LOAD CALCULATION OF NUTRIENTS IN CEDAR RIVER WATERSHED IN SUPPORT OF IOWA’S NUTRIENT REDUCTION STRATEGY

Surface water from eighteen (18) sites within the Cedar River watershed in Iowa were sampled for total suspended sediments (TSS), dissolved nitrogen (N) and total phosphorus (TP) from April through October of 2014. The selected sites are located from Charles City all the way to LaPorte City, Iowa. The Cedar River has a total drainage area of 7,815 mi², where 81% of all land use is agriculture. Iowa farm fields are known sources of nutrients that are responsible for developing hypoxia in the Gulf of Mexico. In recent years, the 12 states along the Mississippi River have taken important initiatives against loss of nutrients from their agricultural fields.

Nitrogen loads were high from early to mid-summer due to the rapid conversion of fertilizer nitrogen into nitrate before the crops entered the period of maximum uptake. The levels went up again toward the later part of fall being derived from the residual organic nitrogen converting into nitrate through the process of nitrification. This was also because nitrogen uptake by crops was considerably reduced by mid to late August. The average nitrogen (as nitrate-N) loads in the watershed ranged from 8.4 tons/day in site 17 (Wolf Creek) to 156.6 tons/day in site 13 (Cedar River at Waterloo). The average load over all sites was 45 tons/day. The total loss of nitrogen for the season was calculated as 9450 tons. Loads of TP directly correlate with the transport of TSS in the watershed. Phosphorus is heavily adsorbed to soil particles and is transported to the streams with eroded soils. The TP loads ranged from 0.12 tons/day in site 17 (Wolf Creek) to 3.12 tons/day in site 12 (Cedar River at Cedar Falls). The average load over all sites was 1.05 tons/day. The total loss of phosphorus for the season was calculated as 221 tons. Taking Cedar River as an example, it is clear that rivers in eastern Iowa contribute high amounts of nutrients to the formation of hypoxia in the Gulf of Mexico.