The release of legacy phosphorus from deforestation-derived sediments in shallow, coastal lake Forsyth/Te Roto o Wairewa

ABSTRACT

Phosphorus (P) release from sediments accumulating since catchment deforestation, has been linked to algal blooms which have occurred in Lake Wairewa (Canterbury, NZ) for over a century. However, the mechanisms controlling this internal P loading are poorly understood. To address this issue, we investigated sediment P fractions, P release, dissolved oxygen (DO) and pH at the sediment–water interface, and undertook targeted water sampling. Sediment depth profiles of P fractions indicated surface enrichment (1600 mg P kg⁻¹), which was due to an upward flux of P from legacy sediments deposited over at least the last 100 years. Phosphorus release under reducing conditions (0.75–64 mg P m⁻² d⁻¹) and pH >9.00 (3.27–11.07 mg P m⁻² d⁻¹) indicated the potential for internal loading. Low DO events (<2 mg L⁻¹) in bottom waters were short-lived, and spatially discrete, although longer events (13 days) were observed. pH was variable (6.20–10.80), and prolonged periods of pH >9 (5–6 days) were recorded. Macrophyte beds were key drivers of low DO, elevated pH, and sedimentary P release. The results challenge the paradigm of well mixed shallow lakes and contribute to the knowledge required to understand shallow lake systems impacted by land-use change.

KEYWORDS:

• Phosphorus
• legacy
• release
• sediment
• pH
• oxygen
• redox
• macrophytes
• deforestation

Disclosure statement

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

Additional information

Funding

This research was supported by a University of Canterbury Doctoral Scholarship (to Sean Waters), with additional funding from Environment Canterbury and Meadow Mushrooms. Manuscript preparation was supported by the New Zealand Ministry for Business, Innovation and Employment (Enhancing the health and resilience of New Zealand Lakes, UOWX1503). We thank Environment Canterbury for providing supporting long-term monitoring data, and Ngai Tahu, Wairewa Runanga and Kinloch Farm for access to the lake.