Impact of Artificial Aeration on Nutrients in a Small Eutrophic Lake
Balangoda, Anusha Rupika
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The Heinrich- Martin Dam Impoundment (HMDI), located in northcentral LaMoure County, North Dakota, is an important water body for fishing, boating, and other recreational activities. To eliminate the thermal stratification and low dissolved oxygen (DO) zone near the bottom, an artificial aeration system was installed and operated by the North Dakota Game and Fish Department (NDG&F). This study was conducted to investigate whether aeration improved water quality of HMDI and to evaluate aeration as a tool to effectively manage eutrophication in lakes of this type. Field monitoring and laboratory analyses were conducted during three consecutive summers, in 2010, 2011 and 2012, to evaluate the impact of aeration on the spatial and temporal variation of physical, chemical, and biological water quality. Variables monitored included total and dissolved forms of inorganic nitrogen (N) and phosphorus (P), chlorophyll a, turbidity, water temperature and dissolved oxygen (DO). Field sampling was carried out during aerated conditions in 2010 and 2012 and under non-aerated conditions in 2011. The study revealed no significant differences between aeration and non-aeration conditions in soluble reactive phosphorus (SRP), total phosphorus (TP), or total nitrogen (TN) concentrations in the water column. In contrast, mean ammonia-nitrogen concentration at the near-bottom layers during non-aerated conditions decreased significantly under artificial aeration, while mean nitrate and nitrite concentrations increased significantly under aeration. Under aerated conditions, dissolved inorganic nutrients, TN, TP, temperature, and algae were homogenously distributed throughout the water column. Aeration expanded aerobic habitats for fish and distributed bio-available nutrients, stimulating algal growth throughout the water column. These results indicate that the existing aeration system vertically mixed nutrients throughout the water column of the HMDI. Chlorophyll a results showed that aeration distributed algae throughout the water column and circulated available nutrients for their growth. In addition, the results indicated that aeration improved water quality as measured by Secchi depth, turbidity, DO and algal biomass based on recommended levels by NDDoH. In a nitrogen-limited, phosphorus-rich water body, like HMDI, lowering phosphorus (P) load rather than nitrogen (N) load is recommended as a means of reducing algal biomass.