Limnology of Lake Alexandrina, River Murray, South Australia, and the effects of nutrients and light on the phytoplankton

Abstract

Lake Alexandrina is a shallow, well-mixed freshwater system at the mouth of the River Murray. Median water renewal time is 109 days. The lake is characterized by high turbidity and high light extinction, with mean values of 93 NTU and 6.54 In units m^-1, respectively. Background extinction is high and the algal biomass contributes little to light extinction. Changes in turbidity reflect disproportionate contributions from the Murray and its major tributary the Darling. Based on total phosphorus and total Kjeldahl nitrogen levels, the lake is chemically eutrophic-hypertrophic but the nitrogen to phosphorus ratio is low and levels of filterable and soluble reactive phosphorus and especially NO₃/NO₂-N are often low, suggesting that most of the nutrients are in particulate form and perhaps unavailable for plant growth. From 1975 to 1978, the ulotrichous green alga Planctonema lauterbornii generally accounted for more than 95% of algal cells. It was growing under conditions of low average irradiance in the mixed water column. Chlorophyll a varied seasonally, with peak values in late spring. The summer drop in chlorophyll a correlated with low availability of nutrients, especially NO₃/NO₂- N. Low light availability appeared to preclude blue-green algae during the present study although blooms were recorded in 1965-1967 and 1972-1973.