Lake Erie. Image Credit: Wikimedia Commons Phosphorus that entered Lake Erie decades ago is resurfacing during storms. In Lake Erie, which belongs to the Great Lakes of North America, the fertiliser phosphorus that entered the lake years ago is resurfacing again. Recent research suggests storms are stirring up phosphorus stored in lakebed sediments for decades.In a study published in Environmental Science & Technology, researchers found that legacy phosphorus may keep contributing to algal blooms despite efforts to curb agricultural runoff. In particular, just one instance of sediment resuspension made the release of bioavailable phosphorus occur at a rate 22 to 256 times higher than that of the aerobic diffusive flux.Old fertiliser is still present below the lakePhosphorus is a critical nutrient that is required for plants to grow. Phosphorus is commonly found in fertilizers used in agriculture. Large quantities of phosphorus entered Lake Erie for decades via the rivers flowing from farmlands, especially from the Maumee River watershed.While policies have decreased the external supply of phosphorus over the years, the phosphorus that entered the lake decades ago has not been lost. On the contrary, it has settled on the lake bottom, forming what scientists refer to as “legacy phosphorus.”As per a new research study conducted by scientists from Cleveland State University and the University of Wisconsin-Milwaukee, available through ACS Publications, the sediments below the lake act more like a nutrient bank that can be accessed any time the lakebed is disturbed.The paper reports that the team combined field measurements with sediment resuspension experiments across Lake Erie, finding that disturbed bottom sediments can quickly elevate both legacy phosphorus and bioavailable phosphorus in the water column. It also notes that the effect is system-wide, with suspended particles and near-bottom waters acting together to mobilise stored phosphorus rather than leaving it locked away.Storms are bringing phosphorus into the lakeRather than slowly leaching nutrients into the water, storms churn the bottom of the lake through physical stirring, bringing sediment into suspension. This is called sediment resuspension, and it releases phosphorus, which is usable by algae.The scientists used samples taken on-site along with satellite data provided by the European Space Agency’s Sentinel-3 satellites to analyse one moderate storm in May 2023. They determined that the storm mobilised approximately 40,314 kg of total phosphorus. Out of that, there were 12,665 kg of bioavailable phosphorus – phosphorus readily used by algae.The scientists estimate that this one event contributed 4.8 per cent of the spring total phosphorus loading target and 7 per cent of the spring soluble reactive phosphorus loading target for the Maumee River basin. These are significant amounts, taking into consideration that they were nutrients that were stored in the lake and not brought into the lake in any other way. 
Lake Erie. Image Credit: Wikimedia Commons
Relevance to harmful algal bloomsLake Erie has seen repeated harmful algal blooms for many years. he bloom usually involves cyanobacteria, also known as blue-green algae, that flourish when phosphorus levels rise. Some blooms can cause toxins to form that harm human health by poisoning water supplies, wildlife, and recreational activities.For decades, phosphorus input into the lake from farms has been the target of scientists who want to reduce this pollutant due to its known role as a key factor in causing eutrophication. However, the recent results seem to reveal the underestimation of internal phosphorus recycling processes. According to the researchers, sediment mixing recycles previously deposited phosphorus, making it available again during storms. It may provide one of the explanations for the occurrence of particularly dangerous blooms despite low levels of phosphorus input compared to expectations.Moreover, the increase in storm intensity as a result of climate change may lead to greater sediment disturbance and thus make internal phosphorus loading more challenging in the future.Satellite technology played a key role in understanding the unseen mechanismPhosphorus loading rates within the western basin of Lake Erie were calculated based on satellite data of the Sentinel-3 Ocean and Land Colour Instrument. Satellite images enabled scientists to detect storm-driven changes in suspended sediment and calculate phosphorus loading across a large area of the lake.Additionally, scientists studied the radioactive isotope beryllium-7. Their findings suggest the surface sediments are repeatedly disturbed, allowing legacy phosphorus to cycle back into the lake. According to the authors, this research presents one of the first calculations of internal phosphorus loading from sediment resuspension in western Lake Erie.Implications of the research findingsAccording to the researchers, nutrient management plans should consider not only the phosphorus going into lakes currently, but also historical phosphorus already present in the sediment beneath the lake.Though limiting agricultural runoff is very important, the research shows that it will take much longer than initially thought to clean up the lakes since lakes still recycle nutrients that have been deposited decades earlier. The authors claim that considering resuspension of sediments when predicting the occurrence of blooms will make the forecasts more accurate and will allow for an evaluation of the success of nutrient reduction programs.Studying legacy pollution alongside storm events is increasingly important for Lake Erie and other freshwater lakes.