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• Fate of heavy metals in the Magela Creek system, northern Australia: II Experiments with plastic enclosures placed in Island Billabong during the 1980 Dry season - limnology and phytoplankton (PDF 1.49 MB)

About the report

Two 5 m plastic cylinders (enclosures) were used to isolate parts of Island Billabong so that the influence of added heavy metals on the phytoplankton could be assessed. A mixture of the metals manganese, cadmium, copper, lead and zinc was added to one enclosure and the other was kept as a control. The changes in physico-chemical qualities, phytoplankton species and numbers, and concentrations of the metals were monitored over a six week period from 6 November 1980.

Temperature and dissolved oxygen measurements indicate that the water in the control enclosure simulated the water in the billabong. In the early part of the study, the pH values were similar in both, but there were considerable differences towards the end of the experiment.

In situ measurement of absolute fluorescence was found to be a poor indicator of algal cell numbers, probably because the algal species present fluctuate considerably in their relative abundance and different species give different fluorescence signals.

Species and total numbers of algae present showed that, in the first half of the study, the control enclosure behaved like the billabong. After Day 15, the billabong and the enclosure differed markedly in both cell concentrations and species composition. Cell numbers stayed approximately constant in the control enclosure, but marked changes in the species composition were noted, with the diatom Eunotia zaawninensie (Cabesiz), Körner becoming increasingly dominant.

The metalls added to the metal-loaded enclosure caused a significant reduction in the cell numbers, particularly during the first three days of the experiment. The most affected of the algae was the diatom E. sasurninensis whose numbers were reduced by over two orders of magnitude. Numbers began to increase gain around Day 15 of the experiment so that after Day 36 of the study both the numbers of phytoplankton and the species composition were similar to those in the control enclosure.

The metals added to Island Billabong waters were relatively rapidly removed from the water column and had little long-term effect on species composition or abundance.