New Zealand Journal of Marine and Freshwater Research abstracts

---

Oxygen consumption in the epilimnia and hypolimnia of two eutrophic, warm-monomictic lakes

S. F. Mitchell

Carolyn W. Burns

Department of Zoology, University of Otago, Box 56, Dunedin, New Zealand

Abstract Oxygen consumption was estimated for two eutrophic New Zealand lakes using a simple 2-layer model, from estimates of net changes in oxygen, phytoplankton photosynthesis, inflow and outflow, diffusion, eddy diffusion, and changes in the depth of the thermocline. Of the total oxygen consumption, 75-85% occurred in the epilimnia. Epilimetic oxygen consumption per m³ and per m² in Lake Johnson were higher than in Lake Hayes, reflecting known differences in the trophic status of the lakes, but estimates for the hypolimnia of the two lakes were similar. Daily hypolimnetic areal deficits were sensitive to both the depths and time intervals chosen.

In the epilimnia, monthly oxygen production by photosynthesis averaged 1.1 and 1.8 times the initial oxygen content in Lakes Hayes and Johnson respectively. Diffusion appeared to be of similar magnitude to photosynthesis in the isothermal period, but was less important during stratification. Losses to the hypolimnia by eddy diffusion were less than 10% of photosynthesis. Photosynthesis in the hypolimnia contributed 30-40% of the oxygen consumed there and eddy diffusion supplied a further 20%.

In Lake Johnson a 3-5 m stratum of water in the metalimnion became anoxic for 2-3 months each summer while there were still up to 5.2 g.rer³ of oxygen remaining in the hypolimnion. Metalimnetic anoxia is attributed largely to oxygen consumption in situ, resulting from the highly eutrophic state of the lake and intense thermal stratification. Zooplankton respiration accounted for less than 10% of the maximum net decline in metalimnetic oxygen in 1970-71, but was more important in the other 2 years.

Changes in the oxygen content of the hypolimnion indicate that Lake Hayes has become more eutrophic since 1954-55. In Lake Johnson low oxygen concentrations at the autumn overturn (<5 g.nr³) represent a threat to the trout population.

New Zealand Journal of Marine & Freshwater Research, 1979, 13 (3): 427-441
Received 5 March 1979; revision received 30 April 1979

PDF file of entire paper: medium quality (1293K); (scanned from paper original: notes about this process)

---

This year's abstracts | Journal home page | All abstracts | Publishing home page