A Comparative study of the limnology of three maar lakes in western Victoria. I. Physiography and physicochemical features

Abstract

Lakes Purrumbete, Bullenmerri and Gnotuk are three typical maar lakes of volcanic origin in western Victoria. They are of similar size and altitude and are 45, 66 and 18.5 m deep respectively. The crater diameter is large (c. 3 km).

Lake Purrumbete is fresh (0.4‰), Lake Bullenmerri slightly saline (8‰) and Lake Gnotuk saline (58‰); major ions are sodium and chloride. Salinity is highest in mid-autumn and lowest in late spring. Annual salinity fluctuation in Lakes Bullenmerri and Gnotuk is significantly less than in shallow saline lakes and there is evidence of secular increase in salinity in these lakes.

Transparency is greatest in late autumn-early winter and least in late spring-summer. On the average Lake Purrumbete water is most transparent, then Lake Gnotuk and finally Lake Bullenmerri.

Surface water temperature ranges from c. 10 to 20º C. The lakes are cheimomictic and mean annual heat budget is 70, 80 and 56 kJcm^-2 for Lakes Purrembete, Bullenmerri and Gnotuk respectively.

Surface waters are always well oxygenated but the hypolimnia are anoxic by the close of stagnation. Hypolimnetic areal deficit is greatest in Lake Bullenmerri and least in Lake Gnotuk. Metalimnetic oxygen deficiencies usually occur in Lakes Purrumbete and Bullenmerri.

The water in all three lakes is alkaline (pH ~8.5); pH values are higher in summer than in winter, the difference being less in the saline lakes.

Sediments are similar in the lakes, the littoral being sandy and the profundal muddy. In the profundal, organic matter content is 35, 23 and 22 % in Lakes Purrumbete, Bullenmerri and Gnotuk respectively. Redox potential is least in the sublittoral and profundal where it is $160, + 183 and + 145 mV in Lakes Purrembete, Bullenmerri and Gnotuk respectively.

Though there are minor differences in physicochemical features between the lakes, these are unimportant compared with the salinity difference.