Lake Okareka, New Zealand
Lake Okareka is a 330 ha lake on the North Island of New Zealand located in a rich dairying region. Between August 2005 and March 2007, three separate applications of Phoslock totalling 60 tonnes were undertaken by Environmental Bay of Plenty, the regional environment authority in an effort to remove 0.1 tonnes of phosphorus from the lake per year for three years. The applications were undertaken over a surface area of 170 hectares, encompassing the deeper basin of the lake approximately within the 20m contour.
This action was considered necessary by the Lake Okareka Working Party as part of the Lake Okareka Action Plan as the lake was showing increasing evidence of depleted oxygen levels in the bottom water towards the end of summer and nutrients were being released from sediments. Hypolimnetic discharge was initially considered, however this was discounted due to its cost.
Comprehensive monitoring of the lake before and after the Phoslock treatments has shown that the Phoslock addition has reduced the release of phosphorus from the bottom sediments by the targeted 100 kg / year. An additional benefit of the applications has been a decrease in the rate of de-oxygenation of the bottom waters. No adverse effects of the treatment have been noted in the lake chemistry or fish and koura health.