As freshwater lakes dry out, turtles normally relocate to another water body or aestivate in moist sediment. However, at times of extreme drought, toxins can concentrate and acute toxicity events may occur. Such an incident may lead to potential extinction in a water body where remaining individuals of a threated or vulnerable species are present. In May, 2005, mass deaths of the turtle Emydura macquarii macquarii and accompanying water quality were investigated in Lake Numalla in the RAMSAR protected Currawinya Lakes, south western Queensland.
Indications of toxicity in the lake began as early as 2002 with a kill of the Common Carp and progressed with various turtle and bird kills until April 2005 when no further live turtles were detected in the lake. The number of dead Em. m. macquarii in May 2005 was estimated as greater than 8,600. The carcasses were in a drying water body of high conductivity, high pH, elevated ammonia, fluoride and boron. The stench of hydrogen sulfide was also present. These conditions can produce fluoride or boron bioaccumulation in the biota and the release of pulses of ammonia or hydrogen sulfide can be responsible for sudden deaths. A bloom of the cyanobacterium Limnospira maxima was also present although no common cyanobacterial toxins were detected. The presence of novel cyanotoxins cannot be ruled out.
Severe drought can also lead to reduced food supply, but events with the sudden death of organisms in a short space of time should be taken into account in the management of critical water bodies and organisms in the Australian environment of cycles of flood and drought.