BIOMASS OVERGROWTH IN LAKE WATER CAUSED BY SLOW RELEASE OF DISSOLVED NITROGEN FROM SEDIMENT PORES

The negative environmental impact of a nitrogen trap formed in sediments was studied in Silver Lake of Iowa. The lake cannot support its designated uses as a recreational water body. Nuisance aquatic plants grow in the lake water in the summer time, which lowers the dissolved oxygen content in water (< 2.0 mg/L). The objective of this project was to investigate the slow release of dissolved nitrogen from the sediment pores as a result of chemical changes in water.

Sediment cores were taken from 20 sites along 5 transects in the lake. In the top 5 cm of the sediments, the pore water nitrogen ranges between 1.8 and 733.1 µg of nitrate per gm of sediments. The average concentration is 94 µg/gm. Vertically, nitrate concentrations were measured at 90 μg/gm at 0-10 cm, 95 μg/gm at 10-20 cm, and 19 μg/gm at 20-30 cm. The sharp decline in nitrate below the 20 cm depth in the sediment is attributed to biochemical reduction of nitrate through denitrification in relatively older, much anoxic sediments. The above results indicate that sediments in Silver Lake are heavily contaminated with N trapped in the pore water. The primary sources of N are the surrounding croplands and an active hog lot on the southeastern lakeshore. The average rate of sedimentation in the lake has been 1 cm/year in the last 32 years. Upon sedimentation, the pore water N is slowly released to the lake water, thereby dramatically limiting the lake's capability to process incoming nutrients. The mass distribution of N in the lake was estimated as 3.6 tons (65%) in bottom sediments, 0.17 tons (3%) in suspended particulates, and 1.83 tons (32%) in the dissolved phase. In addition to N, it was also found that the lake has high amount of P in the bottom sediments. Some of the recommendations made through this study include dredging the top 25 cm of lake sediments, applying buffer strips along the lake's northern and eastern shorelines, and reducing the application of N-P-K fertilizers.