Lakes are one of the most important components of the global carbon cycle and have the capacity to be a sink of carbon as they potentially can store large amounts of carbon in their sediments. Despite this capacity, lakes are mostly supersaturated in carbon dioxide and methane, resulting in the release of these greenhouse gases to the atmosphere.
We hypothesize that high carbon dioxide and methane emissions, de-oxygenation and eutrophication, in freshwater lakes are significantly influenced by groundwater seeping out along channels and banks. This was investigated by conducting two field campaigns at Manly Dam, Manly Vale, NSW in 2019 to collect surface water and groundwater samples in wet and dry conditions. A surface water survey of radon, carbon dioxide, methane and water quality parameters were conducted on each field campaign and grab samples were collected for DIC, DOC, nitrate, ammonia, and phosphate measurements. A range of groundwater samples were also collected and analysed for the same suit of parameters. All samples were also analysed by the size exclusion technique, Liquid Chromatography-Organic Carbon Detection (LC-OCD), for organic matter characterisation.
The results showed that areas of groundwater discharge tended to show elevated levels of nutrients (predominately nitrate and ammonia) and greenhouse gases. This work demonstrated that influence that groundwater discharge had on the carbon and nutrient dynamics, and greenhouse gas emissions. Overall this confirmed our hypothesis that the high carbon dioxide and methane emissions in lakes are significantly influenced by groundwater seeping out along channels and banks.