Lake Burley Griffin is an artificial urban lake located near the geographic centre of Canberra, Australia. During the warmer months the lake is often closed because of cyanobacteria blooms which are linked to nutrient concentration sin the lake. While previous studies have investigated nutrient dynamics in the lake, these are only partially relevant because of major changes to urban catchments and infrastructure have altered the nutrient loading to the lake. An assessment of nutrient dynamics is required to identify in-lake nutrient loading and associated factors.
Water samples were collected weekly during warmer months (October-April) and monthly during colder months (May-September) at different depths across five sites over a year. Samples were analysed for dissolved nutrients (i.e., ammonia, filterable reactive phosphorus, and nitrate/nitrite), total nutrients (i.e., total nitrogen and total phosphorus), and total organic carbon using standard analytical methods. At all in-lake sites, water physicochemical parameters were measured at 1 m depth intervals during each sampling visit to relate with nutrient data. Loggers were deployed to continuously measure temperature and dissolved oxygen, to identify lake transition patterns. Storm sampling was conducted across five inflows and the outflow during large rain events (>10 mm/day).
Lake began stratifying in mid-October when the daily average water temperature reached 20°C. Stratification persisted throughout summer until mid-autumn, with occasional partial or complete mixing events. These mixing events were driven by rainfall, inflow discharge, and/or sudden drops in air temperature. However, some large rain events did not always break the thermal profile, likely due to warm water delivered by the inflows. In-lake total nutrient concentrations showed a strong association with rainfall, while some higher concentrations were linked to small rain events following dry periods. Inflow nutrient concentrations, particularly phosphorus during event flows, exceeded the threshold levels for algal growth. When the lake was stratified, high dissolved nutrient concentrations in the hypolimnion were indicative of internal loading. This was evident for ammonia, where increases in ammonia concentration were related to decreases in dissolved oxygen.
Results from ongoing monitoring indicate that in-lake nutrient dynamics are driven by event flows and internal loading. Nutrient enrichment could exacerbate algal blooms, given the lake’s average residence time of over two months. A detailed assessment of nutrient dynamics and associated physical parameters will provide a comprehensive understanding of the lake, aiding in the prediction of nutrient variability under future climate-driven rainfall changes.