A comparative limnological study was conducted at three east central Illinois farm ponds between March 15 and September 18, 1970, to determine the incidence of nitrogen and phosphorus in the ponds resulting from agricultural sources. All ponds had corn, soybean, winter wheat cropping systems on their watersheds. Corn accounted for the greatest fertilizer application. One pond had significant numbers of livestock on its watershed that contributed high amounts of nitrogen and phosphorus to its waters. Limnological parameters measured at the surface, mid depth, and bottom of a ten foot water column in each pond were water temperature, dissolved oxygen, carbon dioxide, pH, alkalinity, total hardness, nitrate, nitrite, and ortho-phosphate. Seasonal nitrate levels were found with maximum concentrations during the spring, but throughout the study period peak levels were associated with rainfall and runoff. Very high levels of ortho-phosphate were found in the pond with livestock on its watershed. The oxygen regime of the ponds was greatly influenced by thermal stratification and the phytoplankton and aquatic plants growing in the ponds. Severe oxygen depletion and one fish kill resulted from algal die-offs during the summer. Several limnological factors were found to interact in the release of ortho-phosphate from deposited silt and organic sediment on the bottoms. Nitrate absorbed by the biota was later released at the bottom as ammonia
