Denitrification capacity of lake sediments across a gradient of catchment land use in Rotorua, New Zealand

Intensification of agriculture in lake catchments of the Rotorua region has accelerated eutrophication in some lakes, while others remain minimally impacted. Seasonal patterns of hypoxia, ammonium (NH₄⁺) release from sediments, nitrate (NO₃^-) generation and subsequent NO₃^- loss suggest that denitrification plays an important role in nitrogen cycling in these lakes, many of which exhibit nitrogen-limited primary production. Preliminary data indicates that water column NO₃^- is positively related to catchment pasture (r²=0.67, p=0.013, n=10), however, the influence of N-loading on sediment denitrification is unknown. We measured denitrification enzyme activity (DEA) in 10 Rotorua lakes that varied widely in catchment pasture to determine if there is a link between agricultural land use in the lake catchments, N loading, and sediment denitrification capacity. We collected sediment cores for DEA measurements over 3-weeks in spring 2008. Lake Okaro, with 96% catchment pasture, exhibited approximately 50 times higher DEA than Lake Tikitapu, with 3% catchment pasture, (378.5 ± 25.2 vs. 7.2 ± 1.3 mg N gAFDM^-1 d^-1, respectively). We will measure DEA again post summer stratification to determine if agricultural land-use influences sediment denitrification consistently across seasons. Understanding lake sediment denitrification capacity across an agricultural gradient will aid in management for improved water quality.