Limnology of four groundwater-fed saline lakes in South-sestern Australia

Salinity, temperature, dissolved oxygen (DO), pH and total alkalinity (TA) were measured in four saline lakes of Yalgorup National Park, Western Australia, primarily over an 18-month period, July 1985 to January 1987, but also during 1987 and in 1988. These lakes are shallow (<3 m) ground-water sinks with no surface drainage. Rainfall and hence ground-water inflow to the lakes was highly seasonal and occurred mainly between May and October. Lakes Hayward, North Newnham and South Newnham were consistently hypersaline (e.g. Hayward 61-214 g L-1) and Hayward and North Newnham were stratified from autumn to early summer. The bottom layer of water in Hayward was usually supersaturated (to 430%) with respect to DO, because of the photosynthetic activity of the benthic microbial communities (BMC). South Newnham did not stratify in 1985, but did so briefly in 1987 after a BMC developed. The salinity of Lake Pollard varied from 19 to 51 g L-1 and the lake did not stratify at all. During spring, extensive growth of the charophyte Lamprothamnium papulosum across the sediments in Lake Pollard increased DO (from c. 100% to c. 140% saturation) and pH (from c. 8.5 to c. lo), but lowered specific TA (from 0.26 to 0.075 meq L-1 per unit salinity); later removal of the L. papulosum by swans reduced DO to 50% saturation and pH to 7.5, and increased specific TA to 0.15 meq L-1 per unit salinity. It is apparent that the processes controlling Hayward, North Newnham and South Newnham are similar and are based on the activities of the BMC. South Newnham is at an earlier stage of evolution. However, Pollard is controlled primarily by L. papulosum growth and its subsequent removal by swans; this indicates a different evolutionary path for this lake.