Can we restore waterfowl population by restoring aquatic macrophytes? Experiments in a New Zealand lake

Mitchell, S. F. and Wass, R.T. (1997) Can we restore waterfowl population by restoring aquatic macrophytes? Experiments in a New Zealand lake. MAGYAR VÍZIVAD KÖZLEMÉNYEK, 3. pp. 361-362. ISSN 1416-1389 (nyomtatott); 1419-6107 (elektronikus)

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Abstract

The black swan population of Hawksbury Lagoon, a very shallow (z = 40 cm) eutrophic coastal New Zealand lake, collapsed in April 1993. After being stable for more than 3 years at a mean of 250 birds (10 ha-1) it declined to below 10 in a 2 week period with the advent of an algal bloom, higher water levels, increased turbidity of the water, and loss of aquatic macrophytes. In August (Southern spring) an attempt was made to restore the population by lowering the water level to allow more light to reach the lake bottom, and reestablish the benthic flora on which the swans feed. This attempt enjoyed a spectacular success initially, with a progressive increase in the biomass of benthic algae to 30 gnr2 by February, and a tightly coupled increase in the swan population to 25 ha'1, the highest ever recorded for the lake This success was, alas, short-lived. The benthic light climate deteriorated again in association with high phytoplankton biomasses and sediment resuspension by wind, the benthic flora again collapsed, and the swans departed. Swan grazing did not contribute significantly to this loss of vegetation. A model for sediment resuspension suggests that the benthic algal biomass almost reached the threshold necessary to suppress wave resuspension of sediment, and the experiment will be repeated in 1994, in an effort to switch the ecosystem to a state in which benthic plants are dominant and the swan population is high.

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