A scientific paper published in 1989 reports on surveys carried out at Lake Pollard, within the Peel-Yalgorup Wetland System, from 1985 to 1987. The authors observed large numbers of Black Swans feeding at the lake over spring and early summer with up to 2,000 birds recorded during visits in both the 1985-86 and 1986-87 seasons. The attraction was the dense growth of a species of benthic algae known as Foxtail Stonewort (Lamprothamnium sp.) that was recorded as covering over 90% of the lake bed in November and December. Well-known as a food resource for swans, the Stonewort supported this large population of Black Swans for months. Today, there’s no Foxtail Stonewort growing on the bed of Lake Pollard, and no swans. So what’s happened?
As part of PHCC’s Australian Government-funded Wetlands and People project, a comprehensive Yalgorup Lakes Surface Water Quality Monitoring program was initiated in 2019. In addition to monthly water quality and water level checks, a review of past water quality surveys was also carried out with the aim of identifying changes over time. The following November salinity readings, expressed as grams per litre (g/L) of salts, have been recorded for Lake Pollard: 1984 = 24 g/L; 2008 = 33 g/L; 2013 = 44 g/L; 2019 = 70 g/L; and 2020 = 80 g/L. For comparison, the salinity of seawater is approximately 35 g/L. Even allowing for some variation between seasons in terms of the lake level, and hence the dilution by fresh water from rainfall, there has clearly been a dramatic increase in the salinity of the water at Lake Pollard. Stonewort is a robust algae that can grow in saline water, but when that water gets to more than twice the salinity of seawater (approximately 70 g/L) its environmental tolerance is exceeded and it can no longer survive. Our measurements show that this tolerance limit has been exceeded in 2019 and 2020 and this is therefore the most likely reason why there is no Stonewort growing on the bed of the lake and therefore no food to attract Black Swans.
Lake Pollard is not the only lake within the Yalgorup Lakes system for which the salinity is increasing: for example the salinity of Lake Clifton has increased over the past four decades to the point that it is now three times the salinity of sea water. PHCC is concerned that this is detrimentally affecting the Threatened Ecological Community of thrombolites that grow in the lake. This water quality monitoring program is helping us to develop a better concept of the processes at play at the Yalgorup lakes, and together with groundwater assessments that PHCC is undertaking at Lake Clifton will help us develop management actions to protect the thrombolites.
The water quality monitoring program is being delivered by PHCC through funding from the Australian Government’s National Landcare Program.