The role of turbidity in the limnology of Lake Alexandrina, River Murray, South Australia; comparisons between clear and turbid phases

Abstract

In the first months of 1981, the characteristically highly turbid Lake Alexandrina cleared somewhat so that turbidity dropped from a previous mean value (1975-1978) of 93 NTU to 9 NTU, the Secchi transparency and euphotic depth increased from previous means of 19 cm and 0.8 m to 72 cm and 3.1 m, and the light extinction coefficient fell from 6.54 to 1.2 In units m-1. Low flows in the River Murray, especially low contributions from the Darling River, and high salinity appeared to be the major factors responsible for the clearing. During the clear phase, nutrient levels fell, the normal summer peak of the ulotrichous green alga Planctonema lauterbornei failed to occur, there was a late summer-autumn bloom of blue greens especially Nodularia and Anabaena, and densities of the large microcrustaceans Boeckella triarticulata and Daphnia carinata were low. After flushing, the lake turbidity rose to previous high levels and over the period October 1981-April 1982 P. lauterbornei again dominated the phyto- plankton, chlorophyll a biomass reached 67 mg m-3, and the zooplankton community returned to its normal pattern of seasonality and abundance. The role of turbidity in controlling the physicochemical and biological conditions in the lake is discussed.