Submitted on: 20 July 2002.
Revised on: 23 August 2002.
Accepted on: 20 September 2002.
___________________JOURNAL OF FRESHWATER ECOLOGY---TAYLOR & FRANCIS INC
Submitted on: 10 October 1999.
Revised on: 01 December 1999.
Accepted on: 20 January 2000.
___________________JOURNAL OF FRESHWATER ECOLOGY---TAYLOR & FRANCIS INC
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