoration at the University of Waikato.