Mapping of accumulated nitrogen in the sediment pore water of a eutrophic lake in Iowa, USA

A large pool of nitrogen in the sediment pore fluid of a eutrophic lake in Iowa, USA, was mapped in this study. Previously, the lake had supported fishing and boating, but today it no longer supports its designated uses as a recreational water body. In the top 5 cm of the lake bottom, the pore water nitrogen ranges between 3.1 and 1,250 μg/cm³ of sediments, with an average of 160.3 μg/cm³. Vertically, nitrate concentrations were measured as 153 μg/cm³ at 0–10 cm, 162 μg/cm³ at 10–20 cm, and 32 μg/cm³ at 20–30 cm. Nitrate mass distribution was quantified as 3.67 × 10³ kg (65%) in the bottom sediments, 172 kg (3%) in suspended particulates, and 1.83 × 10³ kg (32%) in the dissolved phase. Soil runoff nutrients arrive at the lake from the heavily fertilized lands in the watershed. Upon sedimentation, a large mass of nitrogen desorbs from mineral particles to the relatively immobile pore fluid. Under favorable conditions, this nitrogen diffuses back into the water column, thereby dramatically limiting the lake’s capability to process incoming nutrients from farmlands. Consequently, a condition of oxygen deficiency disrupts the post-season biological activities in the lake.