Aspects of the physics, chemistry, and phytoplankton biology of Lake Taupo

Abstract

Lake Taupo was sampled monthly from August 1974 to March 1976 at three sites where the depth was about 100 m. The lake stratifies in summer and mixes freely in winter, when water temperatures drop to 10–11°C. Oxygen depletion in bottom waters towards the end of the stratification period amounts to about 4 g.m ‐3 . Water transparency, as measured by Secchi disc, ranges from 11 m to 21 m. The pH is typically between 7 and 8; extreme values are 6.8 and 8.5. Major ion concentrations total about 2.3 eq.m ‐3 . Anion concentrations follow the pattern HCO 3 ‐ > Cl ‐ > SO 4 2‐ , and cations Na + > Ca 2+ > Mg 2+ >K + . The major ions varied little seasonally, spatially, or with depth, except bicarbonate, for which bottom concentrations were 1–2 g.m ‐3 higher than at the surface. Reactive silicon concentrations were 9.2–10.3 g.m ‐3 ; there was no apparent relationship with diatom populations. Total dissolved iron concentrations ranged from 2 mg.m ‐3 to 8 mg.m ‐3 , and manganous‐manganese from 5 mg.m ‐3 to 10 mg.m ‐3 , with no discernible pattern. The distributions of reactive phosphorus and nitrate‐nitrogen developed pronounced seasonal chemoclines which were readily interpretable by reference to the state of lake stratification and phytoplankton populations. Ammonium‐nitrogen concentrations in the epilimnion tended to increase in summer; this could not be explained. Chlorophyll concentrations in the top 30 m of the lake varied between 0.65 mg.m ‐3 and 2.9 mg.m ‐3 , with a pronounced winter peak associated with a phytoplankton population dominated by Melosira granulata . Twenty‐four genera of algae appeared in cell counts.