Phytoplankton Production and Winter Mixing: Contrasting Effects in Two Oligotrophic Lakes

Abstract

SUMMARY (1) Two deep oligotrophic lakes in the centre of North Island, New Zealand, shared several limnological features but differed in their seasonal patterns of algal production. (2) Lake Waikaremoana followed, in its thermal stratification, the classic pattern for temperate warm-monomictic lakes. During winter circulation, light availability to the phytoplankton was reduced by a factor of at least 13 relative to that in spring and summer, and there was no increased supply of phosphorus, which later controlled the amount of growth of its phytoplankton. Concentrations of algal biomass and algal production rates were predictably low, but rose rapidly with increased irradiance in spring. (3) Lake Taupo experienced similar thermal stratification and mixing, but concentrations of algal biomass and algal photosynthetic production rose to maxima for the year during winter isothermy when temperatures were at their annual minimum and when mixing extended to the bottom of the lake. Winter circulation brought an increased supply of phosphorus and, more importantly, nitrogen which later controlled the amount of algal growth in the euphotic zone. This increased availability of potentially limiting nutrients offset the effects of low light availability and low temperature. (4) Lake Taupo, and other nearby lakes which behave similarly, form intermediates between temperate lakes with typical winter minima of algal production and many tropical lakes which experience maximum phytoplankton production during storminduced mixing.