Sediments of Lake Rotorua as sources and sinks for plant nutrients

Lake Rotorua, on the Central Volcanic Plateau of the North Island of New Zealand, is a third‐class lake of 81km 2 and 10.7 m average depth. The lake is eutrophic and in recent years has produced blue‐green algal blooms of potential nuisance proportions. By a mass balance approach, nutrient exchanges between lake sediments and the water above were quantified. Phosphorus released from temporarily deoxygenated sediments was estimated at 20–40 mg.m‐2.d −1 and nitrogen at 250–530 mg.m −2 .d −1 . Phosphorus sedimentation was less than 1 mg.m −2 d −1 when the sediment surface was oxidised. A massive net loss of nitrogenous material from lakewater to sediments and/or the atmosphere, equivalent to 20 mg.m −2 .d −l , may contribute to the shortage of nitrogen exhibited by the algae of this lake.