Influence of hypolimnetic stirring and underflow on the limnology of Lake Rotoiti, New Zealand

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Abstract
The limnology of Lake Rotoiti (North Island) is profoundly modified by the effects of an underflowing density current and geothermally induced hypolimnetic stirring. The underflowing density current results in the periodic and seasonally intermittent intrusion of eutrophic water from nearby Lake Rotorua into the eastern basin of Lake Rotoiti. Although this water is causing the long‐term degradation of the water quality of the lake (Vincent et al. 1984, 1991), the immediate impact has both beneficial as well as detrimental effects on the lake. The intruding water initiated a midwater column algal bloom through advection of an algal inoculum from Lake Rotorua but reduced the rate of hypolimnetic deoxygenation through advection of oxygen. Geothermally induced hypolimnetic stirring tended to destabilise the water column, disrupting early stratification. The intermittent cycles of stratification and mixing transported oxygen and solar energy downwards, reducing hypolimnetic deoxygenation while rapidly heating the hypolimnion in spring. When stratification became established, the stirring maintained an isothermal hypolimnion and assisted the rapid dispersion of nutrients released from the sediments throughout the hypolimnion. Consequently, the processes of hypolimnetic oxygen depletion, nitrification of ammonia to nitrate, and the subsequent denitrification occurred uniformly throughout the hypolimnion rather than with intensity progressively decreasing from the sediments upwards.